High-Resolution Geochemical Records in the Inner Shelf Mud Wedge of the East China Sea and Their Indication to the Holocene Monsoon Climatic Changes and Events

WANG Longsheng, ZHOU Bin, ZHENG Bang, WANG Ke, MEI Xi, WANG Qing, WANG Xiaohui, and ZHENG Hongbo

High-Resolution Geochemical Records in the Inner Shelf Mud Wedge of the East China Sea and Their Indication to the Holocene Monsoon Climatic Changes and Events

WANG Longsheng1), 3), 7), *, ZHOU Bin2), *, ZHENG Bang2), WANG Ke4), MEI Xi5), WANG Qing1), WANG Xiaohui1), and ZHENG Hongbo6)

1),264025,2),,210023,3),,,264003,4),,,1130033,5),,,266071,6),,650091,7),,,710061,

The inner shelf mud wedge (ISMW) located in the East China Sea (ECS) is the fine-grained sedimentary area with high sedimentation rate and has provided an ideal study area for understanding the East Asian Summer Monsoon (EASM) evolution during the Holocene. In this paper, we presented the high-resolution geochemical data of the sediments from the core MD06-3040 in the ISMW of the ECS determined by X-ray fluorescence core scanning (XRF-CS) analysis, a high-resolution, continuous, and multi- element method. Geochemical and factor analysis results reveal that the variations of elemental compositions (Al, Si, K, Ti, Fe), the elemental ratios of Al/Zr, Ca/Ti and Rb/Sr, and the factor scores (F1) are correlated with the changes of the EASM during the period of 6000–1300calyrBP. The higher values of geochemical compositions indicating the terrigenous inputs implied the intensification of anthropogenic activities after 1300calyrBP. Meanwhile, the significant decrease of most geochemical compositions and the F1 factor scores during 4500–3500 calyrBP and 1700–1500calyrBP, within the dating errors, coincided with the weak EASM events (presumably drought and cold events). The spectral analysis results of K concentrations, Al/Zr ratios and F1 factor scores show the millennial and centennial climatic fluctuations, which are consistent with other marine sedimentary records in the adjacent areas. All the findings show that the geochemical compositions of sediments from core MD06-3040 are influenced by the EASM evolution, the variable El Ni?o/Southern Oscillation (ENSO) and the local oceanic thermohaline circulation (., Kuroshio Current). These results are greatly helpful in uncovering the forcing mechanism of the monsoonal climate in the east China over the Holocene and also contribute to the understanding of EASM variability.

Holocene climatic event; East Asian Summer Monsoon; XRF core scanning; inner shelf mud wedge

1 Introduction

X-ray fluorescence core scanning (XRF-CS) is a convenient and nondestructive method to determine the geochemical variations of unprocessed sediments for a wide variety of research topics such as paleoenvironmental reconstruction over various timescales, stratigraphic correlation, sedimentology and high-resolution time series analysis (Tian., 2011; Hennekamand de Lange, 2012; Liang., 2012; Chawchai., 2016; Martin-Puertas., 2017;Wang., 2018c). Many significant findings about the nature of abrupt climate changes have been described based on the high-resolution geochemical recordsobtained by XRF-CS analyses (Lamy., 2004; Yancheva., 2007; Spofforth., 2008; L?wemark., 2010; Liang., 2012). For example, Gallet. (1996) used the Rb/Sr ratios from XRF analyses to identify the loess and paleosol sequence of Luochuan profile in China. Brown. (2007) revealed that the changes in sediment input of the tropical Africa were linked to movement of the Intertropical Convergence Zone (ITCZ) over the last 55kyr. Wang. (2018b) advanced Quaternary stratigraphy and paleo- ceanographic reconstruction of Arctic Ocean.

