Chemical Weathering Intensity and Terrigenous Flux in South China during the Last 90,000 Years—Evidence from Magnetic Signals in Marine Sediments

Introduction

Weathering intensity and continental erosion determine the geological progression in near-surface spaces, and these are related to tectonic uplift and climate change (Clift and Giosan, 2014; Zhao and Zheng, 2015). Most eroded terrigenous materials formed by the weathering process are eventually deposited in the oceans and these deposits reveal aspects of the weathering intensity and clastic flux sediments from rivers (Wei et al., 2006). Climate has typically been regarded as the primary determinant of weathering and erosion on a glacial-interglacial time scale. Faster chemical weathering and erosion correlates with more humid, warm climates whereas weaker weathering and less erosion correlates with drier climates (Clift, 2006; Clift et al., 2014; Zhao and Zheng, 2015). In South China, the Asian monsoon has played an important role in the weathering process and river inputs from land to marine after the Cenozoic era. Intense summer monsoons can increase chemical weathering (Derry and France-Lanord, 1996), as well as physical erosion rates. However, some researchers argue that strong physical erosion may be related to a cold climate owing to faster run-off in the absence of drainage capture (Clift et al., 2014). Major and trace elements taken from sediments of the South China Sea have been used to study links between weathering and monsoon paleo-intensity (Wei et al., 2004, 2006; Hu et al., 2012, 2013). However, these studies are inadequate to clarify the relationships between chemical weathering, physical erosion, and monsoon variations.

We used core sediments collected from the north part of the South China Sea to examine evidence for changes in physical erosion and chemical weathering in the last 90,000 years. Magnetic properties IRM_AF80mT (Saturation Isothermal Remanent Magnetization after 80 mT alternation field demagnetization) associated with the element ratios Al₂O₃/TiO₂ and the Chemical Index of Alteration (CIA) were used as the chemical weathering proxies while volume susceptibility, TiO₂ content and ARM/SIRM represent clastic flux which was related to the physical erosion.

Lithology, Sampling, and Methods

A core STD111 (latitude/longitude: 20.0193°N, 116.3596°E, water depth 1142 m) was collected with a gravity piston corer by the Guangzhou Marine Geological Survey in March 2012 during a geological survey of the north part of the South China Sea (SCS). The core was 410 cm long, and taken from the north-west in the continental margin of SCS about 135 km from the mouth of the Pear River (Figure 1). The sediments were segmented in five layers: 0–55 cm, composed mainly of clay silt, with an increase in fine particles toward the surface; silty fine sand layer (55–165 cm) is clearly coarser than the next 165–355 cm layer which consisted of clay silt; silty fine sand is the major sediment of the 355–395 cm layer and silts dominated components of the 395–410 cm layer.

FIGURE 1

Figure 1. Geography location of the core in South China Sea (red circle). Color scale represents the water depth (m).

The core was split and one half was continuously sampled by pushing a pottery cube box (1*1*1 cm³) into the split face of the core section. A total of 278 samples were collected. Magnetic measurements on all discrete samples were performed at 2G-760 Enterprises high-resolution cryogenic magnetometer. Natural remanent magnetization (NRM) was measured and demagnetized stepwise using peak alternating fields (AF) 2, 5, 7.5, 10 mT and then in 5 mT steps up to a peak field of 60 mT and in 10 mT steps up to a peak field of 80 mT. Anhysteretic remanent magnetization (ARM), acquired in a 0.05 mT bias field and a 10–80 mT peaks AF in 10 mT steps respectively, was then subsequently demagnetized at peak fields of 20, 40, 60, and 80 mT. Isothermal remanent magnetization (IRM) was produced in a steady dc field of 100 mT and then AF demagnetized using the same field steps as for ARM demagnetization. Hysteresis measurements were performed in fields of up to 1 T using a MicroMag 2900 alternating field gradient force magnetometer. Magnetic fabric measurement for all samples was done using a Kappabridge KLY-3S. All the magnetic experiments were carried out at the Paleomagnetism Laboratory of the Institute of Earth Sciences, Chinese Academy of Science (CAS), Xian and the Paleomagnetism and Geochronology Laboratory of the Institute of Geology and Geophysics (IGG), CAS, Beijing (Hysteresis measurements).

The foraminifers were selected from the sediments of the upper 200 cm of sample to measure the radio-carbon age in Beta Analytic Inc. The foraminifers were first washed for 30–50 s, using an ultrasonic cleaner, and then we selected clean and unfilled samples of the same species. Delta-R 65 ± 47 was used to correct the local marine reservoir, and calendar years were calculated from the adjusted conventional ages referencing Marine09 curve. The geochemistry data of the oxygen isotope of G.ruber, and the content of TiO₂ Al₂O₃ and CIA were obtained from a report by the Guangzhou Marine Geological Survey.

