Edited by: Liangcheng Tan, Chinese Academy of Sciences, China
Reviewed by: Guanghui Dong, Lanzhou University, China; Ruoyu Sun, Tianjin University, China
This article was submitted to Quaternary Science, Geomorphology and Paleoenvironment, a section of the journal Frontiers in Earth Science
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As a toxic and harmful global pollutant, mercury enters the environment through natural sources, and human activities. Based on large numbers of previous studies, this paper summarized the characteristics of mercury deposition and the impacts of climate change and human activities on mercury deposition from a global perspective. The results indicated that global mercury deposition changed synchronously, with more accumulation during the glacial period and less accumulation during the interglacial period. Mercury deposition fluctuated greatly during the Early Holocene but was stable and low during the Mid-Holocene. During the Late Holocene, mercury deposition reached the highest value. An increase in precipitation promotes a rise in forest litterfall Hg deposition. Nevertheless, there is a paucity of research on the mechanisms of mercury deposition affected by long-term humidity changes. Mercury accumulation was relatively low before the Industrial Revolution ca. 1840, while after industrialization, intensive industrial activities produced large amounts of anthropogenic mercury emissions and the accumulation increased rapidly. Since the 1970s, the center of global mercury production has gradually shifted from Europe and North America to Asia. On the scale of hundreds of thousands of years, mercury accumulation was greater in cold periods and less in warm periods, reflecting exogenous dust inputs. On millennial timescales, the correspondence between mercury deposition and temperature is less significant, as the former is more closely related to volcanic eruption and human activities. However, there remains significant uncertainties such as non-uniform distribution of research sites, lack of mercury deposition reconstruction with a long timescale and sub-century resolution, and the unclear relationship between precipitation change and mercury accumulation.
As a toxic and harmful global pollutant, mercury enters the environment through natural sources (volcanic eruptions, oceans, soil, and forests, etc.), and human activities (fossil fuel combustion; gold, silver, and mercury mining; non-ferrous metal smelting; etc.;
In recent years, there have been many studies reconstructing atmospheric mercury deposition during historical periods using the natural archives. These studies focused mainly on the Northern Hemisphere, whereas studies in the Southern Hemisphere were limited to Chile (
This paper identified and analyzed relevant studies on atmospheric mercury deposition regarding climate change and human activities from a global perspective, discussing the prospects for future research. The global mercury deposition history reflected the global climate change and regional human activities during historical periods. Our work is conducive to the systematic understanding of mercury deposition and its influencing factors. On this basis, new methods and perspectives can be developed to conduct more in depth research.
The data used in this study were mainly based on peer-reviewed papers and the data for Dajiu Lake Basin from our research group. The authors searched for documents in Elsevier SDOL, Nature, Science, Google Scholar, and the CNKI database, among others, using keywords such as “mercury deposition/mercury accumulation,” “climate change,” and “anthropogenic activities/human activities.” Following the principles of a uniform spatial distribution and an extensive timescale, 36 sets of mercury deposition data from 60 papers were selected, covering the history of atmospheric mercury deposition on six continents (Asia, Europe, Africa, North America, South America, and Antarctica) recorded by natural archives (fen peat, lacustrine deposits, ice cores, and marine deposits, etc.) from 670 ka BP to now, as shown in
Information of global mercury deposition research spots.
