Transect variations and controlling factors of redox-sensitive trace element compositions of surface sediments in the South China Sea

Journal Pre-proof Transect variations and controlling factors of redox-sensitive trace element compositions of surface sediments in the South China Sea Jun Cheng, Yi Huang, Shuhong Wang, Li Miao, Wen Yan PII:

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https://doi.org/10.1016/j.csr.2019.103978

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CSR 103978

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Continental Shelf Research

Received Date: 21 April 2019 Revised Date:

19 September 2019

Accepted Date: 27 September 2019

Please cite this article as: Cheng, J., Huang, Y., Wang, S., Miao, L., Yan, W., Transect variations and controlling factors of redox-sensitive trace element compositions of surface sediments in the South China Sea, Continental Shelf Research, https://doi.org/10.1016/j.csr.2019.103978. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.

Transect variations and controlling factors of redox-sensitive trace element compositions of surface sediments in the South China Sea CHENG Jun1,2,3, HUANG Yi1,2,3, WANG Shuhong1,3*, MIAO Li1,3, YAN Wen1,2,3 1

Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese

Academy of Sciences, Guangzhou 510301, China

2

University of Chinese Academy of Sciences, Beijing 100049, China

3

Innovation Academy of South China Sea Ecology and Environmental Engineering, Chinese Academy of Sciences,

Guangzhou 510301, China

*

Corresponding author: [email protected]

Abstract: Signatures of redox-sensitive trace elements (RSTEs; e.g., V, Cr, U, and Mo) preserved in marine sediments provide unique clues to the nature and changes of ocean oxygenation and redox states. There have, however, been few studies of the RSTE geochemistry of marine sediments in the South China Sea (SCS). Here we present the results of a study of concentrations of major elements Al2O3, TiO2, Fe2O3, CaO, and MnO and trace elements V, Cr, U, Mo, Sr, and Ba in 75 surface sediment samples collected along four longitudinal and latitudinal transects in the SCS with the aim of identifying the transect variations and main controlling factors of RSTE compositions as well as their redox indications. Variations in sediment V, Cr, and U concentrations along each transect are similar to those of Al2O3, TiO2, and Fe2O3 but are the inverse of CaO variations. Variations in Mo concentrations differ from those of V, Cr, U, Al2O3, TiO2, Fe2O3, and CaO, but are similar to those of MnO. V, Cr, and U in sediments are derived mainly from terrigenous materials, with significant proportions stored in detrital particulate matter rather than being deposited as redox-related precipitation under anoxic conditions. Sediment V, Cr, and U concentrations are diluted by biogenic carbonates. There is little detrital Mo. Although most samples are enriched in Mo relative to upper continental crust, the degree of enrichment is lower than that observed under anoxic or euxinic conditions and is due mainly to the adsorption by Mn-oxyhydroxides. The little or none of authigenic RSTE component in the surface sediments 1

indicates the current generally oxic SCS seafloor environment, which is consistent with the higher seafloor dissolved oxygen content in the SCS. The assessment of detrital particulate flux and authigenic component of specific RSTEs is necessary for reliable redox interpretations.

Keywords: Redox-sensitive trace elements; Surface sediments; Transect variations; Controlling factors; South China Sea

1. Introduction The solubility of redox-sensitive trace elements (RSTEs), such as V, Cr, U and Mo, is strongly influenced by local redox conditions. Under oxic conditions, RSTEs in marine environments occur mainly as dissolved ionic species. Upon entering reducing (anoxic or euxinic) conditions, the dissolved ionic species are reduced and/or converted to more insoluble particulates (Table 1), with resulting enrichment in surface marine sediments (Algeo and Maynard, 2004; Cole et al., 2017; Tribovillard et al., 2006). The direct link between RSTE solubility and redox state means that their enrichment in marine sediments, particularly of V, Cr, U, and Mo, can act as a useful indicator of redox conditions. Therefore, RSTE concentrations in modern and ancient marine sedimentary systems have been used extensively to reconstruct or trace marine redox conditions as they are potential archives of seawater redox history (Li et al., 2018; Partin et al., 2013; Reinhard et al., 2013; Ruebsam et al., 2017; Sahoo et al., 2012; Scott et al., 2008; Zhang et al., 2016). As the largest semi-enclosed marginal sea in the western Pacific, the South China Sea (SCS) is a major regional sink for terrigenous materials from adjacent continents and islands (Liu and Stattegger, 2014; Liu et al., 2016), and produces large quantities of authigenic deposits (Wei et al., 2004). Sediments in such areas are reliable recorders of information, such as environmental conditions, provenance, and source area weathering. Over the past few decades, the geochemistry of marine sediments in the SCS has received considerable attention, particularly concerning rare earth element compositions (Li et al., 2016; Liu et al., 2013a; Liu et al., 2015; Liu et al., 2013b; Wang et al., 2016; Wang et al., 2015; Wang et al., 2014; Wei et al., 2006; Wei et al., 2004; Zhou et al., 2004; Zhu et al., 2007), but few studies have focused on RSTE geochemistry, which may provide an improved understanding of the redox environment of the SCS. A recent study 2

reconstructed changes in deep-water oxygenation of the SCS since the last glacial period using RSTE core data (Li et al., 2018), demonstrating the value of RSTE geochemistry in marine sediment studies. In this study, surface sediment samples were collected from four longitudinal and latitudinal transects in the SCS. Compositional data for selected major elements (Al2O3, TiO2, Fe2O3, CaO, and MnO) and trace elements (V, Cr, U, Mo, Sr, and Ba)were used to explore the transect variations and controlling factors of RSTE (i.e., V, Cr, U, and Mo) compositions, and then their redox indications are compared with published seafloor dissolved oxygen data.

2. Materials and analytical methods 2.1 Sample collection A total of 75 surface sediment samples were collected in the SCS by using grab samplers and box samplers during four cruises (in 2009, 2010, 2011, and 2012) of the research vessel SHIYAN 3 of the SCS Institute of Oceanology. The longitude and latitude of sites were given in Table A1. The sampling sites formed three latitudinal transects and one longitudinal transect as shown in Fig. 1. Twenty samples were collected along transect T1 (18°N) from sites KJ01–KJ20; 23 along transect T2 (sites KJ21–KJ43; 113°E); 19 from transect T3 (KJ57–KJ75; 10°N, including KJ57– 59 further to the southeast); and 13 from transect T4 (KJ44–KJ56; 6°N). The collection sites are scattered on continental slopes and shelves, and in deep basins (Fig. 1).

