237 Paleoclimatology/Paleooceanography

Tuesday, 21 October 2014
1:00 PM - 5:00 PM
223/224

QUANTITATIVE CONSTRAINTS ON THE EXTENT OF MIDDLE PROTEROZOIC OCEAN ANOXIA FROM MODELING OF THE OCEANIC MASS BALANCE OF RHENIUM

Paper number: 237-12
Program: Technical Sessions
Time: 3:45 PM – 4:00 PM

Authors

Abstract

Accumulating geochemical evidence points to a Middle Proterozoic ocean that was redox-stratified, with oxygenated surface waters, euxinic mid-depth waters at sites of high primary productivity along ocean margins, and weakly oxygenated or ferruginous deep waters. However, the extent of anoxic seafloor at this time remains poorly constrained. Recent modeling of Cr concentrations in organic-rich mudrocks suggest pervasive anoxia, but is associated with large uncertainties arising from a strong detrital influence, whereas a relatively simple model for U concentrations suggests less extensive anoxia. To improve quantitative estimates of Middle Proterozoic ocean anoxia, we turned to the redox-sensitive metal rhenium and its concentration in organic-rich mudrocks. Relative to most other redox-sensitive trace metals, Re has higher enrichment factors (low detrital contribution) in organic-rich sediments underlying O2-poor bottom waters. In addition, Re burial is independent of H2S availability. These properties render the element an effective proxy of global marine redox distribution.

To interpret ancient ocean redox conditions, we constructed an oceanic mass balance model for Re based on modern source and sink fluxes estimated from a compilation of Re burial rates observed in modern marine basins. In addition, we have compiled a large database of Re concentrations in anoxic organic-rich mudrocks. The latter compilation reveals an expansion of marine anoxia following the relatively oxygenated conditions of the Great Oxidation Event. This is quantitatively confirmed by modeling Re authigenic enrichments in mudrocks by using the recent, advanced mass-balance approach developed for the Cr and Mo records (Reinhard et al., 2013; PNAS 110, 5357-5362). Using our model, we find that widespread anoxia (a minimum of 40-50% of the seafloor area) is required to achieve the observed average Re concentration in Middle Proterozoic mudrocks. Given that the Mo record is consistent with 1-10% of euxinic seafloor, our Re model suggests that ferruginous conditions may have been more pervasive in the Middle Proterozoic ocean than previously estimated.