a1 | The Upper Fremont Glacier | Wyoming, United States | 44°20′02″ N 111°36′49″ W | 4100 | Ice core | 160 | Isotope and chemical dating | 1720–1993 AD | |
a2 | San Francisco Bay | United States | 37°40′00″ N 122°25′00″ W | / | Coastal salt marsh | 1.6 | 137Cs, 210Pb | 1850–2000 AD | |
a3 | Southern Baffifin Island | Arctic Canada | 63°57′53″ N 68°15′42″ W | 1825 | Firn | 22.7 | δ18O | 1970–2010 AD | |
a4 | The Bay of Fundy | Canada | 45°12′36″ N 66°54′12″ W | / | Peat bogs, Lake sediments, Coastal salt marsh | / | 137Cs, 210Pb | 1800–2000 AD | |
a5 | Lake Ontario | Canada | 49°48′00″ N 93°48′00″ W | / | Lake sediments | / | 137Cs, 210Pb | 1850–2000 AD | |
a6 | Ellesmere Island | The Canadian High Arctic Archipelago | 78°43′00″ N 74°27′00″ W | 323 | Lake sediments | 12.1 | 137Cs, 210Pb | 1850–2000AD | |
a7 | Caribou Bog | Central Maine, United States | 44°58′97″ N 68°48′35″ W | 80 | Peat bogs | 5 | 210Pb, 14C | 8000 BC–2000 AD | |
a8 | Lost Lake | Wyoming, United States | 43°46′55″ N 110°06′00″ W | 2889 | Lake sediments | 21 | 137Cs, 210Pb | 1350–2000 AD | |
a9 | Oyster Point | California, United States | 45°25′28″ N 111°20′28″ W | / | Coastal salt marsh | 3 | 14C | 1700–2000 AD | |
a10 | Arlberg Bog | Minnesota, United States | 46°56′00″ N 92°41′00″ W | / | Peat bogs | 0.6 | 137Cs, 210Pb | 1700–2000 AD | |
a11 | Narraguinnep Reservoir | Colorado, United States | 40°31′26″ N 106°58′43″ W | 2035 | Lake sediments | 10 | 137Cs | 1950–2000 AD | |
a12 | Patroon Reservoir | New York, United States | 42°41′05″ N 73°47′21″ W | / | Lake sediments | 3 | / | 1955–2000 AD | |
b1 | Etang de la Gruère | Swiss Jura Mountains | 47°15′34″ N 07°03′51″ E | 1005 | Peat bogs | 6.5 | 210Pb, 241Am, 14C | 12500 BC–2000 AD | |
b2 | Franches Montagnes | Swiss Jura Mountains | 47°14′23″ N 07°02′57″ E | 1020 | Peat bogs | 0.8 | 210Pb, 14C | 1200 BC–2000 AD | |
b3 | Lake Lucerne | Central Switzerland | 46°13′25″ N 07°40′28″ E | 2661 | Lake sediments | 1.2 | 14C | 14315 BC–2000 AD | |
b4 | Roñanzas Bog | Asturias, Spain | 43°20′13″ N 04°51′01″ W | / | Peat bogs | 2 | / | 6000 BC–2000 AD | |
b5 | Galicia | Northwest Spain | 43°32′00″ N 07°34′00″ W | / | Peat bogs | 2.5 | 14C | 2635 BC–1995 AD | |
b6 | Lake Montcortès | Pyrenees, Spain | 42°19′00″ N 00°19′00″ E | 1031 | Lake sediments | 1.1 | 210Pb, 14C | 1386–2010 AD | |
b7 | Chao de Lamoso bog | Xistral Mountains, northwest Spian | 43°14′35″ N 09°05′45″ W | 1039 | Peat bogs | 1 | 210Pb | 1825–2000 AD | |
b8 | Portlligat Bay | Iberian Peninsula | 42°17′32″ N 03°17′28″ E | / | Coastal salt marsh | 5 | 14C | 2315 BC–2000 AD | |
b9 | Storelung Mose | Denmark | 55°15′23″ N 10°15′22″ E | / | Peat bogs | 1 | 14C | 2000 BC–2000 AD | |
b10 | Sandhavn | Southern Greenland | 59°59′54″ N 44°46′36″ W | / | Peat bogs | 0.4 | 210Pb, 14C | 1270–2000 AD | |
b11 | Raeburn Flow | Scotland, United Kingdom | 55°02′04″ N 03°06′27″ W | / | Peat bogs | 3.6 | 210Pb, 14C | 1385 BC–2005 AD | |
b12 | Six ombrotrophic bogs | Norway | 58°–69° N 04°–12° E | / | Peat bogs | 1 | 210Pb, 14C | 2000 BC–2000 AD | |
b13 | Svalbard | Norwegian Arctic | 80°03′05″ N 17°37′26″ E | / | Lake sediments | / | 210Pb | 1800–1995 AD | |
b14 | Store Mosse | South-central Sweden | 57°15′00″ N 13°55′00″ E | / | Peat bogs | 0.4 | 137Cs, 210Pb | 1860–2000 AD | |
