Diatom silicon isotope ratios in Quaternary research: Where do we stand?

Abstract

Silicon stable isotope ratios (expressed as δ³⁰Si) in biogenic silica have been widely used as a proxy for past and present biogeochemical cycling in both marine and lacustrine settings, in particular for nutrient utilization reconstructions. Yet an analysis of publication trends suggests a significant decline in the application of δ³⁰Si to Quaternary science questions in the last five years. At the same time as δ³⁰Si proxy applications have decreased, we are learning more about its complexities: an expanding body of work is highlighting biases, caveats or complications involved in the application of δ³⁰Si-based approaches to the sediment record. These include the demonstration of species-specific silicon isotope fractionation factors (i.e. ‘vital effects’) or the potential for Fe or other trace metals to influence silicon isotope fractionation. Others have inferred the potential of biogenic silica dissolution to alter an initial δ³⁰Si value, or questioned the preservation of the initial δ³⁰Si through early diagenetic processes more generally. Another challenge receiving more attention is centered around deconvolving a δ³⁰Si-value into a signal reflecting biological productivity and a signal reflecting changes in the δ³⁰Si of dissolved silicon driven by whole-system and/or circulation changes. Finally, a number of studies focus on analytical difficulties, especially during sample preparation related to achieving and demonstrating a contaminant free biogenic silica. These challenges lead us to posit that the Quaternary science community is moving away from silicon isotope proxies because they are losing confidence in their reliability and usefulness. Here, focusing on the diatoms – the dominant biosilicifiers in both lakes and the ocean – we synthesize progress in understanding nuances and caveats of δ³⁰Si-based proxies in order to answer whether the fall-off in δ³⁰Si-based Quaternary research is warranted. We suggest that with some simple steps that can be readily implemented, and with the closing of key knowledge gaps, there is no reason to believe silicon isotopes do not have a promising future in the Quaternary sciences.