Relating the composition of continental margin surface sediments from the Ross Sea to the Amundsen Sea, West Antarctica, to modern environmental conditions

WANG Jiakai, LI Tiegang, TANG Zheng, XIONG Zhifang, LIU Yanguang, CHEN Zhihua & CHANG Fengming,4

Relating the composition of continental margin surface sediments from the Ross Sea to the Amundsen Sea, West Antarctica, to modern environmental conditions

WANG Jiakai1,2, LI Tiegang2,3,4*, TANG Zheng3,4, XIONG Zhifang3,4, LIU Yanguang3,4, CHEN Zhihua3,4& CHANG Fengming1,2,4

1Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China;2University of Chinese Academy of Sciences, Beijing 100049, China;3Key Laboratory of Marine Geology and Metallogeny, First Institute of Oceanography, Ministry of Natural and Resources, Qingdao 266061, China;4Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237, China

Investigating the multiple proxies involving productivity, organic geochemistry, and trace element (TE) enrichment in surface sediments could be used as paleoenvironment archives to gain insights into past and future environmental conditions changes. We present redox-sensitive TEs (Mn, Ni, Cu, U, P, Mo, Co, V, Zn, and Cd), productivity-related proxies (total organic carbon and opal), and total nitrogen and CaCO3contents of bulk surface sediments of this area. The productivity proxies from the shelf and coastal regions of the Ross and the Amundsen seas showed that higher productivity was affiliated with an area of nutrient-rich deep water upwelling. The upwelling of weakly corrosive deep water may be beneficial for preserving CaCO3,while highly corrosive dense water, if it forms on the shelf near the coastal region (coastal polynya), could limit the preservation of CaCO3in modern conditions. There were no oxic or anoxic conditions in the study area, as indicated by the enrichment factors of redox-sensitive TEs (Mn, Co, and U). The enrichment factor of Cd, which is redox-sensitive, indicated suboxic redox conditions in sediment environments because of high primary productivity and organic matter preservation/decomposition. The enrichment factors of other redox-sensitive TEs (P, Ni, Cu, V, and Zn) and the correlations between the element/Ti ratio with productivity and nutrient proxies indicated that the organic matter decomposed, and there was massive burial of phytoplankton biomass. There was variation in the enrichment, such that sediments were enriched in P, Mo, and Zn, but depleted in Ni, Cu, and V.

redox, nutrient, productivity, shelf and coastal region, offshore of Ruppert and Hobbs coasts, Ross Sea, Amundsen Sea

1 Introduction

Multiple proxies, including indexes, concentrations, and ratios of trace elements (TE) related to productivity and nutrients in shelf and coastal sediments, have been used to trace past relationships between sedimentary redox conditions, ocean circulation, and primary productivity (Calvert and Pedersen, 1993; Crusius et al., 1996; Morford et al., 2001; McKay et al., 2007). Changes in TEs accompaniedby changes in productivity- and nutrient-related proxies at a single location are usually interpreted to indicate variations in ocean circulation and biogeochemical cycling during glacial/interglacial periods, and so provide insights into global climate dynamics (Wehausen and Brumsack, 2000; Pailler et al., 2002; Chang et al., 2015; Dou et al., 2015). Researchers have previously used multiple proxies to study the Mediterranean Sea and Cariaco Basin, and have reported that anoxic conditions are likely to occur within oxygen minimum zones, where there are massive burials of organic matter and bottom currents that have been relatively stagnant since ancient times (Thomson et al., 1995; Passier et al., 1999; Warning and Brumsack, 2000; Yarincik et al., 2000; Arnaboldi and Meyers, 2007). Additionally, there have been various studies of the eastern Pacific, where coastal upwelling zones drive high primary productivity (Nameroff et al., 2002; Nameroff et al., 2004; McManus et al., 2006; Chang et al., 2015). Researchers have also studied the redox chemistry of sediments, primary productivity, and nutrient levels of modern shelves and coastal regions of many parts of the South Pacific sector of the Southern Ocean (Abelmann and Gersonde, 1991; DeMaster et al., 1992; Isla et al., 2004; Baldi et al., 2010; Wagner et al., 2013; Wagner et al., 2015), but few researchers have reported information about the shelf and coastal areas of the Ross and Amundsen seas, apart from Kaiser et al. (2009), who studied shelf sediments from offshore of the Ruppert and Hobbs coasts and the Amundsen Sea. The unique oceanographic and geological setting of this region could mean that the concentrations of non-diagenetic TEs and the contents of productivity-related proxies in bulk sediments differ from those in other shelf and coastal regions. There are various reasons why the shelf and coastal sediments along Ross Sea and Amundsen Sea are different from those from other shelves and coastal regions worldwide, as follows. In this region, the sea ice and insolation vary considerably between seasons, which may affect the seasonal phytoplankton growth (Smith and Nelson, 1985; Smith and Gordon, 1997; Massom and Stammerjohn, 2010) and trigger rapid settling of biogenic organic particles on the seafloor. The nutrients in the water column could be depleted because of these seasonal variations in the primary productivity and regional variations in the nutrient pool (Smith et al., 2006), such that there would be limited export of nutrients with organic particles absorbed to the sediments. The organic matter recycling process in this region might be slow, because of the low temperature of the upper water mass (Gillooly et al., 2001), which would promote the settling of organic matter on the seafloor. This region is adjacent to a fast-changing marine-based ice sheet that includes the Ross Ice Shelf and the glaciers adjacent to the Amundsen Embayment. The detritus-ladened glacier extends to the coastal region and so delivers terrigenous debris to the shelf and slope region (Mosola and Anderson, 2006), which obscures the enrichment signal of non-diagenetic TEs. The various well-ventilated water masses present in this area, including Antarctic slope currents, Circumpolar Deep Water (CDW), Modified Circumpolar Deep Water (MCDW), and newly formed dense shelf water in coastal polynya and shelf-break regions (Budillon and Spezie, 2004; Assmann and Timmermann, 2005; Gordon et al., 2009; Padman et al., 2009), may inhibit the enrichment or remobilization of TEs under reducing conditions (Scholz et al., 2011).

Using multiple proxies to examine sediment paleo-redox conditions, nutrient levels, and productivity, researchers have explained variations in ventilation and productivity that were associated with past climate change (Chang et al., 2015; Wagner and Hendy, 2017). Therefore, investigations of past marine sediment records could help to predict future environmental change in the coastal and shelf region in the area from the Ross Sea to the Amundsen Sea. An earlier step involves investigating the TE concentrations in, and chemical characteristics of, surface sediments and then interpreting the TE enrichment of sediments with depth. Therefore, in this study, we measured the concentrations of a range of TEs, namely manganese (Mn), Nickel (Ni), Copper (Cu), uranium (U), phosphorus (P), molybdenum (Mo), cobalt (Co), cadmium (Cd), vanadium (V), zinc (Zn), barium (Ba), and titanium (Ti); the concentrations of total organic carbon (TOC) and nitrogen (TN), and the opal and CaCO3concentrations in surface sediments from 14 sediment cores extracted from 3 subregions along the Ross Sea and the Amundsen Sea, namely the Ross Sea Embayment, the Ruppert and Hobbs coasts, and the Amundsen Sea Embayment. We then analyzed the data to determine how the ventilation, and nutrient and organic carbon levels affected the surface sediment composition. The data reflect how the state of the high-latitude cryospheric environment has changed with change in climate in recent years, and could highlight the need to consider this region in global debates about climate change.