The Yangtze River has discharged a substantial amount of terrigenous sediments into the East China Sea (ECS), with an average annual sedimentary loading of 48×107tons. During the last 7000yr, nearly 70% of the particles from the Yangtze River have deposited in Yangtze River Delta, and the remaining (about 30%) were transported southward, and settled in a distinct mud sedimentary area along the entire inner shelf mud wedge (ISMW) of the ECS called ‘East China Sea inner shelf mud’ (Milliman and Meade, 1983; Liu., 2006). The provenance ofsediments, and environmental and climatic changes record- ed in the ISMW of the ECS have been discussed (Xiao., 2006; Liu., 2007; Xu., 2012; Wu., 2016; Bi., 2017; Liu., 2018; Zheng., 2018). The grain size compositions (Wang., 2014) and clay mineral assemblages (Fang., 2018) suggested that most terrigenous sediments in the ISMW were originated from the Yangtze River, with minor from the Taiwan Island during the entire Holocene. Based on sedimentology, geochemistry, paleoceanography, mineralogy and modeling studies, the paleoenvironmental changes of the ECS were generally discussed (Liu., 2006; Xiao., 2006; Wang., 2007; Xu., 2009a, 2009b; Liu., 2010; Xu., 2010; Wang and Li, 2014; Wu., 2016; Liu., 2018; Ding, 2019; Li and Zhang, 2020). Geochemical compositions have been widely used in researches about marine productivity, sediment provenance and paleoenvironmental evolution. For example, Sr/ Ca can be used to study the paleo-marine productivity (Heather., 2000), the ratios of Rb/Sr, Ti/Al, K/Ti and K/Al can indicate the chemical weathering and the variation of precipitation (Wei., 2006; Jin., 2015; Yang, 2015). Although a lot of climatic indicators have been widely used in the paleoenvironmental studies in the ECS, the driving mechanism of paleoclimatic evolution is still uncertain and disputable due to their different work regions and limitations (Xiao., 2006; Bi., 2017; Wang., 2020). More effective and definite climatic indicators are needed to meet the requirementsof practical research works. Thus, high-resolution geoche- mical studies are necessary to understand the paleoenviron- mental change and reconstruct the evolution of the EASM in the ECS. In this paper, we presented high-resolution re- sults of XRF-CS elemental analyses for the sediments in core MD06-3040 collected from the ISMW of ECS in the Holocene and compared the geochemical data with previous paleoenvironmental indicators to find their correlation with EASM precipitation. We also discussed the variability of Yangtze River-derived sediments coupled with changes of EASM.

2 Regional Setting

The ISMW in the ECS extends southward about 1.0× 106m from the Yangtze River Mouth to the Taiwan Strait (Qin., 1987). Coastal currents influencing the inner shelf mud wedge of ECS include the northward flowing Kuroshio Current (KC), Taiwan Warm Current (TWC), Zhejiang-Fujian Coastal Current (ZFCC), the southward flowing Yellow Sea Warm Current (YSWC), Changjiang (Yangtze River) Dilute Water (CDW) and Jiangsu Coastal Current (JCC) (Lee and Chao, 2003) (Fig.1). The ISMW in the ECS is composed of clayey silt and occurs as a clinoform zone, which dips approximately 50m in water depth. Modern sedimentation rates of the ISMW in the ECS range from 0.8 to 2cmyr?2, which are very high (Huh and Su, 1999; Liu., 2007). The average annual atmospheric temperature is 17.1℃. The ECS was affected by the subtropical high, with a maximum atmospheric tem- perature of 28.6℃ in summer, and influenced by the Siberian High in winter, with a minimum atmospheric temperature of 4.8℃. The EASM has an important impact on the precipitation of the ECS. The average annual precipitation is 1157cmyr?2. Winter and summer precipitation account for 18% and 54%, respectively (Tong and Cheng, 1981).

Fig.1 Location map of the study region (left), with arrows showing Westerlies, EASM (East Asian summer monsoon), and ISM (Indian summer monsoon). The location of inner shelf mud wedge of East China Sea (right), with arrows showing the Changjiang Dilute Water (CDW), the Chinese Coastal Current (CCC), the Zhejiang Fujian Coastal Current (ZFCC), the Jiangsu Coastal Current (JCC), the Yellow Sea Warm Current (YSWC), and the Taiwan Warm Current (TWC).