2.2 Analytical methods Before the elemental analysis, freeze-dried samples were dried in a hot-air oven at 70 °C. Approximately 2 g dried samples were crushed into powder using an agate mortar. For the major elements (Al2O3, TiO2, Fe2O3, CaO, and MnO, etc.) analysis, 0.1 g powdered samples were completely digested with a concentrated acid mixture (HF:HNO3:HClO4=4:4:1) in low-pressure Teflon digestion vessels and heated on a ceramic hot plate at 120 °C until the mixed solutions were completely dried. Subsequently, 2 % HNO3 was added to the vessels. Finally, the resulting solutions were transferred into plastic bottles, diluted with purified water to a total mass of 40 g 3

achieved. The solutions were analysed on an inductively coupled plasma atomic emission spectrometry (ICP-AES; Perkin-Elmer Model Optima 4300DV) in the Institute of Oceanology, Chinese Academy of Sciences (CAS). For the trace elements (V, Cr, U, and Mo, etc.) analysis, 40 mg powdered samples were with super-puregrade HF, HNO3 and HClO4 at a ratio of 1:2:2 in high-pressure tightly closed Teflon bombs and heated on a hot plate at 150 °C for 24 h to ensure complete digestion. Subsequently, the sample solutions were evaporated to dryness at 120 °C in an open evaporation block. The final residues were dissolved again in 3 % HNO3. Finally, the resulting solutions were transferred into plastic bottles, and purified water was added until a total mass of 40 g achieved. The solutions with rhodium added as an internal standard were analysed on an inductively coupled plasma-mass spectrometry (ICP-MS; Elan DRC II) in the Institute of Oceanology, CAS. For reliable analytical results, standard samples (GBW07316, GBW07315, GBW07311, GBW07309, GBW07307, GBW07306, BCR-2, and BHVO-2) and replicate samples were analysed together with the samples. The analytical accuracy and precision were better than 10 %.

3. Results 3.1 Major element compositions The selected major element compositions of surface sediments in the SCS are listed in Table A1, with ranges, averages, and coefficients of variation (CV = standard deviation/mean %) given in Table 2. Major element compositions of surface sediments in the SCS are characterized by predominant Al2O3 and CaO, lower Fe2O3, and minor TiO2 and MnO concentrations. The average Al2O3 concentrations for transects T1, T2, T3, and T4 are 13.1, 11.2, 9.27, and 10.4 wt.%, respectively (Table 2), which are lower than the values of both the upper continental crust from Rudnick and Gao (2014) (UCC which we refer to throughout this study; 15.4 wt.%) and Post-Archaean Australian Average Shale from Taylor and Mclennan (1985) (PAAS; 18.9 wt.%). The four transects have higher CaO concentrations than both UCC (3.59 wt.%) and PAAS (1.30 wt.%), with averages of 8.17, 17.6, 20.9, and 10.3 wt.% for T1, T2, T3, and T4, respectively (Table 2). These results are consistent with previous studies indicating that terrigenous materials 4

and carbonates are the major components of SCS sediments (Wei et al., 2004). CV values are in the order MnO > CaO > Al2O3 ≅ TiO2 ≅ Fe2O3, with MnO and CaO compositions thus changing more sharply between sites within transects.

3.2 RSTE compositions The RSTE compositions of surface sediments in the SCS are listed in Table A1, and RSTE ranges, averages, and CV values in each transect are shown in Table 2. Average concentrations of V, Cr, U, and Mo within transects T1, T2, T3, and T4 are: 111, 89.1, 76.9, and 88.7 µg g–1; 72.9, 60.5, 53.2, and 60.6 µg g–1; 2.21, 1.88, 1.94, and 2.29 µg g–1; and 4.40, 6.01, 4.25, and 4.14 µg g–1 (Table 2). Average Cr and U concentrations in each transect are lower than UCC values (92 and 2.7 µg g–1, respectively); transects T2–T4 have lower average V concentrations than UCC (97 µg g–1) and average Mo concentrations of all four transects are higher than that of UCC (1.1 µg g–1). In all four transects, CV values of V, Cr, and U are similar, while that of Mo is far higher, indicating Mo concentrations changing more markedly between locations.

3.3 Transect concentration variations of RSTEs and major elements 3.3.1 Transect T1 Transect T1, in the northern SCS, extends from southeastern Hainan Island to the Eastern Sub-basin (ESB) (Fig. 1). V, Cr, U, Al2O3, TiO2, and Fe2O3 display similar trends in concentration across the transect. In both the western continental shelf and ESB, their concentrations first increase and then decrease from west to east, with lowest values occurring on the slope. Mo and MnO display opposite concentrations at some individual sites, but have similar trends overall. CaO concentrations display a trend opposite to that of V, Cr, U, Al2O3, TiO2, and Fe2O3, with the highest CaO concentration occurring on the slope and lowest in the ESB (Fig. 2a).

3.3.2 Transect T2 Transect T2 extends from the northern Xisha Islands to the southern Nansha Islands, with its middle section being in the Southwestern Sub-basin (SSB) (Fig. 1). V, Cr, U, Al2O3, TiO2, and Fe2O3 display similar compositional trends along the transect, with values increasing north to 5

south in both the northern and southern slopes, and with highest values in the SSB. The CaO trend is opposite to those of the aforementioned six elements. Mo and MnO display similar variations, with lowest concentrations in the SSB (Fig. 2b).

3.3.3 Transect T3 The western end of transect T3 is close to the Mekong estuary, the middle section is in the Nansha Islands, and the eastern end is adjacent to Palawan Island (Fig. 1). V, Cr, U, Al2O3, TiO2, and Fe2O3 exhibit similar concentration trends along the transect, with higher values on the western slope and lower values on the eastern slope, again opposite to the trend in CaO concentrations. Mo and MnO trends are again similar (Fig. 3a).

3.3.4 Transect T4 Transect T4 is in the southern SCS, and extends from the eastern Sunda Shelf to the southern continental slope (Fig. 1). V, Cr, U, Al2O3, TiO2, and Fe2O3 display similar concentration trends along T4, with higher concentrations on the southern slope than on the Sunda Shelf. Trends in CaO concentrations are largely unrelated to those of the aforementioned six elements, while Mo and MnO again display similar trends (Fig. 3b). Transect concentration variations in RSTEs and major elements are summarized as follows: V, Cr, U, Al2O3, TiO2, and Fe2O3 display similar trends, generally opposite to that of CaO, and Mo and MnO display similar trends. However, there are variations within transects. For example, although Mo and MnO trends are similar along T1, they display opposite concentrations at some individual sites. The opposing CaO trend is not as obvious in transect T4 as in the other transects.