2 Regional setting

The study area stretches across the South Pacific sector of the Southern Ocean, from the Ross Sea to the Amundsen Sea (Figure 1). Because of its wide shelf region, polynya adjacent to the coastal region and ice shelf edge that trap sediment (DeMaster et al., 1992), and tendency for biomass blooms (Ditullio, 2000), the continental shelf adjacent to the South Pacific differs from other continental shelves worldwide. The topography of the seafloor in the study area changes rapidly from the continental shelf to the continental slope, and has several steep shelf breaks. The seafloor of the Ross Sea and the Amundsen Sea shelf region has been eroded by ice sheets that have advanced and retreated across the continental shelf, thereby removing sediments from many areas and carving numerous submarines through the basin, which has resulted in a bumpy bathymetry (Nitsche et al., 2007). Additionally, the isostatic depression of Antarctica by the West Antarctic Ice Sheet may promote the deposition of debris from basal marine ice sheets and ice shelves close to their grounding line (i.e., the junction between the bedrock, water, and ice sheet) in the Ross and Amundsen seas (Anderson et al., 1980; Anderson et al., 1992; Lowe and Anderson, 2002). This subglacial erosion, with the almost year-round cover of sea ice and massive icebergs in the Amundsen Sea, may lead to infrequent abundances of soft sediments and isopods (generally macrobenthic) (Kaiser et al., 2009).

Figure 1 Locations of surface sediment sampling sites (see Table 1 for the longitude, latitude, and depth), and the phosphate and nitrate contents in the surface ocean in the study area. The blue dashed line boxes Ⅰ, Ⅱ, and Ⅲ represent the Ross Sea, offshore of Ruppert and Hobbs coasts, and the Amundsen Sea sampling areas, respectively. The black dashed line indicates the Amundsen Sea polynya boundary (Kim et al., 2015). The phosphate and nitrate concentration data in the surface ocean are from the Word Ocean Atlas 2018 (Boyer, 2018). MCDW represents modified Circumpolar Deep Water and CDW represents Circumpolar Deep Water.

Meltwater discharge and iceberg-rafting debris sediments to the shelf and coastal regions may be controlled or influenced by global climatic change (Bronselaer et al., 2018). As the annual mean air temperature and/or precipitation increase, the deep water intrusion into the shelf region increases, leading to higher surface ablation, increases in basal melting and iceberg calving, and increases in meltwater and debris inputs (Jacobs and Comiso, 1997; Schmidtko et al., 2014; Dow et al., 2018). Furthermore, terrigenous detritus makes up a significant proportion of the sediment in the Ross and Amundsen embayments directly adjacent to the front of the ice sheet (including glaciers and ice shelves) (Sandroni and Talarico, 2006; Damiani and Giorgetti, 2008).

This study mainly considers the CDW and MCDW masses and the newly formed dense shelf water on the shelf region (Figure 1a). The warm, nutrient-rich, and well-oxygenated CDW, the most prominent water mass that flows along the shelf break, influences the shelf region of the Ross Sea and the Amundsen Sea. The CDW is a relatively warm, salty, and nutrient-rich deep water mass (Jacobs et al., 1970; Jacobs, 1985) that can extend to a depth of about 4000 m (Budillon and Spezie, 2004). The MCDW, which is the primary source of nutrients, salt, and heat to the Ross Sea shelf region, forms when the incoming CDW is modified near the Ross Sea shelf region (Jacobs et al., 1970; Jacobs, 1985; Picco, 2000; Budillon and Spezie, 2004; Hiscock, 2004; Smith et al., 2006). The MCDW flows south onto the shelf along the western side of the Ross Sea and is modified further by cold and newly formed dense shelf water (Orsi and Wiederwohl, 2009) that forms in the coastal polynya and the edge of the ice shelf in the Ross Sea (Figure 1a) (Gordon et al., 2009; Padman et al., 2009; van Wijk and Rintoul, 2014; Gordon et al., 2015).

The phosphate and nitrate contents in the surface water in the Ross Sea and the Amundsen Sea area range from 1.4 to 1.8 μmol·kg?1and from 20 to 25 μmol·kg?1, respectively (Figures 1b and 1c). The phosphate and nitrate contents generally follow a similar symmetrical distribution pattern, and are high near the shelf break and low in the coastal region of the Ross and Amundsen seas, apart from the nitrate content between the Ross and Amundsen seas (Figure 1c).

The fluxes of organic carbon and biogenic silica (opal) produced in the surface water in the Ross Sea were estimated at about 142 and 121 g·m?2·a?1, respectively (Nelson et al., 1996). However, because large amounts of biogenic silica are preserved in the Southern Ocean, organic carbon and biogenic silica accumulation in bottom sediment were estimated at only 0.25 and 6.97 g·m?2·a?1, respectively (Nelson et al., 1996). The sinking fluxes of organic carbon and biogenic silica at a depth of 400 m in the sea ice region on the Amundsen Sea shelf region were estimated at about 2.2 and 9.4 g·m?2·a?1, respectively (Kim et al., 2015).

3 Materials and methods

In this study, 14 surface samples were collected during the 36th Chinese National Antarctic Research Expedition (aboard R/V) using a box corer at sampling stations between 72°S and 76°S, and 165°E and 110°W in the Ross Sea and the Amundsen Sea area. Information about the sampling positions is presented in Table 1 and Figure 1a.Most of the samples were clayey silty sand with weak-medium plasticity and non-weak viscosity, and contained gravel grains between 2 and 10 mm. Biological disturbances were apparent in surface samples collected from the shelf and coastal region of the Ross and Amundsen seas but there were no apparent biological disturbances in the sediment samples collected between 150°W and 140°W offshore off the Ruppert and Hobbs coasts. All the experiments and measurements were performed at the Key Laboratory of Marine Sedimentology and Environmental Geology of the First Institute of Oceanography. The original samples were freeze-dried and ground into 200-mesh particles with an agate mortar. The samples were split and then were dried at 105℃ and 60℃, in preparation for analyzing the element and total carbon concentrations, respectively.

In preparation for trace element analysis, about 0.05 g of dried sample was weighed into a polytetrafluoroethylene flask along with 3 mL hydrofluoric acid (HF) and 1 mL nitric acid (HNO3). The flask was placed on an electric heating plate for sealing and was heated at 170℃ for 48 h. After the flask had cooled, the lid was opened and 1 mL of perchloric acid (HClO4) was added, and then the sample was steam dried on an electric heating plate at 170℃. Nitric acid (5 mL) with a concentration of 1:1 was added to the dried sample for extraction. The sample was then transferred to a colorimetric tube, made up to a constant volume of 25 mL, and tested on an inductively coupled plasma mass spectrometer (ICP-MS, X SeriesⅡ, ThermoFisher, USA). The accuracy of repeated measurements was better than 2%.

Table 1 Surface sediment sampling sites and the water depths at these sites

In preparation for measuring the total carbon, TOC and TN, about 30 mg of dried ground sample was weighed and placed in a tin boat. A small amount (2 g) of the original sample was also weighed and placed in a 10 mL centrifuge tube with 0.2 mL of 1 mol·L?1hydrochloric acid and heated in a water bath for 1 h. A further 0.2 mL of hydrochloric acid was added and then the sample was heated for a further 12 h in a water bath. The acid was washed with 25 mL of ultrapure water three times until neutral, and then dried in a water bath at 60℃. Once dry, the sample was ground, and about 30 mg was placed in a tin boat. The total and organic carbon contents of the sample were measured on an element analyzer (Vario EL Ⅲ, Elmentar, Germany). The total inorganic carbon was calculated as the difference between the total carbon and the TOC, and the CaCO3concentrations were calculated by multiplying the total inorganic carbon concentrations by 8.33 (Kim et al., 2020, Kim et al., 2015). A CaCO3concentration expressed as 0% indicates that the measured value of the TOC was greater than the total carbon in these samples, and may reflect systematic errors during testing.