Reflectance spectra for several samples were analyzed in a Perkin-Elmer Lambda 950 spectrophotometer with a diffuse reflectance attachment (reflectance sphere) from 350–1000 μm in 1 nm steps at Sun Yat-Sen University. Sample preparation and analysis followed the procedures described in Balsam and Deaton (1991).

Magnetic Properties

Temperature-dependent susceptibility curves detected magnetite, which rapidly decreased at around 580°C on all heating curves (Figure 2A; Dunlop and Özdemir, 1997; Deng et al., 2004). Minor decreases at ~450°C on the heating curves for 15, 95, and 350 cm samples suggest the presence of maghemite (Dunlop and Özdemir, 1997; Deng et al., 2000). All cooling curves are above heating curves, indicating enhanced magnetic susceptibility arising from the conversion of Fe-clay minerals to new magnetite during the heating process (Deng et al., 2000). The hysteresis parameters indicate that the low-coercivity minerals are predominant magnetic components (Figures 2B–D) (Day et al., 1977; Dunlop, 2002). The loops close above 300 mT (Figure 2B), and the coercivity of remanence (Bcr) is ~35 mT (Figure 2C). Hysteresis ratios seem to cluster tightly within the pseudo-single domain (PSD) grain size region (Figure 2D).

FIGURE 2

Figure 2. Rock magnetic properties for some represented samples. (A) Temperature-dependent susceptibility variations. 1–11:15.27 cm represents the samples number and the depth of sample respectively at core profile. (B) Hysteresis loops (corrected for paramagnetism), (C) back field curves of SIRM and magnetic grain size following Dunlop (2002). (D) SP-superparamagnetic; SD-single domain; PSD- pseudosingle domain and MD-multidomain.

Second derivative results of diffuse reflectance spectra exhibited minimum and maximum absorption bands between 420 and 460 nm, and between 550 and 570 nm respectively, which represent goethite and hematite (Torrent and Barron, 2002; Figure 3). However, the intensity of goethite and hematite for sample 2–24 was weaker than samples 4–48 and 6–6–3.

FIGURE 3

Figure 3. Second derivative curves for selected samples.

The grain size proxies of ARM_(20–60mT)/SIRM_(20–60mT), ARM/κ and SIRM/κ all show obvious variations down core (Figure 4). Their values display four segments: two lows from 0 to 50 cm and 170 to 360 cm suggest relative coarse magnetic grain while two high intervals from 50 to 170 cm and 360 to 410 cm show the relative fine grain (Liu et al., 2012). However, the magnetic grain sizes contrast to the whole sediments. While the magnetic grain is coarse, the sediments are characterized by the clay-silt particles. The coercivity proxies of IRM_AF80mT/SIRM and S₃₀₀ ratio have a similar pattern with grain size as well as the susceptibility, SIRM and ARM. However, the IRM_AF80mT/SIRM is opposite the ARM_AF60mT/ARM changes.

FIGURE 4

Figure 4. (A) Downcore variations in magnetic parameters of magnetic susceptibility, grain size proxies (SIRM/κ, ARM/κ, ARM/SIRM) and (B) relatively coercivity variations proxies (NRM_AF60mT/NRM, ARM_AF60mT/ARM, IRM_AF60mT/SIRM, and S₃₀₀ ratio). Dotted lines divided several significant staged variations of the magnetic properties of sediments. Lithology of the core was listed at left.

Magnetic Fabric

Magnetic lineation (L) for most of samples was weak (less than 1.08). Nevertheless, they displayed strong staged variations as well as anisotropy degree (P) and volume susceptibility (Figure 5), indicating that high susceptibility is consistent with weak L and P. Inclinations of the minimum susceptibility axes (K_min) were shallow when P values were strong, suggesting a disturbance of the original sedimentary texture. However, most of samples with weak P had high K_min closing to vertical. Declinations of the maximum susceptibility axes tend to fall along one direction. In order to discuss the alignment direction of magnetic minerals, declination of K_max was corrected basing on the declination of NRM. We assume that the declination of NRM for top sediments should be consistent with the modern geomagnetic field. After correction, it shown that Kmax aligned around 310° direction.

FIGURE 5

Figure 5. Downcore variations of magnetic fabric parameters (A–D) and the rose projection of the declinations of the maximum susceptibility axes in primary measured data (E) and after-corrected (F) using the NRM. Lithology of the core was listed at left.

