<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE article PUBLIC "-//NLM//DTD Journal Publishing DTD v2.3 20070202//EN" "journalpublishing.dtd">
<article xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" article-type="research-article" dtd-version="2.3" xml:lang="EN">
<front>
<journal-meta>
<journal-id journal-id-type="publisher-id">Front. Mar. Sci.</journal-id>
<journal-title>Frontiers in Marine Science</journal-title>
<abbrev-journal-title abbrev-type="pubmed">Front. Mar. Sci.</abbrev-journal-title>
<issn pub-type="epub">2296-7745</issn>
<publisher>
<publisher-name>Frontiers Media S.A.</publisher-name>
</publisher>
</journal-meta>
<article-meta>
<article-id pub-id-type="doi">10.3389/fmars.2023.1107530</article-id>
<article-categories>
<subj-group subj-group-type="heading">
<subject>Marine Science</subject>
<subj-group>
<subject>Original Research</subject>
</subj-group>
</subj-group>
</article-categories>
<title-group>
<article-title>Controls on shallow gas distribution, migration, and associated geohazards in the Yangtze subaqueous delta and the Hangzhou Bay</article-title>
</title-group>
<contrib-group>
<contrib contrib-type="author">
<name>
<surname>Song</surname>
<given-names>Lei</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<uri xlink:href="https://loop.frontiersin.org/people/2113859"/>
</contrib>
<contrib contrib-type="author" corresp="yes">
<name>
<surname>Fan</surname>
<given-names>Daidu</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
<xref ref-type="author-notes" rid="fn001">
<sup>*</sup>
</xref>
<uri xlink:href="https://loop.frontiersin.org/people/1415204"/>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Su</surname>
<given-names>Jianfeng</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<xref ref-type="aff" rid="aff3">
<sup>3</sup>
</xref>
<uri xlink:href="https://loop.frontiersin.org/people/2159474"/>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Guo</surname>
<given-names>Xingjie</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<xref ref-type="aff" rid="aff4">
<sup>4</sup>
</xref>
</contrib>
</contrib-group>
<aff id="aff1">
<sup>1</sup>
<institution>State Key Laboratory of Marine Geology, Tongji University</institution>, <addr-line>Shanghai</addr-line>, <country>China</country>
</aff>
<aff id="aff2">
<sup>2</sup>
<institution>Laboratory of Marine Geology, Qingdao National Laboratory for Marine Science and Technology</institution>, <addr-line>Qingdao</addr-line>, <country>China</country>
</aff>
<aff id="aff3">
<sup>3</sup>
<institution>Zhoushan Field Scientific Observation and Research Station for Marine Geo-hazards, China Geological Survey</institution>, <addr-line>Zhoushan</addr-line>, <country>China</country>
</aff>
<aff id="aff4">
<sup>4</sup>
<institution>Shanghai Institute of Geological Survey</institution>, <addr-line>Shanghai</addr-line>, <country>China</country>
</aff>
<author-notes>
<fn fn-type="edited-by">
<p>Edited by: Tim Rixen, Leibniz Centre for Tropical Marine Research (LG), Germany</p>
</fn>
<fn fn-type="edited-by">
<p>Reviewed by: Peter Feldens, Leibniz Institute for Baltic Sea Research (LG), Germany; Qiliang Sun, China University of Geosciences Wuhan, China; Jian Hua Gao, Nanjing University, China</p>
</fn>
<fn fn-type="corresp" id="fn001">
<p>*Correspondence: Daidu Fan, <email xlink:href="mailto:ddfan@tongji.edu.cn">ddfan@tongji.edu.cn</email>
</p>
</fn>
<fn fn-type="other" id="fn002">
<p>This article was submitted to Coastal Ocean Processes, a section of the journal Frontiers in Marine Science</p>
</fn>
</author-notes>
<pub-date pub-type="epub">
<day>20</day>
<month>02</month>
<year>2023</year>
</pub-date>
<pub-date pub-type="collection">
<year>2023</year>
</pub-date>
<volume>10</volume>
<elocation-id>1107530</elocation-id>
<history>
<date date-type="received">
<day>25</day>
<month>11</month>
<year>2022</year>
</date>
<date date-type="accepted">
<day>02</day>
<month>02</month>
<year>2023</year>
</date>
</history>
<permissions>
<copyright-statement>Copyright &#xa9; 2023 Song, Fan, Su and Guo</copyright-statement>
<copyright-year>2023</copyright-year>
<copyright-holder>Song, Fan, Su and Guo</copyright-holder>
<license xlink:href="http://creativecommons.org/licenses/by/4.0/">
<p>This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.</p>
</license>
</permissions>
<abstract>
<p>Shallow gas is generally extensively distributed in the Holocene muddy sediments and gas seepage has been increasingly reported to induce geohazards in coastal seas, but controls on gas distribution and migration remain elusive. This study explores gas distribution and migration in the Yangtze subaqueous delta and the Hangzhou Bay using high-resolution acoustic profiles and core data. Shallow gas is widely detected by the common presence of acoustic anomalous reflections including enhanced reflection, gas chimney, bright spot, acoustic blanking, and acoustic turbidity. The gas front depth is generally less than 17.5&#xa0;m, and is meanly shallower in the Hangzhou Bay than in the Yangtze subaqueous delta because of relatively shallower water depth and coarser Holocene sediments in the Hangzhou Bay. Shallow gas is inferred to be a biogenic product, and its distribution is highly contingent on the Holocene stratal thickness and water depth. Active gas migration and seepages are evident, and recently increasing occurrences of gas seepage can be ascribed to global warming and seabed erosion due to sediment deficit. The findings warn us to pay more attention to the positive feedback loops of gas seepages with global warming and seabed erosion for the associated geohazard prediction and reduction, typically in the highly developed coastal regions.</p>
</abstract>
<kwd-group>
<kwd>shallow gas</kwd>
<kwd>acoustic reflection</kwd>
<kwd>gas distribution</kwd>
<kwd>seabed instability</kwd>
<kwd>Yangtze Delta</kwd>
</kwd-group>
<counts>
<fig-count count="12"/>
<table-count count="2"/>
<equation-count count="0"/>
<ref-count count="85"/>
<page-count count="15"/>
<word-count count="7255"/>
</counts>
</article-meta>
</front>
<body>
<sec id="s1" sec-type="intro">
<label>1</label>
<title>Introduction</title>    <p>Shallow gas usually refers to the gas accumulated in the sub-bottom sediments within 1,000 m of depth (<xref ref-type="bibr" rid="B15">Fleischer et&#xa0;al., 2001</xref>). According to gas formation mechanisms, shallow gas can be divided into biogenic gas and thermogenic gas (<xref ref-type="bibr" rid="B16">Floodgate and Judd, 1992</xref>; <xref ref-type="bibr" rid="B78">Ye et&#xa0;al., 2003</xref>). Shallow gas is usually a biogenic product in shallow water sediments (<xref ref-type="bibr" rid="B33">Li et&#xa0;al., 2010</xref>), of which methane (CH<sub>4</sub>) is the primary component (<xref ref-type="bibr" rid="B21">Hovland and Judd, 1988</xref>; <xref ref-type="bibr" rid="B23">Hu et&#xa0;al., 2012</xref>). The gas can migrate upward from the host sediments to produce seepages, potentially forming pockmarks on the seafloor and gas plumes in the water column (<xref ref-type="bibr" rid="B4">Chen et&#xa0;al., 2017</xref>; <xref ref-type="bibr" rid="B2">Chen et&#xa0;al., 2020</xref>). If the gas escapes further from seawater into the atmosphere, it should accelerate global warming significantly because CH<sub>4</sub> is a greenhouse gas with 20&#x2013;40 times higher radiative efficiency than CO<sub>2</sub> (<xref ref-type="bibr" rid="B31">Letcher, 2019</xref>; <xref ref-type="bibr" rid="B26">IPCC, 2021</xref>). In the past few decades, the content of CH<sub>4</sub> in the atmosphere has been increasing at a rate of approximately 0.5%&#x2013;1% per year (<xref ref-type="bibr" rid="B47">Rasmussen and Khalil, 1986</xref>; <xref ref-type="bibr" rid="B26">IPCC, 2021</xref>). Annual global CH<sub>4</sub> emission from natural geological sources is estimated at 18&#x2013;63 Mt, and marine gas leakage contributes 5&#x2013;10 Mt annually (<xref ref-type="bibr" rid="B10">Etiope and Schwietzke, 2019</xref>; <xref ref-type="bibr" rid="B9">Etiope et&#xa0;al., 2019</xref>). Approximately 20% of total CH<sub>4</sub> emission to the atmosphere is inferred to be sourced from shallow coastal seas, consequently attracting increased research concerns (<xref ref-type="bibr" rid="B15">Fleischer et&#xa0;al., 2001</xref>; <xref ref-type="bibr" rid="B27">Ja&#x15b;niewicz et&#xa0;al., 2019</xref>).</p>
<p>Seismic and acoustic explorations have been widely used for investigating gas-related anomalous reflections and mapping gas distribution in sub-bottom sediments (<xref ref-type="bibr" rid="B78">Ye et&#xa0;al., 2003</xref>; <xref ref-type="bibr" rid="B23">Hu et&#xa0;al., 2012</xref>; <xref ref-type="bibr" rid="B7">Cukur et&#xa0;al., 2013</xref>; <xref ref-type="bibr" rid="B50">Schneider von Deimling et&#xa0;al., 2013</xref>: <xref ref-type="bibr" rid="B22">Hu et&#xa0;al., 2016</xref>; <xref ref-type="bibr" rid="B77">Yang et&#xa0;al., 2019</xref>). Because gas-charged sediments can effectively absorb and scatter the energy of sound waves, which rapidly attenuate inner gas-charged sediments along the vertical direction (<xref ref-type="bibr" rid="B21">Hovland and Judd, 1988</xref>; <xref ref-type="bibr" rid="B78">Ye et&#xa0;al., 2003</xref>; <xref ref-type="bibr" rid="B5">Coughlan et&#xa0;al., 2021</xref>; <xref ref-type="bibr" rid="B59">Toker and Tur, 2021</xref>; <xref ref-type="bibr" rid="B74">Yang et&#xa0;al., 2022b</xref>), stratal structures usually display acoustic anomalous reflections including acoustic turbidity, acoustic blanking, enhanced reflection, bright spot, gas chimney, and pockmark (<xref ref-type="bibr" rid="B78">Ye et&#xa0;al., 2003</xref>; <xref ref-type="bibr" rid="B61">Visnovitz et&#xa0;al., 2015</xref>; <xref ref-type="bibr" rid="B5">Coughlan et&#xa0;al., 2021</xref>; <xref ref-type="bibr" rid="B59">Toker and Tur, 2021</xref>). Shallow gas has been surveyed to be widely distributed in coastal zones, such as the Gulf of Mexico, the Baltic Sea, and the East China Sea (<xref ref-type="bibr" rid="B21">Hovland and Judd, 1988</xref>; <xref ref-type="bibr" rid="B84">Zhang et&#xa0;al., 2004</xref>; <xref ref-type="bibr" rid="B85">Zhang et&#xa0;al., 2008</xref>; <xref ref-type="bibr" rid="B37">Lin et&#xa0;al., 2015</xref>; <xref ref-type="bibr" rid="B39">Liu et&#xa0;al., 2019</xref>; <xref ref-type="bibr" rid="B2">Chen et&#xa0;al., 2020</xref>). Abundant organic matter and suitable environmental conditions are conceivably vital for microbial growth to generate biogenic gas (<xref ref-type="bibr" rid="B37">Lin et&#xa0;al., 2015</xref>; <xref ref-type="bibr" rid="B14">Feng, 2017</xref>). Shallow biogenic gas generally appears when the Holocene sediments reach a certain thickness (<xref ref-type="bibr" rid="B18">Garc&#xed;a-Garc&#xed;a et&#xa0;al., 2007</xref>; <xref ref-type="bibr" rid="B17">Flury et&#xa0;al., 2016</xref>; <xref ref-type="bibr" rid="B2">Chen et&#xa0;al., 2020</xref>). However, most of the CH<sub>4</sub> produced in shallow sediments can be consumed by anaerobic oxidation of methane (AOM) when CH<sub>4</sub> diffuses upwards (<xref ref-type="bibr" rid="B42">Mogoll&#xf3;n et&#xa0;al., 2012</xref>; <xref ref-type="bibr" rid="B43">Mogoll&#xf3;n et&#xa0;al., 2013</xref>). It is generally believed that sulfate is the most important electron acceptor in AOM, and the zone where sulfate reduction and AOM occur most strongly is called the sulfate-methane transition zone (SMTZ) (<xref ref-type="bibr" rid="B43">Mogoll&#xf3;n et&#xa0;al., 2013</xref>; <xref ref-type="bibr" rid="B17">Flury et&#xa0;al., 2016</xref>). CH<sub>4</sub> produced in organic-rich sediments below the sulfate reduction zone may cause the dissolved CH<sub>4</sub> to become oversaturated in pore water, consequently forming gas bubbles that accumulate to produce shallow gas in the sediments (<xref ref-type="bibr" rid="B15">Fleischer et&#xa0;al., 2001</xref>).</p>
<p>Shallow gas has been reported to be widely distributed in the post-LGM (last glacial maximum) strata in the Yangtze Delta and the Hangzhou Bay (<xref ref-type="bibr" rid="B34">Li et&#xa0;al., 2008</xref>; <xref ref-type="bibr" rid="B22">Hu et&#xa0;al., 2016</xref>; <xref ref-type="bibr" rid="B72">Xu et&#xa0;al., 2017</xref>; <xref ref-type="bibr" rid="B64">Wang et&#xa0;al., 2018</xref>). Thick muddy strata are indicated to be rich in organic matter and formed in an anoxic setting, favoring biogenic gas generation (<xref ref-type="bibr" rid="B37">Lin et&#xa0;al., 2015</xref>; <xref ref-type="bibr" rid="B80">Zhang and Lin, 2017</xref>). The top border of shallow gas is named the gas front, and its depth generally varies in a wide range from a few meters to tens of meters below the seafloor (<xref ref-type="bibr" rid="B78">Ye et&#xa0;al., 2003</xref>; <xref ref-type="bibr" rid="B22">Hu et&#xa0;al., 2016</xref>; <xref ref-type="bibr" rid="B64">Wang et&#xa0;al., 2018</xref>). Though a few studies have been carried out for acoustic identification and morphological characteristics classification of shallow gas in the Yangtze subaqueous delta and the Hangzhou Bay (<xref ref-type="bibr" rid="B22">Hu et&#xa0;al., 2016</xref>; <xref ref-type="bibr" rid="B64">Wang et&#xa0;al., 2018</xref>), quantitative analyses of gas distribution and front depth have been little reported. <xref ref-type="bibr" rid="B2">Chen et&#xa0;al. (2020)</xref> proposed that the thickness of the Holocene mud wedge is a key factor to determine the front depth of shallow gas, but its internal logic and other factors need additional exploration.</p>
<p>The gas-charged sediments have long been considered marine geologic hazards in terms of several aspects. The shear strength of sub-bottom sediments will be reduced obviously after filling with dissolved CH<sub>4</sub>, and they are more susceptible to liquefaction under the influence of currents and waves (<xref ref-type="bibr" rid="B51">Sills and Wheeler, 1992</xref>; <xref ref-type="bibr" rid="B62">Wang and Liu, 2016</xref>; <xref ref-type="bibr" rid="B52">Song et&#xa0;al., 2021</xref>). Typically, sediment liquefaction likely occurs during extreme events such as typhoons, producing the collapse of marine constructions (<xref ref-type="bibr" rid="B57">Sumer et&#xa0;al., 2006</xref>; <xref ref-type="bibr" rid="B63">Wang et&#xa0;al., 2016</xref>; <xref ref-type="bibr" rid="B62">Wang and Liu, 2016</xref>; <xref ref-type="bibr" rid="B66">Wang et&#xa0;al., 2019</xref>; <xref ref-type="bibr" rid="B65">Wang et&#xa0;al., 2020</xref>). Meanwhile, sediment deficit due to dam constructions and other anthropogenic activities has caused severe erosion in the Yangtze subaqueous delta and the Hangzhou Bay (<xref ref-type="bibr" rid="B70">Xie et&#xa0;al., 2013</xref>; <xref ref-type="bibr" rid="B19">Guo et&#xa0;al., 2021</xref>), which has been accused to activate gas seepages because of thinning low permeability layer above gas-charged sediments (<xref ref-type="bibr" rid="B2">Chen et&#xa0;al., 2020</xref>). Considering the shallow burial depth of shallow gas in the study area, the effect of seabed erosion on gas preservation needs further investigation.</p>
<p>Therefore, this study is dedicated to identifying shallow gas-related anomalous acoustic reflections and mapping the gas front depth in the Yangtze subaqueous delta and the northern Hangzhou Bay, where the sedimentation regime has recently shifted from rapid deposition into severe erosion due to sediment deficit (<xref ref-type="bibr" rid="B70">Xie et&#xa0;al., 2013</xref>; <xref ref-type="bibr" rid="B19">Guo et&#xa0;al., 2021</xref>). Then, acoustic and core data are combined to probe the important influencing factors on gas distribution patterns. Ultimately, potential geological hazards related to recent shallow gas activities will be discussed.</p>
</sec>
<sec id="s2">
<label>2</label>
<title>Study area</title>
<p>The study area consists of the Yangtze subaqueous delta and the Hangzhou Bay (<xref ref-type="fig" rid="f1">
<bold>Figure&#xa0;1</bold>
</xref>). The Yangtze Estuary is influenced by semidiurnal tides, and the average tidal range is 2.6&#xa0;m with a maximum tidal range of 4.6&#xa0;m (<xref ref-type="bibr" rid="B11">Fan et&#xa0;al., 2004</xref>; <xref ref-type="bibr" rid="B12">Fan et&#xa0;al., 2017</xref>). The estuary is influenced more frequently by wind waves and less by swells. The Hangzhou Bay features a funnel shape with a rapid landward increase in tidal ranges (<xref ref-type="bibr" rid="B44">Ni et&#xa0;al., 2003</xref>; <xref ref-type="bibr" rid="B70">Xie et&#xa0;al., 2013</xref>; <xref ref-type="bibr" rid="B71">Xie et&#xa0;al., 2017</xref>). Due to the effect of the Coriolis force, the high tide level on the north shore is much higher than that on the south shore, while the low tide level on the south shore is higher than that on the north shore, resulting in a greater tidal difference between the opposite shores of the Hangzhou Bay (<xref ref-type="bibr" rid="B76">Yang et&#xa0;al., 2011</xref>; <xref ref-type="bibr" rid="B40">Luan et&#xa0;al., 2021</xref>). The bay is mainly influenced by wind waves, and large waves are induced by typhoons in summer and cold fronts in winter (<xref ref-type="bibr" rid="B38">Liu, 2019</xref>).</p>
<fig id="f1" position="float">
<label>Figure&#xa0;1</label>
<caption>