To determine the biogenic silica concentrations, the silica was extracted by alkaline, and the silica concentrations in the extract were determined by molybdate-blue spectrophotometry (ultraviolet and visible spectrophotometer, TU-1810PC, Beijing General Instrument Co. LTD). The biogenic opal concentration was calculated by multiplying the biogenic silica concentration by 2.4, and the biogenic opal (SiO2+nH2O)/Si ratio was determined by assuming that nH2O accounted for 10% of biogenic SiO2(Mortlock and Froelich, 1989).

4 Results and discussion

4.1 Concentrations of productivity and nutrient- related proxies

The proxies from individual sites along the shelf and coastal region from the Ross Sea to the Amundsen Sea may be modified considerably by bottom current redistribution processes (Abelmann and Gersonde, 1991) that transport large quantities of biological/detrital particles to the deep channel or the low bathymetry shelf region (Isla et al., 2004), which act as the ‘sediment trap’. This means that the correlation between the production of biological particles in surface water and the preservation concentration in the sediments directly below is much weaker than it otherwise might be.Additionally, the uncertainty in the provenance and the chemical composition of reflux detrital particles may lead to errors in estimating the enrichment of non-diagenetic components. Therefore, to obtain robust conclusions, the proxy data were divided to describe three different continental shelf and coastal areas, namely the Ross Sea, offshore from the Rupert and Hobbs coasts, and the Amundsen Sea.

The total contents of all the productivity and nutrition-related proxy indicators are shown in Table 2 and their distribution in surface sediments is shown in Figure 2. Our results show that the contents of the productivity- related proxies (Ba, TOC, and opal) were high in the Ross Sea and the Amundsen Sea, and the TOC and opal contents reached 0.62% and 15.30% at site R1-05 on the Ross Sea shelf region, respectively. The contents of the productivity- related proxies were lowest in the sediment from the region offshore from the Ruppert and Hobbs coasts, and had minimum TOC and opal contents of 0.09% and 3.55% at site RA1-00 on the bottom continental slope, respectively. This distribution is consistent with the results of a previous study (Arrigo and van Dijken, 2003), who also found that the primary productivity was highest in the shelf and coastal region along the Ross Sea to the Amundsen Sea, and suggested that it was affiliated with phytoplankton blooms in the coastal polynya.

Table 2 Element concentration data for the bulk surface samples

Note: Productivity, nutrient, and redox show in subtitle represent the productivity-related proxies, nutrient-related proxy, and redox-related proxies

A trace element, Ba mainly occurs as the mineral barite (BaSO4) in the marine environment. Previous studies suggested a potential relationship between the Ba flux and the TOC/opal flux into the seafloor (Bishop, 1988; Dymond, 1992; Francois et al., 1995; Bonn et al., 1998). Our results showed that the Ba concentration distribution in the bulk sediments was different from the TOC/opal and other productivity proxies. The Spearman Rank correlation coefficients () between the Ba and opal and TOC contents were ?0.387 and 0.306, respectively, and the Spearman Rank correlation coefficients () for the relationship between the Ba/Ti and the opal and TOC contents were 0.178 and 0.424, respectively. This decoupling between Ba or Ba/Ti and other productivity proxies may reflect the different degrees to which opal and TOC are preserved in these three sub-regions, and will be discussed later. Moreover, all the Spearman Rank correlation coefficients for Ba/Ti and the opal and TOC contents were higher than those for the Ba concentration and the opal and TOC contents, which may indicate that the detrital dilution effect contributes to the decoupling of the Ba content and the productivity in bulk sediments around the Antarctic.

The ratio of the annual export flux of solid opal to organic carbon in the euphotic zone of the Southern Ocean is generally about 2.5 (Jennings et al., 1984; DeMaster et al., 1992). However, researchers have reported that this ratio can be more than 30 in deep sediments (DeMaster et al., 1992, Nelson et al., 1996). This difference between the organic carbon and silicon ratios and the absence of a relationship between biogenic silica-rich sediment and high primary productivity areas mean that opals are not a reliable indicator of productivity (Pondaven et al., 2000). Our results showed that there was a significant correlation between the bulk sediment TOC content and the opal concentration in the study region, with a Spearman Rank correlation coefficient () of 0.568 (Table 3), and may be related to the productivity in the study area. However, it is noteworthy that the value offor the relationship between the TOC and opal contents (was close to 0.5) was lower than that between TOC and TN (=0.949), and may indicate that the conditions for preserving organic matter are relatively poor in the Ross and the Amundsen seas shelf region, where the higher oxygen-rich water ventilation is not conducive to preserving organic matter (Kim et al., 2020). Moreover, the slight increase in the TOC content that corresponded to the significant increase in the opal concentrations in sediments from offshore of the Ruppert and Hobbs coasts (Figures 2 and 3a) may reflect the ‘opal paradox’ (Pondaven et al., 2000), which indicates higher preservation of opal than organic carbon in this region. This index should therefore be used with caution and should be combined with other bulk sedimentary proxies to make a holistic judgment about the region’s productivity, especially the sedimentary environment offshore from the Ruppert and Hobbs coasts.

Table 3 Spearman Rank Correlation Coefficients of Ti normalized trace elements, productivity, and concentrations of nutrient-related proxies

Notes: (1) The correlation was significant at the 0.01 level (double-tailed) in bold;

(2) The correlation was significant at the 0.05 level (double-tailed) in bold and in italic.

Similar to the productivity-related proxies, nutrient- related proxies (TN) were also significantly correlated with the TOC content, with avalue of 0.949, but were less strongly correlated with the opal concentration, with avalue of 0.450. Moreover, the significant increase in the opal concentration in sediment offshore from the Ruppert and Hobbs coasts was not accompanied by a corresponding increase in the TN concentration (Figure 3c). This suggests that the size of the nutrient pool may control the primary productivity in the Ross Sea and Amundsen Sea shelf and coastal region. However, there were minor variations in the TOC and TN contents in the sediments from offshore from the Ruppert and Hobbs coasts that were not present in the shelf and coastal sediment in the Ross and Amundsen seas (Figures 3b and 3c). These variations may reflect a shrinking of the nutrient pool caused by (1) the lower effective use of nutrients, which meant that less nutrients were absorbed from the surface water by the sediment, or (2) the relative stability of the nutrient pool in the water mass offshore from the Ruppert and Hobbs coasts caused by a lack of nutrients from other nutrient pools. These interpretations are preliminary hypotheses, which need to be tested further against data for the δ15N in different components of surface sediments (bulk surface sediment, diatom shells, acid-insoluble organic carbon, or apatite).

Figure 3 Scatter distribution of the TOC, TN, and opal contents in bulk surface sediments.

4.2 Distribution of CaCO3

Studies have shown that CaCO3content peaks observed in the past in the sediments from the South Pacific sector of Southern Ocean usually corresponded to an enriched layer of foraminifera (mainly sinistral) in Antarctic continental sediment and represented higher paleo-productivity (Hillenbrand et al., 2002; Hillenbrand et al., 2009). However, the premise for this suggestion is that the CaCO3content in the bulk surface sediments of the Southern Ocean is influenced by carbonate (including coccolith and foraminifer) productivity, opal dilution, and its preservation efficiency (Howard and Prell, 1994). Kim et al. (2020) suggested that the CaCO3was preserved in the Southern Ocean shelf and coastal sediment because (1) there was limited intrusion of a corrosive bottom water mass, (2) high-energy bottom currents restricted the fine material deposition, and (3) reduced formation of corrosive dense water mass. The infrequent preservation, or absence, of CaCO3in surface sediment may also reflect a decrease in organic carbon preservation of deep sea sediement and [CO32?]in deep water, leading to the further potential dissolution of CaCO3(Yu et al., 2014). In contrast, a large supply of organic matter may lead to the preservation of large amounts of CaCO3(Sayles et al., 2001). Additionally, CaCO3was not preserved in surface sediment below the calcium carbonate compensation depth (CCD).