b15 | Marano and Grado Lagoon | Northern Adriatic Sea | 40°12′–45°53′ N 12°13′–19°34′ E | / | Coastal salt marsh | 1.1 | 137Cs, 210Pb | 1650–2000 AD | |
b16 | Misten peat bog | Eastern Belgium | 50°38′28″ N 03°11′17″ E | / | Peat bogs | 1 | 210Pb, 14C | 431–2011 AD | |
b17 | Brdy Hills | The Czech Republic | 49°42′42″ N 13°52′30″ E | / | Peat bogs | 0.38 | 210Pb | 1807–2003 AD | |
b18 | Multiple lakes | Across England | 50°–55° N 04° W–02° E | 3–244 | Lake sediments | 1.5 | 137Cs, 210Pb, 241Am | 1850–2010 AD | |
c1 | Tanghongling | Heilongjiang Province, northeast China | 46°42′–48°40′ N 129°05′–129°55′ E | / | Peat bogs | 1.05 | 210Pb, 14C | 4480 BC–2000 AD | |
c2 | Dajiu Lake Basin | Hubei Province, China | 31°29′27″ N 109°59′45″ E | 1760 | Peat bogs | 2.97 | 14C | 14141 BC–2004 AD | |
c3 | Motianling mountain | Western Great Hinggan Mountains, China | 46°39′–47°39′ N 119°28′–121°23′ E | 1200 | Peat bogs | 0.78 | 137Cs, 210Pb | 1820–2005 AD | |
c4 | Tibetan Plateau | Southwest China | 28°41′–37°17′ N 85°23′–100°16′ E | 2813–4652 | Lake sediments | 0.4 | 137Cs, 210Pb, 241Am, 226Ra | 1830–2010 AD | |
c5 | Qinghai Lake | Northeast Tibetan Plateau, China | 36°24′00″ N 100°09′00″ E | / | Lake sediments | 0.205 | 137Cs, 210Pb, 241Am, 226Ra | 1860–2000 AD | |
c6 | Guangjin Islan | South China Sea | 16°27′07″ N 111°42′05″ E | / | Eggshells | 1.05 | 210Pb, 14C | 1280–2000 AD | |
c7 | Xisha Islands | South China Sea | 15°47′–17°08′ N 110°10′–112°55′ E | / | Coral sand | 0.95 | 210Pb, 14C | 1300–2000 AD | |
c8 | Lake Sayram | Xinjiang Province, China | 44°30′–44°42′ N 81°05′–81°15′ E | 2072 | Lake sediments | 0.3 | 210Pb | 1810–2010 AD | |
c9 | Hani peat bog | Jilin Province, China | 42°12′50″ N 126°31′05″ E | 882–900 | Peat bogs | 9 | 14C | 11927 BC–2010 AD | |
c10 | Okinawa Trough | East China Sea | 27°29′50″ N 126°41′16″ E | / | Marginal pelagic sediments | 4.95 | 14C | 17990 BC–2010 AD | |
c11 | East China Sea | China | 26°–32° N 120°–126° E | / | Marginal pelagic sediments | 1.4 | 137Cs, 210Pb | 1913–2015 AD | |
c12 | Shanghai | China | 31°26′17″ N 121°23′02″ E | / | Lake sediments | 0.35 | 137Cs, 210Pb | 1750–2010 AD | |
c13 | Chao Lake | Anhui Province, China | 31°25′–31°43′ N 117°16′–117°51′ E | / | Lake sediments | 0.3 | 210Pb | 1890–2009 AD | |
c14 | Hongyuan | Sichuan Province, China | 32°46′46″ N 102°30′58″ E | 3510 | Peat bogs | 0.25 | 210Pb | 1850–2006 AD | |
c15 | Huguangyan Maar Lake | Guangdong Province, China | 21°09′00″ N 110°17′00″ E | / | Lake sediments | 0.94 | 137Cs, 14C | 766–2005 AD | |
c16 | Huguangyan Maar Lake | Guangdong Province, China | 21°09′00″ N 110°17′00″ E | / | Lake sediments | 1.175 | 137Cs, 210Pb, 14C | 1350–2004 AD | |
d1 | Lake Titicaca region | Bolivia | 15°50′–16°13′ S 68°03′–68°17′ W | 3760–4040 | Peat bogs | 1.63 | 14C | 11430 BC–2014 AD | |
d2 | Lake Chungará | Chile | 18°15′07″ S 69°09′47″ W | 4520 | Lake sediments | 1.46 | 137Cs, 210Pb | 686 BC–2014 AD | |
d3 | Lake Hambre | Patagonia, Chile | 53°36′13″ S 70°57′08″ W | 80 | Lake sediments | 13.94 | 14C | 14722 BC–2008 AD | |
d4 | The Gran Campo bog | Magellanic Moorlands, Chile | 52°47′26″ S 72°56′37″ W | / | Peat bogs | 1.5 | 210Pb, 14C | 466 BC–1995 AD | |