<p>Geographical locations of the Yangtze Delta and the Hangzhou Bay <bold>(A)</bold>, and the distribution of shallow acoustic survey profiles and cores <bold>(B)</bold>. Light green dashed lines with numbers denote isobaths in meters.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-10-1107530-g001.tif"/>
</fig>
<p>Deep incised valleys of a few tens of meters were formed at the present Yangtze Delta and the Hangzhou Bay during the LGM when the sea level dropped &gt;130&#xa0;m below the present sea level (<xref ref-type="bibr" rid="B34">Li et&#xa0;al., 2008</xref>; <xref ref-type="bibr" rid="B35">Li et&#xa0;al., 2014</xref>; <xref ref-type="bibr" rid="B56">Su et&#xa0;al., 2020</xref>). These paleo-incised valleys experienced rapid filling after the post-LGM transgression (<xref ref-type="bibr" rid="B34">Li et&#xa0;al., 2008</xref>; <xref ref-type="bibr" rid="B81">Zhang et&#xa0;al., 2013a</xref>; <xref ref-type="bibr" rid="B37">Lin et&#xa0;al., 2015</xref>; <xref ref-type="bibr" rid="B80">Zhang and Lin, 2017</xref>; <xref ref-type="bibr" rid="B56">Su et&#xa0;al., 2020</xref>). Transgressive surface (TS) is considered as the bottom interface during initial transgression, and maximum flooding surface (MFS) was formed during maximum transgression, occurring at 7.5 ka BP in the present Yangtze Delta area (<xref ref-type="bibr" rid="B35">Li et&#xa0;al., 2014</xref>; <xref ref-type="bibr" rid="B55">Su et&#xa0;al., 2017</xref>). The transgressive system tract (TST) represents deposition occurring between TS and MFS, and the highstand system tract (HST) is formed above MFS. The HST is further subdivided into early highstand system tract (E-HST) and late highstand system tract (L-HST) using the isochron of 2 kyr BP. Thick Holocene fine-grained sediments bear high TOC content of 0.54%&#x2013;1.13% (<xref ref-type="bibr" rid="B67">Wang et&#xa0;al., 2012</xref>; <xref ref-type="bibr" rid="B79">Zhan et&#xa0;al., 2012</xref>), favoring shallow biogenic gas generation (<xref ref-type="bibr" rid="B36">Lin et&#xa0;al., 2010</xref>; <xref ref-type="bibr" rid="B81">Zhang et&#xa0;al., 2013a</xref>; <xref ref-type="bibr" rid="B37">Lin et&#xa0;al., 2015</xref>).</p>
<p>Annual average runoff of the Yangtze River at the Datong gauging station varied insignificantly in the past 60 years (<xref ref-type="bibr" rid="B40">Luan et&#xa0;al., 2021</xref>; Yang et&#xa0;al., 2022). However, the suspended sediment discharge has decreased dramatically by approximately 70% after the operation of the Three Gorges Dam (TGD) (<xref ref-type="bibr" rid="B19">Guo et&#xa0;al., 2021</xref>). Accordingly, the Yangtze subaqueous delta has shifted from rapid siltation to remarkable erosion, and erosion has recently expanded toward the Hangzhou Bay, which formerly receives a huge sediment supply from the Yangtze Estuary through strong tides (<xref ref-type="bibr" rid="B70">Xie et&#xa0;al., 2013</xref>; <xref ref-type="bibr" rid="B71">Xie et&#xa0;al., 2017</xref>).</p>
</sec>
<sec id="s3">
<label>3</label>
<title>Data and methods</title>
<p>Two acoustic surveys were carried out in July 2011 and August 2012, respectively. Acoustic profiles in the Yangtze subaqueous delta were collected in 2011 by the EdgeTech 3200XS Sub-Bottom Profiling System with the frequency 0.5&#x2013;8 kHz at a ship speed of 4&#x2013;5 kn (<xref ref-type="fig" rid="f1">
<bold>Figure&#xa0;1B</bold>
</xref>). It can penetrate 50&#xa0;m or more below the seafloor, with a good vertical resolution of 0.12&#xa0;m. In 2012, the GEO-SPARK2000 Electric Spark System was employed to collect acoustic profiles in the Hangzhou Bay with the frequency 8 kHz at a ship speed of 5 kn, fired at 400-ms intervals with the power of 2,000 J. Theoretically, it can detect the stratigraphic information range of 60 to 70&#xa0;m below the seafloor. Towing observation operation and differential global positioning system were adopted in two surveys.</p>
<p>SonarWiz 5 software was applied to process all acoustic data (<ext-link ext-link-type="uri" xlink:href="https://chesapeaketech.com/">https://chesapeaketech.com/</ext-link>). After gain adjustment, automatic TVG compensation, and filtering technology, all profiles achieve the most effective visualization effect, typically reducing the banding phenomena due to discontinuities at the swath edges. After the interpretation of anomalous acoustic reflections, the gas front was identified and marked in each profile. To calculate the sediment thickness conveniently, the acoustic wave propagation velocity in sediments is set to 1,500 m/s (<xref ref-type="bibr" rid="B2">Chen et&#xa0;al., 2020</xref>). The distribution of shallow gas front depths was mapped using DIVA gridding interpolation in professional Ocean Data View software. DIVA, short for data-interpolating variational analysis, is a software tool dedicated to the spatial interpolation of <italic>in situ</italic> oceanographic data on a regular grid in an optimal way (<ext-link ext-link-type="uri" xlink:href="https://www.seadatanet.org/Software/DIVA">https://www.seadatanet.org/Software/DIVA</ext-link>) (<xref ref-type="bibr" rid="B1">Beckers et&#xa0;al., 2014</xref>).</p>
<p>Stratal comparison in the study area was conducted using data from eight cores obtained from published papers (<xref ref-type="table" rid="T1">
<bold>Table&#xa0;1</bold>
</xref>). Published <sup>14</sup>C ages were converted into calendar years BP using the Calib Rev. 8.2 program (<ext-link ext-link-type="uri" xlink:href="http://calib.qub.ac.uk/calib/calib.html">http://calib.qub.ac.uk/calib/calib.html</ext-link>), and marine biological samples were calibrated with the Marine20 curve and plant debris or organic-rich sediments with the IntCal20 curve (<xref ref-type="bibr" rid="B54">Stuiver and Reimer, 1993</xref>; <xref ref-type="bibr" rid="B56">Su et&#xa0;al., 2020</xref>). Meanwhile, the marine radiocarbon reservoir effect (&#x394;<italic>R</italic>) of &#x2212;96 &#xb1; 60 yr was adopted here (<xref ref-type="bibr" rid="B53">Southon et&#xa0;al., 2002</xref>; <xref ref-type="bibr" rid="B56">Su et&#xa0;al., 2020</xref>). After that, the calibrated <sup>14</sup>C ages under 2&#x3c3; confidence intervals were selected and took their median ages as the final calibrated ages (<xref ref-type="table" rid="T2">
<bold>Table&#xa0;2</bold>
</xref>).</p>
<table-wrap id="T1" position="float">
<label>Table&#xa0;1</label>
<caption>
<p>Summary information of cores used in this study (see <xref ref-type="fig" rid="f1">
<bold>Figure&#xa0;1B</bold>
</xref> for core locations).</p>
</caption>
<table frame="hsides">
<thead>
<tr>
<th valign="middle" align="center">Core</th>
<th valign="middle" align="center">Water depth (m)</th>
<th valign="middle" align="center">Length (m)</th>
<th valign="middle" align="center">Lon. (E)</th>
<th valign="middle" align="center">Lat. (N)</th>
<th valign="middle" align="center">Source</th>
</tr>
</thead>
<tbody>
<tr>
<td valign="middle" align="center">HZK11</td>
<td valign="middle" align="center">&#x2212;10.8</td>
<td valign="middle" align="center">60.8</td>
<td valign="middle" align="center">122&#xb0;04.586&#x2019;</td>
<td valign="middle" align="center">30&#xb0;41.942&#x2019;</td>
<td valign="middle" align="center">
<xref ref-type="bibr" rid="B64">Wang et&#xa0;al., 2018</xref>
</td>
</tr>
<tr>
<td valign="middle" align="center">HZK12</td>
<td valign="middle" align="center">&#x2212;10.8</td>
<td valign="middle" align="center">96.9</td>
<td valign="middle" align="center">121&#xb0;41.300&#x2019;</td>
<td valign="middle" align="center">30&#xb0;46.198&#x2019;</td>
<td valign="middle" align="center">
<xref ref-type="bibr" rid="B64">Wang et&#xa0;al., 2018</xref>
</td>
</tr>
<tr>
<td valign="middle" align="center">HZK15</td>
<td valign="middle" align="center">&#x2212;13.0</td>
<td valign="middle" align="center">53.6</td>
<td valign="middle" align="center">121&#xb0;36.327&#x2019;</td>
<td valign="middle" align="center">30&#xb0;35.079&#x2019;</td>
<td valign="middle" align="center">
<xref ref-type="bibr" rid="B64">Wang et&#xa0;al., 2018</xref>
</td>
</tr>
<tr>
<td valign="middle" align="center">HZK16</td>
<td valign="middle" align="center">&#x2212;10.0</td>
<td valign="middle" align="center">123.0</td>
<td valign="middle" align="center">121&#xb0;55.132&#x2019;</td>
<td valign="middle" align="center">30&#xb0;34.706&#x2019;</td>
<td valign="middle" align="center">
<xref ref-type="bibr" rid="B64">Wang et&#xa0;al., 2018</xref>
</td>
</tr>
<tr>
<td valign="middle" align="center">YD0901</td>
<td valign="middle" align="center">&#x2212;21.0</td>
<td valign="middle" align="center">65.2</td>
<td valign="middle" align="center">122&#xb0;30.011&#x2019;</td>
<td valign="middle" align="center">31&#xb0;11.029&#x2019;</td>
<td valign="middle" align="center">
<xref ref-type="bibr" rid="B55">Su et&#xa0;al., 2017</xref>
</td>
</tr>
<tr>
<td valign="middle" align="center">YD0902</td>
<td valign="middle" align="center">&#x2212;23.0</td>
<td valign="middle" align="center">69.1</td>
<td valign="middle" align="center">122&#xb0;44.348&#x2019;</td>
<td valign="middle" align="center">30&#xb0;56.999&#x2019;</td>
<td valign="middle" align="center">
<xref ref-type="bibr" rid="B56">Su et&#xa0;al., 2020</xref>
</td>
</tr>
<tr>
<td valign="middle" align="center">YD0903</td>
<td valign="middle" align="center">&#x2212;36.0</td>
<td valign="middle" align="center">60.2</td>
<td valign="middle" align="center">122&#xb0;54.550&#x2019;</td>
<td valign="middle" align="center">30&#xb0;53.905&#x2019;</td>
<td valign="middle" align="center">
<xref ref-type="bibr" rid="B55">Su et&#xa0;al., 2017</xref>
</td>
</tr>
</tbody>
</table>
</table-wrap>
<table-wrap id="T2" position="float">
<label>Table&#xa0;2</label>
<caption>
<p>
<sup>14</sup>C age data selected from seven cores in the Yangtze subaqueous delta and the Hangzhou Bay (see <xref ref-type="fig" rid="f1">
<bold>Figure&#xa0;1B</bold>
</xref> for core locations).</p>
</caption>
<table frame="hsides">
<thead>
<tr>
<th valign="middle" rowspan="2" align="center">Core ID</th>
<th valign="middle" rowspan="2" align="center">Depth (m)</th>
<th valign="middle" rowspan="2" align="center">Dating material</th>
<th valign="middle" rowspan="2" align="center">
<sup>14</sup>C age (yr BP)</th>
<th valign="top" colspan="3" align="center">Calibrated age (cal yr BP)</th>
<th valign="middle" rowspan="2" align="center">Source</th>
</tr>
<tr>
<th valign="top" align="center">2&#x3c3; range</th>
<th valign="top" align="center">Prob.</th>
<th valign="top" align="center">Median</th>
</tr>
</thead>
<tbody>
<tr>
<td valign="middle" align="center">HZK11</td>
<td valign="middle" align="center">2.22</td>
<td valign="middle" align="center">Bivalve shell</td>
<td valign="middle" align="center">950 &#xb1; 30</td>
<td valign="middle" align="center">313&#x2013;634</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">474</td>
<td valign="middle" align="center">
<xref ref-type="bibr" rid="B64">Wang et&#xa0;al., 2018</xref>
</td>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">6.5</td>
<td valign="middle" align="center">Gastropod shell</td>
<td valign="middle" align="center">1,510 &#xb1; 30</td>
<td valign="middle" align="center">793&#x2013;1,185</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">989</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">9.6</td>
<td valign="middle" align="center">Gastropod shell</td>
<td valign="middle" align="center">2,260 &#xb1; 30</td>
<td valign="middle" align="center">1,588&#x2013;2,016</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">1,802</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">11.1</td>
<td valign="middle" align="center">Plant material</td>
<td valign="middle" align="center">8,030 &#xb1; 40</td>
<td valign="middle" align="center">8,750&#x2013;9,020</td>
<td valign="middle" align="center">0.98</td>
<td valign="middle" align="center">8,885</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">13.03</td>
<td valign="middle" align="center">Plant material</td>
<td valign="middle" align="center">8,170 &#xb1; 30</td>
<td valign="middle" align="center">9,010&#x2013;9,149</td>
<td valign="middle" align="center">0.71</td>
<td valign="middle" align="center">9,080</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">14.58</td>
<td valign="middle" align="center">Plant material</td>
<td valign="middle" align="center">8,290 &#xb1; 40</td>
<td valign="middle" align="center">9,191&#x2013;9,427</td>
<td valign="middle" align="center">0.84</td>
<td valign="middle" align="center">9,309</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">20.5</td>
<td valign="middle" align="center">Plant material</td>
<td valign="middle" align="center">8,620 &#xb1; 40</td>
<td valign="middle" align="center">9,532&#x2013;9,682</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">9,607</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center">HZK15</td>
<td valign="middle" align="center">11.2</td>
<td valign="middle" align="center">Bivalve shell</td>
<td valign="middle" align="center">7,800 &#xb1; 40</td>
<td valign="middle" align="center">7,988&#x2013;8,369</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">8,179</td>
<td valign="middle" align="center">
<xref ref-type="bibr" rid="B64">Wang et&#xa0;al., 2018</xref>
</td>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">11.5</td>
<td valign="middle" align="center">Bivalve shell</td>
<td valign="middle" align="center">6,520 &#xb1; 40</td>
<td valign="middle" align="center">6,690&#x2013;7,142</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">6,916</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">26.7</td>
<td valign="middle" align="center">Bivalve shell</td>
<td valign="middle" align="center">8,580 &#xb1; 40</td>
<td valign="middle" align="center">8,940&#x2013;9,394</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">9,167</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">29.15</td>
<td valign="middle" align="center">Bivalve shell</td>
<td valign="middle" align="center">9,330 &#xb1; 40</td>
<td valign="middle" align="center">9,806&#x2013;10,313</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">10,060</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">32.8</td>
<td valign="middle" align="center">Bivalve shell</td>
<td valign="middle" align="center">9,250 &#xb1; 40</td>
<td valign="middle" align="center">9,709&#x2013;10,206</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">9,957</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center">HZK16</td>
<td valign="middle" align="center">9.5</td>
<td valign="middle" align="center">Bivalve shell</td>
<td valign="middle" align="center">7,640 &#xb1; 30</td>
<td valign="middle" align="center">7,824&#x2013;8,206</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">8,015</td>
<td valign="middle" align="center">
<xref ref-type="bibr" rid="B64">Wang et&#xa0;al., 2018</xref>
</td>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">23.76</td>
<td valign="middle" align="center">Bivalve shell</td>
<td valign="middle" align="center">9,140 &#xb1; 50</td>
<td valign="middle" align="center">9,550&#x2013;10,103</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">9,827</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">27.5</td>
<td valign="middle" align="center">Bivalve shell</td>
<td valign="middle" align="center">9,550 &#xb1; 50</td>
<td valign="middle" align="center">10,156&#x2013;10,628</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">10,392</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">27.82</td>
<td valign="middle" align="center">Bivalve shell</td>
<td valign="middle" align="center">9,640 &#xb1; 50</td>
<td valign="middle" align="center">10,227&#x2013;10,740</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">10,484</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">29.9</td>
<td valign="middle" align="center">Bivalve shell</td>
<td valign="middle" align="center">9,800 &#xb1; 40</td>
<td valign="middle" align="center">10,487&#x2013;11,049</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">10,768</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center">YD0901</td>
<td valign="middle" align="center">1.57</td>
<td valign="middle" align="center">Mollusk shell</td>
<td valign="middle" align="center">765 &#xb1; 30</td>
<td valign="middle" align="center">131&#x2013;492</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">312</td>
<td valign="middle" align="center">
<xref ref-type="bibr" rid="B55">Su et&#xa0;al., 2017</xref>
</td>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">4.01</td>
<td valign="middle" align="center">Mollusk shell</td>
<td valign="middle" align="center">790 &#xb1; 30</td>
<td valign="middle" align="center">151&#x2013;502</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">327</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">7.13</td>
<td valign="middle" align="center">Mollusk shell</td>
<td valign="middle" align="center">1,435 &#xb1; 30</td>
<td valign="middle" align="center">725&#x2013;1,109</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">917</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">7.7</td>
<td valign="middle" align="center">Mollusk shell</td>
<td valign="middle" align="center">1,690 &#xb1; 30</td>
<td valign="middle" align="center">986&#x2013;1,345</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">1,166</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">13.5</td>