The CaCO3contents of sediments varied considerably through the region. Sediment from coastal polynya of the Ross Sea (Terra Nova polynya, 0% in both sites R1-02 and R1-05) and Amundsen Sea (Amundsen Sea polynya, 0.03% and 0% in sites A11-01 and A11-02, respectively), and offshore of the Ruppert and Hobbs coasts (0.07%, 0.37 0%, 0.55%, and 0.11% in sites RA3-02, RA3-03, RA2-01, RA2-01A, and RA1-00, respectively), had extremely low concentrations of CaCO3or no CaCO3. Sediments from the Amundsen shelf region (0.59%, 0.61% in sites A4-03 and A3-01, respectively) have higher CaCO3content than those above. The CaCO3contents were the highest near the shelf break of all study sites (1.38%, 9.53%, and 4.58% in sites R1-07, R1-10, and A11-04, respectively). Except for the Ross Sea shelf region, low CaCO3concentrations were generally accompanied by high opal concentrations (Figure 2). An earlier study showed significant fluxes of CaCO3moving towards the seafloor in the modern Ross Sea (Collier et al., 2000) but low fluxes of CaCO3settling in the modern Amundsen Sea shelf region (sea ice region) (Kim et al., 2015). However, because of intensive biogeochemical processes in the modern Southern Ocean south of the polar front, the coastal waters are closer to becoming corrosive than elsewhere in the ocean ( Orr et al., 2005; McNeil and Matear, 2008; McNeil et al., 2010). Moreover, Tamura et al. (2008) reported that the annual sea-ice production was highest in the Ross Sea (Terra Nova polynya), and high in the Amundsen Sea coastal polynya, and these areas were associated with dense water formation (Massom and Stammerjohn, 2010). Additionally, the CaCO3contents in the three sub-regions were not correlated with the other productivity proxies in our study (Table 2). Therefore, we suggest that the distribution of CaCO3may be related to (1) the newly formed dense corrosive shelf water, which limits the preservation of CaCO3in the coastal polynya region of the Ross Sea and the Amundsen Sea; (2) the low CaCO3production in surface water of the modern Amundsen Sea sea-ice zone and offshore from the Ruppert and Hobbs coasts; (3) sediment located below the local modern CCD and/or a decrease in the preservation of organic carbon in deep sediments (Figure 2); (4) the intrusion of weakly corrosive CDW/MCDW along the shelf break region in the Ross Sea and the Amundsen Sea; or (5) a dilution effect related to the high opal preservation.

4.3 Biochemical behavior of TEs and redox sensitive variations in TEs

The total concentrations of all the proxies are presented in Table 2. The TE enrichment was determined by normalizing the bulk sediment TE concentrations to Ti and then comparing with the detritus background. Here, to compare the enrichment of the TEs, the TE concentrations are given in the form of enrichment factors (EFs), calculated as follows:

EF= (/)sample/(/)PAAS,

whereandrepresent the weight concentrations of the elements X and Ti, respectively, and the PAAS value was from Mclennan (2001).are useful for rapid estimates if the TEs are from authigenic or detrital sources.values larger than 1.0 suggest TE enrichment relative to the detrital background value (Xiong et al., 2012). Similar results may be obtained by normalizing with Al, as the excess Al may be captured by sink organic-sourced particles, leading to a possible excess of Al (Murray and Leinen, 1993, 1996). This method should be used with caution because it may lead to false normalization of sediment that receives large quantities of detrital material and organic-sourced particles. Thus, the discussion and data presentation here are based on data normalized with Ti. The distribution of the EFs in surface sediment is shown in Figure 4.

The Southern Ocean has long been considered an unlikely environment for TE accumulation because the seafloor is well ventilated and the sedimentation rates are low (Crusius and Thomson, 2000). In line with this earlier finding, the sediments in the three subregions were weakly enriched by Mn, Ni, Cu, Co, U, and V. However, sediments confined to specific areas were highly enriched by TEs, reflecting the high primary productivity. This limited enrichment was obvious in areas such as the shelf break and coastal polynya region where elements including P, Zn, Mo, and Cd may be associated with organic matter complexes or decomposition (Tribovillard et al., 2006). Manganese has a limited role as a redox substitute (Calvert and Pedersen, 1993). However, its unique geochemical behavior means it plays a prominent role in the enrichment of TEs (Ni, Cu, and Co) from oxic water to sediments, resulting in co-authigenic sedimentation processes (Tribovillard et al., 2006). Ni, Co, and Cu enrichment may occur in anoxic to euxinic conditions, by reducing the low state ion and combining with S to precipitate at the water-sediment interface (Tribovillard et al., 2006). There may also be U and V enrichment in sediment under anoxic conditions, through reduction reactions and adsorption or precipitation as UO2(uraninite, the most frequent form), U3O7,or U3O8(Morford et al., 2001; McManus et al., 2005), with insoluble matter forming when reduced ion V(III) complexes with hydroxyl (Breit and Wanty, 1991). Thus, there was no enrichment of Mn, Ni, Cu, Co, U, and V (Figure 4), and significant enrichment of Cd, in the three subregions, which suggests the sedimentary environment in the modern Southern Ocean was neither oxic nor anoxic (Tribovillard et al., 2006) and that there were mild or strong suboxic conditions in the shelf and coastal environments (Calvert and Pedersen, 1993; Rosenthal et al., 1995; Morford et al., 2001; Chaillou et al., 2002), respectively.

The preservation of organic matter did not seem to be important for the enrichment of Mn, Cu, U, and Co, as these TEs were not significantly correlated with the TOC content (Figure 5, Table 3). However, the distribution pattern of the enrichment factors of TEs such as Mn, Ni, Cu, U Co, V, and Cd indicates that there was widespread preservation of TOC, as also discussed in section 4.1. We suggest that the sedimentary redox chemistry may be strongly influenced by the flux of organic matter into the sediments and the decomposition/remineralization of this organic matter, which then affects the accumulation of TEs by driving the consumption of oxidants and maintaining the suboxic conditions in the shelf and coastal sediment. Moreover, we suggest that the difference in the degree of decay of organic matter between the shelf and coastal sediments in the Ross Sea, Amundsen Sea, and offshore from the Ruppert and Hobbs coasts may mean that the TE enrichment differs among the subregions. For example, the Cd concentration was correlated with the TOC content, with avalue of 0.739. However, the high TOC content may reflect relatively well preserved organic carbon in the Ross Sea and the Amundsen Sea shelf, and coastal sediment may drive the Cd enrichment.

In contrast, the strengthened decomposition/ remineralization of organic matter (lower TOC content) accompanied by theCdvalue below 1 in sediment from offshore of the Ruppert and Hobbs coasts may indicate the release of TEs complexed with organic carbon into pore water (Figures 2, 4). This process has been observed in various continental margin sediments in the South Pacific sector of the Southern Ocean (Wagner et al., 2013, 2015). Notably, although the EFs of Mn were less than 1 in all three subregions, theMnwas higher in the sediment from the Ross Sea coastal polynya (Terra Nova Bay) than at other sites, which may reflect the formation of oxygen-rich dense water that flushing the adjacent shelf region, led to the penetration of oxygen to the sediment porewater and the oxidation of Mn2+in pore water. Moreover, since Holocene, the sediment preserving the last deglacial and/or early Holocene relict manganese oxide during near modern subsequent sediment and then burial in the sub-surface sediments under relative oxic porewater conditions (Pavia et al., 2021), which may help to explain the high value (close to 1) for theMnin the surface sediment from the Ross Sea polynya.

Figure 5 Scatter distribution of the TOC content with Mn/Ti, Cu/Ti, U/Ti, and Co/Ti in bulk surface sediments. The data points of sites A3-01 and R1-05 are shaded gray.