3 Materials and Methods

Two parallel sediment cores, MD06-3039 (core depth: 8.11m, 121?46?91?E, 27?43?36?N) and MD06-3040 (depth: 19.39m, 121?46?88?E, 27?43?36?N), were collected in close proximity from the ISMW in the ECS at a water depth of 47m in 2006 (Fig.1). The lithostratigraphic units in the core MD06-3040 were described by Wang. (2014). Core MD06-3040 is mainly divided into three li- thological units according to visual observation, geochemi- cal compositions and grain size compositions (Wang., 2014), clay mineral compositions (Fang., 2018), and magnetic properties (Zheng., 2010). The age model of the core MD06-3040 based on eleven AMS14C ages of bivalve shells has been built (Wang., 2014; Kajita., 2018). The detailed descriptions of lithology and14C ages are shown in Fig.2. There is no sign of hiatus in core MD06-3040, so it can provide the continuous paleo- environmental record since the early Holocene.

The core MD06-3040 was scanned by using an Avaatech III X-ray fluorescence core scanner at the Key Laboratory of Surficial Geochemistry of Ministry of Education, Nanjing University. The Avaatech X-ray fluorescence core scan- ner generated three types of output: high-resolution optical pictures, chromaticity data for the RGB and CIE L*- a*-b* color spaces at a resolution of 70μm and element signal values. The instrumental setup was as follows: a 5- kV tube voltage and a 10-kV tube voltage with no filter were used to analyze the light elements (., Ca, Al, Si, Ca and Fe), and a 30-kV tube voltage with a Pb filter and a 50-kV tube voltage with a Cu filter were used for the determination of the heavy elements (., Zr, Sr and Rb). Signal intensity of the element was expressed as counts per second (cps), which can provide semi-quantitative information about each elemental concentration of the diffe- rent sediments (Francus., 2009; L?wemark., 2011).

Fig.2 Comprehensive profile of core MD06-3040 including 14C ages (Wang et al., 2014; Kajita et al., 2018), grain size (Wang et al., 2014), magnetic susceptibility (Zheng et al., 2010), lithology profile and description, sedimentary units.

4 Results

In continental shelf sediments, Ca may be derived from terrestrial input or marine biogenesis. Generally, the biogenetic Ca is higher than those from detrital inputs (Arz., 2001; Carlson., 2008; Kwiecien., 2008; Jorry., 2011). The Ca values in the core MD06-3040 sediments range from 8120 to 19864cps, with a mean of 12086cps. Unit 1 (19.3–18.3m) has the highest Ca contents (mean of 12957cps), and Unit 3 (15.86–0m) has the lowest Ca contents (mean of 11957cps) (Table 1). Fe, Ti, Al, Si and K are commonly the major components of terrigenous detritus (Arz., 2001, 2003). The Fe, Ti, Al, Si and K values range from 29329 to 45114cps, 3298 to 4978cps, 876 to 2743cps, 7626 to 20748cps and 7977 to 13604cps, respectively. The highest values of Al, Si, K, Ti and Fe occurred in Unit 3, and the lowest values occurred in Unit 1 (Fig.3).

The Pearson coefficient of determination (2) is an effective parameter to assess inter-elemental associations and provide more details about different sedimentary components. The concentrations of major elements Fe, Ti, Al, Si and K in core MD06-3040 sediments are correlated (2>0.57), indicating the downcore variation of a terrigenous fraction (Table 2). Al has a weak mobility and is non-active in the weathering process. It is difficult to be carried away by fluids (Wehausen and Brumsack, 2002). Compared with Al, Zr exists in zircon which has strong weathering resistance. Al/Zr is commonly used to reflect monsoon changes(Wei., 2006). The behavior of K, Al, Ti and Fe in se- diments is well known and they can be used as indicative of terrigenous components (Vidal., 2002; Wehausen and Brumsack, 2002; Grüetzner., 2003). A strong correlation between the ratios of Al/Zr and the concentrations of Fe, Ti, Al, Si and K suggests that the ratios of Al/Zr can also be used as the indicator of terrigenous detrital input(Fig.4). The correlation between these major elements and elemental ratios can also be found in elemental pair diagrams (Fig.5).