<td valign="middle" align="center">Mollusk shell</td>
<td valign="middle" align="center">3,085 &#xb1; 35</td>
<td valign="middle" align="center">2,631&#x2013;3,054</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">2,843</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">14.8</td>
<td valign="middle" align="center">Mollusk shell</td>
<td valign="middle" align="center">3,670 &#xb1; 35</td>
<td valign="middle" align="center">3,327&#x2013;3,765</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">3,546</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">20.78</td>
<td valign="middle" align="center">Mollusk shell</td>
<td valign="middle" align="center">5,470 &#xb1; 40</td>
<td valign="middle" align="center">5,558&#x2013;5,958</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">5,758</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">32.77</td>
<td valign="middle" align="center">Mollusk shell</td>
<td valign="middle" align="center">6,310 &#xb1; 40</td>
<td valign="middle" align="center">6,438&#x2013;6,887</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">6,663</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">33.21</td>
<td valign="middle" align="center">Mollusk shell</td>
<td valign="middle" align="center">6,350 &#xb1; 40</td>
<td valign="middle" align="center">6,484&#x2013;6,935</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">6,710</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center">YD0902</td>
<td valign="middle" align="center">8.17</td>
<td valign="middle" align="center">Snail</td>
<td valign="top" align="center">764 &#xb1; 36</td>
<td valign="middle" align="center">659&#x2013;731</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">695</td>
<td valign="middle" align="center">
<xref ref-type="bibr" rid="B48">Ren et&#xa0;al., 2019</xref>
</td>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">11.13</td>
<td valign="middle" align="center">Shell</td>
<td valign="top" align="center">1,543 &#xb1; 37</td>
<td valign="middle" align="center">842&#x2013;1,238</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">1,040</td>
<td valign="middle" align="center">
<xref ref-type="bibr" rid="B56">Su et&#xa0;al., 2020</xref>
</td>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">14.07</td>
<td valign="middle" align="center">Shell debris</td>
<td valign="top" align="center">2,416 &#xb1; 39</td>
<td valign="middle" align="center">1,773&#x2013;2,251</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">2,012</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">19.51</td>
<td valign="middle" align="center">Sediments</td>
<td valign="top" align="center">4,703 &#xb1; 44</td>
<td valign="middle" align="center">5,320&#x2013;5,427</td>
<td valign="middle" align="center">0.56</td>
<td valign="middle" align="center">5,374</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">23.21</td>
<td valign="middle" align="center">Sediments</td>
<td valign="top" align="center">8,791 &#xb1; 56</td>
<td valign="middle" align="center">9,592&#x2013;9,963</td>
<td valign="middle" align="center">0.83</td>
<td valign="middle" align="center">9,778</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">40.83</td>
<td valign="middle" align="center">Sediments</td>
<td valign="top" align="center">10,325 &#xb1; 61</td>
<td valign="middle" align="center">11,930&#x2013;12,474</td>
<td valign="middle" align="center">0.98</td>
<td valign="middle" align="center">12,202</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center">YD0903</td>
<td valign="middle" align="center">4.45</td>
<td valign="middle" align="center">Mollusk shell</td>
<td valign="middle" align="center">2,490 &#xb1; 30</td>
<td valign="middle" align="center">1,882&#x2013;2,309</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">2,096</td>
<td valign="middle" align="center">
<xref ref-type="bibr" rid="B55">Su et&#xa0;al., 2017</xref>
</td>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">9.59</td>
<td valign="middle" align="center">Mollusk shell</td>
<td valign="middle" align="center">3,200 &#xb1; 35</td>
<td valign="middle" align="center">1,748&#x2013;3,175</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">2,462</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">18.55</td>
<td valign="middle" align="center">Mollusk shell</td>
<td valign="middle" align="center">6,020 &#xb1; 40</td>
<td valign="middle" align="center">6,153&#x2013;6,570</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">6,362</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">22.04</td>
<td valign="middle" align="center">Mollusk shell</td>
<td valign="middle" align="center">7,400 &#xb1; 40</td>
<td valign="middle" align="center">7,587&#x2013;7,952</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">7,770</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">34.93</td>
<td valign="middle" align="center">Mollusk shell</td>
<td valign="middle" align="center">11,350 &#xb1; 60</td>
<td valign="middle" align="center">12,601&#x2013;13,007</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">12,804</td>
<td valign="middle" align="center"/>
</tr>
<tr>
<td valign="middle" align="center"/>
<td valign="middle" align="center">47.58</td>
<td valign="middle" align="center">Mollusk shell</td>
<td valign="middle" align="center">13,090 &#xb1; 83</td>
<td valign="middle" align="center">14,603&#x2013;15,363</td>
<td valign="middle" align="center">1</td>
<td valign="middle" align="center">14,983</td>
<td valign="middle" align="center"/>
</tr>
</tbody>
</table>
</table-wrap>
</sec>
<sec id="s4" sec-type="results">
<label>4</label>
<title>Results</title>
<sec id="s4_1">
<label>4.1</label>
<title>Anomalous acoustic reflections of shallow gas in sediments</title>
<p>Acoustic reflection characteristics of gas-charged sediments are different from gas-free sediments. Gas-charged sediments can significantly absorb and scatter the energy of sound waves, making it difficult to reflect the information of stratal internal structures. Through interpretation and classification of anomalous acoustic reflections in the study area, the anomalies include bright spots, enhanced reflections, gas chimneys, acoustic blanking, acoustic turbidity, phase reversal, and pockmark (<xref ref-type="fig" rid="f2">
<bold>Figure&#xa0;2</bold>
</xref>). Among them, the first five anomalous acoustic reflections were used to identify the presence of shallow gas in the strata.</p>
<fig id="f2" position="float">
<label>Figure&#xa0;2</label>
<caption>
<p>Identification and interpretation of main anomalous acoustic reflections in acoustic profiles, such as bright spot, enhanced reflection, polarity reversal, and acoustic blanking <bold>(A)</bold>; gas chimney, pockmark, and acoustic turbidity <bold>(B)</bold>.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-10-1107530-g002.tif"/>
</fig>
<p>Bright spots are characterized by strong amplitude reflection at the gas front, featuring the polarity reversal of in-phase axes of normal seismic unit interfaces (<xref ref-type="fig" rid="f2">
<bold>Figure&#xa0;2A</bold>
</xref>). Generally, the extension of bright spots in the study area is less than 1&#xa0;km, occurring within 10&#xa0;m below the seafloor. If gas chimneys were found below bright spots, they should be caused by gas migration from the deeper sediments (<xref ref-type="fig" rid="f2">
<bold>Figure&#xa0;2A</bold>
</xref>).</p>
<p>Enhanced reflections show similar strong amplitude characteristics to bright spots, but their extension can be significantly longer than bright spots (<xref ref-type="fig" rid="f2">
<bold>Figure&#xa0;2A</bold>
</xref>). Enhanced reflections can extend more than 10&#xa0;km in length, showing continuous irregular strong reflections (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3</bold>
</xref>). It is generally believed that enhanced reflections should occur at the topmost gas-charged organic-rich sediments in the sand (<xref ref-type="bibr" rid="B28">Judd and Hovland, 2007</xref>). Due to the buoyancy of gas, its upward migration and accumulation should produce the irregular shape of the gas front in the inconsistent sediment lithology and permeability. The depth of enhanced reflections varies greatly from 2&#xa0;m to more than 10&#xa0;m below the seafloor (<xref ref-type="fig" rid="f2">
<bold>Figures&#xa0;2A</bold>
</xref>, <xref ref-type="fig" rid="f3">
<bold>3</bold>
</xref>), but is mostly distributed within 10&#xa0;m.</p>
<fig id="f3" position="float">
<label>Figure&#xa0;3</label>
<caption>
<p>A shallow acoustic profile across core HZK11 to show irregular enhanced reflections by the presence of gas in shallow sediments in the Hangzhou Bay. GBZ is short for the gas-bearing zone. See <xref ref-type="fig" rid="f1">
<bold>Figure&#xa0;1B</bold>
</xref> for the locations of the profile and the core.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-10-1107530-g003.tif"/>
</fig>
<p>Formation of the gas chimney is generally thought to be associated with gas migration upwards (<xref ref-type="bibr" rid="B25">Hustoft et&#xa0;al., 2010</xref>; <xref ref-type="bibr" rid="B7">Cukur et&#xa0;al., 2013</xref>). The gas chimney is characterized as a narrow area of vertical disturbances, where the reflection phase is undular and distorted (main performance is acoustic turbidity) compared with the surrounding horizontal and vertical gas-free sediments (<xref ref-type="bibr" rid="B78">Ye et&#xa0;al., 2003</xref>; <xref ref-type="bibr" rid="B7">Cukur et&#xa0;al., 2013</xref>; <xref ref-type="bibr" rid="B59">Toker and Tur, 2021</xref>) (<xref ref-type="fig" rid="f2">
<bold>Figures&#xa0;2</bold>
</xref>, <xref ref-type="fig" rid="f4">
<bold>4</bold>
</xref>). They serve as gas migration pathways from deep gas-charged sediments upwards. When the gas migrated upwards to reach but not penetrate the seafloor, mound-shaped features could be formed on the seabed (<xref ref-type="fig" rid="f5">
<bold>Figures&#xa0;5</bold>
</xref>, <xref ref-type="fig" rid="f6">
<bold>6</bold>
</xref>). When the gas could penetrate the seabed and escape, the gas plume was observed in the water column (<xref ref-type="fig" rid="f6">
<bold>Figure&#xa0;6</bold>
</xref>). Gas seepages result in the formation of submarine pockmarks. When submarine pockmarks were observed, evidence of gas migration was found in their underlain sediments (<xref ref-type="fig" rid="f2">
<bold>Figure&#xa0;2B</bold>
</xref>).</p>
<fig id="f4" position="float">
<label>Figure&#xa0;4</label>
<caption>
<p>A shallow acoustic profile across core HZK16 to show different shapes of abnormal acoustic reflections by the presence of gas in shallow sediments in&#xa0;the Hangzhou Bay <bold>(A)</bold>, and interpretation of the presence of shallow gas in the acoustic profile <bold>(B)</bold>. See <xref ref-type="fig" rid="f1">
<bold>Figure&#xa0;1B</bold>
</xref> for the locations of the profile and&#xa0;the core.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-10-1107530-g004.tif"/>
</fig>
<fig id="f5" position="float">
<label>Figure&#xa0;5</label>
<caption>
<p>A shallow acoustic profile across core YD0901 to show gas-related anomalous reflections in the Yangtze subaqueous delta <bold>(A)</bold>, and interpretation of the profile <bold>(B)</bold>. See <xref ref-type="fig" rid="f1">
<bold>Figure&#xa0;1B</bold>
</xref> for the locations of the profile and the core. The purple line marks the gas front. TS, Transgressive surface; TST, Transgressive system tract; MFS, Maximum flooding surface; E-HST, Early highstand system tract; L-HST, Late highstand system tract.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-10-1107530-g005.tif"/>
</fig>
<fig id="f6" position="float">
<label>Figure&#xa0;6</label>
<caption>
<p>A shallow acoustic profile across cores YD0902 and YD0903 to show gas-related anomalous reflections in the Yangtze subaqueous delta <bold>(A)</bold>, and interpretation of the profile <bold>(B)</bold>. See <xref ref-type="fig" rid="f1">
<bold>Figure&#xa0;1B</bold>
</xref> for the locations of the profile and the cores. The purple line marks the gas front. TS, Transgressive surface; TST, Transgressive system tract; MFS, Maximum flooding surface; E-HST, Early highstand system tract; L-HST, Late highstand system tract.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-10-1107530-g006.tif"/>
</fig>
<p>Acoustic turbidity is characterized by amorphous low-amplitude chaotic reflection, and the phase axis is chaotic and difficult to track (<xref ref-type="fig" rid="f2">
<bold>Figures&#xa0;2B</bold>
</xref>, <xref ref-type="fig" rid="f5">
<bold>5</bold>
</xref>, <xref ref-type="fig" rid="f6">
<bold>6</bold>
</xref>). Acoustic turbidity is commonly observed below enhanced reflections, bright spots, or inside gas chimneys (<xref ref-type="fig" rid="f2">
<bold>Figures&#xa0;2</bold>
</xref>, <xref ref-type="fig" rid="f5">
<bold>5</bold>
</xref>, and <xref ref-type="fig" rid="f6">
<bold>6</bold>
</xref>). As long as the sediment contains 1% of gas content, acoustic turbidity may appear (<xref ref-type="bibr" rid="B13">Fannin, 1980</xref>). The extension varies greatly from tens of meters inside gas chimneys to several kilometers below irregularly enhanced reflections (<xref ref-type="fig" rid="f2">
<bold>Figures&#xa0;2A</bold>
</xref>, <xref ref-type="fig" rid="f3">
<bold>3</bold>
</xref>). Acoustic blanking shows phase axis reflection suddenly becoming weak or even disappearing (<xref ref-type="fig" rid="f2">
<bold>Figure&#xa0;2A</bold>
</xref>). For acoustic turbidity and acoustic blanking, the internal structure of the strata is difficult to distinguish. The extension length of acoustic blanking can reach up to tens of kilometers (<xref ref-type="fig" rid="f5">
<bold>Figures&#xa0;5</bold>
</xref>, <xref ref-type="fig" rid="f6">
<bold>6</bold>
</xref>).</p>
<p>Regular gas-related reflections with clear borders can be divided into curtain-shaped, columnar-shaped, and chimney-shaped reflections according to their morphological characteristics (<xref ref-type="fig" rid="f4">
<bold>Figure&#xa0;4</bold>
</xref>). Curtain-shaped reflection having nearly vertical sidewalls refers to the horizontal extension area with a certain width of &gt;1&#xa0;km, and the gas front is nearly parallel to the bedding with the burial depth of 5&#x2013;10 m below the seafloor. Columnar-shaped reflection having similar gas front and sidewall structures with the curtain-shaped reflection refers to the gas-bearing disturbance area with a limited horizontal width of 0.5&#x2013;1.0 km, and the gas front depth is generally not more than 5&#xa0;m. Compared with curtain-shaped and columnar-shaped reflections, chimney-shaped reflection is characterized by a smaller horizontal extension of &lt;0.5&#xa0;km and much shallower gas front, which can indicate the stronger upward migration and accumulation of gas.</p>
</sec>
<sec id="s4_2">
<label>4.2</label>
<title>Distribution and migration of shallow gas in sediments</title>
<p>The burial depth of the shallow gas front in the study area was mapped according to new sub-bottom acoustic data in this study and those by <xref ref-type="bibr" rid="B3">Chen et&#xa0;al. (2022)</xref>. Shallow gas is widely distributed in the study area with the gas front depth ranging from 0 to 17.5&#xa0;m. In comparison, gas front depths varying from 5 to 15&#xa0;m in the Yangtze subaqueous delta are relatively deeper than those from 2 to 7.5&#xa0;m in the Hangzhou Bay (<xref ref-type="fig" rid="f7">
<bold>Figure&#xa0;7</bold>
</xref>).</p>
<fig id="f7" position="float">
<label>Figure&#xa0;7</label>
<caption>
<p>The contour map of burial depths of the shallow gas front in the Yangtze subaqueous delta and the Hangzhou Bay. The data of gas front depths in the northern Yangtze subaqueous delta are retrieved from <xref ref-type="bibr" rid="B3">Chen et&#xa0;al. (2022)</xref>. Dashed yellow lines indicate the acoustic profiles acquired in this study, and solid yellow lines show the acoustic profiles investigated by <xref ref-type="bibr" rid="B3">Chen et&#xa0;al. (2022)</xref>. Dashed black lines represent the isobaths.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-10-1107530-g007.tif"/>
</fig>
<p>According to acoustic profiles collected in the Yangtze subaqueous delta in 2011 (<xref ref-type="fig" rid="f5">
<bold>Figures&#xa0;5</bold>
</xref>, <xref ref-type="fig" rid="f6">
<bold>6</bold>
</xref>), wide acoustic blanking zones were commonly found below the seafloor. Acoustic turbidity and acoustic basement could be seen between the two acoustic blanking zones (<xref ref-type="fig" rid="f6">
<bold>Figure&#xa0;6</bold>
</xref>). Seaward termination of acoustic blanking zones is present where the Holocene sediment thickness is 14&#xa0;m (<xref ref-type="fig" rid="f5">
<bold>Figure&#xa0;5</bold>
</xref>) and 23.7&#xa0;m (<xref ref-type="fig" rid="f6">
<bold>Figure&#xa0;6</bold>
</xref>), respectively. Because the high-altitude paleo-topography interrupts continuous variation of the Holocene sediment thickness, the distribution of acoustic blanking zones is disconnected until the Holocene sediment thickness reaches ~20.7 m (<xref ref-type="fig" rid="f6">
<bold>Figure&#xa0;6</bold>
</xref>). In the southern Yangtze subaqueous delta, gas front depths of two acoustic profiles show their different landward-changing patterns (<xref ref-type="fig" rid="f5">