NRM Directions and Relative Intensity

All samples subjected to AF demagnetization were analyzed using principal component analysis (Kirschvink, 1980) (Figures 7A–D). Samples with shallow K_min (less than 60°) were removed. Only 183 of the total 277 samples were used to analyze ChRM. The AF demagnetization results of selected samples are presented in Figure 6. The NRM median destructive field (MDF) varied between 10 and 35 mT for all samples, and NRM intensities for most of samples were almost completely demagnetized in fields of 80 mT. After 60 mT AF, the remanence of most samples was < 30%. The orthogonal demagnetization diagrams show that secondary magnetization components are either absent or easily removed by stepwise alternating field demagnetization between 5 and 10 mT. Further steps revealed a highly stable magnetization direction. The characteristic remanent magnetization (ChRM) has been derived using principle component analysis (Kirschvink, 1980). Inclinations and declinations of the profile were plotted as a function of depth (Figures 7B,C). Samples with MAD values > 6° were also removed (Figure 7D).

FIGURE 6

Figure 6. Representative NRM vector plots for typical samples. Open circles (cross) indicate projection onto the horizontal (vertical) plane (top).

FIGURE 7

Figure 7. NRM properties as a function a core depth. NRM intensities (A), ChRM directions (B,C) and associated maximum angular deviation (MAD) (D), as well as the relative intensity proxy NRM_(20–40mT)/ARM_(20–40mT) (E).

Although hematite, goethite, and maghemite exist in the sediments, magnetite was the main contributor to NRM and ARM as suggested by several coercivity proxies. SIRM, ARM, volume susceptibility, and IRM_AF80mT/SIRM of samples varied within one order of magnitude. These properties met the requirements for reconstructing relative paleointensity using the normalized NRM (King et al., 1983; Tauxe and Wu, 1990). The NRM AF pattern was similar to ARM but opposite that of SIRM, e.g., NRM_AF60mT/NRM agree well with ARM_AF60mT/ARM, suggesting ARM has remanence carriers similar to NRM. In order to reduce the influence of a mineral mixture with different coercivities to relative intensity, we used the remanence vector difference between 20 and 40 mT demagnetized field for NRM and ARM to reconstruct the relative paleointensity proxy, which was PRI = NRM_(20–40mT)/ARM_(20–40mT). The RPI curves as a function of core depth are shown in Figure 7E.

Discussion

Refining the Chronology of the Profile

The radio-carbon ages of five foraminifer samples suggested the deposition ages of top 130 cm (Table 1). The oxygen isotope results of G. ruber provided a general chronology frame for the sediments relating to other oxygen isotope stratigraphy, such as ODP 1144 (Bühring et al., 2004) and SPECMAP curves (Imbrie and McIntyre, 2006; Figure 8A). However, a few samples introduced large uncertainties and reduced the ability to construct an accurate profile chronology. Fortunately, the relative paleo-intensity proxy can also serve to establish the correlation although some ambiguities remain. However, it is possible to integrate multi-proxies synchronously, such as radio-carbon ages, the oxygen isotope stratigraphy, and relative paleointensity (Laj et al., 2004; Yang et al., 2009; Figure 8A). We can correlate the pattern between our data and reference curves. The resulting ambiguities can be greatly reduced if both the isotope oxygen and relative paleointensity records can be correlated from different proxies simultaneously. The exact correlation strategy is shown in Figure 8. The age model of the reference ODP 1144 core was constructed on the oxygen isotopes of G. ruber and four AMS ¹⁴C dates on G. ruber and G. sacculifer mixtures by Bühring et al. (2004), which provided a reliable chronology. The reference paleointensity curve NH-PIS was presented by Yang et al. (2009).

TABLE 1

Table 1. Radiocarbon dates for sediments.

FIGURE 8

Figure 8. Oxygen isotope values correlation with SPECMAP (Imbrie and McIntyre, 2006) and oxygen isotope stratigraphy of ODP1144 site (Bühring et al., 2004). (A), Calibrated AMS¹⁴C ages were also listed along depth; In (B), the relative paleointensity variations was comparison with the standard curve GLOPIS-75 (Laj et al., 2004) and the stacked curve of the north of South China Sea (Yang et al., 2009). Red dotted lines indicate the tied points synchronous in relative intensity and oxygen isotope. In (C), the correlated age-depth model was listed referring to the oxygen and relative intensity. Triangles represent the AMS¹⁴C ages.

The chronology of our profile was constrained by eight tied points integrating three AMS¹⁴C ages, suggesting that the sediments in the ~410 cm long record the deposition of about 90 kyr. The AMS¹⁴C age 28.565 kyr clearly departs from the general deposition rate trend, so we omitted it when constructing the age-depth profile model. Constrained by this age-depth model, the deposition rate was generally consistent with a mean value 4.8 cm/kyr.

