Germanium/silicon and silicon isotope fractionation by marine diatoms and sponges and utility as tracers of silicic acid utilization

• Main Author: Sutton, Jill
• Format: Thesis
• Language: English
• Published: The Australian National University 2011
• Subjects: QH344.3.A6 S88 2011; Silicon Isotopes; Germanium; Diatoms Antarctic Ocean; Sponges Antarctic Ocean; Silicon cycle Biogeochemistry Antarctic Ocean; Silicic acid; Antarctic; Southern Ocean; The Antarctic; Pacific; Antarctic Ocean; Antarc*
• Online Access: https://dx.doi.org/10.25911/5d626d2b8770e; https://openresearch-repository.anu.edu.au/handle/1885/149899

This research endeavoured to lay the groundwork in understanding the oceanic cycling of silicon (Si) during the past, with a particular focus on the Southern Ocean, using a multi-proxy and multi-organism approach. Two oceanographic tracers, one based on the fractionation of natural abundances of stable Si isotopes and the other based on the germanium:silicon ratio (Ge/Si), were examined in siliceous sponges and diatoms and their surrounding seawater. Specifically, this thesis attempts to validate the use of the Si isotope composition and Ge/Si, measured in siliceous organisms, as proxies for Si utilisation in order to test questions such as the Silicic Acid Leakage Hypothesis (SALH). The investigations examine (1) Ge/Si fractionation in diatoms, (2) Si isotope fractionation in sponges, (3) Si utilisation over the past 120,000 years in a low Si environment based on Si isotope and Ge/Si signatures of fossil diatoms, and (4) the utility of Ge/Si and Si isotope composition for estimating Si utilisation in paleoceanographic reconstructions. (1) In the modem global ocean, Ge/Si fractionation occurs as a result of subtle differences in the uptake of germanium and silicon via diatoms in surface waters. Temporal changes in seawater Ge/Si are likely to be faithfully recorded by diatoms in areas that do not drop below a seawater Si concentration that leads to a reduction in diatom growth; (2) A new model is proposed by which Si isotope fractionation in marine sponges is regulated by the Si influx to efflux ratio which, in tum, is controlled by the external Si concentration of seawater. The model indicates that Si isotope fractionation associated with uptake transport is constant at -1.34 per mil whereas fractionation during spicule formation increases as a function of external Si concentration. The model predicts that the observed Si isotope fractionation in sponges will never exceed -6.02 permil; (3) The comparison of two geochemical proxies for Si utilisation, Ge/Si and Si isotope composition of diatoms, for sediment core E33-22 located north of the Antarctic Polar Front (APF) in the Pacific sector of the Southern Ocean provides geochemical evidence that seawater Si concentrations do not change over glacial-interglacial periods of the late Quaternary. The estimated Si concentrations, ranging from 0 to 3 micro mol L-l, were used to elucidate the decoupling between the Ge/Si and Si isotope composition of diatoms. The temporal fluctuations in Si concentration are small and do not meet expectations of the SALH where a major assumption is that Si "leaked" northward of the APF into the sub-Antarctic zone; (4) This thesis suggests that the reconstruction of paleo-Si concentrations comparing two geochemical proxies for Si utilisation, Ge/Si and Si isotope composition of siliceous organisms, is more accurate than if used alone. Further, the reconstruction of deep-water Si concentrations based on sponge spicule analyses provides more useful information on whole ocean changes than simply examining surface water variation provided by the Ge/Si or Si isotope composition of diatoms.