<bold>Figures&#xa0;5</bold>
</xref>, <xref ref-type="fig" rid="f6">
<bold>6</bold>
</xref>). The gas front depth increases gradually landward along the acoustic profile across core YD0901 (<xref ref-type="fig" rid="f5">
<bold>Figure&#xa0;5</bold>
</xref>), while it increases landward first but ends with a landward decreasing trend along the acoustic profile across cores YD0902 and YD0903 (<xref ref-type="fig" rid="f6">
<bold>Figure&#xa0;6</bold>
</xref>).</p>
<p>In the northern Hangzhou Bay, a series of intermittent narrow acoustic blanking zones and acoustic turbidity zones were found (<xref ref-type="fig" rid="f4">
<bold>Figure&#xa0;4</bold>
</xref>). A few acoustic turbidity zones indicate that gas can migrate upwards from the deeper sediments (<xref ref-type="fig" rid="f4">
<bold>Figure&#xa0;4</bold>
</xref>). In this area, different characteristics of narrow acoustic blanking zones are associated with the gas distribution difference. If the locations of acoustic blanking zones or gas fronts were closely connected to enhanced reflections, they should have a high concentration of shallow gas with a remarkable tendency of upward migration (<xref ref-type="fig" rid="f3">
<bold>Figures&#xa0;3</bold>
</xref>, <xref ref-type="fig" rid="f4">
<bold>4</bold>
</xref>). However, some stratal internal structures can be diagnosed in the weak acoustic blanking zones that are not spatially interwoven with enhanced reflections, thus hinting at a relatively low concentration of shallow gas (<xref ref-type="fig" rid="f4">
<bold>Figure&#xa0;4</bold>
</xref>).</p>
<p>Gas plumes were observed in the water column, and some mounds were found on the seafloor in the Yangtze subaqueous delta (<xref ref-type="fig" rid="f5">
<bold>Figures&#xa0;5</bold>
</xref>, <xref ref-type="fig" rid="f6">
<bold>6</bold>
</xref>). The gas plumes rose ~5 m above the seafloor, and below the seafloor, gas chimney and acoustic turbidity could be identified (<xref ref-type="fig" rid="f5">
<bold>Figure&#xa0;5</bold>
</xref>). The mounds were approximately 1&#x2013;2 m high, and their formation was also closely related to gas migration (<xref ref-type="fig" rid="f5">
<bold>Figures&#xa0;5</bold>
</xref>, <xref ref-type="fig" rid="f6">
<bold>6</bold>
</xref>). Pockmarks, with a maximum diameter of ~500 m, were observed in the Jinshan Deep Trough and near Yehuangpan island and Xiaojishan island in the northern Hangzhou Bay, and gas-related acoustic blanking and acoustic turbidity were well developed below the pockmarks. Meanwhile, pockmarks generally are accompanied by enhanced reflections and gas chimneys (<xref ref-type="fig" rid="f8">
<bold>Figure&#xa0;8</bold>
</xref>). The pockmarks with diameters exceeding 20&#xa0;m were also found in the Yangtze subaqueous delta (<xref ref-type="fig" rid="f8">
<bold>Figure&#xa0;8</bold>
</xref>).</p>
<fig id="f8" position="float">
<label>Figure&#xa0;8</label>
<caption>
<p>Gas-seepage-related structures in the Yangtze subaqueous delta and the Hangzhou Bay: <bold>(A)</bold> pockmarks detected by side-scan sonar (modified from <xref ref-type="bibr" rid="B3">Chen et&#xa0;al., 2022</xref>), and <bold>(B&#x2013;D)</bold> seabed depressions induced by gas seepage activities disclosed by acoustic profiles.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-10-1107530-g008.tif"/>
</fig>
</sec>
</sec>
<sec id="s5" sec-type="discussion">
<label>5</label>
<title>Discussion</title>
<sec id="s5_1">
<label>5.1</label>
<title>Possible source for shallow gas</title>
<p>Shallow gas can be generated by biogenic (generally &#x3b4;<sup>13</sup>C<sub>CH4</sub> ~ &#x2212;110&#x2030;&#x2212;60&#x2030;, &#x3b4;D<sub>CH4</sub> &lt;110&#x2030;) or thermogenic processes (generally &#x3b4;<sup>13</sup>C<sub>CH4</sub> &gt;60&#x2030;, &#x3b4;D<sub>CH4</sub> &gt;110&#x2030;) (<xref ref-type="bibr" rid="B69">Whiticar, 1999</xref>; <xref ref-type="bibr" rid="B78">Ye et&#xa0;al., 2003</xref>; <xref ref-type="bibr" rid="B14">Feng, 2017</xref>). The low &#x3b4;<sup>13</sup>C values and heavy hydrocarbon contents of shallow gas in core sediments retrieved from the Yangtze delta plain (<xref ref-type="bibr" rid="B81">Zhang et&#xa0;al., 2013a</xref>; <xref ref-type="bibr" rid="B14">Feng, 2017</xref>) and the southern coastal plain of Hangzhou Bay (<xref ref-type="bibr" rid="B73">Yang et&#xa0;al., 2021</xref>) indicate its biogenic origin.</p>
<p>In the Yangtze subaqueous delta and the Hangzhou Bay, shallow gas can be detected extensively as shown by acoustic blanking and acoustic turbidity (<xref ref-type="fig" rid="f4">
<bold>Figures&#xa0;4</bold>
</xref>&#x2013;<xref ref-type="fig" rid="f6">
<bold>6</bold>
</xref>). The gas front is distributed in the Holocene sediments of HST and TST, and its burial depth is mostly shallower than 17.5&#xa0;m (<xref ref-type="fig" rid="f7">
<bold>Figure&#xa0;7</bold>
</xref>). Meanwhile, anomalous acoustic reflections were not observed in the thin Holocene sediments. Thus, the shallow gas is presumed to be biogenic gas within the Holocene deltaic or estuarine sediments.</p>
<p>The generation of shallow biogenic gas requires abundant organic matter in sediments to be used by methanogenic bacteria (<xref ref-type="bibr" rid="B37">Lin et&#xa0;al., 2015</xref>; <xref ref-type="bibr" rid="B14">Feng, 2017</xref>). The content of total organic carbon (TOC) in cores CJK08, YD0901 (<xref ref-type="bibr" rid="B67">Wang et&#xa0;al., 2012</xref>), and ZK9 (<xref ref-type="bibr" rid="B79">Zhan et&#xa0;al., 2012</xref>) is generally &gt;0.5%, high enough for the occurrence of microbial methanogenesis (<xref ref-type="bibr" rid="B49">Rice and Claypool, 1981</xref>). In comparison, the upper Holocene sediments in the Yangtze subaqueous delta usually have much higher TOC content (TOC ~ 0.54%&#x2013;1.13%, <xref ref-type="bibr" rid="B67">Wang et&#xa0;al., 2012</xref>; <xref ref-type="bibr" rid="B79">Zhan et&#xa0;al., 2012</xref>) than those in the Yangtze delta plain (<xref ref-type="bibr" rid="B82">Zhang et&#xa0;al., 2013b</xref>). However, shallow gas is little found in the former strata because therein CH<sub>4</sub> production could be mostly consumed by sulfate reduction in marine sediments (<xref ref-type="bibr" rid="B42">Mogoll&#xf3;n et&#xa0;al., 2012</xref>; <xref ref-type="bibr" rid="B43">Mogoll&#xf3;n et&#xa0;al., 2013</xref>; <xref ref-type="bibr" rid="B17">Flury et&#xa0;al., 2016</xref>). The gas front has often been detected to locate near the bottom boundary of the delta-front facies (<xref ref-type="fig" rid="f5">
<bold>Figures&#xa0;5</bold>
</xref>, <xref ref-type="fig" rid="f6">
<bold>6</bold>
</xref>), indicating that shallow gas should be sourced from the deeper Holocene sediments by upward migration and accumulation in the Yangtze subaqueous delta and the Hangzhou Bay.</p>
</sec>
<sec id="s5_2">
<label>5.2</label>
<title>Controlling factors of gas distribution</title>
<p>Shallow gas was usually found in relatively thicker Holocene strata as shown by large-scale distributions of acoustic blanking and acoustic turbidity (<xref ref-type="fig" rid="f5">
<bold>Figures&#xa0;5</bold>
</xref>, <xref ref-type="fig" rid="f6">
<bold>6</bold>
</xref>). It might disappear as the Holocene strata suddenly taper out (<xref ref-type="fig" rid="f6">
<bold>Figure&#xa0;6</bold>
</xref>). Previous studies also concluded that the Holocene stratal thickness plays a key role in controlling shallow gas generation and accumulation (<xref ref-type="bibr" rid="B18">Garc&#xed;a-Garc&#xed;a et&#xa0;al., 2007</xref>; <xref ref-type="bibr" rid="B17">Flury et&#xa0;al., 2016</xref>; <xref ref-type="bibr" rid="B2">Chen et&#xa0;al., 2020</xref>). Part of the reason is that thicker fine-grained Holocene sediments with a higher abundance of organic matter tend to generate more CH<sub>4</sub>, consequently increasing the upward CH<sub>4</sub> flux to shift the SMTZ towards the shallow depth (<xref ref-type="bibr" rid="B17">Flury et&#xa0;al., 2016</xref>). Moreover, enhanced methanogenesis should produce CH<sub>4</sub> supersaturation in sediments to generate CH<sub>4</sub> gas bubbles (<xref ref-type="bibr" rid="B18">Garc&#xed;a-Garc&#xed;a et&#xa0;al., 2007</xref>; <xref ref-type="bibr" rid="B17">Flury et&#xa0;al., 2016</xref>), causing its underlying gas-charged strata to not show clear internal stratal structures because of acoustic anomalous reflections.</p>
<p>Increasing water depth may increase the solubility of CH<sub>4</sub> in interstitial water, making it more difficult for biogenic CH<sub>4</sub> to form gas bubbles in shallow sediments (<xref ref-type="bibr" rid="B60">Ulyanova et&#xa0;al., 2012</xref>). This is shown by the acoustic profile across cores YD0902 and YD0903 where shallow gas could be detected in the Holocene strata with its thickness exceeding 20.7&#xa0;m at the water depth of ~10 m, but the stratal thickness should exceed 23.7&#xa0;m at the water depth of ~20 m (<xref ref-type="fig" rid="f6">
<bold>Figure&#xa0;6</bold>
</xref>). However, at the water depth of ~20 m along the acoustic profile across core YD0901, shallow gas was observed when the Holocene stratal thickness just exceeded 14.0&#xa0;m (<xref ref-type="fig" rid="f5">
<bold>Figure&#xa0;5</bold>
</xref>). It is therefore suggested that water depth is an influencing factor but not a single determining factor on the shallow gas distribution in shallow sediments.</p>
<p>Once the gas is generated, it tends to move upward due to buoyancy. However, upward gas migration requires overcoming the capillary force and the overburden pressure of overlying strata and water mass. Typically, the higher capillary force indicates the lower permeability of the sediment (<xref ref-type="bibr" rid="B36">Lin et&#xa0;al., 2010</xref>; <xref ref-type="bibr" rid="B46">Qu et&#xa0;al., 2013</xref>; <xref ref-type="bibr" rid="B82">Zhang et&#xa0;al., 2013b</xref>). In general, coarse-grained sediments are more beneficial for the gas surpassing capillary forces to migrate upward. The overburden pressure reduces as the water depth and/or the overlying strata thickness decrease. In the Yangtze subaqueous delta, the gas front was usually observed to locate in the delta-front facies (<xref ref-type="fig" rid="f5">
<bold>Figures&#xa0;5</bold>
</xref>, <xref ref-type="fig" rid="f6">
<bold>6</bold>
</xref>), and its burial depth might decrease towards the shallower water zone or the thinner stratal thickness of silt-dominated delta-front facies (<xref ref-type="fig" rid="f6">
<bold>Figures&#xa0;6</bold>
</xref>, <xref ref-type="fig" rid="f7">
<bold>7</bold>
</xref>, <xref ref-type="fig" rid="f9">
<bold>9</bold>
</xref>).</p>
<fig id="f9" position="float">
<label>Figure&#xa0;9</label>
<caption>
<p>Core correlation profiles in the northern Hangzhou Bay and the Yangtze subaqueous delta show detailed information on lithology, sedimentary facies, chronology, and system tracts. The Holocene stratal base at ca. 11.7 ka BP was deduced from <sup>14</sup>C ages data of cores HZK11, HZK15, and HZK16 (<xref ref-type="bibr" rid="B64">Wang et&#xa0;al., 2018</xref>), and cores YD0901, YD0902, and YD0903 (<xref ref-type="bibr" rid="B55">Su et&#xa0;al., 2017</xref>; <xref ref-type="bibr" rid="B48">Ren et&#xa0;al., 2019</xref>; <xref ref-type="bibr" rid="B56">Su et&#xa0;al., 2020</xref>). See <xref ref-type="fig" rid="f1">
<bold>Figure&#xa0;1B</bold>
</xref> for the locations of the cores. TST, Transgressive system tract; MFS, Maximum flooding surface; E-HST, Early highstand system tract; L-HST, Late highstand system tract.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-10-1107530-g009.tif"/>
</fig>
<p>Compared with the Yangtze subaqueous delta, the gas front in the Hangzhou Bay was much shallower as a whole (<xref ref-type="fig" rid="f7">
<bold>Figure&#xa0;7</bold>
</xref>). Firstly, the mean water depth in the Hangzhou Bay is shallower than that in the Yangtze subaqueous delta, potentially leading to lower CH<sub>4</sub> solubility for the easier formation of CH<sub>4</sub> gas bubbles in the shallower sediments. Then, well-sorted estuarine sediment by intense tidal currents in the northern Hangzhou Bay should increase sediment permeability to ease gas upward migration (<xref ref-type="fig" rid="f9">
<bold>Figure&#xa0;9</bold>
</xref>). Moreover, recently increasing seabed erosion has been extensively observed in the northern Hangzhou Bay due to a sharp decrease in sediment input from the Yangtze River after the TGD construction (<xref ref-type="bibr" rid="B70">Xie et&#xa0;al., 2013</xref>; <xref ref-type="bibr" rid="B71">Xie et&#xa0;al., 2017</xref>; <xref ref-type="bibr" rid="B19">Guo et&#xa0;al., 2021</xref>). The seabed erosion rate in the northern Hangzhou Bay was about &#x2212;0.4 m/yr during 2010&#x2013;2014 (<xref ref-type="bibr" rid="B71">Xie et&#xa0;al., 2017</xref>), and the total scouring thickness at the Jinshan Deep Trough exceeded 2.4&#xa0;m during the recent 30 years from 1989 to 2018 (<xref ref-type="bibr" rid="B83">Zhang et&#xa0;al., 2021</xref>). The seabed erosion has greatly reduced the thickness of cap sediments above the shallow gas, consequently lessening the overburden pressure of the gas to activate gas seepages in the Hangzhou Bay (<xref ref-type="bibr" rid="B2">Chen et&#xa0;al., 2020</xref>).</p>
<p>Gas front depth becoming shallower due to seabed erosion can also be observed in the Yangtze subaqueous delta (<xref ref-type="fig" rid="f6">
<bold>Figures&#xa0;6</bold>
</xref>, <xref ref-type="fig" rid="f10">
<bold>10</bold>
</xref>). According to the acoustic surveys, the gas front depth near YD0902 changed slightly from 9.7&#xa0;m in July 2011 to 9.4&#xa0;m in November 2017, which may be related to temperature changes (<xref ref-type="bibr" rid="B41">Martinez-Carre&#xf1;o and Garcia-Gil, 2013</xref>). According to the acoustic profiles, the average water depth on the west side was approximately 8.8&#xa0;m in July 2011, and became 11.6&#xa0;m in November 2017, indicating a seabed erosion thickness of approximately 2.8&#xa0;m. Meanwhile, shallow gas front depth became shallow up to 3&#xa0;m or even more at the inner Yangtze subaqueous delta (<xref ref-type="fig" rid="f6">
<bold>Figures&#xa0;6</bold>
</xref>, <xref ref-type="fig" rid="f10">
<bold>10</bold>
</xref>). A mound was found with its landward occurrence of a shallow acoustic turbidity zone in the seismic profile of 2017, both indicating upward gas migration activities (<xref ref-type="fig" rid="f10">
<bold>Figure&#xa0;10</bold>
</xref>). Meanwhile, the occurrence of huge negative topography at the inner delta front zone in 2017 was considered to be related with recently increasing seabed erosion in the Yangtze subaqueous delta with a water depth of 5&#x2013;20 m because of sediment deficit (<xref ref-type="bibr" rid="B76">Yang et&#xa0;al., 2011</xref>; <xref ref-type="bibr" rid="B2">Chen et&#xa0;al., 2020</xref>; <xref ref-type="bibr" rid="B19">Guo et&#xa0;al., 2021</xref>).</p>
<fig id="f10" position="float">
<label>Figure&#xa0;10</label>
<caption>
<p>A shallow acoustic profile showing gas-related anomalous reflections in the Yangtze subaqueous delta. The purple line marks the gas front. The original acoustic profile was reproduced from <xref ref-type="bibr" rid="B3">Chen et&#xa0;al. (2022)</xref>.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-10-1107530-g010.tif"/>
</fig>
<p>In summary, gas distribution and accumulation in shallow deltaic/estuarine sediments are primarily determined by the stratal thickness and permeability of the Holocene sediments and water depth (<xref ref-type="bibr" rid="B42">Mogoll&#xf3;n et&#xa0;al., 2012</xref>; <xref ref-type="bibr" rid="B43">Mogoll&#xf3;n et&#xa0;al., 2013</xref>; <xref ref-type="bibr" rid="B17">Flury et&#xa0;al., 2016</xref>; <xref ref-type="bibr" rid="B2">Chen et&#xa0;al., 2020</xref>). Thicker Holocene strata and shallower water depth are favorable for gas generation and accumulation. Overburden pressure and low permeability of sediments above the shallow gas have significant effects on gas migration. Recently, gas migration is highly enhanced by increasing seabed erosion at the inner Yangtze delta front and the northern Hangzhou Bay due to sediment deficit, potentially triggering gas seepage and seabed instability events. Thus, more attention should be paid to this new situation for potential geological hazardous occurrences.</p>
</sec>
<sec id="s5_3">
<label>5.3</label>
<title>Implications of shallow gas-associated geological hazards</title>
<sec id="s5_3_1">
<label>5.3.1</label>
<title>Gas seepage</title>
<p>Distinct gas seepage phenomena, such as gas plumes and pockmarks, have been commonly observed in the study area (<xref ref-type="fig" rid="f5">
<bold>Figures&#xa0;5</bold>
</xref>, <xref ref-type="fig" rid="f6">
<bold>6</bold>
</xref>, <xref ref-type="fig" rid="f8">
<bold>8</bold>
</xref>, <xref ref-type="fig" rid="f10">
<bold>10</bold>