4. Discussion 4.1 Main factors controlling RSTE compositions 4.1.1 Terrigenous input and detrital contribution Mobilization of RSTEs occurs during oxidative chemical weathering of the UCC, with weathering and erosion products including dissolved ions and detrital particulate components of 6

RSTEs being delivered to the oceans by rivers (Cole et al., 2017). RSTEs in marine sediments thus include both detrital particulate components inherited from the terrigenous materials and authigenic components formed by precipitation of dissolved metal ions in reducing environments. It is only concentrations of the authigenic component that vary in response to changing redox conditions, with detrital particulate fractions being unrelated to redox states. The continents and islands surrounding the SCS are characterized by intense weathering and rapid erosion, and have well-developed riverine transport systems, including several of the world’s largest rivers (i.e., the Pearl River, the Red River, and the Mekong River) and numerous smaller mountain rivers in Taiwan, Luzon, Malay Peninsula, Kalimantan, and Sumatra (Zhao et al., 2018). Terrigenous materials are transported into the SCS at up to 700 Mt yr–1 (Liu et al., 2016), with such a background terrigenous detrital input adding to the complexity of interpretation of sediment systems and requiring thorough assessment of terrigenous RSTE inputs. Weathering products constitute most of the detrital sediment component in the SCS. The behaviors of elements are different during weathering processes. Experimentally determined element migration rankings in chemical weathering are: Mg > Ca ≈ Rb > K > Na > Sr > Al > Zr > Fe ≈ Ti (Lo et al., 2017). Previous studies have also found that elements such as Al, Fe, Ti, and Zr are conservative during chemical weathering and resistant to leaching, and are therefore enriched in weathering products (Nesbitt and Markovics, 1997). Al and Ti are also immobile during transportation, sedimentation, and diagenesis (Böning et al., 2004; Calvert and Pedersen, 1993; Tribovillard et al., 2006). Therefore, Al and Ti can be regarded as powerful detrital tracers due to their overwhelmingly detrital origin. The transect concentration variations of V, Cr, U, Al2O3, and TiO2 are essentially the same, whereas Mo variations differ from those of Al2O3 and TiO2 (Figs 2 and 3). Cross-plots of RSTEs versus Al2O3 and TiO2 concentrations (Fig. 4a, b) indicate that V, Cr, and U are positively correlated with Al2O3 and TiO2 (R2 = 0.50–0.85), but there is no significant correlation between Mo and Al2O3 or TiO2 (R2 = 0.03–0.08). This may suggest significant terrigenous detrital contributions to V, Cr, and U, but little to Mo. To exclude the effects of detrital background of RSTEs as well as compare their authigenic enrichment degrees, an enrichment factor (EF) was defined as: 7

XEF = (X/Al)Sample/(X/Al)Reference.

(1)

where X and Al represent the weight concentrations of elements X and Al, respectively. Reference compositional values, including the PAAS from Taylor and Mclennan (1985), the UCC from McLennan (2001), and the UCC from Rudnick and Gao (2014), were chosen differently in studies of marine sediments and authigenic carbonates associated with hydrocarbon seeps by different researchers (Algeo and Tribovillard, 2009; Hu et al., 2014; Li et al., 2017; Palomares et al., 2012; Wang et al., 2016). The respective RSTE reference concentrations of the above three sources are listed in Table 1. PAAS has been widely used as a reference for fine-grained siliciclastic sediments or sedimentary rocks rich in organic matter, such as grey and black shales. However, the comparison to shale values may raise some complications when sediments or sedimentary rocks are poor in organic matter (Van der Weijden, 2002). Rudnick and Gao (2014) derived their own best estimate of average crustal composition based on consideration of PAAS compositions proposed by Taylor and Mclennan (1985) and UCC compositions suggested by McLennan (2001). The most recent estimate of UCC composition presented by Rudnick and Gao (2014) was adopted here for use in Equation (1). Considering the likelihood of non-detrital Al occurrence in sediments in the open and highly productive ocean (Murray and Leinen, 1996), Al was replaced by Ti in a modified Equation (1): XEF = (X/Ti)Sample/(X/Ti)UCC

(2)

If XEF < 1, element X is depleted relative to the UCC, and if XEF > 1, it is enriched. A detectable authigenic enrichment corresponds to XEF > 3, and a substantial authigenic enrichment to XEF > 10 (Algeo and Tribovillard, 2009). The EF results of RSTEs are listed in Table A1. In the SCS, average values of VEF, CrEF, and UEF are 1.28, 0.91, and 1.12, respectively, while the average MoEF value is as high as 6.40 (i.e., 3 < MoEF < 10). Transect trends in V, Cr, and U concentrations are obviously different to their respective EF trends (Fig. 5), while Mo and MoEF trends are very similar. All these aforementioned results support there being a greater detrital contribution to V, Cr, and U than Mo in surface sediments of the SCS.

4.1.2 Dilution by biogenic carbonate 8

The SCS is in a tropical region with carbonate-rich deposits of biological origin from the Xisha and Nansha islands. The average CaO composition of our samples is 14.6 wt.% (Table 2), much higher than that of the UCC (3.59%). Unlike biophilic trace elements, such as Sr and Ba, many trace elements, including V, Cr, U, and Mo, show no evidence of a bio-nutrient role in modern marine systems (Algeo and Maynard, 2004; Algeo and Tribovillard, 2009; Morford and Emerson, 1999; Tribovillard et al., 2006), as supported by the low correlation coefficients between the RSTEs and Sr or Ba (Table 3). RSTE concentrations are very low in biogenic carbonate, with increasing concentrations of CaO diluting those of V, Cr, U, and Mo in the sediments (Li et al., 2017; Liu et al., 2015; Tribovillard et al., 2006). However, V, Cr, and U concentrations are negatively correlated with CaO concentrations (r = -0.75, -0.77 and -0.69, respectively; Table 3 and Fig. 4c), whereas Mo has no CaO correlation (r= -0.20; Table 3 and Fig. 4c), further distinguishing Mo from the former three elements and indicating that a large portion of the Mo in the surface sediments is not of detrital origin. For V, Cr, and U, the detrital contribution and calcium carbonate dilution may account for their respective transect trends. The shelf in transect T1 and southern slope in transect T4 are close to the southern China continent and Kalimantan Island, respectively, and high input fluxes of terrigenous materials result in higher concentrations of V, Cr, and U in the sediments. The slope in transect T1 and northern and southern slopes in transect T2 are in coral reef islands (i.e., Xisha and Nansha Islands), with high carbonate production, which lowers V, Cr, and U sediment concentrations. Carbonate is partially dissolved in the ESB and SSB (with average CaO concentrations of 5.21 and 1.90 wt.%, respectively) due to the water depth exceeding the carbonate compensation depth. Consequently, the weakened dilution effect and convergence of terrigenous materials in the basins lead to higher V, Cr, and U sediment concentrations. The depositional model changes significantly from west to east along transect T3. Sediments on the western shelf and slope are dominated by terrigenous materials derived from the Mekong River, and those on the eastern slope comprise mainly biogenic carbonate. Sediment V, Cr, and U concentrations on the western shelf and slope are thus higher than on the eastern slope. The low V, Cr, and U concentrations on the western Sunda Shelf in transect T4 may reflect a small detrital sediment component. 9