Table 1 Comparison of the range and mean value of geochemical compositions and factor analysis results (F1) for sediments in different units

Fig.3 Variation of Al, Si, K, Ti, and Fe with depth for sediments in each unit (Black lines are the original values, with red lines smoothed by a 9-point running average).

Table 2 Pearson correlation coefficients and results of factor analysis for sediments from core MD06-3040

Notes: PVE, percent of variance explained; CPVE, cumulative percent of variance explained.

Fig.4 Variation of Al/Zr, Rb/Sr, Sr/Al, Ca/Ti and factor analysis results (F1) with depth for sediments in each unit (Black lines are the original values, with red lines smoothed by a 9-point running average).

Fig.5 Correlation diagrams of selected major elements in core MD06-3040 sediments (number of data: 1904).

5 Discussion

The large accommodation for river-derived sediments provided by sea-level changes may have an important influence for the formation of the ISMW in the ECS (Chen., 2000; Berne., 2002; Wang., 2014). The high contents of silt with the thin interbedded sandy silt layer (Wang., 2014), the coarse magnetic fraction (Zheng., 2010) and the high Fe, Ti, Al, Si, K values (this study) in sediments below 15.86m (before 7500calyrBP) of core MD06-3040 were controlled by the transgression. Fang(2018) presented a high-resolution clay mineral study to the sediments from core MD06-3040 and semi-quantitatively evaluated the terrigenous contributionsfrom various potential sources throughout the entire Holocene. Provenance analysis suggested that most fine-grained terrigenous sediments originated from the Yangtze River, with minor sediments derived from Taiwan Island and negligible sediments from nearby Zhejiang and Fujian Pro- vinces. In addition, the quasi-bimodal grain size distribution showed that the sediments in the core MD06-3040 from 7500 to 6000calyrBP are influenced by the local rivers such as Qiantang, Min and Ou Rivers, which was also supported by the clay mineral assemblages (Bi, 2017; Fang., 2018), the crystallinity index (CI) of quartz and electron spin resonance (ESR) signal intensity in the 16–63μm fraction (Wang., 2020). All these showed that the sediments in the core MD06-3040 are mainly from the Yangtze River since 6000calyrBP, and the influence of the local rivers can be neglected. Thus, in order to eliminate the influence caused by the provenance differences, we mainly discussed the relationships between the geochemical characteristics of the sediments in the core MD06-3040 and the environmental changes under the control of the EASM since 6000calyrBP.

Factor analysis is a useful technique to combine many variables into several potential components that form the basis of multivariate data (Reimann., 2002; Yao., 2012). The factor analysis in sedimentary geochemistry has already been used to distinguish the end-member se- dimentary components and their respective compositions (Ziegler and Murray, 2007; Yao, 2012). The results (Table 2) shows that the elements of Al, Si, S, Zr, Cl, Ca, Ti and Fe were dominated by two principal components (F1 and F2) that account for 74.108% of the total variances. F1 account for 59.589% of the total variances with high positive loadings of Al (0.933), Si (0.893), K (0.896), Ti (0.807) and Fe (0.719). The elements such as Si, Al and Fe mainly hosted in the terrigenous clasts and clay minerals. The strong hydrodynamic conditions are conducive to the enrichment of Si, Al and Fe in the sediments (Dou., 2012). The behavior of K, Al, Ti and Fe in sediments is well known to be indicative of terrigenous components (Vidal., 2002; Grüetzner., 2003). Therefore, the F1 factor is interpreted as the terrigenous components in the sediments. The F2 factor explains 14.51% of the total variance and shows high positive factor loading of S (0.643) and relatively high negative factor loading of Ca (?0.733), indicating a variance related to biogenic component (Yao, 2012).