</xref>). Gas seepage is closely related to gas migration in sediments. Below the gas plumes and pockmarks, gas chimneys should be visibly observed. The occurrences of mounds on the seabed and gas chimneys below the seafloor show their clear spatial overlap. The mounds are where the gas is trapped below the seabed sediments, conceivably being the initial form of pockmarks (<xref ref-type="bibr" rid="B21">Hovland and Judd, 1988</xref>; <xref ref-type="bibr" rid="B29">Koch et&#xa0;al., 2015</xref>; <xref ref-type="bibr" rid="B45">Nyman et&#xa0;al., 2020</xref>). When the gas pressure exceeds the overburden pressure in shallow sediments because of excessive gas accumulation or seabed erosion, it will cause the seabed sediments to dome upward. Once the accumulation pressure of the gas is high enough within the mounds, the gas will burst into the water column and the companion sediments will be carried away. Afterward, the positive topography of the mound structure should be replaced by the negative topography of pockmark structures (<xref ref-type="bibr" rid="B4">Chen et&#xa0;al., 2017</xref>).</p>
<p>The behavior and location of SMTZ are closely linked to the methanogenic efficiency and the gas front depth in shallow sediments. Data of <xref ref-type="bibr" rid="B17">Flury et&#xa0;al. (2016)</xref> clearly showed the changes of gas front depths and the SMTZ in gas-charged sediments and gas-free sediments, and the shallowing SMTZ promoted gas generation and accumulation in shallower sediments (<xref ref-type="fig" rid="f11">
<bold>Figure&#xa0;11</bold>
</xref>). <xref ref-type="bibr" rid="B43">Mogoll&#xf3;n et&#xa0;al. (2013)</xref> also proposed that the gas front depth should take a key role in controlling CH<sub>4</sub> and sulfate diffusion fluxes toward the SMTZ. The shallower the gas front depth becomes, the greater the diffusion flux of CH<sub>4</sub> is. The latter has the potential to shift the SMTZ upward. In other words, the depth of SMTZ and the gas front depth are closely coupled. As discussed previously, seabed erosion might cause a significant change in both the gas front depths and the SMTZ locations. Since the rapid development of dam projects and soil-water conservation projects in the Yangtze River catchment, the sediment discharge into the estuary has dramatically decreased (<xref ref-type="bibr" rid="B19">Guo et&#xa0;al., 2021</xref>; <xref ref-type="bibr" rid="B40">Luan et&#xa0;al., 2021</xref>; <xref ref-type="bibr" rid="B75">Yang et&#xa0;al., 2022a</xref>). The mean sediment discharge during the first decade after the closure of the Three Gorges Dam in 2003 dropped to a relatively low level (145 Mt yr<sup>&#x2212;1</sup>), which was only approximately 30% of the value in 1950&#x2013;1968 (<xref ref-type="bibr" rid="B73">Yang et&#xa0;al., 2021</xref>). Shallow water areas between 5&#xa0;m and 20&#xa0;m in the Yangtze subaqueous delta and the northern Hangzhou Bay have experienced remarkable erosion (<xref ref-type="bibr" rid="B70">Xie et&#xa0;al., 2013</xref>; <xref ref-type="bibr" rid="B19">Guo et&#xa0;al., 2021</xref>). From 1989 to 2018, the volume of seabed erosion in the whole Jinshan Deep Trough area was nearly 162.7&#xd7;10<sup>6</sup> m<sup>3</sup>, and the average erosion thickness exceeded 2.4&#xa0;m (<xref ref-type="bibr" rid="B83">Zhang et&#xa0;al., 2021</xref>). Some pockmarks and mounds were observed in these eroded areas, indicating that the gas migration and seepages responded quickly to the recently increasing seabed erosion (<xref ref-type="fig" rid="f8">
<bold>Figure&#xa0;8</bold>
</xref>).</p>
<fig id="f11" position="float">
<label>Figure&#xa0;11</label>
<caption>
<p>The chirp acoustic profile of gas-charged and gas-free sediment intersection area in the Aarhus Bay <bold>(A)</bold>, sulfate (light blue solid line), dissolved CH<sub>4</sub> (blue solid line), and CH<sub>4</sub> solubility (blue dashed line) profiles in different sediments <bold>(B&#x2013;D)</bold> (after <xref ref-type="bibr" rid="B17">Flury et&#xa0;al., 2016</xref>).</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-10-1107530-g011.tif"/>
</fig>
<p>The gas front depth could fluctuate over different time scales in response to the change in environmental factors, including porewater salinity, sediment temperature, and seafloor hydrostatic pressure (<xref ref-type="bibr" rid="B68">Wever et&#xa0;al., 2006</xref>; <xref ref-type="bibr" rid="B8">Diez et&#xa0;al., 2007</xref>; <xref ref-type="bibr" rid="B41">Martinez-Carre&#xf1;o and Garcia-Gil, 2013</xref>). Over a seasonal scale, <xref ref-type="bibr" rid="B58">Sun et&#xa0;al. (2018)</xref> reported that the concentration of CH<sub>4</sub> in bottom water in summer and autumn was significantly higher than that in spring and winter in the East China Sea. The seasonal variability of gas front depths and gas seepages was also observed in the R&#xed;a de Vigo by means of more gas plumes and shallower gas front depth in summer than in winter (<xref ref-type="bibr" rid="B41">Martinez-Carre&#xf1;o and Garcia-Gil, 2013</xref>). This could be explained by the fact that the higher sediment temperature decreases the solubility of CH<sub>4</sub> in interstitial water and increases gas pressure in sediments, consequently leading to more CH<sub>4</sub> escaping from shallow sediments during the warm season.</p>
<p>Moreover, global warming may trigger more CH<sub>4</sub> escaping from marine sediments. CH<sub>4</sub> is estimated to be approximately 20&#x2013;40 times more effective in causing global warming than CO<sub>2</sub> (<xref ref-type="bibr" rid="B31">Letcher, 2019</xref>; <xref ref-type="bibr" rid="B26">IPCC, 2021</xref>). Thus, more caution should be paid to the loop of positive feedback between increasing global warming and more shallow gas seepages from the world&#x2019;s coastal areas.</p>
</sec>
<sec id="s5_3_2">
<label>5.3.2</label>
<title>Seabed instability</title>
<p>Marine sediments containing dissolved CH<sub>4</sub> can cause a reduction of soil strength, becoming fragile to trigger seabed erosion and seafloor construction subsidence (<xref ref-type="bibr" rid="B24">Huang and Han, 2020</xref>; <xref ref-type="bibr" rid="B32">Li, 2020</xref>). It is generally believed that the SMTZ depth determines the upper limit of dissolved CH<sub>4</sub> diffusion in shallow sediments. The gas front depth was observed to range from 2 to 5&#xa0;m in the Hangzhou Bay (<xref ref-type="fig" rid="f7">
<bold>Figure&#xa0;7</bold>
</xref>), suggesting that the dissolved CH<sub>4</sub> could even be found in the upper 2-m sub-bottom sediments. As discussed above, the occurrence of seabed erosion will lead to strengthening gas upward migration and shallowing the SMTZ. Continuous erosion due to sediment deficit in the study area should have induced an upward shift of the SMTZ, as indicated by the presence of dissolved CH<sub>4</sub> in shallower marine sediments (<xref ref-type="bibr" rid="B62">Wang and Liu, 2016</xref>; <xref ref-type="bibr" rid="B52">Song et&#xa0;al., 2021</xref>). Meanwhile, shallower sediments containing dissolved CH<sub>4</sub> will further enhance seabed erosion. Thus, positive feedback of seabed erosion induced by external factors (e.g., sediment deficit due to river damming projects) and internal factors (e.g., reduced soil strength by the presence of dissolved CH<sub>4</sub>) could produce severe erosion, gas migration, and seepage events even under small wave conditions (<xref ref-type="fig" rid="f12">
<bold>Figure&#xa0;12A</bold>
</xref>).</p>
<fig id="f12" position="float">
<label>Figure&#xa0;12</label>
<caption>
<p>Potential geological hazards and disasters induced by the seabed instability of shallow gas-charged sediments in the highly developed coastal seas under small waves <bold>(A)</bold> and storm waves <bold>(B)</bold>.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-10-1107530-g012.tif"/>
</fig>
<p>The Yangtze Delta and the Hangzhou Bay are subject to storm impacts, including typhoons in summer and cold fronts in winter. Storm waves can cause sediment liquefication and submarine failures due to the sharp increase of pore pressure in water-saturated gassy sediments, leading to geological hazards or even geological disaster events (<xref ref-type="fig" rid="f12">
<bold>Figure&#xa0;12B</bold>
</xref>) (<xref ref-type="bibr" rid="B62">Wang and Liu, 2016</xref>; <xref ref-type="bibr" rid="B30">Kramer et&#xa0;al., 2017</xref>; <xref ref-type="bibr" rid="B66">Wang et&#xa0;al., 2019</xref>; <xref ref-type="bibr" rid="B20">Gupta et&#xa0;al., 2022</xref>). In the Hangzhou Bay, the oil and gas pipelines were reported to be destroyed a few times during typhoon events as indicated by newly produced large pockmarks (<xref ref-type="bibr" rid="B6">Cui et&#xa0;al., 2014</xref>; <xref ref-type="bibr" rid="B32">Li, 2020</xref>). Sometimes, the pipelines can be broken by liquefaction-induced subsidence as shown by the increased burial depth of oil pipeline reaching up to 2&#xa0;m after the strike of typhoon &#x201c;SWAN&#x201d; in the Hangzhou Bay. More frequent and powerful storms are expected in the warming earth; thus, the instability of shallow gas-charged sediments in the highly developed coastal seas deserves more attention to build up early warming capability for hazard and disaster prediction and reduction.</p>
</sec>
</sec>
</sec>
<sec id="s6" sec-type="conclusions">
<label>6</label>
<title>Conclusion</title>
<p>Shallow gas is explored and has been found to extensively exist in the Holocene muddy sediments in the Yangtze subaqueous delta and the Hangzhou Bay as shown by the common presence of acoustic anomalous reflections including enhanced reflection, gas chimney, bright spot, acoustic blanking, and acoustic turbidity. The gas front depth does not exceed 17.5&#xa0;m, and it is generally much shallower in the Hangzhou Bay than that in the Yangtze subaqueous delta because the former has, on average, smaller water depth and coarser sediments than the latter. Shallow gas is a biogenic production, and its distribution is primarily determined by the Holocene stratal thickness and water depth. Shallow gas migration is mainly controlled by overburden pressure and sediment permeability. Gas migration and seepage phenomenon (such as gas chimney, mound, pockmark, and gas plumes) are common in the study area, which are clearly enhanced by recently increasing seabed erosion because of sediment deficit. Positive feedback loops of gas migration with global warming and seabed erosion will expose coastal society to more frequent and more intense coastal geological hazards induced by shallow gas seepages. This study improves our understanding of the controlling mechanisms of dynamic changes of shallow gas in coastal seas under internal and external factors, highlighting the urgent requirement to build up early warming capability to monitor and predict shallow gas activities and associated hazards.</p>
</sec>
<sec id="s7" sec-type="data-availability">
<title>Data availability statement</title>
<p>The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding author.</p>
</sec>
<sec id="s8" sec-type="author-contributions">
<title>Author contributions</title>
<p>LS collected and analyzed the data and wrote the manuscript. DF conceived the idea, designed the experiments, analyzed the data, and revised the manuscript. JS and XG collected and analyzed the data, and revised the manuscript. All authors contributed to the article and approved the submitted version.</p>
</sec>
</body>
<back>
<sec id="s9" sec-type="funding-information">
<title>Funding</title>
<p>This study was supported by the Innovation Program of Shanghai Municipal Education Commission (2021-01-07-00-07-E00093), the National Natural Science Foundation of China (NSFC-41976070 and 42206062), the Open Foundation of Zhoushan Field Scientific Observation and Research Station for Marine Geo-hazards, China Geological Survey (ZSORS-22-11), and the Fundamental Research Funds for the Central Universities (No. ZD-21-202101).</p>
</sec>
<sec id="s10" sec-type="COI-statement">
<title>Conflict of interest</title>
<p>The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.</p>
</sec>
<sec id="s11" sec-type="disclaimer">
<title>Publisher&#x2019;s note</title>
<p>All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.</p>
</sec>
<ref-list>
<title>References</title>
<ref id="B1">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Beckers</surname> <given-names>J. M.</given-names>
</name>
<name>
<surname>Barth</surname> <given-names>A.</given-names>
</name>
<name>
<surname>Troupin</surname> <given-names>C.</given-names>
</name>
<name>
<surname>Alvera-Azc&#xe1;rate.</surname> <given-names>A.</given-names>
</name>
</person-group> (<year>2014</year>). <article-title>Some approximate and efficient methods to assess error fields in spatial gridding with DIVA (Data interpolating variational analysis)</article-title>. <source>J. Atmospheric Oceanic Technol.</source> <volume>31</volume>, <fpage>515</fpage>&#x2013;<lpage>530</lpage>. doi: <pub-id pub-id-type="doi">10.1175/JTECH-D-13-00130.1</pub-id>
</citation>
</ref>
<ref id="B2">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Chen</surname> <given-names>Y. F.</given-names>
</name>
<name>
<surname>Deng</surname> <given-names>B.</given-names>
</name>
<name>
<surname>Zhang</surname> <given-names>J.</given-names>
</name>
</person-group> (<year>2020</year>). <article-title>Shallow gas in the Holocene mud wedge along the inner East China Sea shelf</article-title>. <source>Mar. Pet. Geol.</source> <volume>114</volume>, <fpage>104233</fpage>. doi: <pub-id pub-id-type="doi">10.1016/j.marpetgeo.2020.104233</pub-id>
</citation>
</ref>
<ref id="B3">
<citation citation-type="book">
<person-group person-group-type="author">
<name>
<surname>Chen</surname> <given-names>Y. F.</given-names>
</name>
<name>
<surname>Deng</surname> <given-names>B.</given-names>
</name>
<name>
<surname>Zhang</surname> <given-names>W. G.</given-names>
</name>
<name>
<surname>Zhang</surname> <given-names>G. L.</given-names>
</name>
</person-group> (<year>2022</year>). <source>Required data for paper publication</source> (<publisher-name>Figureshare, v4</publisher-name>). doi:&#xa0;<pub-id pub-id-type="doi">10.6084/m9.Figureshare.14768967.v4</pub-id>
</citation>
</ref>
<ref id="B4">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Chen</surname> <given-names>S. S.</given-names>
</name>
<name>
<surname>Sun</surname> <given-names>Q. L.</given-names>
</name>
<name>
<surname>Lu</surname> <given-names>K.</given-names>
</name>
<name>
<surname>Hovland</surname> <given-names>M.</given-names>
</name>
<name>
<surname>Li</surname> <given-names>R. H.</given-names>
</name>
<name>
<surname>Luo</surname> <given-names>P.</given-names>
</name>
</person-group> (<year>2017</year>). <article-title>Anomalous depressions in the northern yellow Sea basin: Evidences for their evolution processes</article-title>. <source>Mar. Pet. Geol.</source> <volume>84</volume>, <fpage>179</fpage>&#x2013;<lpage>194</lpage>. doi: <pub-id pub-id-type="doi">10.1016/j.marpetgeo.2017.03.030</pub-id>
</citation>
</ref>
<ref id="B5">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Coughlan</surname> <given-names>M.</given-names>
</name>
<name>
<surname>Roy</surname> <given-names>S.</given-names>
</name>
<name>
<surname>O&#x2019;Sullivan</surname> <given-names>C.</given-names>
</name>
<name>
<surname>Clements</surname> <given-names>A.</given-names>
</name>
<name>
<surname>O&#x2019;Toole</surname> <given-names>R.</given-names>
</name>
<name>
<surname>Plets</surname> <given-names>R.</given-names>
</name>
</person-group> (<year>2021</year>). <article-title>Geological settings and controls of fluid migration and associated seafloor seepage features in the north Irish Sea</article-title>. <source>Mar. Pet. Geol.</source> <volume>123</volume>, <fpage>104762</fpage>. doi: <pub-id pub-id-type="doi">10.1016/j.marpetgeo.2020.104762</pub-id>
</citation>
</ref>
<ref id="B6">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Cui</surname> <given-names>Z. K.</given-names>
</name>
<name>
<surname>Cai</surname> <given-names>C. L.</given-names>
</name>
<name>
<surname>Shi</surname> <given-names>J.</given-names>
</name>
<name>
<surname>Zhang</surname> <given-names>Y. B.</given-names>
</name>
</person-group> (<year>2014</year>). <article-title>Geohazard factors and their impact on submarine pipeline safety in the East China Sea</article-title>. <source>Mar. Geol. Front.</source> <volume>30</volume> (<issue>01</issue>), <fpage>48</fpage>&#x2013;<lpage>54</lpage>.</citation>
</ref>
<ref id="B7">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Cukur</surname> <given-names>D.</given-names>
</name>
<name>
<surname>Krastel</surname> <given-names>S.</given-names>
</name>
<name>
<surname>Tomonaga</surname> <given-names>Y.</given-names>
</name>
<name>
<surname>&#xc7;a&#x11f;atay</surname> <given-names>M. N.</given-names>
</name>
<name>
<surname>Meydan</surname> <given-names>A. F.</given-names>
</name>
</person-group> (<year>2013</year>). <article-title>Seismic evidence of shallow gas from lake van, eastern Turkey</article-title>. <source>Mar. Pet. Geol.</source> <volume>48</volume>, <fpage>341</fpage>&#x2013;<lpage>353</lpage>. doi: <pub-id pub-id-type="doi">10.1016/j.marpetgeo.2013.08.017</pub-id>
</citation>
</ref>
<ref id="B8">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Diez</surname> <given-names>R.</given-names>
</name>
<name>
<surname>Garc&#xed;a-Gil</surname> <given-names>S.</given-names>
</name>
<name>
<surname>Dur&#xe1;n</surname> <given-names>R.</given-names>
</name>
<name>