Phosphorus plays a vital role in many metabolic processes (Tribovillard et al., 2006). Usually, P is released from decaying organic matter below the sediment-water interface, and P could be trapped within the sediment during this process of P remineralization (Span et al., 1992). Previous studies have reported that some TEs (including Cd, Mo, U, and Zn) were significantly enriched in phosphorites relative to their average shale background value (Prév?t and Lucas, 1980, 1986; Jarvis, 1995; Nathan et al., 1997). Middelburg and Comans (1991) reported that Cd and U were strongly enriched in phosphorites. However, these TEs may not necessarily be located in the apatite structure itself and may be associated with other minerals (mainly sulfides) and organic matter (Tribovillard et al., 2006). Overall, the P/Ti was negatively correlated with productivity and nutrient-related proxies, withvalues of 0.582 and 0.671 for TN and P/Ti, and for TOC and P/Ti, respectively. Thevalues between P and the other TEs were also low (Table 3). Therefore, the P enrichment may reflect the high degree of organic decomposition in deep sediment from off the Ruppert and Hobbs coasts and it may not be in the form of apatite.

Mo can be removed by organic detritus or trapped by pyrite from the water column. It is abundant relative to the biological requirements of its distribution and contrasts sharply with the nutrient-like distribution characteristics of many other TEs that are essential for organisms (Collier, 1985). Additionally, Ni behaves as a nutrient-like TE, and, as Ni complexes with organic matter, Ni will be scavenged in the water column (Tribovillard et al., 2006). Previous experiments in the Southern Ocean have suggested that Zn is a co-limiting nutrient, primarily through the regulation of silicic acid uptake by diatoms in the South Pacific sector of the Southern Ocean (Franck et al., 2000, Cochlan et al., 2002). Thus, Zn may to be added to the sediment in association with planktonic materials (Francois, 1988) and it may also exhibit the behavior of a nutrient-like TE.

Moreover, under moderate reducing conditions, V(V) will transfer to V(IV) and form insoluble hydroxides VO(OH)2when organic acids are present (Tribovillard et al., 2006). Sayles et al. (2001) interpreted pore water data and suggested that the enrichment of Mo in the Pacific sector did not imply that the surface sediment environment was fully anoxic, which is consistent with our interpretation about no anoxic conditions in the study area. Therefore, this enrichment pattern of Mo may be related to organic carbon export and decomposition associated with nutrient use by phytoplankton rather than the sediment redox conditions. Furthermore, we also suggest that, through the decomposition of organic matter, TEs (Zn, Ni, and V) could be transferred into the pore water and thereby contribute to the distribution pattern of Zn, Ni, and V. This may help explain (1) the ρ values of 0.576, 0.539, and 0.625 between TOC and Zn/Ti, Ni/Ti, and V/Ti, respectively; (2) the ρ values of 0.627, 0.618, and 0.595 between TN and Zn/Ti, Ni/Ti, and V/Ti, respectively (Figure 6, Table 3); and (3) the absence of any enrichment of Ni and V. Moreover, there was significant enrichment of Zn (Figure 4), which may relate to the massive burial of planktonic materials in these three subregions (Francois, 1988).

5 Summary and conclusions

In this study, we reported the TE concentrations in continental margin surface sediments from three subregions, namely the Ross Sea, offshore from the Ruppert and Hobbs coasts, and the Amundsen Sea. The main findings of this study are as follows:

(1) Regardless of the potential differences in the degree of preservation of TOC/opal, our data suggest that of these three sub-regions, the primary productivity should be highest in the Amundsen Sea. Productivity proxies, including opal, TOC, and TN, could indicate the spatial variation in primary productivity in the seas around the margins of the Antarctic. We reduced the potential bias from the opal by coupling the opal concentration with the TOC and TN concentrations, even though the opal preservation may be much higher than the TOC preservation in these subregions.

Figure 6 Scatter distribution of the TN and TOC contents with Mo/Ti, P/Ti, Zn/Ti, V/Ti, Ni/Ti, and Cd/Ti in bulk surface sediments. The data points for sites A3-01 and R1-05 are shaded gray.

(2) The CaCO3content in the three subregions represents the degree of preservation and mainly reflects the occupation of newly formed dense shelf water and the upwelling of CDW/MCDW in the shelf region of the Ross Sea and the Amundsen Sea and the decomposition of organic carbon in sediment from the area offshore of the Ruppert and Hobbs coasts.

(3) The EF values for Mn, Ni, Cu, U, Co, and V were lower than 1 in all the subregions, meaning that they were not significantly enriched. This indicates that the conditions in these subregions were neither oxic nor anoxic. The EF values of P, Mo, and Zn were either greater than or approximately equal to 1 in the subregions. These values may reflect the decomposition/preservation of organic carbon and settling of biogenic particles and the nutrient supply status. There was significant enrichment of Cd in the shelf and coastal region of the Ross Sea and the Amundsen Sea, which may reflect the high organic carbon flux and/or that the conditions were conducive to preservation.

(4) Overall, the TOC, opal, and CaCO3concentrations in the sediments were moderately elevated, which, along with the TE concentrations, suggests that the sedimentary redox conditions are mainly controlled by the organic carbon flux or preservation and that the TE enrichment was not blurred by the ventilation of oxygen-rich newly formed water or upwelling of deep water. Our results suggest that the remineralization of sedimentary organic matter may promote stable redox conditions in the sediments across the study area, and may also control the sedimentary redox chemistry in other sectors of the Southern Ocean (Wagner et al., 2013, 2015).

Notably, the surface sediments examined in this study were mainly from the continental shelf and coastal areas, and the chemical composition of sediments from the central and inner shelf might be different from the composition of sediments from the top and bottom of the continental slope. Sediments from these other areas should be surveyed in the future to facilitate further investigations of the relationships between multiple proxies and the environmental conditions, thereby improving our understanding of paleoceanographic environments.

Acknowledgments We thank the members of the 36th Chinese National Antarctic Research Expedition for retrieving the sediment samples. This work was financially supported by National Polar Special Program “Impact and Response of Antarctic Seas to Climate Change” (Grant nos. IRASCC 01-03-02 and 02-03), was supported by the Basic Scientific Fund for National Public Research Institutes of China (Grant no. 2019Q09), and the National Natural Science Foundation of China (Grant nos. 41976080 and 41406220). We appreciate Dr. Yuanyuan Feng as reviewer, one anonymous reviewer, and Guest Editor Prof. Rujian Wang for their constructive comments that have further improved the manuscript.

Abelmann A, Gersonde R. 1991. Biosiliceous particle flux in the Southern Ocean. Mar Chem, 35(1-4): 503-536, doi:10.1016/S0304-4203(09) 90040-8.

Anderson J B, Kurtz D D, Domack E W, et al. 1980. Glacial and glacial marine sediments of the Antarctic continental shelf. J Geol, 88(4): 399-414, doi:10.1086/628524.

Anderson J B, Shipp S S, Bartek L R, et al. 1992. Evidence for a grounded ice sheet on the Ross Sea continental shelf during the Late Pleistocene and preliminary paleodrainage reconstruction. Contributions to Antarctic Research III. Washington, D. C.: American Geophysical Union, 39-62, doi:10.1029/ar057p0039.

Arnaboldi M, Meyers P A. 2007. Trace element indicators of increased primary production and decreased water-column ventilation during deposition of latest Pliocene sapropels at five locations across the Mediterranean Sea. Palaeogeogr Palaeoclimatol Palaeoecol, 249(3-4): 425-443, doi:10.1016/j.palaeo.2007.02.016.

Arrigo K R, van Dijken G L. 2003. Phytoplankton dynamics within 37 Antarctic coastal polynya systems. J Geophys Res, 108(C8): 3271, doi:10.1029/2002jc001739.

Assmann K M, Timmermann R. 2005. Variability of dense water formation in the Ross Sea. Ocean Dyn, 55(2): 68-87, doi:10.1007/ s10236-004-0106-7.