d5 | Lake Futalaufquen | Patagonia, Chile | 42°49′00″ S 71°43′00″ W | 518 | Lake sediments | 0.79 | 137Cs | 400–2000 AD | |
d6 | Pinheiro Mire | Minas Gerais, Brazil | 18°03′44″ S 43°39′42″ W | 1230–1270 | Peat bogs | 2.2 | 14C | 54990 BC–2010 AD | |
d7 | Yanacocha | Southeast Peru | 13°56′42″ S 70°52′30″ W | 4910 | Lake sediments | 3.33 | 210Pb, 14C | 10290 BC–2010 AD | |
d8 | Huancavelica | Central Peru | 14°51′27″ S 75°24′38″ W | / | Lake sediments | / | 210Pb, 14C | 2800 BC–2000 AD | |
e1 | Berg River | South Africa | 32°47′26″ S 18°12′05″ E | / | Coastal salt marsh | 0.3 | 210Pb | 1900–2007 AD | |
e2 | Lake Tanganyika | East Africa | 04°40′–06°34′ S 29°37′–29°59′ W | / | Lake sediments | 0.28 | 210Pb, 14C | 1600–2000 AD | |
f1 | King George Island | Western Antarctica | 62°11′57″ S 59°58′48″ W | / | Seal hairs | 0.425 | 137Cs, 14C | 18–2002 AD | |
f2 | Adélie Basin | Southern Antarctica | 66°12′53″ S 140°26′17″ E | / | Diatom ooze sediments | 170 | Diatom fossils | 6600 BC–2000 AD | |
f3 | Dome C | Antarctica | 75°06′00″ S 123°21′00″ E | 3233 | Ice core | 3062.4 | δ18O | 652000 BC–2000 AD | |
f4 | Dome C | Antarctica | 77°39′00″ S 124°10′00″ E | 3240 | Ice core | 905 | δ18O | 31710 BC–1990 AD |
According to the global studies on atmospheric mercury deposition, the authors found that the site distribution of global mercury deposition studies was characterized by an overall pattern of “more in the north and fewer in the south”; that is, the research areas were mostly distributed in the Northern Hemisphere, such as Europe (including Belgium, the Czech Republic, Denmark, Greenland, Ireland, Norway, Scotland, Spain, Sweden, and Switzerland), North America (including the United States and Canada), and China (including the Northeast Hani, Shennongjia Dajiu Lake Basin, Greater Khingan Range, Lesser Khingan Range, Sichuan Hongyuan, Huguangyan Maar Lake, Chao Lake, Tibetan Plateau, Xinjiang Mount Tianshan, East China Sea, Yellow Sea, and South China Sea), while in the Southern Hemisphere, the areas were distributed only in South America (including Chile, Peru, Brazil, and Bolivia), Africa (including the Berg River of South Africa and Lake Tanganyika in East Africa), and some parts of Antarctica, as shown in
Distribution of mercury deposition research spots. 1. Bering Sea (
At the global scale, atmospheric mercury deposition is closely related to climate variability (such as temperature and humidity changes) and human activities (such as mining, metal smelting, and other industrial activities). Due to the interaction between these factors and regional differences in human activities, the mercury deposition changes in different regions showed both certain commonalities and relatively large differences through time. Based on previous studies, the history of global mercury deposition was divided into the following four stages:
The Dome C ice core in Antarctica provided a mercury deposition time series for past 670 ka (
Mercury deposition and temperature change in Antarctica since 670,000 a BP. The data were based on the research results of Petru Jitaru et al. in Dome C ice core of Antarctica, the red curve represents δD (‰), indicating the temperature change (
Changes of mercury deposition in different time scales around the world.
