Variations of diatom opal Ge/Si in Prydz Bay, East Antarctica
Introduction
Germanium (Ge) has chemical properties similar to silicon (Si) and follows analogous geochemical pathways (Froelich et al., 1985). Both elements are delivered to the ocean by riverine input of dissolved or dissolvable phase derived from continental weathering and hydrothermal discharge, and removed from the ocean by formation and burial of biogenic opal (DeMaster, 1981; Froelich et al., 1985). Variations in seawater Ge/Si reflect changes in the mass balance (input versus output) of Ge and Si, which is recorded in siliceous skeletons deposited on the seafloor (Froelich et al., 1992; Bareille et al., 1998). Thus opal Ge/Si could monitor changes in the relative rate of continental weathering, hydrothermal processes, oceanic Si cycle,and surface water productivity over geological time (Shemesh et al., 1988; Froelich et al., 1989; Mortlock et al., 1991; Mortlock et al., 1993; Bareille et al., 1998; Lin and Chen, 2002). Glacial-interglacial fluctuations in opal Ge/Si have been reported from the Pacific (Froelich et al., 1989) and Atlantic sectors (Mortlock et al., 1991) of the Southern Ocean. These variations have been suggested to reflect rapid global changes in continental weathering (Froelich et al., 1992), or that low Ge/Si in glacial diatoms represents low biosiliceous productivity and incomplete Si consumption, leading to a discrimination of Ge against Si (Froelich et al., 1989; Mortlock et al., 1991).The possibility of discrimination between Ge and Si during frustule construction, relative to seawater Ge/Si has been further demonstrated by models, but its impact was not found to be significant (Ellwood and Maher, 2003; Ellwood et al., 2006; Sutton et al., 2010). More recently, preferential sequestration of Ge in surficial organic-rich sediments with dissolved Fe-rich pore waters was detected (Hammond et al., 2000; McManus et al., 2003; Baronas et al., 2019), indicating that early diagenesis of opals may decouple Ge from Si and provide the “missing sink” of Ge, which is speculated to be partly responsible for glacial–interglacial changes in oceanic opal Ge/Si (Hammond et al., 2004; King et al., 2000; Baronas et al., 2016). Therefore, identification and quantification of biological fractionation and authigenic Ge-sink are critical for accurately interpreting any changes in opal Ge/Si ratio and its usefulness as a paleo-proxy.
Previous explanations for temporal variations in opal Ge/Si have focused on relative strength of primary sources (Froelich et al., 1992; Lin and Chen, 2002). Alternatively, the cause may be variations in the relative strength of two sinks for Ge (opal and non-opal) in marginal sediments (Hammond et al., 2004; Baronas et al., 2016). Prydz Bay, the largest shelf sea on the East Antarctic margin, is an appropriate place to study Ge sinks and Ge/Si variations because biogenic opal is the dominant form of export production (Sun et al., 2003; Sun et al., 2016a). The bay is bounded to the south by the Amery Ice Shelf (AIS), which is the largest ice shelf in East Antarctica and buttresses the Lambert Glacier drainage system (Passchier, 2011). The glacier system is predominantly responsible for the terrigenous supply to Prydz Bay and adjacent areas (Borchers, 2010). Unlike other areas, glacial debris delivered by ice shelves and drifting icebergs potentially plays an important role in the GeSi mass balance and seawater Ge/Si in the bay. As an effective proxy, opal Ge/Si could offer some insights into the relationship between Ge and Si as well as Si cycling, which is the key to understanding the contribution of Prydz Bay to the Southern Ocean carbon pump. Here we present the first opal Ge/Si from Prydz Bay, and compare it with records from the other areas of the world ocean. We discuss the causes of the observed variations, which should be carefully considered by using opal Ge/Si as a tracer for Si cycling in high latitude marginal seas.
Section snippets
Study area and sampling
Prydz Bay is the third largest shelf sea of the Antarctic margin. It is bounded to the south by the AIS and extends northwards to the oceanic domain, where is dominated by a broad, deep and eastward-flowing Antarctic Circumpolar Current (ACC) north of the Antarctic Divergence. The shelf region is covered by seasonal sea ice and bounded to the east by Princess Elizabeth Land and to the west by MacRobertson Land (Fig. 1). The surface circulation is characterized by a closed cyclonic gyre (Prydz
Ge/Si in diatom frustules
The opal Ge/Si (all ratios here are given in μmol mol−1) in the surface sediments ranged from 0.48 to 0.77 (average 0.58 ± 0.10, Table 2). The (Ge/Si)opal of the continental samples were generally lower than those of the oceanic samples (P3–6 and P3–9) with the exception of Station IS-11 (Fig. 2). For core P3–16, (Ge/Si)opal ranged from 0.50 to 0.78 (average 0.62 ± 0.09), values in core sections 5-6 cm and 7-8 cm were elevated (Fig. 3). As the core represents the past 128 years (average
Sedimentary constituents and impact on data quality
Sponges were rarely observed in the surface sediments of the bay. If there were any sponge skeletal fragments or spicules visible under the microscope, they were picked out. A small number of radiolarians were observed in the bulk sediments, however they were usually >50 μm (mostly 50–300 μm, not published data), and should have been removed during physical cleaning. Microscopic observations showed that the cleaned samples were pure diatom opal (Sun et al., 2016b). Furthermore, sodium carbonate
Conclusions
Ge/Si in diatom opal from Prydz Bay sediments ranged from 0.48 to 0.78 (average 0.60 ± 0.09). Opal Ge/Si in the open sea area and at the edge of Amery Ice Shelf (IS-11) was comparable to the ratio of modern oceanic seawater, while values were relatively low on the continental shelf. Authigenic precipitation of Ge was generally observed in the bay, and accounted for 20% ~ 52% of the total Ge in bulk sediments. Authigenic Ge precipitation was generally controlled by Fe-rich minerals, but it was
Declaration of Competing Interest
None.
Acknowledgments
This study was funded by National Natural Science Foundation of China (No. 41206182; 41076134; 41976227) and The Response and Feedback of the Southern Ocean to Climate Change (RFSOCC2020-2025). Thanks for the comments from the anonymous reviewers which have helped us much to improve our manuscript.
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