Silver as a Novel Tracer for Late Quaternary Southern Ocean Biological and Geophysical Processes.

Abstract

The global air-sea CO2 balance is set within the Southern Ocean through deep water formation, upwelling, and the biological pump. Diatoms dominate Southern Ocean primary productivity and export large amounts of Corg. However, past changes in Southern Ocean biological pump strength are presently not well understood because conventional proxies for export production can be biased by particle degradation. This dissertation explores Ag as a novel, potentially more robust proxy for diatom (paleo)productivity. The reliability of Ag and its delivery mechanism to sediments are investigated in Southern Ocean sediments and by laboratory studies on pure diatom cultures. To test whether sedimentary Ag enrichment occurs underlying oceanic regions of high diatom productivity, Ag and redox-sensitive trace metal fluxes were measured alongside conventional productivity proxy fluxes in Southern Ocean surface sediments. In the open ocean Pacific sector, increased Ag and productivity proxy fluxes coincide with the highly productive opal belt. Along the West Antarctic Peninsula continental margin, high regional surface productivity and nonlithogenic trace metal concentrations strongly suggest suboxic sediments maintained by seasonally elevated export production. Having demonstrated that Ag records diatom productivity patterns, Ag and redox-sensitive trace metals were measured in two South Atlantic cores to determine whether glacial-age deep waters contained greater CO2/lower O2 concentrations. Results reveal a shift in pore water chemistry from poorly oxygenated during the Last Glacial Maximum and deglaciation, to well oxygenated during the late Holocene. Sedimentary suboxia resulting from high Corg flux and lower bottom water O2 concentrations implies that glacial Lower Circumpolar Deep Water was more isolated and stored more CO2 compared to present day. Accurately interpreting Ag as a paleoproxy requires understanding how diatom debris delivers Ag to sediments. To probe this mechanism, diatoms were fed Ag in culture and subsequently analyzed in bulk and as single cells to determine Ag storage location. Results indicate that diatoms store Ag in organic matter with little to no Ag in frustules. Although biogenic particle degradation and Ag scavenging during particle water column transit likely affect the amount of Ag delivered to sediments, Ag shows considerable potential to serve as a reliable qualitative proxy for diatom (paleo)productivity.