Post YD cooling, in general there was warming in to an interglacial climate. However, in the Early Holocene, the climate was still unstable and there were many climate oscillations sourced in the North Atlantic area, including the 11.1 ka BP event, 10.3 ka BP event, 9.4 ka BP event, 8.2 ka BP event, and possibly other climate variability (
Many studies showed that the climate in the Mid-Holocene was generally warm and humid (
The Late Holocene is a stage when human civilization has developed rapidly and human activities have had a profound impact on natural systems. Since the Industrial Revolution, the population has increased dramatically and the impact of human activities on earth system reached an unprecedented magnitude. Under the multiple effects of natural systems and human activities, regional and global atmospheric mercury deposition changed significantly and the rate of atmospheric mercury accumulation increased to its highest value during historical periods. Atmospheric mercury deposition between 4.2 and 0 ka BP in all regions of the world continued to rise (
Through the comparison of atmospheric mercury deposition data at a global scale, it could be inferred that the characteristics of mercury deposition in European countries represented by Switzerland, Greenland, Spain, and Belgium, among others, are similar to those of the North American countries represented by the United States and Canada. Specifically, beginning with the Industrial Revolution in the mid-19th century, the mercury deposition rate increased rapidly, and mercury accumulation reached a peak value in the mid- and late 20th century and then showed a downward trend. With the increasing emphasis on the environmental pollution caused by a large number of mercury emissions in European and North American countries, measures were taken in various regions to reduce mercury emissions (
In studies around the world, the impact of climate change on mercury deposition has been the focus of researchers with various outcomes. Most scholars believe that a cold and dry climate is conducive to mercury accumulation, while a warm and humid climate limits the accumulation of mercury (
Mercury emitted from anthropogenic and natural sources to the atmosphere eventually falls back to the surface through dry and wet deposition. Dry deposition refers to the deposition of aerosol particles. An aerosol is a gas dispersion composed of solid or liquid particles suspended in a gas medium, which has a complex chemical composition (
Atmospheric mercury deposition is a process through which various forms of mercury are removed from the atmosphere. The dry deposition of mercury mainly includes the direct deposition of Hg(0) and RGM, which occurs throughout the year as long as it does not rain heavily. The forms of mercury in wet deposition tend to be soluble and granular Hg(II). Due to the long-distance diffusion and water solubility of gaseous Hg(II), most of the mercury is deposited via Hg(II) dissolved in atmospheric water or adsorbed on the surface of raindrop particles (
At present, the deposition mechanism and long-term monitoring sites of mercury wet deposition flux in many countries are well established. As one of the countries emitting the most mercury in the world, China has yet to establish a relatively complete monitoring system for wet mercury deposition. Additionally, the measurement and evaluation of dry mercury deposition are mostly local or achieved by atmospheric models, and there are few published relevant records.
To evaluate the impact of temperature change on mercury deposition, the mercury deposition records of Maine of the United States, southeastern Peru, the Swiss Jura Mountains, Patagonia of Chile, the Shennongjia Dajiu Lake Basin of China, Maranhão of Brazil, Dome C of Antarctica, and Minas Gerais of Brazil were selected for comparison, as shown in
The relationship between mercury deposition and temperature change.
The main effect of precipitation on mercury deposition is that an increase in precipitation promotes a rise in forest litterfall Hg deposition. Terrestrial vegetation often represents the first ecosystem compartment with which new atmospheric Hg interacts following deposition. It was recently demonstrated that a portion of newly wet-deposited Hg(II) may not initially pass directly through the forest canopy to the forest floor, but rather is retained over the growing season, only to be deposited later with litterfall (
According to a large number of previous studies, atmospheric mercury deposition caused by human activities began approximately 3,500 years ago. Before industrialization, gold, silver, and mercury mining and the widespread use of cinnabar were the main sources of anthropogenic mercury. After industrialization, coal combustion, mercury mining, non-ferrous metal smelting, liquid mercury production, steel manufacturing, cement manufacturing, gold mining, waste incineration, and the development of the chlor-alkali industry led to a sharp increase in anthropogenic emissions of mercury, which in turn caused a sharp increase in mercury deposition.
With the development of human civilization, mercury is being increasingly used in all aspects of society. Cinnabar has a bright red color and never fades, so it has long been used as a pigment. According to the literature, the use of cinnabar can be traced back to the Shang Dynasty (∼1600–1046 BC) in China, and some characters engraved with cinnabar were found on unearthed animal bones or turtle shells (
At approximately 1400 BC, the Huancavelica cinnabar mine in Peru began to be mined. During the period of rapid development and expansion of mining and metallurgy in the Andes (∼500–1000 AD, ∼1000–1400 AD), mercury emissions increased continuously (
Relationship between mercury deposition and human activities since 4000 a BP.
