Climate/ocean dynamics and possible atmospheric mercury depletion events during the Late Sturtian deglaciation

Abstract

Late Sturtian deglaciation, resulting in development of oxic conditions and deposition of banded iron formations (BIFs), was a pivotal period in evolution of global ocean chemistry. The driving forces behind these Neoproterozoic BIFs and climatic/oceanic changes remain controversial, however. Here we use mercury (Hg) to elucidate the cause(s) of these dramatic changes to earth biogeochemical cycles. In the Guandong section, South China, we observe anomalously low Hg concentrations and Hg to total organic carbon (Hg/TOC) ratios in the Late Sturtian BIFs, suggesting a period of limited continental volcanism during the Late Sturtian deglaciation. We hypothesize that instead of a major LIP event, long-lasting and subdued submarine volcanism slowly increased atmospheric CO2 to trigger the greenhouse climate that caused deglaciation. Negative shifts of Δ199Hg in the studied section suggest that deglaciation was accompanied by atmospheric mercury depletion events (AMDEs). Extremely low TOC levels in the studied section may suggest low oceanic productivity, and this may support a non-biological cause of oceanic oxygenation during the Late Sturtian glaciation, through the release of photochemically produced H2O2 in melting glaciers.