<surname>Vilas</surname> <given-names>F.</given-names>
</name>
</person-group> (<year>2007</year>). <article-title>Gas accumulations and their association with particle size distribution patterns in the r&#xed;a de arousa seabed (Galicia, NW spain): An application of discriminant analysis</article-title>. <source>Geo-Mar. Lett.</source> <volume>27</volume>, <fpage>89</fpage>. doi: <pub-id pub-id-type="doi">10.1007/s00367-007-0064-4</pub-id>
</citation>
</ref>
<ref id="B9">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Etiope</surname> <given-names>G.</given-names>
</name>
<name>
<surname>Ciotoli</surname> <given-names>G.</given-names>
</name>
<name>
<surname>Schwietzke</surname> <given-names>S.</given-names>
</name>
<name>
<surname>Schoell</surname> <given-names>M.</given-names>
</name>
</person-group> (<year>2019</year>). <article-title>Gridded maps of geological methane emissions and their isotopic signature</article-title>. <source>Earth Syst. Sci. Data</source> <volume>11</volume> (<issue>1</issue>), <fpage>1</fpage>&#x2013;<lpage>22</lpage>. doi: <pub-id pub-id-type="doi">10.5194/essd-11-1-2019</pub-id>
</citation>
</ref>
<ref id="B10">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Etiope</surname> <given-names>G.</given-names>
</name>
<name>
<surname>Schwietzke</surname> <given-names>S.</given-names>
</name>
</person-group> (<year>2019</year>). <article-title>Global geological methane emissions: An update of top-down and bottom-up estimates</article-title>. <source>Elementa-Sci. Anthrop.</source> <volume>7</volume>, <fpage>47</fpage>. doi: <pub-id pub-id-type="doi">10.1525/elementa.383</pub-id>
</citation>
</ref>
<ref id="B11">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Fan</surname> <given-names>D.</given-names>
</name>
<name>
<surname>Li</surname> <given-names>C.</given-names>
</name>
<name>
<surname>Wang</surname> <given-names>D.</given-names>
</name>
<name>
<surname>Wang</surname> <given-names>P.</given-names>
</name>
<name>
<surname>Archer</surname> <given-names>A. W.</given-names>
</name>
<name>
<surname>Greb</surname> <given-names>S.</given-names>
</name>
</person-group> (<year>2004</year>). <article-title>Morphology and sedimentation on open-coast intertidal flats of the changjiang delta, China</article-title>. <source>J. Coast. Res.</source> <volume>43</volume>, <fpage>23</fpage>&#x2013;<lpage>35</lpage>.</citation>
</ref>
<ref id="B12">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Fan</surname> <given-names>D.</given-names>
</name>
<name>
<surname>Wu</surname> <given-names>Y.</given-names>
</name>
<name>
<surname>Zhang</surname> <given-names>Y.</given-names>
</name>
<name>
<surname>Burr</surname> <given-names>G.</given-names>
</name>
<name>
<surname>Huo</surname> <given-names>M.</given-names>
</name>
<name>
<surname>Li</surname> <given-names>J.</given-names>
</name>
</person-group> (<year>2017</year>). <article-title>South flank of the Yangtze delta: Past, present, and future</article-title>. <source>Mar. Geol.</source> <volume>392</volume>, <fpage>78</fpage>&#x2013;<lpage>93</lpage>. doi: <pub-id pub-id-type="doi">10.1016/j.margeo.2017.08.015</pub-id>
</citation>
</ref>
<ref id="B13">
<citation citation-type="book">
<person-group person-group-type="author">
<name>
<surname>Fannin</surname> <given-names>N.</given-names>
</name>
</person-group> (<year>1980</year>). <article-title>The use of regional geological surveys in the North Sea and adjacent areas in the recognition of offshore hazards</article-title>. <source>Safety in Offshore Drilling: The Role of Gas Surveys</source>, <publisher-name>Kluwer Academic Publishers</publisher-name>, <publisher-loc>Dordrecht</publisher-loc>.</citation>
</ref>
<ref id="B14">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Feng</surname> <given-names>X.</given-names>
</name>
</person-group> (<year>2017</year>). <article-title>Geochemical characterization of shallow Holocene biogenic gas source rocks in the coastal plain of Jiangsu and Zhejiang</article-title>. <source>Nanjing University (in Chinese with English abstract)</source>.</citation>
</ref>
<ref id="B15">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Fleischer</surname> <given-names>P.</given-names>
</name>
<name>
<surname>Orsi</surname> <given-names>T. H.</given-names>
</name>
<name>
<surname>Richardson</surname> <given-names>M. D.</given-names>
</name>
<name>
<surname>Anderson</surname> <given-names>A. L.</given-names>
</name>
</person-group> (<year>2001</year>). <article-title>Distribution of free gas in marine sediments: A global overview</article-title>. <source>Geo-Mar. Lett.</source> <volume>21</volume>, <fpage>103</fpage>&#x2013;<lpage>122</lpage>. doi: <pub-id pub-id-type="doi">10.1007/s003670100072</pub-id>
</citation>
</ref>
<ref id="B16">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Floodgate</surname> <given-names>G. D.</given-names>
</name>
<name>
<surname>Judd</surname> <given-names>A. G.</given-names>
</name>
</person-group> (<year>1992</year>). <article-title>The origins of shallow gas</article-title>. <source>Cont. Shelf Res.</source> <volume>12</volume> (<issue>10</issue>), <fpage>1145</fpage>&#x2013;<lpage>1156</lpage>. doi: <pub-id pub-id-type="doi">10.1016/0278-4343(92)90075-U</pub-id>
</citation>
</ref>
<ref id="B17">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Flury</surname> <given-names>S.</given-names>
</name>
<name>
<surname>R&#xf8;y</surname> <given-names>H.</given-names>
</name>
<name>
<surname>Dale</surname> <given-names>A. W.</given-names>
</name>
<name>
<surname>Fossing</surname> <given-names>H.</given-names>
</name>
<name>
<surname>T&#xf3;th</surname> <given-names>Z.</given-names>
</name>
<name>
<surname>Spiess</surname> <given-names>V.</given-names>
</name>
<etal/>
</person-group>. (<year>2016</year>). <article-title>Controls on subsurface methane fluxes and shallow gas formation in Baltic Sea sediment (Aarhus bay, Denmark)</article-title>. <source>Geochim. Cosmochim. Acta</source> <volume>188</volume>, <fpage>297</fpage>&#x2013;<lpage>309</lpage>. doi: <pub-id pub-id-type="doi">10.1016/j.gca.2016.05.037</pub-id>
</citation>
</ref>
<ref id="B18">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Garc&#xed;a-Garc&#xed;a</surname> <given-names>A.</given-names>
</name>
<name>
<surname>Orange</surname> <given-names>D. L.</given-names>
</name>
<name>
<surname>Miserocchi</surname> <given-names>S.</given-names>
</name>
<name>
<surname>Correggiari</surname> <given-names>A.</given-names>
</name>
<name>
<surname>Langone</surname> <given-names>L.</given-names>
</name>
<name>
<surname>Lorenson</surname> <given-names>T. D.</given-names>
</name>
<etal/>
</person-group>. (<year>2007</year>). <article-title>What controls the distribution of shallow gas in the Western Adriatic Sea</article-title>? <source>Cont. Shelf Res.</source> <volume>27</volume> (<issue>3-4</issue>), <fpage>359</fpage>&#x2013;<lpage>374</lpage>. doi: <pub-id pub-id-type="doi">10.1016/j.csr.2006.11.003</pub-id>
</citation>
</ref>
<ref id="B19">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Guo</surname> <given-names>X. J.</given-names>
</name>
<name>
<surname>Yan</surname> <given-names>X. X.</given-names>
</name>
<name>
<surname>Zheng</surname> <given-names>S. W.</given-names>
</name>
<name>
<surname>Wang</surname> <given-names>H. M.</given-names>
</name>
<name>
<surname>Yin</surname> <given-names>P.</given-names>
</name>
</person-group> (<year>2021</year>). <article-title>Characteristics of high-resolution subaqueous micro-topography in the jinshan deep trough and its implications for riverbed deformation, hangzhou bay, China</article-title>. <source>Estuar. Coast. Shelf Sci.</source> <volume>250</volume>, <fpage>107147</fpage>. doi: <pub-id pub-id-type="doi">10.1016/j.ecss.2020.107147</pub-id>
</citation>
</ref>
<ref id="B20">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Gupta</surname> <given-names>S.</given-names>
</name>
<name>
<surname>Schmidt</surname> <given-names>C.</given-names>
</name>
<name>
<surname>B&#xf6;ttner</surname> <given-names>C.</given-names>
</name>
<name>
<surname>R&#xfc;pke</surname> <given-names>L.</given-names>
</name>
<name>
<surname>Hartz</surname> <given-names>E. H.</given-names>
</name>
</person-group> (<year>2022</year>). <article-title>Spontaneously exsolved free gas during major storms as an ephemeral gas source for pockmark formation</article-title>. <source>Geochem. Geophys. Geosyst.</source> <volume>23</volume>, <elocation-id>e2021GC010289</elocation-id>. doi: <pub-id pub-id-type="doi">10.1029/2021GC010289</pub-id>
</citation>
</ref>
<ref id="B21">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Hovland</surname> <given-names>M.</given-names>
</name>
<name>
<surname>Judd</surname> <given-names>A. G.</given-names>
</name>
</person-group> (<year>1988</year>). <article-title>Seabed pockmarks and seepages: Impact on geology, biology and the marine environment</article-title>. <source>Graham Trotman</source>. <volume>244</volume>, <fpage>590</fpage>&#x2013;<lpage>591</lpage>. doi:&#xa0;<pub-id pub-id-type="doi">10.13140/RG.2.1.1414.1286</pub-id>
</citation>
</ref>
<ref id="B22">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Hu</surname> <given-names>X. Q.</given-names>
</name>
<name>
<surname>Gu</surname> <given-names>Z. F.</given-names>
</name>
<name>
<surname>Zhang</surname> <given-names>X. H.</given-names>
</name>
<name>
<surname>Zhao</surname> <given-names>L. H.</given-names>
</name>
<name>
<surname>Xing</surname> <given-names>Z. H.</given-names>
</name>
</person-group> (<year>2016</year>). <article-title>Seismic shape features and distribution of shallow gas in the sea area off the Yangtze river estuary</article-title>. <source>Mar. Geol. Quat. Geol.</source> <volume>36</volume> (<issue>01</issue>), <fpage>151</fpage>&#x2013;<lpage>157</lpage>.</citation>
</ref>
<ref id="B23">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Hu</surname> <given-names>Y.</given-names>
</name>
<name>
<surname>Li</surname> <given-names>H. D.</given-names>
</name>
<name>
<surname>Xu</surname> <given-names>J.</given-names>
</name>
</person-group> (<year>2012</year>). <article-title>Shallow gas accumulation in a small estuary and its implications: A case history from in and around xiamen bay</article-title>. <source>Geophys. Res. Lett.</source> <volume>39</volume>, <fpage>L24605</fpage>. doi: <pub-id pub-id-type="doi">10.1029/2012GL054478</pub-id>
</citation>
</ref>
<ref id="B24">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Huang</surname> <given-names>Y.</given-names>
</name>
<name>
<surname>Han</surname> <given-names>X.</given-names>
</name>
</person-group> (<year>2020</year>). <article-title>Features of earthquake-induced seabed liquefaction and mitigation strategies of novel marine structures</article-title>. <source>J. Mar. Sci. Eng.</source> <volume>8</volume> (<issue>5</issue>), <fpage>310</fpage>. doi: <pub-id pub-id-type="doi">10.3390/jmse8050310</pub-id>
</citation>
</ref>
<ref id="B25">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Hustoft</surname> <given-names>S.</given-names>
</name>
<name>
<surname>B&#xfc;nz</surname> <given-names>S.</given-names>
</name>
<name>
<surname>Mienert</surname> <given-names>J.</given-names>
</name>
</person-group> (<year>2010</year>). <article-title>Three-dimensional seismic analysis of the morphology and spatial distribution of chimneys beneath the nyegga pockmark field, offshore mid-Norway</article-title>. <source>Basin Res.</source> <volume>22</volume>, <fpage>465</fpage>&#x2013;<lpage>480</lpage>. doi: <pub-id pub-id-type="doi">10.1111/j.1365-2117.2010.00486.x</pub-id>
</citation>
</ref>
<ref id="B26">
<citation citation-type="book">
<person-group person-group-type="author">
<collab>IPCC</collab>
</person-group> (<year>2021</year>). <source>Climate change 2021: The physical science basis: Contribution of working group I to the sixth assessment report of the intergovernmental panel on climate change</source> (<publisher-loc>Cambridge</publisher-loc>: <publisher-name>Cambridge University Press</publisher-name>).</citation>
</ref>
<ref id="B27">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Ja&#x15b;niewicz</surname> <given-names>D.</given-names>
</name>
<name>
<surname>Klusek</surname> <given-names>Z.</given-names>
</name>
<name>
<surname>Brodecka-Goluch</surname> <given-names>A.</given-names>
</name>
<name>
<surname>Bola&#x142;ek</surname> <given-names>J.</given-names>
</name>
</person-group> (<year>2019</year>). <article-title>Acoustic investigations of shallow gas in the southern Baltic Sea (Polish exclusive economic zone): a review</article-title>. <source>Geo-Mar Lett.</source> <volume>39</volume>, <fpage>1</fpage>&#x2013;<lpage>17</lpage>. doi: <pub-id pub-id-type="doi">10.1007/s00367-018-0555-5</pub-id>
</citation>
</ref>
<ref id="B28">
<citation citation-type="book">
<person-group person-group-type="author">
<name>
<surname>Judd</surname> <given-names>A.</given-names>
</name>
<name>
<surname>Hovland</surname> <given-names>M.</given-names>
</name>
</person-group> (<year>2007</year>). <source>Seabed fluid flow: The impact on geology, biology and the marine environment</source> (<publisher-name>Cambridge University Press</publisher-name>), <fpage>475</fpage>.</citation>
</ref>
<ref id="B29">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Koch</surname> <given-names>S.</given-names>
</name>
<name>
<surname>Berndt</surname> <given-names>C.</given-names>
</name>
<name>
<surname>Bialas</surname> <given-names>J.</given-names>
</name>
<name>
<surname>Haeckel</surname> <given-names>M.</given-names>
</name>
<name>
<surname>Crutchley</surname> <given-names>G.</given-names>
</name>
<name>
<surname>Papenberg</surname> <given-names>C.</given-names>
</name>
<etal/>
</person-group>. (<year>2015</year>). <article-title>Gas-controlled seafloor doming</article-title>. <source>Geology</source> <volume>43</volume> (<issue>7</issue>), <fpage>571</fpage>&#x2013;<lpage>574</lpage>. doi: <pub-id pub-id-type="doi">10.1130/G36596.1</pub-id>
</citation>
</ref>
<ref id="B30">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Kramer</surname> <given-names>K.</given-names>
</name>
<name>
<surname>Holler</surname> <given-names>P.</given-names>
</name>
<name>
<surname>Herbst</surname> <given-names>G.</given-names>
</name>
</person-group> (<year>2017</year>). <article-title>Abrupt emergence of a large pockmark field in the German bight, southeastern north Sea</article-title>. <source>Sci. Rep.</source> <volume>7</volume> (<issue>1</issue>), <fpage>5150</fpage>. doi:&#xa0;<pub-id pub-id-type="doi">10.1038/s41598-017-05536-1</pub-id>
</citation>
</ref>
<ref id="B31">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Letcher</surname> <given-names>T. M.</given-names>
</name>
</person-group> (<year>2019</year>). <article-title>Why do we have global warming</article-title>? <source>Manage. Glob. Warm.</source> <volume>1</volume>, <fpage>3</fpage>&#x2013;<lpage>15</lpage>. doi:&#xa0;<pub-id pub-id-type="doi">10.1016/B978-0-12-814104-5.00001-6</pub-id>
</citation>
</ref>
<ref id="B32">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Li</surname> <given-names>X. F.</given-names>
</name>
</person-group> (<year>2020</year>). <article-title>Damage and repair of subsea pipelines in shallow gas areas</article-title>. <source>Oil. Gas. Stor. Transp.</source> <volume>11</volume>, <fpage>1310</fpage>&#x2013;<lpage>1315</lpage>.</citation>
</ref>
<ref id="B33">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Li</surname> <given-names>P.</given-names>
</name>
<name>
<surname>Du</surname> <given-names>J.</given-names>
</name>
<name>
<surname>Liu</surname> <given-names>L. J.</given-names>
</name>
<name>
<surname>Cao</surname> <given-names>C. X.</given-names>
</name>
<name>
<surname>Xu</surname> <given-names>Y. Q.</given-names>
</name>
</person-group> (<year>2010</year>). <article-title>Distribution characteristics of offshore shallow gas in China</article-title>. <source>Chin. J. Geol. Hazards. Prev.</source> <volume>21</volume> (<issue>01</issue>), <fpage>69</fpage>&#x2013;<lpage>74</lpage>.</citation>
</ref>
<ref id="B34">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Li</surname> <given-names>C.</given-names>
</name>
<name>
<surname>Fan</surname> <given-names>D.</given-names>
</name>
<name>
<surname>Yang</surname> <given-names>S.</given-names>
</name>
<name>
<surname>Cai</surname> <given-names>J.</given-names>
</name>
</person-group> (<year>2008</year>). <article-title>Characterization and formation of the late quaternary incised valley sequence in the estuarine delta region of China</article-title>. <source>J. Paleogeogr.</source> <volume>10</volume> (<issue>1</issue>), <fpage>87</fpage>&#x2013;<lpage>97</lpage>. doi:&#xa0;<pub-id pub-id-type="doi">10.2110/pec.06.85.0141</pub-id>
</citation>
</ref>
<ref id="B35">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Li</surname> <given-names>G. X.</given-names>
</name>
<name>
<surname>Li</surname> <given-names>P.</given-names>
</name>
<name>
<surname>Liu</surname> <given-names>Y.</given-names>
</name>
<name>
<surname>Qiao</surname> <given-names>L. L.</given-names>
</name>
<name>
<surname>Ma</surname> <given-names>Y. Y.</given-names>
</name>
<name>
<surname>Xu</surname> <given-names>J. S.</given-names>
</name>
<etal/>