We selected the K, the ratios of Al/Zr and factor F1 to compared their variation with those of the reconstructed summer rainfall at Xinjie site in the lower reaches of Yang- tze River (Lu, 2019), the annual sea surface temperature (SST) derived by the linear transfer function FP-12E in the Okinawa Through (Jian, 2000) and the number of ENSO events per century (Moy., 2002). These results revealed that higher values of K, Al/Zr and F1 factor scores correspond to the higher precipitation and temperature during 6000–3500calyrBP, and the lower va- lues of these geochemical compositions correspond to the lower precipitation and temperature during 3500–1300calyrBP. The previous studies showed that the precipitation and temperature in the ECS are mainly controlled by the change of EASM (Liu, 2006; Wang., 2018d). All these suggested a relatively strong EASM period during the period 6000–3500calyrBP, indicated by higher va- lues of K (mean of 11467cps), Al/Zr (mean of 1.03) and F1 factor scores (mean of 0.47). The decrease of K (mean of 10432cps), Al/Zr (mean of 0.98) and F1 factor scores (mean of ?0.82) during the period 3500–1300calyrBP suggest a relatively weak EASM period in the Yangtze drainage. Since 1300calyrBP, the temperature and rainfallkept at lower levels. However, a rapid increase in K (meanof 10760cps), Al/Zr (mean of 0.96), and F1 (mean of ?0.54) during this period indicated that the impact of human activities (., deforestation, agriculture, and soil erosion) cannot be ignored (Fig.6), which is also supported by the high values of magnetic susceptibility (Zheng., 2010) and organic geochemical records (Zheng., 2018). Significant variability in the geochemical compositions indicates the variation of EASM since 6000calyrBP, besides the notable decreases at 4500–3500calyrBP and 1700–1500calyrBP. The abrupt geochemical changes at these periods, within dating errors, are correlated with the weak EASM events documented by grain size parameters (Wang., 2014) and magnetic parameters in the adjacent area (Zheng., 2010). In the Yangtze River Basin, several terrestrial monsoon records such as lake-level changes and speleothems have shown the weak EASM events and abrupt cool and dry shifts during the same periods (Wang., 2005; Hu., 2008; Innes., 2014; Wang., 2018a). These findings show that EASM precipitation of Yangtze drainage should be a prevailing factor for the deposition of Yangtze-derived sediments in the ECS’s ISMW. The periods with the low values of geo- chemical compositions also coincide with globally recognized events such as ice-rafted debris events documented in North Atlantic (Bond., 2001), cold events in the subtropical Africa (deMenocal., 2000), and the weakening of the Kuroshio Current (Jian., 2000) (Fig.6). Although Stanley. (1999), Zhang. (2005) and Wang. (2018d) proposed that there were floods due to climate and sea-level change from 4500 to 3500calyrBP in sub-humid regions, and the climate change was characterized by high variability with the occasional incidence of both droughts and floods. In this study, the well-dated and continuous geochemical records suggested that the climate showed a dry and cold trend in the lower reaches of the Yangtze River from 4500–3500calyrBP with a relatively large fluctuation, which was also supported by the-alkanes indicators (Zheng, 2018). Several terrestrial monsoon records obtained in southern-central China, such as speleothems, lake levels and sediment cores, have indicated the dry and cold shifts in southeast China during 4500–3500calyrBP (An, 2000; Chen, 2005; Ma, 2009; Innes, 2014). Meanwhile, the numbers of cultural sites in the Yangtze River Delta decrease during 4500–3500calyrBP (Zhang, 2005), which is consistent with the weak EASM events shown by the geochemical data. In addition, the notable decrease of some characteristic geochemical compositions can well correspond with the low ENSO events, which indicated that the EASM are readily influenced by ENSO through the strength of the subtropical high in the western Pacific region, and ENSO can possibly serve as the physical mechanism of the extreme climatic and flooding events in the Yangtze River Basin (Moy., 2002). A recent study suggests a close relationship between the large floods along the Yangtze River in 1998 and ENSO event (Wei, 2014). These results suggest that the changes of runoff in the Yangtze River catchment were sensitive to the ENSO events to vary at the sub-millennial timescale (Moy, 2002; Marchitto, 2010).

