Elsevier

Marine Chemistry

Volume 227, 20 December 2020, 103879
Marine Chemistry

Variations of diatom opal Ge/Si in Prydz Bay, East Antarctica

https://doi.org/10.1016/j.marchem.2020.103879Get rights and content

Highlights

  • (Ge/Si)opal in the open sea area and at the edge of Amery Ice Shelf (AIS) are comparable to modern oceanic ratio.

  • Oceanic water supplement and high glacial debris input control (Ge/Si)opal in the open sea and AIS edge areas respectively.

  • Authigenic precipitation of Ge is responsible for the low ratios on the shelf region.

  • Authigenic Ge sink is controlled by precipitation Fe-rich minerals, but also influenced by opal delivery in the centre bay.

Abstract

Diatom frustules record the variations of seawater Ge/Si, a ratio related to continental weathering, early diagenesis, and biological productivity. To investigate the geochemistry of germanium and the possibility of using opal Ge/Si as a proxy of changes in Ge and Si budgets in high latitude Antarctic marginal sea, nine surface sediment samples and one sediment core were collected from Prydz Bay during Chinese Antarctic Expeditions from 2005 to 2009. Diatom opal was separated and purified from sediment samples, and opal Ge/Si was determined. The results showed that (Ge/Si)opal ranged from 0.48 to 0.78 μmol mol−1 (0.60 ± 0.09 μmol mol−1) in Prydz Bay. (Ge/Si)opal from sites in the open sea and at the edge of the Amery Ice Shelf are comparable to modern oceanic seawater, while it is relatively low on the continental shelf. The variations in (Ge/Si)opal were not significantly influenced by biological fractionation during diatom uptake, but were related to changes in external input or authigenic loss of Ge. An authigenic sink of Ge was detected at all sites and accounted for 20% ~ 52% of Ge in bulk sediments. Authigenic precipitation of Ge is generally controlled by precipitation of Fe-rich minerals, but is also influenced by biogenic opal delivery in the centre of the bay where siliceous productivity is extremely high. The higher (Ge/Si)opal relative to those on the continental shelf is suggested to be caused by Ge supplementation from oceanic water in the open sea area and by high input of glacial debris from ice-melting at the edge of the Amery Ice Shelf.

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 Gesingle bondSi 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.

References (82)

  • M. Gasparon et al.

    Geogenic sources and sinks of trace metals in the Lasermann Hills, East Antarctica: natural processes and human impact

    Appl. Geochem.

    (2006)
  • R.R. Haese et al.

    The early diagenesis of iron in pelagic sediments: a multidisciplinary approach

    Earth Planet. Sci. Lett.

    (1998)
  • D.E. Hammond et al.

    Diagenetic fractionation of Ge and Si in reducing sediments: the missing Ge sink and a possible mechanism to cause glacial/interglacial variations in oceanic Ge/Si

    Geochim. Cosmochim. Acta

    (2000)
  • J.R. Hawkings et al.

    Biolabile ferrous iron bearing nanoparticles in glacial sediments

    Earth Planet. Sci. Lett.

    (2018)
  • K.R. Hendry et al.

    The role of sea ice formation in cycling of aluminum in northern Marguerite Bay, Antarctica

    Estuar. Coast. Shelf Sci.

    (2010)
  • S.L. King et al.

    Early diagenesis of germanium in sediments of the Antarctic South Atlantic: in research of the missing Ge sink

    Geochim. Cosmochim. Acta

    (2000)
  • B. Kuvaas et al.

    Glaciomarine turbidite and current controlled deposits in Prydz Bay, Antarctica

    Mar. Geol.

    (1992)
  • D. Lannuzel et al.

    Distribution of dissolved and particulate metals in Antarctic sea ice

    Mar. Chem.

    (2011)
  • H.L. Lin et al.

    A late Pliocene diatom Ge/Si record from the Southeast Atlantic

    Mar. Geol.

    (2002)
  • J. McManus et al.

    Diagenetic Ge-Si fractionation in continental margin environments: further evidence for a nonopal Ge sink

    Geochim. Cosmochim. Acta

    (2003)
  • R.A. Mortlock et al.

    A simple method for the rapid determination of biogenic opal in pelagic marine sediments

    Deep-Sea Res.

    (1989)
  • R.J. Murnane et al.

    Germanium geochemistry in the Southern-California borderlands

    Geochim. Cosmochim. Acta

    (1989)
  • R.A. Nunes Vaz et al.

    Physical oceanography of the Prydz Bay region of Antarctic waters

    Deep-Sea Res.

    (1996)
  • S. Passchier et al.

    Pliocene—Pleistocene glaciomarine sedimentation in eastern Prydz Bay and development of the Prydz trough-mouth fan, ODP Sites 1166 and 1167, East Antarctica

    Mar. Geol.

    (2003)
  • O.S. Pokrovsky et al.

    Experimental study of germanium adsorption on goethite and germanium coprecipitation with iron hydroxide: X-ray absorption fine structure and macroscopic characterization

    Geochim. Cosmochim. Acta

    (2006)
  • K. Schmidt et al.

    Zooplankton gut passage monilizes lithogenic iron for ocean productivity

    Cyrrent Biol.

    (2016)
  • A. Shemesh et al.

    Determination of Ge/Si in marine siliceous microfossils: separation, cleaning and dissolution of diatoms and radiolarian

    Mar. Chem.

    (1988)
  • A. Shemesh et al.

    Dissolution and preservation of Antarctic diatoms and the effect on sediment thanatocoenoses

    Quat. Res.

    (1989)
  • N.R. Smith et al.

    Water masses and circulation in the region of Prydz Bay, Antarctica

    Deep-Sea Research, Part A

    (1984)
  • W.P. Sun et al.

    Particulate barium flux and its relationship with export production on the continental shelf of Prydz Bay, East Antarctica

    Mar. Chem.

    (2013)
  • W.P. Sun et al.

    Source composition and seasonal variation of particulate trace element fluxes in Prydz Bay, East Antarctica

    Chemosphere

    (2016)
  • W.P. Sun et al.

    Zn/Si records in diatom opal from Prydz Bay, East Antarctica

    Mar. Geol.

    (2016)
  • S.R. Taylor

    Abundance of chemical elements in the continental crust: a new table

    Geochim. Cosmochim. Acta

    (1964)
  • N. Tribovillard et al.

    Trace metals as paleoredox and paleoproductivity proxies: An update

    Chem. Geol.

    (2006)
  • N. Tribovillard et al.

    Transfer of germanium to marine sediments: insights from its accumulation in radiolarites and authigenic capture under reducing conditions. Some examples through geological ages

    Chem. Geol.

    (2011)
  • C.G. Wheat et al.

    The potential role of ridge-flank hydrothermal systems on oceanic germanium and silicon balances

    Geochim. Cosmochim. Acta

    (2005)
  • A.M. Anders et al.

    Germanium/silicon ratios in the Copper River basin, Alaska: Weathering and partitioning in periglacial versus glacial environments

    J. Geophys. Res.

    (2003)
  • F. Azam

    Silicic acid uptake in diatoms studies with [68Ge] germanic acid as tracer

    Planta

    (1974)
  • F. Azam et al.

    Germanium incorporation into silica of diatom cell walls

    Archiv fuer Mikrobiologie

    (1973)
  • G. Bareille et al.

    A test of (Ge/Si)opal as a paleorecorder of (Ge/Si)seawater

    Geology

    (1998)
  • J.J. Baronas et al.

    A first look at dissolved Ge isotopes in marine sediments

    Front. Earth Sci.

    (2019)
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