4.1.3 Adsorption by Mn-oxyhydroxides Based on the above discussion, Mo can be distinguished from V, Cr, and U. Most samples have MoEF > 1 (62 samples) and the average MoEF reaches 6.40, indicating that a large proportion of Mo in sediments is of non-detrital origin. Non-detrital Mo in sediments is sourced mainly from the conversion of dissolved Mo in the overlying water column or sediment interstitial water. The primary source of dissolved Mo in the modern ocean is riverine molybdate (MoO2–4 ) liberated during oxidative weathering of continental crust (Scott et al., 2008). Previous studies have concluded that MoO2–4 is neither concentrated by plankton nor adsorbed by most types of particles, and it displays little affinity for the surfaces of CaCO3, clay mineral surfaces, or Fe-oxyhydroxides at marine pH values (Tribovillard et al., 2006), as supported by the low correlation coefficients between Mo and other elements including Sr, Ba, CaO, and Fe2O3 (r = -0.18, -0.36, -0.20, and -0.16, respectively; Table 3 and Fig. 6a). By contrast, MoO 2–4

is easily adsorbed by

Mn-oxyhydroxides from the overlaying water column in well-oxygenated environments and then buried in sediments (Scott et al., 2008; Tribovillard et al., 2006; Zheng et al., 2000). Under anoxic or euxinic conditions with free hydrogen sulphide (i.e., H2S and HS-), MoO2–4 is readily converted to particle-reactive thiomolybdates (MoO4-xS2-x , x=0 to 3) that are scavenged from solution by sulphidized (organic S-rich) organic material (Algeo and Tribovillard, 2009; Erickson and Helz, 2000; Tribovillard et al., 2006; Zheng et al., 2000). The question remains as to which of these two processes accounts for the higher Mo enrichment of surface sediments in the SCS relative to the UCC. Algeo and Lyons (2006) proposed a method for estimating the amount of Mo taken up by sediments in anoxic conditions based on the relationship: [Mo]s ≅ [Mo]aq × [TOC]s

(3)

where [Mo]s and [Mo]aq are Mo concentrations in sediment and seawater, respectively, and [TOC]s is the total organic carbon content of sediment. Mo has an average concentration of 105 nmol kg–1 (Emerson and Huested, 1991) in present-day predominantly oxygenated oceans, and exhibits globally uniform seawater concentrations due to its long oceanic residence time of ~780 kyr (Algeo and Tribovillard, 2009). The TOC content of surface sediments in the middle deep-sea 10

basin of the SCS ranges from 0.1 to 1.91 wt.%, with an average of 0.70 wt.% (Cai et al., 2018). If the seafloor environment there was anoxic, the average Mo concentration of surface sediments would be ~74 µg g–1 based on Equation (3). According to a statistical analysis of many samples from different environments by Scott and Lyons (2012), where sulfide and thiomolybdates are present but restricted to the interstitial waters, sediment Mo concentrations are ~10 µg g–1 and rarely exceeds 20 µg g–1. Where hydrogen sulfide is present in the water column throughout the year, sediment Mo concentrations are mostly >60 µg g–1, even reaching 100s µg g–1. However, the range and average concentration of Mo over all sampling sites in our study are 0.27–24.1 and 4.81 µg g–1, respectively, with only 11 samples having concentrations of >10 µg g–1. Such Mo concentrations are not consistent with an anoxic seafloor environment being the cause of Mo enrichment of surface sediments in the SCS. In a wide range of oxygen-rich environments, the ratio of total Mo to total Mn concentrations (Mo/Mn) of sediments is consistently 0.002 (Shimmield and Price, 1986). The SCS Mo/Mn ratio is in the range 0.0001–0.0242 (average = 0.0011) (Table A1). Previous studies have also considered that sediment V/Cr ratios of <2 indicate oxic conditions in the overlying water column (Jones and Manning, 1994; Scheffler et al., 2006). In the SCS, sediment V/Cr ratios are in the range 0.90–2.42 (average = 1.49) (Table A1). These above results may indicate that the SCS seafloor is generally oxic at present. The low sediment detrital Mn concentration is consistent with its lack of correlation with Al2O3 and TiO2 concentrations (r = 0.33 and 0.18, respectively; Table 3). The dominant Mn species in seawater are dissolved forms of Mn2+ and MnCl+, and in oxic settings Mn(II) is readily oxidized to insoluble Mn-oxyhydroxides (i.e., MnO2 and MnOOH), which adsorb MoO2–4 from the water column and become incorporated in sediments (Scott et al., 2008; Tribovillard et al., 2006; Zheng et al., 2000). Although Mo and MnO display opposite concentration at some individual sites in transect T1, overall they display similar transect trends, especially in transects T2–T4, with their correlation (R2 = 0.61) for all samples being shown in Fig. 6b. The Mo enrichment of most sediment samples relative to the UCC is thus mainly caused by its adsorption on Mn-oxyhydroxides.