Fig.6 Variation of proxy parameters for K (a), Al/Zr (b) and factor analysis results (F1 factor scores) (c) smoothed by 9- point running averages with time for sediments from core MD06-3040; (d), the summer rainfall at site Xinjie in the lower reaches of Yangtze River (Lu et al., 2019); (e), the number of ENSO events per 100yr (Moy et al., 2002); (f), the annual SSTderived from the standard errors of the linear transfer function FP-12E in the Okinawa Through (Jian et al., 2000).

In order to further confirm this correlation, the REDFIT 38 was used to perform power spectral analysis (Fig.7) (Schulz and Mudelsee, 2002). The results show that the values of K, Al/Zr and F1 factor scores have a clear periodicity of 2893yr, similar to the results of Kuroshio Current (2560yr) (Jian., 2000), ENSO (2000yr) (Moy., 2002) and magnetic characteristics of sediments from the South Yellow Sea, eastern China (2361yr) (Wang., 2018a). The 1243–1571 yr is another period, close to the 1400–1500yr of the North Atlantic climate (Bianchi and McCave, 1999), the 1500yr of Kuroshio Current (Jian., 2000) and the 1500yr of ENSO (Moy., 2002). The variation of K, Al/Zr and F1 are attributed to the changes of ENSO and oceanic thermohaline circulation. The marked centennial periods at 364 and 521yr are close to the periods of the Kuroshio Current (388yr, 602yr) (Jian., 2000). All these results revealed that the geochemical compositions of sediments in core MD06- 3040 are influenced by the evolution of EASM, the variation of ENSO and local oceanic thermohaline circulation (., Kuroshio Current).

Fig.7 Spectral analysis of K, Al/Zr and factor analysis results (F1) in core MD06-3040 for the past 6000yr. Peaks are labeled with periods in years above 90% (yellow line) and 95% (gray line) confidence levels.

6 Conclusions

High-resolution geochemical compositions of sediments were determined by XRF-CS for the core MD06-3040 from the ISMW of the ECS. Geochemical variations and factor analysis revealed their correlation with the changes in EASM precipitation during the period of 6000–1300calyrBP. After 1300calyrBP, the higher inputs of terrigenous materials implied the intensification of anthropogenic activities. The notable decreases of some geochemical compositions at 4500–3500 and 1700–1500calyrBP, within the dating errors, coincided with the presumable cold and dry geochemical compositionsevents documented in the adjacent area, and were also correlated with many global recognized features. The spectral analysis revealed that the geochemical compositions of core MD06-3040 were influenced by the evolution of EASM, the variation of El Ni?o/Southern Oscillation (ENSO) and local oceanic ther- mohaline circulation (., Kuroshio Current), indicating that the geochemical compositions determined by XRF-CS can provide new insights for paleoenvironmental changes. These findings are important for studying the relationshipsbetween regional systems and global changes in monsoonal climate regions.

Acknowledgements

This research was supported financially by the National Natural Science Foundation of China (Nos. 41991323, 41 702185, 41977378, U1706220), the National Key Basic Research Program of China (No. 2015CB953804), the Na- tural Science Foundation of Shandong Province (No. ZR 2018PD005), the Jiangsu Provincial Basic Research Program Natural Science Foundation General Project of China(No. BK20171340), the Open Foundation of CAS Key La- boratory of Coastal Environmental Processes and Ecolo- gical Remediation, YICCAS (No. 2020KFJJ10), the Open Foundation of State Key Laboratory of Loess and Quater- nary Geology, Institute of Earth Environment, CAS (No. SKLLQG2024), the Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology(No. MGQNLM-KF201704), and the Foundation of School and Land Integration Development in Yantai (No. 2021 XDRHXMQT18).

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September 8, 2020;

February 28, 2021

? Ocean University of China, Science Press and Springer-Verlag GmbH Germany 2021

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(Edited by Chen Wenwen)