</person-group>. (<year>2014</year>). <article-title>Sedimentary system response to the global sea level change in the East China seas since the last glacial maximum</article-title>. <source>Earth-Sci. Rev.</source> <volume>139</volume>, <fpage>390</fpage>&#x2013;<lpage>405</lpage>. doi: <pub-id pub-id-type="doi">10.1016/j.earscirev.2014.09.007</pub-id>
</citation>
</ref>
<ref id="B36">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Lin</surname> <given-names>C. M.</given-names>
</name>
<name>
<surname>Li</surname> <given-names>Y. L.</given-names>
</name>
<name>
<surname>Zhuo</surname> <given-names>H. C.</given-names>
</name>
<name>
<surname>Shurr</surname> <given-names>G. W.</given-names>
</name>
<name>
<surname>Ridgley</surname> <given-names>J. L.</given-names>
</name>
<name>
<surname>Zhang</surname> <given-names>Z. P.</given-names>
</name>
<etal/>
</person-group>. (<year>2010</year>). <article-title>Features and sealing mechanism of shallow biogenic gas in incised valley fills (the qiantang river, eastern china): A case study</article-title>. <source>Mar. Pet. Geol.</source> <volume>27</volume> (<issue>4</issue>), <fpage>909</fpage>&#x2013;<lpage>922</lpage>. doi: <pub-id pub-id-type="doi">10.1016/j.marpetgeo.2009.11.006</pub-id>
</citation>
</ref>
<ref id="B37">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Lin</surname> <given-names>C. M.</given-names>
</name>
<name>
<surname>Zhang</surname> <given-names>X.</given-names>
</name>
<name>
<surname>Xu</surname> <given-names>Z. Y.</given-names>
</name>
<name>
<surname>Deng</surname> <given-names>C. W.</given-names>
</name>
<name>
<surname>Yin</surname> <given-names>Y.</given-names>
</name>
<name>
<surname>Cheng</surname> <given-names>Q. Q.</given-names>
</name>
</person-group> (<year>2015</year>). <article-title>Sedimentary characteristics and accumulation conditions of shallow-biogenic gas for the late quaternary sediments in the changjiang river delta area</article-title>. <source>Adv. Earth Sci.</source> <volume>30</volume> (<issue>05</issue>), <fpage>589</fpage>&#x2013;<lpage>601</lpage>.</citation>
</ref>
<ref id="B38">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Liu</surname> <given-names>Y. F.</given-names>
</name>
</person-group> (<year>2019</year>). <article-title>Recent evolution process and influencing factors of jinshan deep trough in the hangzhou bay</article-title>. <source>East China Normal Univ</source>.</citation>
</ref>
<ref id="B39">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Liu</surname> <given-names>X. Y.</given-names>
</name>
<name>
<surname>Feng</surname> <given-names>X. L.</given-names>
</name>
<name>
<surname>Sun</surname> <given-names>Y. F.</given-names>
</name>
<name>
<surname>Chen</surname> <given-names>Y. L.</given-names>
</name>
<name>
<surname>Tang</surname> <given-names>Q. H.</given-names>
</name>
<name>
<surname>Zhou</surname> <given-names>X. H.</given-names>
</name>
<etal/>
</person-group>. (<year>2019</year>). <article-title>Acoustic and biological characteristics of seafloor depressions in the north yellow Sea basin of China: Active fluid seepage in shallow water seafloor</article-title>. <source>Mar. Geol.</source> <volume>414</volume>, <fpage>34</fpage>&#x2013;<lpage>46</lpage>. doi: <pub-id pub-id-type="doi">10.1016/j.margeo.2019.05.002</pub-id>
</citation>
</ref>
<ref id="B40">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Luan</surname> <given-names>H. L.</given-names>
</name>
<name>
<surname>Ding</surname> <given-names>P. X.</given-names>
</name>
<name>
<surname>Yang</surname> <given-names>S. L.</given-names>
</name>
<name>
<surname>Wang</surname> <given-names>Z. B.</given-names>
</name>
</person-group> (<year>2021</year>). <article-title>Accretion-erosion conversion in the subaqueous Yangtze delta in response to fluvial sediment decline</article-title>. <source>Geomorphology</source> <volume>382</volume>, <fpage>107680</fpage>. doi: <pub-id pub-id-type="doi">10.1016/j.geomorph.2021.107680</pub-id>
</citation>
</ref>
<ref id="B41">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Martinez-Carre&#xf1;o</surname> <given-names>N.</given-names>
</name>
<name>
<surname>Garcia-Gil</surname> <given-names>S.</given-names>
</name>
</person-group> (<year>2013</year>). <article-title>The Holocene gas system of the ria de vigo (NW spain): Factors controlling the location of gas accumulations, seeps and pockmarks</article-title>. <source>Mar. Geol.</source> <volume>344</volume>, <fpage>82</fpage>&#x2013;<lpage>100</lpage>. doi: <pub-id pub-id-type="doi">10.1016/j.margeo.2013.07.012</pub-id>
</citation>
</ref>
<ref id="B42">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Mogoll&#xf3;n</surname> <given-names>J. M.</given-names>
</name>
<name>
<surname>Dale</surname> <given-names>A. W.</given-names>
</name>
<name>
<surname>Fossing</surname> <given-names>H.</given-names>
</name>
<name>
<surname>Regnier</surname> <given-names>P.</given-names>
</name>
</person-group> (<year>2012</year>). <article-title>Timescales for the development of methanogenesis and free gas layers in recently-deposited sediments of arkona basin (Baltic Sea)</article-title>. <source>Biogeosciences</source> <volume>9</volume>, <fpage>1915</fpage>&#x2013;<lpage>1933</lpage>. doi: <pub-id pub-id-type="doi">10.5194/bg-9-1915-2012</pub-id>
</citation>
</ref>
<ref id="B43">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Mogoll&#xf3;n</surname> <given-names>J. M.</given-names>
</name>
<name>
<surname>Dale</surname> <given-names>A. W.</given-names>
</name>
<name>
<surname>Tensen.</surname> <given-names>J. B.</given-names>
</name>
</person-group> (<year>2013</year>). <article-title>A method for the calculation of anaerobic oxidation of methane rates across regional scales: An example from the belt seas and the sound (North Sea&#x2013;Baltic Sea transition)</article-title>. <source>Geo-Mar. Lett.</source> <volume>33</volume>, <fpage>299</fpage>&#x2013;<lpage>310</lpage>. doi: <pub-id pub-id-type="doi">10.1007/s00367-013-0329-z</pub-id>
</citation>
</ref>
<ref id="B44">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Ni</surname> <given-names>Y. Q.</given-names>
</name>
<name>
<surname>Geng</surname> <given-names>Z. X.</given-names>
</name>
<name>
<surname>Zhu</surname> <given-names>J. Z.</given-names>
</name>
</person-group> (<year>2003</year>). <article-title>Study on the hydrodynamic characteristics of hangzhou bay</article-title>. <source>Hydrodyn. Res. Prog.</source> <volume>18</volume> (<issue>4</issue>), <fpage>439</fpage>&#x2013;<lpage>445</lpage>.</citation>
</ref>
<ref id="B45">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Nyman</surname> <given-names>A.</given-names>
</name>
<name>
<surname>Rinne</surname> <given-names>H.</given-names>
</name>
<name>
<surname>Salovius-Lauren</surname> <given-names>S.</given-names>
</name>
<name>
<surname>Vallius</surname> <given-names>H.</given-names>
</name>
</person-group> (<year>2020</year>). <article-title>The distribution and characterization of gas domes in lumparn bay, &#xc5;land islands, northern Baltic Sea</article-title>. <source>J. Mar. Syst.</source> <volume>208</volume>, <fpage>103359</fpage>. doi: <pub-id pub-id-type="doi">10.1016/j.jmarsys.2020.103359</pub-id>
</citation>
</ref>
<ref id="B46">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Qu</surname> <given-names>C. W.</given-names>
</name>
<name>
<surname>Zhang</surname> <given-names>X.</given-names>
</name>
<name>
<surname>Lin</surname> <given-names>C. M.</given-names>
</name>
<name>
<surname>Chen</surname> <given-names>S. Y.</given-names>
</name>
<name>
<surname>Li</surname> <given-names>Y. L.</given-names>
</name>
<name>
<surname>Pan</surname> <given-names>F.</given-names>
</name>
<etal/>
</person-group>. (<year>2013</year>). <article-title>Characteristics of capillary sealing mechanism of late quaternary shallow biogenic gas in the hangzhou bay area</article-title>. <source>Adv. Earth Sci.</source> <volume>28</volume> (<issue>2</issue>), <fpage>209</fpage>&#x2013;<lpage>220</lpage>.</citation>
</ref>
<ref id="B47">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Rasmussen</surname> <given-names>R. A.</given-names>
</name>
<name>
<surname>Khalil</surname> <given-names>M. A.</given-names>
</name>
</person-group> (<year>1986</year>). <article-title>Atmospheric trace gases: Trends and distributions over the last decade</article-title>. <source>Science</source> <volume>232</volume> (<issue>4758</issue>), <fpage>1623</fpage>&#x2013;<lpage>1624</lpage>. doi:&#xa0;<pub-id pub-id-type="doi">10.1126/science.232.4758.1623</pub-id>
</citation>
</ref>
<ref id="B48">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Ren</surname> <given-names>F.</given-names>
</name>
<name>
<surname>Fan</surname> <given-names>D.</given-names>
</name>
<name>
<surname>Wu</surname> <given-names>Y.</given-names>
</name>
<name>
<surname>Zhao</surname> <given-names>Q.</given-names>
</name>
</person-group> (<year>2019</year>). <article-title>The evolution of hypoxia off the changjiang estuary in the last 3000 years: Evidence from benthic foraminifera and elemental geochemistry</article-title>. <source>Mar. Geol.</source> <volume>417</volume>, <fpage>106039</fpage>. doi: <pub-id pub-id-type="doi">10.1016/j.margeo.2019.106039</pub-id>
</citation>
</ref>
<ref id="B49">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Rice</surname> <given-names>D. D.</given-names>
</name>
<name>
<surname>Claypool</surname> <given-names>G. E.</given-names>
</name>
</person-group> (<year>1981</year>). <article-title>Generation, accumulation, and resource potential of biogenic gas</article-title>. <source>AAPG Bulletin.</source> <volume>65</volume> (<issue>1</issue>), <fpage>5</fpage>&#x2013;<lpage>25</lpage>. doi:&#xa0;<pub-id pub-id-type="doi">10.1306/2F919765-16CE-11D7-8645000102C1865D</pub-id>
</citation>
</ref>
<ref id="B50">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Schneider von Deimling</surname> <given-names>J.</given-names>
</name>
<name>
<surname>Weinrebe</surname> <given-names>W.</given-names>
</name>
<name>
<surname>T&#xf3;th</surname> <given-names>Zs.</given-names>
</name>
<name>
<surname>Fossing</surname> <given-names>H.</given-names>
</name>
<name>
<surname>Endler</surname> <given-names>R.</given-names>
</name>
<name>
<surname>Rehder</surname> <given-names>G.</given-names>
</name>
<etal/>
</person-group>. (<year>2013</year>). <article-title>A low frequency multibeam assessment: Spatial mapping of shallow gas by enhanced penetration and angular response anomaly</article-title>. <source>Mar. Pet. Geol.</source> <volume>44</volume>, <fpage>217</fpage>&#x2013;<lpage>222</lpage>. doi: <pub-id pub-id-type="doi">10.1016/j.marpetgeo.2013.02.013</pub-id>
</citation>
</ref>
<ref id="B51">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Sills</surname> <given-names>G. C.</given-names>
</name>
<name>
<surname>Wheeler</surname> <given-names>S. J.</given-names>
</name>
</person-group> (<year>1992</year>). <article-title>The significance of gas for offshore operations</article-title>. <source>Cont. Shelf Res.</source> <volume>12</volume>, <fpage>1239</fpage>&#x2013;<lpage>1250</lpage>. doi: <pub-id pub-id-type="doi">10.1016/0278-4343(92)90083-V</pub-id>
</citation>
</ref>
<ref id="B52">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Song</surname> <given-names>Y. P.</given-names>
</name>
<name>
<surname>Sun</surname> <given-names>Y. F.</given-names>
</name>
<name>
<surname>Song</surname> <given-names>B. H.</given-names>
</name>
<name>
<surname>Dong</surname> <given-names>L. F.</given-names>
</name>
<name>
<surname>Du</surname> <given-names>X.</given-names>
</name>
</person-group> (<year>2021</year>). <article-title>Comparative study on the liquefaction properties of seabed silt under wave loading in the huanghe river delta</article-title>. <source>Acta Oceanol. Sin.</source> <volume>43</volume> (<issue>06</issue>), <fpage>129</fpage>&#x2013;<lpage>138</lpage>.</citation>
</ref>
<ref id="B53">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Southon</surname> <given-names>J.</given-names>
</name>
<name>
<surname>Kashgarian</surname> <given-names>M.</given-names>
</name>
<name>
<surname>Fontugne</surname> <given-names>M.</given-names>
</name>
<name>
<surname>Metivier</surname> <given-names>B.</given-names>
</name>
<name>
<surname>Yim</surname> <given-names>W. W. S.</given-names>
</name>
</person-group> (<year>2002</year>). <article-title>Marine reservoir corrections for the Indian Ocean and southeast Asia</article-title>. <source>Radiocarbon</source> <volume>44</volume>, <fpage>167</fpage>&#x2013;<lpage>180</lpage>.</citation>
</ref>
<ref id="B54">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Stuiver</surname> <given-names>M.</given-names>
</name>
<name>
<surname>Reimer</surname> <given-names>P. J.</given-names>
</name>
</person-group> (<year>1993</year>). <article-title>Extended (super 14) C data base and revised CALIB 3.0 (super 14) C age calibration program</article-title>. <source>Radiocarbon</source> <volume>35</volume> (<issue>1</issue>), <fpage>215</fpage>&#x2013;<lpage>230</lpage>.</citation>
</ref>
<ref id="B55">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Su</surname> <given-names>J.</given-names>
</name>
<name>
<surname>Fan</surname> <given-names>D.</given-names>
</name>
<name>
<surname>Leng</surname> <given-names>W.</given-names>
</name>
<name>
<surname>Chen</surname> <given-names>L.</given-names>
</name>
<name>
<surname>Yin</surname> <given-names>P.</given-names>
</name>
</person-group> (<year>2017</year>). <article-title>Postglacial sequence stratigraphy and sedimentary environment evolution of the Yangtze river subaqueous delta</article-title>. <source>J. Palaeogeogr.</source> <volume>19</volume> (<issue>03</issue>), <fpage>541</fpage>&#x2013;<lpage>556</lpage>.</citation>
</ref>
<ref id="B56">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Su</surname> <given-names>J.</given-names>
</name>
<name>
<surname>Fan</surname> <given-names>D.</given-names>
</name>
<name>
<surname>Liu</surname> <given-names>J.</given-names>
</name>
<name>
<surname>Wu</surname> <given-names>Y.</given-names>
</name>
</person-group> (<year>2020</year>). <article-title>Anatomy of the transgressive depositional system in a sediment-rich tide-dominated estuary: The paleo-Yangtze estuary, China</article-title>. <source>Mar. Pet. Geol.</source> <volume>121</volume>, <fpage>104588</fpage>. doi: <pub-id pub-id-type="doi">10.1016/j.marpetgeo.2020.104588</pub-id>
</citation>
</ref>
<ref id="B57">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Sumer</surname> <given-names>B. M.</given-names>
</name>
<name>
<surname>Truelsen</surname> <given-names>C.</given-names>
</name>
<name>
<surname>Freds&#xf8;e</surname> <given-names>J.</given-names>
</name>
</person-group> (<year>2006</year>). <article-title>Liquefaction around pipelines under waves</article-title>. <source>J. Waterw. Port Coast. Ocean Eng.</source> <volume>132</volume> (<issue>4</issue>), <fpage>266</fpage>&#x2013;<lpage>275</lpage>. doi: <pub-id pub-id-type="doi">10.1061/(ASCE)0733-950X(2006)132:4(266)</pub-id>
</citation>
</ref>
<ref id="B58">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Sun</surname> <given-names>M. S.</given-names>
</name>
<name>
<surname>Zhang</surname> <given-names>G. L.</given-names>
</name>
<name>
<surname>Ma.</surname> <given-names>X.</given-names>
</name>
<name>
<surname>Cao</surname> <given-names>X. P.</given-names>
</name>
<name>
<surname>Mao</surname> <given-names>X. Y.</given-names>
</name>
<name>
<surname>Ye</surname> <given-names>W. W.</given-names>
</name>
<etal/>
</person-group>. (<year>2018</year>). <article-title>Dissolved methane in the East China Sea: Distribution, seasonal variation and emission</article-title>. <source>Mar. Chem.</source> <volume>202</volume>, <fpage>12</fpage>&#x2013;<lpage>26</lpage>. doi: <pub-id pub-id-type="doi">10.1016/j.marchem.2018.03.001</pub-id>
</citation>
</ref>
<ref id="B59">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Toker</surname> <given-names>M.</given-names>
</name>
<name>
<surname>Tur</surname> <given-names>H.</given-names>
</name>
</person-group> (<year>2021</year>). <article-title>Shallow seismic characteristics and distribution of gas in lacustrine sediments at lake er&#xe7;ek, Eastern Anatolia, Turkey, from high&#x2212;resolution seismic data</article-title>. <source>Environ. Earth Sci.</source> <volume>80</volume>, <fpage>727</fpage>. doi: <pub-id pub-id-type="doi">10.1007/s12665-021-10039-4</pub-id>
</citation>
</ref>
<ref id="B60">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Ulyanova</surname> <given-names>M.</given-names>
</name>
<name>
<surname>Sivkov</surname> <given-names>V.</given-names>
</name>
<name>
<surname>Kanapatskij</surname> <given-names>T.</given-names>
</name>
<name>
<surname>Sigalevich</surname> <given-names>P.</given-names>
</name>
<name>
<surname>Pimenov</surname> <given-names>N.</given-names>
</name>
</person-group> (<year>2012</year>). <article-title>Methane fluxes in the southeastern Baltic Sea</article-title>. <source>Geo-Mar Lett.</source> <volume>32</volume>, <fpage>535</fpage>&#x2013;<lpage>544</lpage>. doi: <pub-id pub-id-type="doi">10.1007/s00367-012-0304-0</pub-id>
</citation>
</ref>