4.2 Consistency between authigenic RSTE component concentration and seafloor 11

dissolved oxygen content The above discussion indicates that V, Cr, and U in the studied sediments are sourced predominantly from terrigenous input and stored in detrital particulates, with Mo enrichment relative to its UCC concentration being mainly due to adsorption by Mn-oxyhydroxides. Little or none of the authigenic RSTE component was accumulated under anoxic conditions, which seem to indicate the current generally oxic seafloor environment in the SCS. The SCS is a semi-enclosed marginal sea with the Luzon Strait (LS) connecting to the west Pacific. Previous studies have suggested that SCS Deep Water (SCSDW; below 1500 m) is sourced from Pacific Deep Water (PDW) through LS (Fig. 7a and b). The SCSDW upwells in the southern SCS and forms intermediate waters (SCS Intermediate Water; SCSIW) at depths of 400– 1500 m. Bottom water in the Okinawa Trough (OT) is sourced mainly from North Pacific Tropical Intermediate Water (NPTIW) through the Kerama Gap (KG) (Li et al., 2018). A meridional bathymetric profile (along the white dashed line in Fig. 7a) of dissolved oxygen content from the SCS to the OT was generated using Ocean Data View software (Schlitzer, 2018) based on the World Ocean Atlas (Garcia et al., 2013). Redox conditions can be classified as oxic (>2.0 mL O2 L–1), suboxic (0.2–2.0 mL O2 L–1), and anoxic (<0.2 mL O2 L–1) (Tyson and Pearson, 1991). The dissolved oxygen content of SCSDW exceeds 2.0 mL L–1 (Fig. 7b). The seafloor dissolved oxygen content along transects T1–T3 exceeds 2.0 mL L–1, and transect T4 is more than 0.2 mL L–1 (Fig. 7c~f). The high seafloor dissolved oxygen content thus indicates that the present seafloor redox conditions in the SCS are generally oxic, which shows a good consistency with the redox indications from little or none of authigenic RSTE component accumulation.

5. Conclusion The factors controlling RSTE compositions of surface sediments in the SCS are terrigenous material input, dilution by biogenic carbonate, and adsorption by Mn-oxyhydroxides. The factors controlling compositions of V, Cr, and U in SCS surface sediments are different to those of Mo. The former are derived mainly from terrigenous materials and stored predominantly in detrital particulate matter, rather than redox-related enrichment through precipitation under anoxic 12

conditions. Sediment V, Cr, and U concentrations vary similarly along each of the four transects with similar CV values, and are positively correlated with Al2O3 and TiO2 concentrations. Biogenic carbonates have an obvious dilution effect on V, Cr, and U concentrations, which display negative correlations with CaO. The transect trends of Mo, not consistent with those of V, Cr, U, Al2O3, TiO2, Fe2O3, or CaO, match those of MnO well. Mo has a very small detrital component. Although most samples are enriched in Mo relative to UCC, the degree of enrichment is lower than that observed under anoxic or euxinic conditions and is due mainly to adsorption by Mn-oxyhydroxides. Little or none of the authigenic RSTE component in the studied surface sediments indicates a generally oxic seafloor environment in the SCS, consistent with the higher seafloor dissolved oxygen content. RSTEs in surface sediments have the potential to reflect seafloor environments, but the magnitude of background detrital flux unrelated to redox conditions cannot be ignored. The assessment of detrital particulate flux and authigenic RSTE component is essential in making reliable redox interpretations.

Appendix （See Table A1）

Acknowledgments We thank Gary Fones and another two anonymous reviewers for their helpful comments on this paper. This work was jointly supported by the Natural Science Foundation of China (No. 41576035), the National Special Project on Basic Research of Science and Technology (No. 2008FY110100), the Science and Technology Basic Resource Investigation Program of China (No. 2017FY201403), and the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDA13010102).

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Figure Caption Fig. 1. Sampling sites of surface sediments from four transects in the SCS. Fig. 2. Concentration variations of RSTEs and major elements of surface sediments along Transect T1 and T2. Shelf, slope and deep basin are divided by gray vertical bars. Fig. 3. Concentration variations of RSTEs and major elements of surface sediments along Transect T3 and T4. Shelf and slope are divided by gray vertical bars. Fig. 4. Correlations between the concentration of RSTEs and that of major elements of surface sediments from the four transects in the SCS. (a) Al2O3; (b) TiO2; (c) CaO. Fig. 5. Variations between RSTE concentration and their respective EF of surface sediments from the four transects in the SCS. Fig. 6. Correlations between Mo and major elements of surface sediments from four typical transects in the SCS. (a)Fe2O3, (b) MnO. Fig. 7. (a) The studied latitudinal and longitudinal transects (red dashed line) and flow route of PDW (yellow arrows). (b) The meridional bathymetric profile (along the white dashed line in Figure 7a) of dissolved oxygen content from the SCS to the OT, and schematic diagram of deep circulation was marked with white arrows. (c)–(f) Dissolved oxygen profiles of transects T1–T4, respectively. These profiles were generated by Ocean Data View software (Schlitzer, 2018) based on World Ocean Atlas (Garcia et al., 2013). OT = Okinawa Trough; LS = Luzon Strait; KG = Kerama Gap; NPIW = North Pacific Intermediate Water; NPTIW = North Pacific Tropical Intermediate Water; PDW = Pacific Deep Water; SCSDW = South China Sea Deep Water; SCSIW = South China Sea Intermediate Water.

19

Table 2. Compositions of RSTE and major element in SCS sediments RSTE composition Transects

Major element composition

Parameters V

Cr

U

Mo

Al2O3

TiO2

CaO

Min

48.3

47.3

1.54

0.28

7.07

0.44

Max

157

98.8

2.98

13.4

17.6

Average

111

72.9

2.21

4.40

CV

25.4

20.3

16.5

Min

46.0

28.5

Max

143

Average

a

Fe2O3

MnO

0.69

3.56

0.06

0.87

18.1

7.14

1.86

13.1

0.66

8.17

5.09

0.75

86.3

21.1

19.2

66.3

23.2

74.6

1.13

0.83

5.74

0.21

0.75

2.02

0.11

107

3.35

24.1

18.0

0.79

35.5

6.86

3.00

89.1

60.5

1.88

6.01

11.2

0.46

17.6

4.03

0.98

CV

31.7

37.0

33.1

80.2

32.3

38.2

61.2

35.5

73.6

Min

26.8

13.1

0.70

0.29

2.85

0.10

0.69

0.94

0.06

Max

171

92.1

4.16

16.2

16.5

0.75

38.7

7.82

2.35

Average

76.9

53.2

1.94

4.25

9.27

0.39

20.9

3.50

0.70

CV

54.5

52.6

50.9

116

55.7

63.9

62.4

58.0

98.2

Min

43.4

31.3

1.49

0.27

3.67

0.24

2.47

3.15

0.05

Max

151

84.3

3.08

18.5

15.5

0.68

20.9

5.48

3.77

Average

88.7

60.6

2.29

4.14

10.4

0.48

10.3

4.44

0.89

CV

42.1

34.8

19.9

128

45.3

31.1

42.6

17.2

122

Min

26.8

13.1

0.70

0.27

2.85

0.10

0.69

0.94

0.05

T1+T2+

Max

171

107

4.16

24.1

18.0

0.87

38.7

7.82

3.77

T3+T4

Average

91.7

62.0

2.05

4.81

11.1

0.50

14.6

4.25

0.83

CV

39.3

37.5

33.2

98.8

39.0

42.0

73.8

37.2

91.6

T1

T2

T3

T4

1

a

Total iron as Fe2O3. CV = SD/mean %, CV: coefficient of variation, SD: standard deviation. Major elements in

wt.% and trace elements in µg g–1.

