Opal (Zn/Si) ratios as a nearshore geochemical proxy in coastal Antarctica

Abstract

[1] During the last 50 years, the Antarctic Peninsula has experienced rapid warming with associated retreat of 87% of marine and tidewater glacier fronts. Accelerated glacial retreat and iceberg calving may have a significant impact on the freshwater and nutrient supply to the phytoplankton communities of the highly productive coastal regions. However, commonly used biogenic carbonate proxies for nutrient and salinity conditions are not preserved in sediments from coastal Antarctica. Here we describe a method for the measurement of zinc to silicon ratios in diatom opal, (Zn/Si) opal , which is a potential archive in Antarctic marine sediments. A core top calibration from the West Antarctic Peninsula shows (Zn/Si) opal is a proxy for mixed layer salinity. We present down-core (Zn/Si) opal paleosalinity records from two rapidly accumulating sites taken from nearshore environments off the West Antarctic Peninsula which show an increase in meltwater input in recent decades. Our records show that the recent melting in this region is unprecedented for over 120 years.

A Review of the Stable Isotope Bio-geochemistry of the Global Silicon Cycle and Its Associated Trace Elements

“…However, more recent observations of Zn speciation in the Southern Ocean question this link, because the variability in free Zn 2+ ion concentrations are not captured by diatom bSiO 2 Zn/Si (Baars and Croot, 2011). Field studies support a link between diatom bSiO 2 Zn/Si and productivity and diatom bSiO 2 burial rates, although the mechanism behind this relationship is less clear and could relate to nutrient uptake vs. supply, growth rate or salinity (Hendry and Rickaby, 2008;Andersen et al, 2011). The Zn/Si ratio of diatom bSiO 2 from sediment cores, which appears to be less susceptible to initial alteration of the bSiO 2 as compared to Al/Si (Hendry and Rickaby, 2008), has been used in a small number of studies as a proxy for changes in Zn supply to surface waters in the open ocean and coastal Southern Ocean regions (Ellwood and Hunter, 2000;Hendry and Rickaby, 2008).…”

“…Field studies support a link between diatom bSiO 2 Zn/Si and productivity and diatom bSiO 2 burial rates, although the mechanism behind this relationship is less clear and could relate to nutrient uptake vs. supply, growth rate or salinity (Hendry and Rickaby, 2008;Andersen et al, 2011). The Zn/Si ratio of diatom bSiO 2 from sediment cores, which appears to be less susceptible to initial alteration of the bSiO 2 as compared to Al/Si (Hendry and Rickaby, 2008), has been used in a small number of studies as a proxy for changes in Zn supply to surface waters in the open ocean and coastal Southern Ocean regions (Ellwood and Hunter, 2000;Hendry and Rickaby, 2008). Recent studies indicate that a large proportion of Zn uptake by diatoms is into the organic material in the cell rather than the bSiO 2 , challenging its use as a palaeoceanographic proxy (Twining et al, 2003).…”

Application of Fourier Transform Infrared Spectroscopy (FTIR) for assessing biogenic silica sample purity in geochemical analyses and palaeoenvironmental research

“…Existing research for example, encompassing both high and low latitude regions, has been marked by investigations using diatom δ 15 N and δ 30 Si to study the role of the Southern Ocean biological pump and silicic acid leakage in regulating atmospheric CO 2 over glacial-interglacial cycles (Beucher et al, 2007). Similarly, the use of diatom δ 18 O has led to the detection of hydrological changes in both Africa and Scandinavia (Barker et al, 2007;Jonsson et al, 2010) while diatom Zn/Si ratios are beginning to be developed as proxies in both marine and non-marine settings (Hendry and Rickaby, 2008;Jaccard et al, 2009).…”

“…Increased attention is focused on the potential for geochemical measurements of biogenic silica to be used in palaeoenvironmental research in both continental, riverine, lacustrine and marine settings (e.g., Filippelli et al, 2000;de la Rocha et al, 2000;de la Rocha, 2003de la Rocha, , 2006Derry et al, 2005;Hendry and Rickaby, 2008;Hodson et al, 2008;Opfergelt et al, 2008;Swann et al, 2010). These studies, most commonly involving the analysis of diatoms, plant phytoliths, radiolaria and siliceous sponges, are believed to be particularly important in attempts to better understand the global silicon cycle as well as high latitude environmental change in regions where carbonates are not readily preserved in the sediment record (Conley, 2002;Street-Perrott and Barker, 2008;Leng et al, 2009;Swann and Leng, 2009).…”

“…Recent research has shifted towards employing trace element geochemistry as a measure of contamination. This includes the use of dissolution experiments to take advantage of the different digestion rates for biogenic silica and contaminants (Ellwood and Hunter, 1999;Hendry and Rickaby, 2008) or X-ray Fluorescence (XRF) in which the ionisation of a sample leads to the production of X-rays that are characteristic of the analysed sample (Brewer et al, 2008). Whereas the latter permits direct measurement of contamination by comparing measured oxide concentrations such as Al 2 O 3 and CaO to published diatom concentrations (e.g., Mackay et al, 2011a), the relatively large, 0.1-1.0 g, amounts of material required for a single analysis is often prohibitive for the technique to be used in routine analyses.…”