Baldi F, Marchetto D, Pini F, et al. 2010. Biochemical and microbial features of shallow marine sediments along the Terra Nova Bay (Ross Sea, Antarctica). Cont Shelf Res, 30(15): 1614-1625, doi:10.1016/j.csr. 2010.06.009.

Bishop J K B. 1988. The barite-opal-organic carbon association in oceanic particulate matter. Nature, 332(6162): 341-343, doi:10.1038/332 341a0.

Bonn W J, Gingele F X, Grobe H, et al. 1998. Palaeoproductivity at the Antarctic continental margin: opal and barium records for the last 400 ka. Palaeogeogr Palaeoclimatol Palaeoecol, 139(3-4): 195-211, doi:10.1016/S0031-0182(97)00144-2.

Breit G N, Wanty R B. 1991. Vanadium accumulation in carbonaceous rocks: a review of geochemical controls during deposition and diagenesis. Chem Geol, 91(2): 83-97, doi:10.1016/0009-2541(91) 90083-4.

Bronselaer B, Winton M, Griffies S M, et al. 2018. Change in future climate due to Antarctic meltwater. Nature, 564(7734): 53-58, doi:10.1038/s41586-018-0712-z.

Budillon G, Spezie G. 2004. Thermohaline structure and variability in the Terra Nova Bay polynya, Ross Sea. Antartic Sci, 12(4): 493-508, doi:10.1017/s0954102000000572.

Calvert S E, Pedersen T F. 1993. Geochemistry of Recent oxic and anoxic marine sediments: implications for the geological record. Mar Geol, 113(1-2): 67-88, doi:10.1016/0025-3227(93)90150-T.

Chaillou G, Anschutz P, Lavaux G, et al. 2002. The distribution of Mo, U, and Cd in relation to major redox species in muddy sediments of the Bay of Biscay. Mar Chem, 80(1): 41-59, doi:10.1016/S0304-4203(02) 00097-X.

Chang A S, Pichevin L, Pedersen T F, et al. 2015. New insights into productivity and redox-controlled trace element (Ag, Cd, Re, and Mo) accumulation in a 55 kyr long sediment record from Guaymas Basin, Gulf of California. Paleoceanography, 30(2): 77-94, doi:10.1002/ 2014pa002681.

Cochlan W P, Bronk D A, Coale K H. 2002. Trace metals and nitrogenous nutrition of Antarctic phytoplankton: experimental observations in the Ross Sea. Deep Sea Res Part II Top Stud Oceanogr, 49(16): 3365-3390, doi:10.1016/S0967-0645(02)00088-7.

Collier R, Dymond J, Honjo S, et al. 2000. The vertical flux of biogenic and lithogenic material in the Ross Sea: moored sediment trap observations 1996-1998. Deep Sea Res Part II Top Stud Oceanogr, 47(15-16): 3491-3520, doi:10.1016/S0967-0645(00)00076-X.

Collier R W. 1985. Molybdenum in the Northeast Pacific Ocean. Limnol Oceanogr, 30(6): 1351-1354, doi:10.4319/lo.1985.30.6.1351.

Crusius J, Calvert S, Pedersen T, et al. 1996. Rhenium and molybdenum enrichments in sediments as indicators of oxic, suboxic and sulfidic conditions of deposition. Earth Planet Sci Lett, 145(1-4):

Crusius J, Thomson J. 2000. Comparative behavior of authigenic Re, U, and Mo during reoxidation and subsequent long-term burial in marine sediments. Geochimica Cosmochimica Acta, 64(13): 2233-2242, doi:10.1016/S0016-7037(99)00433-0.

Damiani D, Giorgetti G. 2008. Provenance of glacial-marine sediments under the McMurdo/Ross Ice Shelf (Windless Bight, Antarctica): heavy minerals and geochemical data. Palaeogeogr Palaeoclimatol Palaeoecol, 260(1-2): 262-283, doi:10.1016/j.palaeo.2007.08.010.

DeMaster D, Dunbar R, Gordon L, et al. 1992. Cycling and accumulation of biogenic silica and organic matter in high-latitude environments: the Ross Sea. Oceanography, 5(3): 146-153, doi:10.5670/oceanog. 1992.03.

DiTullio G R, Grebmeier J M, Arrigo K R, et al. 2000. Rapid and early export ofblooms in the Ross Sea, Antarctica. Nature, 404(6778): 595-598, doi:10.1038/35007061.

Dou Y G, Yang S Y, Li C, et al. 2015. Deepwater redox changes in the southern Okinawa Trough since the last glacial maximum. Prog Oceanogr, 135: 77-90, doi:10.1016/j.pocean.2015.04.007.

Dow C F, Lee W S, Greenbaum J S, et al. 2018. Basal channels drive active surface hydrology and transverse ice shelf fracture. Sci Adv, 4(6): 1-10, doi:10.1126/sciadv.aao7212.

Dymond J, Suess E, Lyle M. 1992. Barium in deep-sea sediment: a geochemical proxy for paleoproductivity. Paleoceanography, 7(2): 163-181, doi:10.1029/92pa00181.

Franck M V, Brzezinski M A, Coale K H, et al. 2000. Iron and silicic acid concentrations regulate Si uptake north and south of the Polar Frontal Zone in the Pacific Sector of the Southern Ocean. Deep Sea Res Part II Top Stud Oceanogr, 47(15-16): 3315-3338, doi:10.1016/S0967- 0645(00)00070-9.

Francois R. 1988. A study on the regulation of the concentrations of some trace metals (Rb, Sr, Zn, Pb, Cu, V, Cr, Ni, Mn and Mo) in Saanich Inlet Sediments, British Columbia, Canada. Mar Geol, 83(1-4): 285-308, doi:10.1016/0025-3227(88)90063-1.

Francois R, Honjo S, Manganini S J, et al. 1995. Biogenic barium fluxes to the deep sea: implications for paleoproductivity reconstruction. Global Biogeochem Cycles, 9(2): 289-303, doi:10.1029/95gb00021.

Gordon A L, Huber B A, Busecke J. 2015. Bottom water export from the western Ross Sea, 2007 through 2010. Geophys Res Lett, 42(13): 5387-5394, doi:10.1002/2015gl064457.

Gordon A L, Orsi A H, Muench R, et al. 2009. Western Ross Sea continental slope gravity currents. Deep Sea Res Part II Top Stud Oceanogr, 56(13-14): 796-817, doi:10.1016/j.dsr2.2008.10.037.

Hillenbrand C D, Fütterer D K, Grobe H, et al. 2002. No evidence for a Pleistocene collapse of the West Antarctic Ice Sheet from continental margin sediments recovered in the Amundsen Sea. Geo Mar Lett, 22(2): 51-59, doi:10.1007/s00367-002-0097-7.

Hillenbrand C D, Kuhn G, Frederichs T. 2009. Record of a Mid-Pleistocene depositional anomaly in West Antarctic continental margin sediments: an indicator for ice-sheet collapse? Quat Sci Rev, 28(13-14): 1147-1159, doi:10.1016/j.quascirev.2008.12.010.

Hiscock M R. 2004. The regulation of primary productivity in the Southern Ocean. Ph.D, Duke University.

Howard W R, Prell W L. 1994. Late Quaternary CaCO3production and preservation in the Southern Ocean: implications for oceanic and atmospheric carbon cycling. Paleoceanography, 9(3): 453-482, doi:10.1029/93pa03524.

Isla E, Masqué P, Palanques A, et al. 2004. Sedimentation of biogenic constituents during the last century in western Bransfield and Gerlache Straits, Antarctica: a relation to currents, primary production, and sea floor relief. Mar Geol, 209(1-4): 265-277, doi:10.1016/j.margeo. 2004.06.003.