2

Table 3. Correlation matrix of major and trace elements of surface sediments from four transects in the SCS (The total number of sample is 75) V

a

Cr

U

Mo

Sr

Ba

Al2O3

TiO2

CaO

MnO

V

1.00

Cr

0.92

1.00

U

0.63

0.80

1.00

Mo

0.36

0.24

0.13

1.00

Sr

-0.62

-0.65

-0.43

-0.18

1.00

Ba

0.02

-0.06

-0.32

0.36

0.16

1.00

Al2O3

0.91

0.92

0.71

0.29

-0.63

-0.01

1.00

TiO2

0.86

0.89

0.74

0.17

-0.67

-0.23

0.94

1.00

CaO

-0.75

-0.77

-0.69

-0.20

0.74

0.36

-0.79

-0.87

1.00

MnO

0.39

0.30

0.23

0.78

-0.21

0.34

0.33

0.18

-0.21

1.00

Fe2O3

0.86

0.86

0.68

0.16

-0.70

-0.26

0.91

0.94

-0.89

0.21

Total iron as Fe2O3.

1

Fe2O3a

1.00

Table 1 Geochemical data for the studied elements (modified from Tribovillard et al., 2006) Elements

Main species

Average

Residence

Speciation in

Upper

Upper

Post-Archean

in oxic

concentration

time in

reducing

Continental

Continental

Average

seawater and

in seawater

seawater

conditions

Crust

Crust

Australian

oxidation

–1

(nmol kg )

(kyr)

–1 a

(µg g )

state

H2VO-4/V(V)

Shale(µg g–1)c

(µg g )

VO2-, VO(OH)-3 ,

HVO2-4 and V

–1 b

39.3

50

97

107

150

92

83

110

2.7

2.8

3.1

1.1

1.5

1.0

3840

4100

6000

VO(OH)2/V(IV)

Dominantly CrO2-4 /Cr

Cr(OH)+2 , Cr(OH)3,

(VI) + Cr

Cr(OH)+2 and

4.04

8 (Cr,Fe)(OH)3/Cr

Cr(OH)03

(III)

/Cr(III) UO2(CO3)4-3

U

13.4

400

/U(VI)

UO2, U3O7 or U3O8

MoO2-4 Mo

105

800

Thiomolybdates

/Mo(VI) Ti a

Rudnick et al. (2014)

b

c

McLennan (2001)

Taylor and McLennan (1985)

Table A1 Analytical data of trace elements, major elements and the enrichment factors of redox-sensitive trace elements of surface sediments from four transects in the SCS No.

Longitude(°E)

Latitude(°N)