<ref id="B61">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Visnovitz</surname> <given-names>F.</given-names>
</name>
<name>
<surname>Bodn&#xe1;r</surname> <given-names>T.</given-names>
</name>
<name>
<surname>T&#xf3;th</surname> <given-names>Z.</given-names>
</name>
<name>
<surname>Spiess</surname> <given-names>V.</given-names>
</name>
<name>
<surname>Kud&#xf3;</surname> <given-names>I.</given-names>
</name>
<name>
<surname>Tim&#xe1;r</surname> <given-names>G.</given-names>
</name>
<etal/>
</person-group>. (<year>2015</year>). <article-title>Seismic expressions of shallow gas in the lacustrine deposits of lake balaton, Hungary</article-title>. <source>Near Surf Geophys.</source> <volume>13</volume>, <fpage>433</fpage>&#x2013;<lpage>446</lpage>. doi: <pub-id pub-id-type="doi">10.3997/1873-0604.2015026</pub-id>
</citation>
</ref>
<ref id="B62">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Wang</surname> <given-names>H.</given-names>
</name>
<name>
<surname>Liu</surname> <given-names>H. J.</given-names>
</name>
</person-group> (<year>2016</year>). <article-title>Evaluation of storm wave-induced silty seabed instability and geo-hazards: A case study in the yellow river delta</article-title>. <source>Appl. Ocean Res.</source> <volume>58</volume>, <fpage>135</fpage>&#x2013;<lpage>145</lpage>. doi: <pub-id pub-id-type="doi">10.1016/j.apor.2016.03.013</pub-id>
</citation>
</ref>
<ref id="B63">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Wang</surname> <given-names>H.</given-names>
</name>
<name>
<surname>Liu</surname> <given-names>H. J.</given-names>
</name>
<name>
<surname>Zhang</surname> <given-names>M. S.</given-names>
</name>
<name>
<surname>Wang</surname> <given-names>X. H.</given-names>
</name>
</person-group> (<year>2016</year>). <article-title>Wave-induced seepage and its possible contribution to the formation of pockmarks in the huanghe (Yellow) river delta</article-title>. <source>Chin. J. Oceanol. Limnol.</source> <volume>34</volume> (<issue>1</issue>), <fpage>200</fpage>&#x2013;<lpage>211</lpage>. doi:&#xa0;<pub-id pub-id-type="doi">10.1007/s00343-015-4245-0</pub-id>
</citation>
</ref>
<ref id="B64">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Wang</surname> <given-names>Z. H.</given-names>
</name>
<name>
<surname>Saito</surname> <given-names>Y.</given-names>
</name>
<name>
<surname>Zhan</surname> <given-names>Q.</given-names>
</name>
<name>
<surname>Nian</surname> <given-names>X. M.</given-names>
</name>
<name>
<surname>Pan</surname> <given-names>D. D.</given-names>
</name>
<name>
<surname>Wang</surname> <given-names>L.</given-names>
</name>
<etal/>
</person-group>. (<year>2018</year>). <article-title>Three-dimensional evolution of the Yangtze river mouth, China during the Holocene: Impacts of sea level, climate and human activity</article-title>. <source>Earth-Sci. Rev.</source> <volume>185</volume>, <fpage>938</fpage>&#x2013;<lpage>955</lpage>. doi: <pub-id pub-id-type="doi">10.1016/j.earscirev.2018.08.012</pub-id>
</citation>
</ref>
<ref id="B65">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Wang</surname> <given-names>H.</given-names>
</name>
<name>
<surname>Su</surname> <given-names>L.</given-names>
</name>
<name>
<surname>Zhang</surname> <given-names>M. S.</given-names>
</name>
<name>
<surname>Liu</surname> <given-names>H. J.</given-names>
</name>
</person-group> (<year>2020</year>). <article-title>Effects of storm wave-induced liquefaction on lateral deformation of monopile-type offshore wind turbines in silt seabed</article-title>. <source>J. Mar. Environ. Eng.</source> <volume>10</volume> (<issue>3</issue>), <fpage>225</fpage>&#x2013;<lpage>242</lpage>.</citation>
</ref>
<ref id="B66">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Wang</surname> <given-names>Y.</given-names>
</name>
<name>
<surname>Yan</surname> <given-names>X. X.</given-names>
</name>
<name>
<surname>Wang</surname> <given-names>Z. H.</given-names>
</name>
<name>
<surname>Zhao</surname> <given-names>B. C.</given-names>
</name>
<name>
<surname>Zhan</surname> <given-names>Q.</given-names>
</name>
</person-group> (<year>2019</year>). <article-title>Shallow seismic facies characteristics of the modern underwater delta of the Yangtze river</article-title>. <source>Mar. Geol. Quat. Geol.</source> <volume>39</volume> (<issue>02</issue>), <fpage>114</fpage>&#x2013;<lpage>122</lpage>.</citation>
</ref>
<ref id="B67">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Wang</surname> <given-names>M. J.</given-names>
</name>
<name>
<surname>Zheng</surname> <given-names>H. B.</given-names>
</name>
<name>
<surname>Yang</surname> <given-names>S. Y.</given-names>
</name>
<name>
<surname>Fan</surname> <given-names>D. D.</given-names>
</name>
</person-group> (<year>2012</year>). <article-title>Environmental information in subaqueous Yangtze river delta since Holocene</article-title>. <source>J. Tongji Univ. (Nat. Sci.)</source> <volume>40</volume> (<issue>03</issue>), <fpage>473</fpage>&#x2013;<lpage>477</lpage>.</citation>
</ref>
<ref id="B68">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Wever</surname> <given-names>T. F.</given-names>
</name>
<name>
<surname>L&#xfc;hder</surname> <given-names>R.</given-names>
</name>
<name>
<surname>Knispel</surname> <given-names>U.</given-names>
</name>
</person-group> (<year>2006</year>). <article-title>Potential environmental control of free shallow gas in the seafloor of eckernf&#xf6;rde bay, Germany</article-title>. <source>Mar. Geol.</source> <volume>225</volume>, <fpage>1</fpage>&#x2013;<lpage>4</lpage>. doi: <pub-id pub-id-type="doi">10.1016/j.margeo.2005.08.005</pub-id>
</citation>
</ref>
<ref id="B69">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Whiticar</surname> <given-names>M. J.</given-names>
</name>
</person-group> (<year>1999</year>). <article-title>Carbon and hydrogen isotope systematics of bacterial formation and oxidation of methane</article-title>. <source>Chem. Geol.</source> <volume>161</volume>, <fpage>291</fpage>&#x2013;<lpage>314</lpage>. doi: <pub-id pub-id-type="doi">10.1016/S0009-2541(99)00092-3</pub-id>
</citation>
</ref>
<ref id="B70">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Xie</surname> <given-names>D. F.</given-names>
</name>
<name>
<surname>Pan</surname> <given-names>C. H.</given-names>
</name>
<name>
<surname>Cao</surname> <given-names>Y.</given-names>
</name>
<name>
<surname>Zhang</surname> <given-names>B. H.</given-names>
</name>
</person-group> (<year>2013</year>). <article-title>Decadal variations in the erosion/deposition pattern of the hangzhou bay and their mechanism in recent 50a</article-title>. <source>Acta Oceanol. Sin.</source> <volume>35</volume> (<issue>04</issue>), <fpage>121</fpage>&#x2013;<lpage>128</lpage>.</citation>
</ref>
<ref id="B71">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Xie</surname> <given-names>D. F.</given-names>
</name>
<name>
<surname>Pan</surname> <given-names>C. H.</given-names>
</name>
<name>
<surname>Wu</surname> <given-names>X. G.</given-names>
</name>
<name>
<surname>Gao</surname> <given-names>S.</given-names>
</name>
<name>
<surname>Wang</surname> <given-names>Z. B.</given-names>
</name>
</person-group> (<year>2017</year>). <article-title>Local human activities overwhelm decreased sediment supply from the changjiang river: Continued rapid accumulation in the hangzhou bay-qiantang estuary system</article-title>. <source>Mar. Geol.</source> <volume>392</volume>, <fpage>66</fpage>&#x2013;<lpage>77</lpage>. doi: <pub-id pub-id-type="doi">10.1016/j.margeo.2017.08.013</pub-id>
</citation>
</ref>
<ref id="B72">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Xu</surname> <given-names>Y. S.</given-names>
</name>
<name>
<surname>Wu</surname> <given-names>H. N.</given-names>
</name>
<name>
<surname>Shen</surname> <given-names>J. S.</given-names>
</name>
<name>
<surname>Zhang</surname> <given-names>N.</given-names>
</name>
</person-group> (<year>2017</year>). <article-title>Risk and impacts on the environment of free-phase biogas in quaternary deposits along the coastal region of shanghai</article-title>. <source>Ocean Eng.</source> <volume>137</volume>, <fpage>129</fpage>&#x2013;<lpage>137</lpage>. doi: <pub-id pub-id-type="doi">10.1016/j.oceaneng.2017.03.051</pub-id>
</citation>
</ref>
<ref id="B73">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Yang</surname> <given-names>J. H.</given-names>
</name>
<name>
<surname>Lai</surname> <given-names>X. H.</given-names>
</name>
<name>
<surname>Chen</surname> <given-names>Z. X.</given-names>
</name>
</person-group> (<year>2021</year>). <article-title>Analysis of submarine pockmarked landforms and their genesis in the waters of Qingbang Island, eastern Zhoushan Islands</article-title>. <source>J Appl. Oceanogr</source>. <volume>40</volume> (<issue>02</issue>), <fpage>251</fpage>&#x2013;<lpage>259</lpage>.</citation>
</ref>
<ref id="B74">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Yang</surname> <given-names>X. D.</given-names>
</name>
<name>
<surname>Chun</surname> <given-names>M. H.</given-names>
</name>
<name>
<surname>Luo</surname> <given-names>X. Q.</given-names>
</name>
<name>
<surname>Yao</surname> <given-names>Z. G.</given-names>
</name>
</person-group> (<year>2022</year>b). <article-title>Research on application of seismic attribute analysis in identification of subsea shallow gas</article-title>. <source>Coast. Eng.</source> <volume>01)</volume>, <fpage>26</fpage>&#x2013;<lpage>36</lpage>.</citation>
</ref>
<ref id="B75">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Yang</surname> <given-names>H. F.</given-names>
</name>
<name>
<surname>Li</surname> <given-names>B. C.</given-names>
</name>
<name>
<surname>Yang</surname> <given-names>S. L.</given-names>
</name>
<name>
<surname>Zhang</surname> <given-names>Z. L.</given-names>
</name>
<name>
<surname>Xu</surname> <given-names>K. H.</given-names>
</name>
<name>
<surname>Chen</surname> <given-names>C. P.</given-names>
</name>
<etal/>
</person-group>. (<year>2022</year>a). <article-title>Impacts of large projects on the sediment dynamics and evolution of the hengsha shoal in the Yangtze delta</article-title>. <source>Ocean Eng.</source> <volume>261</volume>, <fpage>112030</fpage>. doi: <pub-id pub-id-type="doi">10.1016/j.oceaneng.2022.112030</pub-id>
</citation>
</ref>
<ref id="B76">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Yang</surname> <given-names>S. L.</given-names>
</name>
<name>
<surname>Milliman</surname> <given-names>J. D.</given-names>
</name>
<name>
<surname>Li</surname> <given-names>P.</given-names>
</name>
<name>
<surname>Xu</surname> <given-names>K.</given-names>
</name>
</person-group> (<year>2011</year>). <article-title>50,000 dams later: Erosion of the Yangtze river and its delta</article-title>. <source>Glob. Planet. Change</source> <volume>75</volume> (<issue>1-2</issue>), <fpage>14</fpage>&#x2013;<lpage>20</lpage>. doi: <pub-id pub-id-type="doi">10.1016/j.gloplacha.2010.09.006</pub-id>
</citation>
</ref>
<ref id="B77">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Yang</surname> <given-names>X. D.</given-names>
</name>
<name>
<surname>Yao</surname> <given-names>Z. G.</given-names>
</name>
<name>
<surname>Chun</surname> <given-names>M. H.</given-names>
</name>
<name>
<surname>Luo</surname> <given-names>X. Q.</given-names>
</name>
</person-group> (<year>2019</year>). <article-title>Research on detection technology of subsea well mouth</article-title>. <source>Coast. Eng.</source> <volume>38</volume> (<issue>03</issue>), <fpage>232</fpage>&#x2013;<lpage>239</lpage>.</citation>
</ref>
<ref id="B78">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Ye</surname> <given-names>Y. C.</given-names>
</name>
<name>
<surname>Chen</surname> <given-names>J. R.</given-names>
</name>
<name>
<surname>Pan</surname> <given-names>G. F.</given-names>
</name>
<name>
<surname>Liu</surname> <given-names>K.</given-names>
</name>
</person-group> (<year>2003</year>). <article-title>A study of formation cause, existing characteristics of the shallow gas and its danger to engineering</article-title>. <source>Donghai Mar. Sci.</source> <volume>01)</volume>, <fpage>27</fpage>&#x2013;<lpage>36</lpage>.</citation>
</ref>
<ref id="B79">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Zhan</surname> <given-names>Q.</given-names>
</name>
<name>
<surname>Wang</surname> <given-names>Z.</given-names>
</name>
<name>
<surname>Xie</surname> <given-names>Y.</given-names>
</name>
<name>
<surname>Xie</surname> <given-names>J.</given-names>
</name>
<name>
<surname>He</surname> <given-names>Z.</given-names>
</name>
</person-group> (<year>2012</year>). <article-title>Assessing C/N and &#x3b4;<sup>13</sup>C as indicators of Holocene sea level and freshwater discharge changes in the subaqueous Yangtze delta, China</article-title>. <source>Holocene</source> <volume>22</volume>, <fpage>697</fpage>&#x2013;<lpage>704</lpage>. doi: <pub-id pub-id-type="doi">10.1177/0959683611423685</pub-id>
</citation>
</ref>
<ref id="B80">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Zhang</surname> <given-names>X.</given-names>
</name>
<name>
<surname>Lin</surname> <given-names>C. M.</given-names>
</name>
</person-group> (<year>2017</year>). <article-title>Characteristics and accumulation model of the late quaternary shallow biogenic gas in the modern changjiang delta area, eastern China</article-title>. <source>Pet. Sci.</source> <volume>14</volume>, <fpage>261</fpage>&#x2013;<lpage>275</lpage>. doi: <pub-id pub-id-type="doi">10.1007/s12182-017-0157-2</pub-id>
</citation>
</ref>
<ref id="B81">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Zhang</surname> <given-names>X.</given-names>
</name>
<name>
<surname>Lin</surname> <given-names>C. M.</given-names>
</name>
<name>
<surname>Gao</surname> <given-names>S.</given-names>
</name>
<name>
<surname>Robert</surname> <given-names>W. D.</given-names>
</name>
<name>
<surname>Qu</surname> <given-names>C. W.</given-names>
</name>
<name>
<surname>Yin</surname> <given-names>Y.</given-names>
</name>
<etal/>
</person-group>. (<year>2013</year>a). <article-title>Sedimentary sequence and distribution pattern of filling in qiantang river incised valley</article-title>. <source>J. Paleogeogr.</source> <volume>15</volume> (<issue>06</issue>), <fpage>839</fpage>&#x2013;<lpage>852</lpage>.</citation>
</ref>
<ref id="B82">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Zhang</surname> <given-names>X.</given-names>
</name>
<name>
<surname>Lin</surname> <given-names>C. M.</given-names>
</name>
<name>
<surname>Li</surname> <given-names>Y. L.</given-names>
</name>
<name>
<surname>Qu</surname> <given-names>C. W.</given-names>
</name>
<name>
<surname>Wang</surname> <given-names>S. J.</given-names>
</name>
</person-group> (<year>2013</year>b). <article-title>Sealing mechanism for cap beds of shallow-biogenic gas reservoirs in the qiantang river incised valley, China</article-title>. <source>Cont. Shelf Res.</source> <volume>69</volume>, <fpage>155</fpage>&#x2013;<lpage>167</lpage>. doi: <pub-id pub-id-type="doi">10.1016/j.csr.2013.09.006</pub-id>
</citation>
</ref>
<ref id="B83">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Zhang</surname> <given-names>X. Y.</given-names>
</name>
<name>
<surname>Dai</surname> <given-names>Z. J.</given-names>
</name>
<name>
<surname>Feng</surname> <given-names>L. X.</given-names>
</name>
<name>
<surname>Huang</surname> <given-names>Z. M.</given-names>
</name>
</person-group> (<year>2021</year>). <article-title>Study of the erosion-siltation state of the Jinshan deep trough on the north coast of Hangzhou Bay</article-title>. <source>Adv Mar. Sci.</source> <volume>39</volume> (<issue>04</issue>), <fpage>535</fpage>&#x2013;<lpage>547</lpage>.</citation>
</ref>
<ref id="B84">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Zhang</surname> <given-names>G. L.</given-names>
</name>
<name>
<surname>Zhang</surname> <given-names>J.</given-names>
</name>
<name>
<surname>Kang</surname> <given-names>Y. B.</given-names>
</name>
<name>
<surname>Liu</surname> <given-names>S. M.</given-names>
</name>
</person-group> (<year>2004</year>). <article-title>Distributions and fluxes of methane in the East China Sea and the yellow Sea in spring</article-title>. <source>J. Geophys. Res.</source> <volume>109</volume>, <fpage>C07011</fpage>. doi: <pub-id pub-id-type="doi">10.1029/2004JC002268</pub-id>
</citation>
</ref>
<ref id="B85">
<citation citation-type="journal">
<person-group person-group-type="author">
<name>
<surname>Zhang</surname> <given-names>G. L.</given-names>
</name>
<name>
<surname>Zhang</surname> <given-names>J.</given-names>
</name>
<name>
<surname>Liu</surname> <given-names>S. M.</given-names>
</name>
<name>
<surname>Ren</surname> <given-names>J. L.</given-names>
</name>
<name>
<surname>Xu</surname> <given-names>J.</given-names>
</name>
<name>
<surname>Zhang</surname> <given-names>F.</given-names>
</name>
</person-group> (<year>2008</year>). <article-title>Methane in the changjiang (Yangtze river) estuary and its adjacent marine area: Riverine input, sediment release and atmospheric fluxes</article-title>. <source>Biogeochemistry</source> <volume>91</volume> (<issue>1</issue>), <fpage>71</fpage>&#x2013;<lpage>84</lpage>. doi: <pub-id pub-id-type="doi">10.1007/s10533-008-9259-7</pub-id>
</citation>
</ref>
</ref-list>
</back>
</article>