Jacobs S S, Amos A F, Bruchhausen P M. 1970. Ross Sea oceanography and Antarctic bottom water formation. Deep Sea Res Oceanogr Abstr, 17(6): 935-962, doi:10.1016/0011-7471(70)90046-X.

Jacobs S S, Comiso J C. 1997. Climate variability in the Amundsen and Bellingshausen seas. J Climate, 10(4): 697-709, doi:10.1175/1520- 0442(1997)010<0697: cvitaa>2.0.co;2.

Jacobs S S, Fairbanks R G, Horibe Y G. 1985. Origin and evolution of water masses near the Antarctic continental margin: evidence from H218O/H216O ratios in seawater. Oceanology of the Antarctic Continental Shelf. Washington, D. C.: American Geophysical Union, 59-85, doi:10.1029/ar043p0059.

Jarvis I, Burnett W C, Nathan Y, et al. 1994. Phosphorite geochemistry: state-of-the-art and environmental concerns. Eclogae Geol Helv, 87(3): 643-700, 10.1111/j.1365-3091.1994.tb01397.x.

Jennings J C, Gordon L I, Nelson D M. 1984. Nutrient depletion indicates high primary productivity in the Weddell Sea. Nature, 309(5963): 51-54, doi:10.1038/309051a0.

Kaiser S, Barnes D K A, Sands C J, et al. 2009. Biodiversity of an unknown Antarctic Sea: assessing isopod richness and abundance in the first benthic survey of the Amundsen continental shelf. Mar Biodivers, 39(1): 27-43, doi:10.1007/s12526-009-0004-9.

Kim M, Hwang J, Kim H J, et al. 2015. Sinking particle flux in the sea ice zone of the Amundsen Shelf, Antarctica. Deep Sea Res Part I Oceanogr Res Pap, 101: 110-117, doi:10.1016/j.dsr.2015.04.002.

Kim S, Lee J I, McKay R M, et al. 2020. Late pleistocene paleoceanographic changes in the Ross Sea – Glacial-interglacial variations in paleoproductivity, nutrient utilization, and deep-water formation. Quat Sci Rev, 239: 106356, doi:10.1016/j.quascirev.2020. 106356.

Lowe A L, Anderson J B. 2002. Reconstruction of the West Antarctic Ice Sheet in Pine Island Bay during the Last Glacial Maximum and its subsequent retreat history. Quat Sci Rev, 21(16-17): 1879-1897, doi:10.1016/S0277-3791(02)00006-9.

Massom R A, Stammerjohn S E. 2010. Antarctic Sea ice change and variability – Physical and ecological implications. Polar Sci, 4(2): 149-186, doi:10.1016/j.polar.2010.05.001.

McKay J L, Pedersen T F, Mucci A. 2007. Sedimentary redox conditions in continental margin sediments (N.E. Pacific)—Influence on the accumulation of redox-sensitive trace metals. Chem Geol, 238(3-4): 180-196, doi:10.1016/j.chemgeo.2006.11.008.

McLennan S M. 2001. Relationships between the trace element composition of sedimentary rocks and upper continental crust. Geochem Geophys Geosyst, 2(4): 2000GC000109, doi:10.1029/ 2000gc000109.

McManus J, Berelson W M, Klinkhammer G P, et al. 2005. Authigenic uranium: relationship to oxygen penetration depth and organic carbon rain. Geochimica Cosmochimica Acta, 69(1): 95-108, doi:10.1016/ j.gca.2004.06.023.

McManus J, Berelson W M, Severmann S, et al. 2006. Molybdenum and uranium geochemistry in continental margin sediments: Paleoproxy potential. Geochimica Cosmochimica Acta, 70(18): 4643-4662, doi:10.1016/j.gca.2006.06.1564.

McNeil B I, Matear R J. 2008. Southern Ocean acidification: a tipping point at 450-ppm atmospheric CO2. PNAS, 105(48): 18860-18864, doi:10.1073/pnas.0806318105.

McNeil B I, Tagliabue A, Sweeney C. 2010. A multi-decadal delay in the onset of corrosive ‘a(chǎn)cidified’ waters in the Ross Sea of Antarctica due to strong air-sea CO2disequilibrium. Geophys Res Lett, 37(19): L19607, doi:10.1029/2010gl044597.

Middelburg J J, Comans R N J. 1991. Sorption of cadmium on hydroxyapatite. Chem Geol, 90(1-2): 45-53, doi:10.1016/0009- 2541(91)90032-M.

Morford J L, Russell A D, Emerson S. 2001. Trace metal evidence for changes in the redox environment associated with the transition from terrigenous clay to diatomaceous sediment, Saanich Inlet, BC. Mar Geol, 174(1-4): 355-369, doi:10.1016/S0025-3227(00)00160-2.

Mortlock R A, Froelich P N. 1989. A simple method for the rapid determination of biogenic opal in pelagic marine sediments. Deep Sea Res A Oceanogr Res Pap, 36(9): 1415-1426, doi:10.1016/0198- 0149(89)90092-7.

Mosola A B, Anderson J B. 2006. Expansion and rapid retreat of the West Antarctic Ice Sheet in eastern Ross Sea: possible consequence of over-extended ice streams? Quat Sci Rev, 25(17-18): 2177-2196, doi:10.1016/j.quascirev.2005.12.013.

Murray R W, Leinen M. 1993. Chemical transport to the seafloor of the equatorial Pacific Ocean across a latitudinal transect at 135°W: Tracking sedimentary major, trace, and rare earth element fluxes at the Equator and the Intertropical Convergence Zone. Geochimica Cosmochimica Acta, 57(17): 4141-4163, doi:10.1016/0016-7037(93) 90312-K.

Murray R W, Leinen M. 1996. Scavenged excess aluminum and its relationship to bulk titanium in biogenic sediment from the central equatorial Pacific Ocean. Geochimica Cosmochimica Acta, 60(20): 3869-3878, doi:10.1016/0016-7037(96)00236-0.

Nameroff T J, Balistrieri L S, Murray J W. 2002. Suboxic trace metal geochemistry in the Eastern Tropical North Pacific. Geochimica Cosmochimica Acta, 66(7): 1139-1158, doi:10.1016/S0016-7037(01) 00843-2.

Nameroff T J, Calvert S E, Murray J W. 2004. Glacial-interglacial variability in the eastern tropical North Pacific oxygen minimum zone recorded by redox-sensitive trace metals. Paleoceanography, 19(1): PA1010, doi:10.1029/2003pa000912.

Nathan Y, Soudry D, Levy Y, et al. 1997. Geochemistry of cadmium in the Negev phosphorites. Chem Geol, 142(1-2): 87-107, doi:10.1016/ S0009-2541(97)00078-8.

Nelson D M, DeMaster D J, Dunbar R B, et al. 1996. Cycling of organic carbon and biogenic silica in the Southern Ocean: estimates of water-column and sedimentary fluxes on the Ross Sea continental shelf. J Geophys Res, 101(C8): 18519-18532, doi:10.1029/96jc01573.

Nitsche F O, Jacobs S S, Larter R D, et al. 2007. Bathymetry of the Amundsen Sea continental shelf: implications for geology, oceanography, and glaciology. Geochem Geophys Geosyst, 8(10): Q10009, doi:10.1029/2007gc001694.

Orr J C, Fabry V J, Aumont O, et al. 2005. Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature, 437(7059): 681-686, doi:10.1038/nature04095.

Orsi A H, Wiederwohl C L. 2009. A recount of Ross Sea waters. Deep Sea Res Part II Top Stud Oceanogr, 56(13-14): 778-795, doi:10.1016/j. dsr2.2008.10.033.