V

Cr

U

Mo

Sr

Ba

Al2O3

TiO2

CaO

Fe2O3a

MnO

Mo/Mn

V/Cr

VEF

CrEF

UEF

MoEF

KJ01 KJ02

119.53

18.04

119.04

18.04

96.0 63.3

54.6 53.5

1.66 2.27

12.0 0.29

589 335

770 298

9.41 16.7

0.57 0.79

9.19 0.98

3.56 6.68

0.06 0.60

0.0242 0.0001

1.76 1.18

1.11 0.53

0.67 0.47

0.69 0.68

12.25 0.21

KJ03

118.55

18.01

143

98.8

2.30

4.42

134

542

15.9

0.87

1.43

6.38

0.65

0.0009

1.45

1.08

0.79

0.63

2.96

KJ04

118.03

18.01

133

92.8

2.46

5.16

137

493

17.1

0.84

0.69

6.99

1.67

0.0004

1.43

1.04

0.77

0.69

3.57

KJ05

117.95

18.02

152

89.1

2.50

13.4

129

602

17.6

0.83

0.74

7.14

0.89

0.0020

1.70

1.21

0.75

0.71

9.39

KJ06

116.98

18.98

157

96.8

2.58

5.73

136

698

15.5

0.73

2.36

6.50

1.01

0.0007

1.62

1.42

0.92

0.84

4.57

KJ07

116.50

18.97

141

85.1

2.23

4.35

176

793

13.3

0.60

10.8

5.23

1.49

0.0004

1.65

1.55

0.99

0.88

4.22

KJ08

116.00

17.98

122

67.6

1.96

4.26

413

745

14.9

0.73

4.08

5.81

0.23

0.0024

1.81

1.10

0.64

0.64

3.40

KJ09

115.48

18.98

130

83.4

2.40

0.74

221

773

12.3

0.56

13.5

4.73

1.10

0.0001

1.56

1.53

1.04

1.02

0.77

KJ10

115.02

17.99

112

66.5

1.87

5.38

516

778

13.7

0.70

8.34

5.23

0.57

0.0012

1.68

1.06

0.66

0.63

4.47

KJ11

114.51

18.03

120

78.9

2.69

2.67

343

652

15.4

0.72

3.12

5.93

1.60

0.0002

1.52

1.10

0.76

0.89

2.16

KJ12

114.03

18.00

135

83.3

2.31

9.01

205

884

12.2

0.61

10.6

4.74

0.74

0.0016

1.62

1.46

0.95

0.90

8.59

KJ13

113.50

18.02

97.5

67.5

2.01

3.17

401

630

10.2

0.44

16.0

3.66

1.86

0.0002

1.44

1.46

1.07

1.08

4.19

KJ14

113.03

18.06

96.3

55.4

1.54

9.58

611

748

10.5

0.46

18.1

3.76

1.05

0.0012

1.74

1.38

0.84

0.79

12.12

KJ15

112.49

18.00

89.7

62.4

1.88

1.02

522

696

11.8

0.56

14.1

4.29

0.34

0.0004

1.44

1.06

0.78

0.80

1.06

KJ16

111.99

17.99

95.4

61.3

1.69

3.59

449

557

10.3

0.49

11.1

3.72

0.31

0.0015

1.56

1.28

0.87

0.82

4.26

KJ17

111.46

17.98

97.3

69.2

2.04

0.91

528

531

11.7

0.60

13.6

4.15

0.23

0.0005

1.41

1.07

0.80

0.81

0.88

KJ18

110.97

18.07

85.7

66.4

2.26

1.63

462

432

11.7

0.65

12.3

4.31

0.33

0.0006

1.29

0.87

0.71

0.82

1.46

KJ19

110.50

17.99

99.0

77.8

2.98

0.48

233

344

14.4

0.83

5.28

5.42

0.09

0.0007

1.27

0.79

0.65

0.85

0.34

KJ20

110.01

18.01

48.3

47.3

2.55

0.28

314

237

7.07

0.60

7.04

3.60

0.07

0.0005

1.02

0.53

0.55

1.01

0.27

KJ21

113.02

17.52

81.4

60.2

1.33

2.18

890

690

8.78

0.39

30.7

3.16

0.39

0.0007

1.35

1.38

1.07

0.81

3.25

KJ22

112.98

17.01

56.3

35.6

1.13

1.28

920

624

6.79

0.30

32.4

2.57

0.23

0.0007

1.58

1.24

0.83

0.89

2.48

KJ23

113.00

16.50

46.2

31.8

1.29

1.07

1450

583

5.74

0.24

33.1

2.11

0.20

0.0007

1.46

1.27

0.92

1.27

2.59

KJ24

112.98

16.03

68.9

43.7

1.38

5.59

801

924

8.58

0.36

24.8

3.14

0.89

0.0008

1.58

1.26

0.84

0.91

9.03

KJ25

113.00

15.49

78.1

52.3

1.57

2.52

681

928

10.0

0.43

22.0

3.78

0.72

0.0005

1.49

1.20

0.85

0.87

3.41

1

Table A1 (continued ) No.

Longitude(°E)

Latitude(°N)

V

Cr

U

Mo

Sr

Ba

Al2O3

TiO2

CaO

Fe2O3a

MnO

Mo/Mn

V/Cr

VEF

CrEF

UEF

MoEF

KJ26 KJ27

113.03 113.05

15.05 14.53

125 79.2

79.6 52.5

2.47 1.69

10.8 3.81

192 579

1189 953

15.0 10.9

0.63 0.42

3.03 16.9

5.53 3.72

2.60 0.57

0.0005 0.0009

1.57 1.51

1.31 1.24

0.88 0.87

0.93 0.95

9.97 5.28

KJ28

113.04

13.97

98.3

61.6

1.78

7.10

601

1088

11.7

0.46

16.9

3.95

0.80

0.0011

1.60

1.41

0.93

0.92

8.98

KJ29

113.01

13.47

104

67.0

2.10

8.68

345

1096

13.2

0.53

7.84

4.71

1.47

0.0008

1.55

1.29

0.88

0.94

9.53

KJ30

114.12

13.70

127

95.2

2.64

3.50

139

628

17.0

0.78

1.84

6.55

0.53

0.0008

1.33

1.07

0.85

0.80

2.61

KJ31

114.06

12.03

137

102

2.73

7.22

120

668

17.4

0.77

0.75

6.69

0.93

0.0010

1.35

1.17

0.92

0.84

5.46

KJ32

112.92

11.98

143

107

2.94

2.39

144

664

18.0

0.79

1.80

6.86

0.41

0.0008

1.33

1.19

0.94

0.88

1.76

KJ33

112.88

11.49

130

98.6

3.35

0.83

183

559

16.1

0.77

3.19

6.10

0.11

0.0010

1.32

1.11

0.89

1.03

0.63

KJ34

114.06

10.80

46.0

28.5

1.16

4.14

1063

1023

5.94

0.21

29.7

2.02

1.06

0.0005

1.62

1.45

0.94

1.31

11.47

KJ35

112.69

10.40

82.6

55.4

1.81

6.99

660

1042

11.0

0.43

20.8

3.96

1.26

0.0007

1.49

1.27

0.90

1.00

9.46

KJ36

112.99

9.80

49.4

33.2

1.13

2.56

1006

685

6.45

0.24

35.5

2.35

0.41

0.0008

1.49

1.36

0.96

1.12

6.21

KJ37

113.01

9.48

68.5

41.2

1.33

5.52

915

965

8.18

0.29

24.7

2.68

0.71

0.0010

1.66

1.56

0.99

1.09

11.07

KJ38

113.01

9.01

77.2

44.6

1.37

6.77

571

898

8.79

0.32

15.3

2.93

0.77

0.0011

1.73

1.59

0.97

1.01

12.31

KJ39

113.06

8.49

82.8

53.1

1.70

11.0

710

1026

10.7

0.40

21.3

3.70

1.54

0.0009

1.56

1.37

0.92

1.01

16.00

KJ40 KJ41

113.03 113.04

7.99 7.51

72.7 89.3

51.5 61.3

1.61 1.89

4.99 8.03

724 587

765 787

9.57 11.6

0.37 0.46

22.6 16.8

3.36 4.14

0.80 1.08

0.0008 0.0010

1.41 1.46

1.30 1.28

0.97 0.93

1.03 0.97

7.85 10.16

KJ42

113.01

7.00

105

63.4

2.19

24.1

497

645

12.0

0.46

11.7

3.94

3.00

0.0010

1.65

1.51

0.96

1.13

30.48

KJ43

113.04

6.49

102

71.7

2.55

7.17

434

650

13.7

0.56

10.6

4.83

1.96

0.0005

1.42

1.20

0.89

1.08

7.45

KJ44

114.01

6.00

117

79.9

2.27

0.82

342

671

14.6

0.53

7.18

4.93

0.21

0.0005

1.47

1.46

1.05

1.02

0.90

KJ45

113.61

6.06

129

84.3

2.62

1.71

331

702

15.5

0.59

6.79

5.13

0.36

0.0006

1.53

1.44

0.99

1.05

1.69

KJ46

112.98

5.99

131

80.6

3.08

18.5

425

675

13.6

0.59

8.63

4.71

3.77

0.0006

1.62

1.46

0.95

1.24

18.24

KJ47

112.51

6.05

68.4

52.5

2.17

0.81

1002

233

9.58

0.46

20.9

3.65

0.12

0.0009

1.30

0.98

0.79

1.12

1.02

KJ48

111.87

5.96

111

81.5

2.74

3.47

384

524

15.1

0.68

8.88

5.48

1.25

0.0004

1.36

1.08

0.83

0.96

2.97

2

KJ49

111.83

6.00

114

79.0

2.76

10.5

385

586

14.9

0.66

8.89

5.31

1.57

0.0009

1.45

1.14

0.83

0.99

9.26

KJ50

111.00

5.98

151

80.1

2.69

9.81

425

577

14.8

0.62

9.54

4.91

2.49

0.0005

1.89

1.61

0.90

1.03

9.21

Table A1 (continued ) No.