Chemical Weathering Intensity and Terrigenous Flux

Marine terrigenous detrital sediments record information from chemical weathering of the continental crust, which is directly related to climate (Clift, 2006; Hu et al., 2012, 2013; Clift et al., 2014; Zhao and Zheng, 2015). The intensity of weathering can be represented by some chemical element ratios in the marine sediments, such as Al/Ti, Al/K (Kronberg et al., 1986; Wei et al., 2004; Clift et al., 2014; Zhao and Zheng, 2015). In South China, climate is influenced significantly by the Asian monsoon, hence the chemical intensity should correspond to monsoon changes. For instance, stronger summer monsoons may increase chemical weathering and the physical erosion rate (Derry and France-Lanord, 1996; Bookhagen et al., 2005; Clift, 2006). However, some data show that the winter monsoon is a major contributor to the general pattern of weathering and paleoclimate (Tamburini et al., 2003; Wei et al., 2004). Studies in Southern China have demonstrated that the wet and warm climate during the interglacial produced stronger chemical weathering, and the dry climate during the glacial period produced less chemical weathering. This indicates that the East Asian monsoon is a key for paleoclimate variation patterns (Wei et al., 2004; Hu et al., 2012; Clift et al., 2014).

On our profile, the diagram of maximum susceptibility axis direction was focused toward 310° (Figure 5F), indicating that the weathered material mainly came from this direction (Parés et al., 2007). Materials transported by the Pear River are main candidates. The terrigenous detritus therefore mostly originated in Southern China. The mineral assemblage, element ratio, and terrigenous flux are influenced by the climate conditions and then chemical weathering intensity.

Proxies listed as the function of age clearly show the staged variations, which were consistent with the climate variations (Figure 9). During a 84–40 kyr interval in which the oxygen isotope values of marine sediments (Bühring et al., 2004) and stalagmite (Wang et al., 2001) as well as the sea surface water temperature (SST; Wei et al., 2007) indicated a warm climate and strong Asian monsoon, high volume susceptibility and TiO₂ content represented greater flux of terrigenous detritus which was consistent with the high ratio of IRM_AF80mT/SIRM. The latter was characterized by some high coercivity minerals, such as hematite coeval with the maghemite derived from the strong chemical weathering. The high element ratios of Al₂O₃/TiO₂ and Chemical Index of Alteration (CIA) are also evidence of the same weathering process (Clift and Giosan, 2014). During this period, magnetic mineral grain size is coarser (low ARM/SIRM), consistent with the high susceptibility values, exhibiting the intensified erosion, and strong flux from river transport.

FIGURE 9

Figure 9. Environmental proxies (Susceptibility, TiO₂ content, IRM_AF80mT/SIRM, ARM/SIRM, P′) as well as the chemical weathering indices TiO₂/Al₂O₃ and CIA were correlated to the climate proxies of δ¹⁸O and SST of ODP1144 site (Bühring et al., 2004; Wei et al., 2007), and stalagmite from Hulu cave (Wang et al., 2001). Dotted lines segment the two high (I, III) and one low (II) chemical weathering stages.

As the climate cooled gradually and had a weak monsoon stage from about 40 kyr to 15 kyr, susceptibility, and low TiO₂ appeared, and the ratios of IRM_AF80mT/SIRM, Al₂O₃/TiO₂ decreased as well as the CIA proxy. These are all indicators of low terrigenous input and weak chemical weathering (Wei et al., 2004; Hu et al., 2012; Clift et al., 2014). High P values exhibit strong physical transport energy during this period.

Our results demonstrate that the chemical weathering intensity, intensified erosion, and clastic influx are closely linked to historic climate conditions. A strong monsoon, intensified chemical weathering and physical erosion led to more detritus being transported into marine deposits.

Summary

On the basis of the chronology of integrating the paleointensity and oxygen isotope, magnetic properties of marine sediments provide a good record of the chemical weathering intensity and clastic flux processes in Southern China. The high ratios of IRM_AF80mT/SIRM and CIA and Al₂O₃/TiO₂ suggest strong chemical weathering during the 84-40 kyr period while low ratios indicate weaker chemical weathering during 40–15 kyr period. This variation is closely correlated with changes in the Asian monsoon pattern.

Author Contributions

All authors listed, have made substantial, direct, and intellectual contribution to the work, and approved it for publication.

Funding

This work was supported by the National Natural Science Foundation of China (grant no. 41072264, 41274072), the Fundamental Research Funds for the Central Universities and the Marine Geological Project (1212011220115).

Conflict of Interest Statement

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.

Acknowledgments

Thanks Mrs. Mei Lu for her help during sampling and experiments. Thanks for good suggestions, to the two reviewers.

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