Since the Industrial Revolution of 1840, coal combustion, non-ferrous metal smelting, and the chlor-alkali industry have become the main sources of anthropogenic mercury. Intensive industrial activities have caused unprecedented mercury pollution. According to previous studies, in Spain, the mercury accumulation after industrialization was 10 times as high as the level before industrialization (
Based on the previous analysis, it could be concluded that the main factors influencing mercury deposition include temperature change, human activities, and volcanic eruption, while the influencing factors vary among phases and timescales. According to
This study summarized the characteristics of mercury deposition from a global perspective by analyzing previous studies on mercury deposition on yearly to 100,000-year timescales. It was found that regarding the accumulation of mercury, there are certain commonalities within the regional range, while there are also differences among regions. In addition, both climate change and human activities have a significant impact on mercury deposition.
The common feature of global mercury deposition in the Holocene is that the accumulation was generally lower before the start of the Industrial Revolution of 1840 and posthaste increased rapidly. Alternatively, due to regional volcanic activities or human activities, such as mining and metallurgy, there are differences in mercury accumulations among regions. It is worth noting that since the 1970s, emission reduction measures have been adopted in Europe and North America to reduce mercury accumulation, and Asia has gradually become the global center of anthropogenic mercury emissions.
On the scale of hundreds of thousands of years, mercury accumulated more in cold periods and less in warm periods. On millennial timescales, the correspondence between mercury deposition and temperature change appears non-significant, and the former is more closely related to volcanic eruption and human activities. An increase in precipitation leads to a rise in forest litterfall Hg deposition. Little research has been performed on the effect of long-term humidity change on mercury deposition, with inconsistent conclusions, so the specific impact of humidity change on mercury deposition is unknown.
The atmospheric mercury deposition caused by human activities can be traced back to 3500 years ago. Before industrialization, gold, silver, and mercury mining and the widespread use of cinnabar were the main anthropogenic mercury sources; after industrialization, coal combustion, non-ferrous metal smelting, waste incineration, and chlor-alkali industry development led to a significant increase in anthropogenic emissions of mercury, which in turn led to a sharp increase in mercury deposition.
Although many studies have been conducted on the relationships of mercury deposition with climate change and human activities, with significant results, there remains some urgent challenges from a global perspective.
The mercury deposition monitoring and research sites are unevenly distributed. The research sites are mostly distributed in the Northern Hemisphere, including Western Europe, North America, and China, while the number of research areas distributed in the Southern Hemisphere is relatively small, located in only a few regions of South America, Africa, and Antarctica. There are no research sites in Eastern Europe, Northern and Central Asia, Northern Africa, or Oceania. In future studies, mercury deposition research in these gap areas can be strengthened to provide data for the global reconstruction of the evolutionary history of mercury deposition.
Most previous studies focused on the history of mercury deposition in the Holocene. There have been few studies on a longer timescale and even fewer studies on the relationship between precipitation change and mercury accumulation. As a result, the impact of climate change, especially humidity change, on the mercury accumulation mechanism has yet to be investigated. Therefore, it is suggested that studies of mercury deposition on a longer timescale and of the impact of precipitation on mercury accumulation be strengthened.
There is a lack of high-resolution mercury deposition reconstruction. In previous studies, fen peat and lacustrine deposits were widely used as natural archives, with few ice core records with high resolution and almost no stalagmite records available. To accurately analyze the relationships of mercury deposition with climate change and human activities, mercury deposition reconstruction at a higher resolution should be performed.
FL and CM designed the research. FL presented a synthesis of the state of the art of mercury deposition worldwide. FL and PZ completed the data collection, analysis, and interpretation. All authors listed have made substantial, direct and intellectual contributions to the work, and approved it for publication. All authors contributed to the article and approved the submitted version.
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.
We are grateful to Renhui Huang and Chujun Yuan for their valuable suggestions about the final version of the manuscript. We also acknowledge Yunkai Deng and Haiyan Li for their assistance with the graphics. Our deepest gratitude goes to the two reviewers and the Associate Editor Liangcheng Tan as well as the Chief Editor Steven L. Forman for their careful work and thoughtful suggestions which have helped improve this manuscript substantially.