Longitude (°E)

Latitude (°N)

V

Cr

U

Mo

Sr

Ba

Al2O3

TiO2

CaO

Fe2O3a

MnO

Mo/Mn

V/Cr

VEF

CrEF

UEF

MoEF

KJ51 KJ52

110.49 110.00

6.02 6.00

92.6 47.1

69.6 43.3

2.30 2.08

4.15 2.46

437 429

492 116

13.0 3.67

0.56 0.32

12.2 11.2

4.86 3.85

0.82 0.33

0.0007 0.0010

1.33 1.09

1.09 0.97

0.86 0.94

0.97 1.54

4.31 4.47

KJ53

109.50

6.00

52.4

38.7

1.97

0.41

375

211

5.64

0.36

9.74

3.85

0.08

0.0006

1.35

0.96

0.75

1.30

0.66

KJ54

109.01

6.00

48.6

31.7

1.85

0.62

610

154

4.29

0.29

16.8

4.70

0.47

0.0002

1.53

1.11

0.76

1.51

1.24

KJ55

108.50

6.01

47.1

34.7

1.68

0.27

509

219

5.52

0.31

10.3

3.21

0.06

0.0005

1.36

1.00

0.78

1.28

0.51

KJ56

108.00

6.00

43.4

31.3

1.49

0.31

123

210

4.57

0.24

2.47

3.15

0.05

0.0008

1.39

1.19

0.91

1.47

0.75

KJ57

108.75

7.00

171

70.5

1.02

1.47

200

357

16.0

0.74

2.96

7.82

0.31

0.0006

2.42

1.52

0.66

0.33

1.16

KJ58

108.99

7.92

29.7

19.3

1.25

0.29

963

148

2.85

0.16

22.0

2.53

0.17

0.0002

1.54

1.22

0.84

1.85

1.05

KJ59

109.19

8.92

111

87.3

3.52

0.41

285

404

15.6

0.75

6.31

5.95

0.08

0.0006

1.27

0.98

0.81

1.11

0.32

KJ60

109.56

9.98

122

92.1

4.16

0.54

342

431

15.7

0.73

7.72

5.34

0.06

0.0012

1.32

1.10

0.88

1.35

0.43

KJ61

110.04

10.03

100

70.2

2.16

4.15

399

661

12.6

0.52

8.96

4.24

1.25

0.0004

1.43

1.27

0.94

0.98

4.64

KJ62

110.48

10.04

115

77.5

2.57

16.2

379

685

14.0

0.58

8.48

4.66

1.79

0.0012

1.49

1.31

0.93

1.05

16.25

KJ63

110.76

10.26

110

82.3

2.84

0.49

330

508

15.0

0.71

7.82

5.74

0.10

0.0006

1.34

1.02

0.81

0.95

0.40

KJ64

111.52

10.00

128

89.8

2.78

15.5

129

959

16.5

0.70

0.69

6.21

2.19

0.0009

1.42

1.21

0.89

0.94

12.88

KJ65

112.10

10.02

114

76.6

2.49

11.3

379

1108

14.2

0.57

8.74

5.23

2.35

0.0006

1.49

1.32

0.93

1.04

11.53

KJ66

112.50

10.00

76.4

48.8

1.61

8.10

739

910

9.85

0.37

24.3

3.26

0.58

0.0018

1.57

1.36

0.92

1.03

12.74

KJ67

113.56

9.92

50.8

33.4

1.06

2.58

907

882

6.28

0.23

31.3

2.28

0.36

0.0009

1.52

1.46

1.01

1.09

6.53

KJ68

114.03

9.86

45.5

31.8

1.13

1.72

1181

1117

5.72

0.19

32.5

1.85

0.52

0.0004

1.43

1.58

1.16

1.41

5.27

KJ69

114.52

10.13

26.8

13.1

1.57

1.98

4690

680

2.99

0.10

36.0

0.94

0.33

0.0008

2.05

1.77

0.91

3.72

11.52

KJ70

115.06

10.06

45.4

28.3

0.94

2.54

1115

1361

5.53

0.19

35.4

1.95

0.48

0.0007

1.60

1.58

1.04

1.17

7.78

KJ71

115.54

10.02

29.2

15.5

1.14

0.94

3169

904

3.46

0.11

37.4

1.19

0.18

0.0007

1.88

1.75

0.98

2.46

4.97

3

a

KJ72

116.02

9.99

35.9

18.6

0.70

7.57

1048

1032

3.40

0.11

29.2

1.09

1.11

0.0009

1.93

2.15

1.18

1.51

40.04

KJ73

116.50

10.02

31.0

24.0

1.60

2.38

2391

604

3.22

0.11

38.7

1.20

0.45

0.0007

1.29

1.86

1.52

3.45

12.59

KJ74

117.01

9.98

69.4

77.4

3.21

1.56

1294

576

7.38

0.29

26.7

2.78

0.40

0.0005

0.90

1.58

1.86

2.62

3.13

KJ75

117.50

10.00

48.8

54.5

1.05

1.13

1083

1225

5.92

0.21

31.2

2.16

0.57

0.0003

0.90

1.53

1.81

1.19

3.13

–1

Total iron as Fe2O3. Major elements in wt.% and trace elements in µg g .

4

Highlights: 1. Redox sensitive trace element (RSTE, herein referred to V, Cr, U and Mo) compositions of 75 surface sediment samples collected from four longitudinal and latitudinal transects in the South China Sea (SCS) are first used to investigate their controlling factors and redox indications. 2. V, Cr and U have a substantial detrital component, while a greater proportion of Mo enrichment in surface sediments in the SCS is due mainly to adsorption by Mn-oxyhydroxides. 3. The current generally oxic seafloor environment in the SCS results in little or no authigenic RSTE component in its surface sediments.