Padman L, Howard S L, Orsi A H, et al. 2009. Tides of the northwestern Ross Sea and their impact on dense outflows of Antarctic Bottom Water. Deep Sea Res Part II Top Stud Oceanogr, 56(13-14): 818-834, doi:10.1016/j.dsr2.2008.10.026.

Pailler D, Bard E, Rostek F, et al. 2002. Burial of redox-sensitive metals and organic matter in the equatorial Indian Ocean linked to precession. Geochimica Cosmochimica Acta, 66(5): 849-865, doi:10.1016/S0016- 7037(01)00817-1.

Passier H F, Bosch H J, Nijenhuis I A, et al. 1999. Sulphidic Mediterranean surface waters during Pliocene sapropel formation. Nature, 397(6715): 146-149, doi:10.1038/16441.

Pavia F J, Wang S Y, Middleton J, et al. 2021. Trace metal evidence for deglacial ventilation of the abyssal Pacific and Southern Oceans. Paleoceanogr Paleoclimatol, 36(9): e2021PA004226, doi:10.1029/ 2021pa004226.

Picco P, Bergamasco A, Demicheli L, et al. 2000. Large-scale circulation features in the central and western Ross Sea (Antarctica)//Faranda F M, Guglielmo L, Ianora A (eds). Ross Sea ecology, 95-105, doi:10.1007/ 978-3-642-59607-0_8.

Pondaven P, Ragueneau O, Tréguer P, et al. 2000. Resolving the ‘opal paradox’ in the Southern Ocean. Nature, 405(6783): 168-172, doi:10.1038/35012046.

Prév?t L, Lucas J. 1980. Behavior of some trace elements in phosphatic sedimentary formations//Bentor Y K. Marine phosphorites— geochemistry, occurrence, genesis. SEPM Society for Sedimentary Geology, 29: 31-39, doi: 10.2110/pec.80.29.0031.

Prév?t L, Lucas J. 1986. Microstructure of apatite-replacing carbonate in synthesized and natural samples. J Sediment Res, 56(1): 153-159, doi:10.1306/212f88aa-2b24-11d7-8648000102c1865d.

Rosenthal Y, Lam P, Boyle E A, et al. 1995. Authigenic cadmium enrichments in suboxic sediments: precipitation and postdepositional mobility. Earth Planet Sci Lett, 132(1-4): 99-111, doi:10.1016/0012- 821X(95)00056-I.

Sandroni S, Talarico F M. 2006. Analysis of clast lithologies from CIROS-2 core, New Harbour, Antarctica—Implications for ice flow directions during Plio-Pleistocene time. Palaeogeogr Palaeoclimatol Palaeoecol, 231(1-2): 215-232, doi:10.1016/j.palaeo.2005.07.031.

Sayles F L, Martin W R, Chase Z N, et al. 2001. Benthic remineralization and burial of biogenic SiO2, CaCO3, organic carbon, and detrital material in the Southern Ocean along a transect at 170° West. Deep Sea Res Part II Top Stud Oceanogr, 48(19-20): 4323-4383, doi:10. 1016/S0967-0645(01)00091-1.

Schmidtko S, Heywood K J, Thompson A F, et al. 2014. Multidecadal warming of Antarctic waters. Science, 346(6214): 1227-1231, doi:10.1126/science.1256117.

Scholz F, Hensen C, Noffke A, et al. 2011. Early diagenesis of redox-sensitive trace metals in the Peru upwelling area—response to ENSO-related oxygen fluctuations in the water column. Geochimica Cosmochimica Acta, 75(22): 7257-7276, doi:10.1016/j.gca.2011.08. 007.

Smith W O, Gordon L I. 1997. Hyperproductivity of the Ross Sea (Antarctica) polynya during austral spring. Geophys Res Lett, 24(3): 233-236, doi:10.1029/96gl03926.

Smith W O, Nelson D M. 1985. Phytoplankton bloom produced by a receding ice edge in the Ross Sea: spatial coherence with the density field. Science, 227(4683): 163-166, doi:10.1126/science.227.4683. 163.

Smith W O, Shields A R, Peloquin J A, et al. 2006. Interannual variations in nutrients, net community production, and biogeochemical cycles in the Ross Sea. Deep Sea Res Part II Top Stud Oceanogr, 53(8-10): 815-833, doi:10.1016/j.dsr2.2006.02.014.

Span D, Dominik J, Loizeau J L, et al. 1992. Phosphorus trapping by turbidites in deep-lake sediments. Chem Geol, 102(1-4): 73-82, doi:10.1016/0009-2541(92)90147-W.

Thomson J, Higgs N C, Wilson T R S, et al. 1995. Redistribution and geochemical behaviour of redox-sensitive elements around S1, the most recent eastern Mediterranean sapropel. Geochimica Cosmochimica Acta, 59(17): 3487-3501, doi:10.1016/0016-7037(95) 00232-O.

Tribovillard N, Algeo T J, Lyons T, et al. 2006. Trace metals as paleoredox and paleoproductivity proxies: an update. Chem Geol, 232(1-2): 12-32, doi:10.1016/j.chemgeo.2006.02.012.

van Wijk E M, Rintoul S R. 2014. Freshening drives contraction of Antarctic Bottom Water in the Australian Antarctic Basin. Geophys Res Lett, 41(5): 1657-1664, doi:10.1002/2013gl058921.

Wagner M, Hendy I L. 2017. Trace metal evidence for a poorly ventilated glacial Southern Ocean. Quat Sci Rev, 170: 109-120, doi:10.1016/j. quascirev.2017.06.014.

Wagner M, Hendy I L, McKay J L, et al. 2013. Influence of biological productivity on silver and redox-sensitive trace metal accumulation in Southern Ocean surface sediments, Pacific sector. Earth Planet Sci Lett, 380: 31-40, doi:10.1016/j.epsl.2013.08.020.

Wagner M, Hendy I L, McKay J L, et al. 2015. Redox chemistry of West Antarctic Peninsula margin surface sediments. Chem Geol, 417: 102-114, doi:10.1016/j.chemgeo.2015.10.002.

Warning B, Brumsack H J. 2000. Trace metal signatures of eastern Mediterranean sapropels. Palaeogeogr Palaeoclimatol Palaeoecol, 158(3-4): 293-309, doi:10.1016/S0031-0182(00)00055-9.

Wehausen R, Brumsack H J. 2000. Chemical cycles in Pliocene sapropel-bearing and sapropel-barren eastern Mediterranean sediments. Palaeogeogr Palaeoclimatol Palaeoecol, 158(3-4): 325-352, doi:10. 1016/S0031-0182(00)00057-2.

Xiong Z F, Li T G, Algeo T, et al. 2012. Paleoproductivity and paleoredox conditions during late Pleistocene accumulation of laminated diatom mats in the tropical West Pacific. Chem Geol, 334: 77-91, doi:10. 1016/j.chemgeo.2012.09.044.

Yarincik K M, Murray R W, Lyons T W, et al. 2000. Oxygenation history of bottom waters in the Cariaco Basin, Venezuela, over the past 578, 000 years: results from redox-sensitive metals (Mo, V, Mn, and Fe). Paleoceanography, 15(6): 593-604, doi:10.1029/ 1999pa000401.

Yu J M, Anderson R, Rohling E. 2014. Deep Ocean carbonate chemistry and glacial-interglacial atmospheric CO2change. Oceanography, 27(1): 16-25, doi:10.5670/oceanog.2014.04.

31 March 2021;

1 September 2021;

30 March 2022

10.13679/j.advps.2021.0005

: Wang J K, Li T G, Tang Z, et al.Relating the composition of continental margin surface sediments from the Ross Sea to the Amundsen Sea, West Antarctica, to modern environmental conditions. Adv Polar Sci, 2022, 33(1): 55-70,doi:10.13679/j.advps.2021.0005

, ORCID: 0000-0002-7119-7897, E-mail: tgli@fio.org.cn