HG CONCENTRATIONS AND MASS INDEPENDENT FRACTIONATION IN MID-PROTEROZOIC MARINE SHALES

Mercury (Hg) concentrations and isotopes are widely used to track the distribution, sources, and cycling of Hg in modern environments. Hg is a photochemically active, redox-sensitive, chalcophilic metal with complex biogeochemistry and displays a wide range of mass-dependent (MDF) and mass-independent (MIF) stable isotopic fractionation. As our understanding of the modern Hg cycle has increased, it is now possible to study ancient Hg cycling through the interpretation of Hg concentrations and isotopic compositions preserved in the rock record. Records of Hg accumulation and isotopic variation in ancient sediments have the potential to track changes in the redox state, biogeochemistry, or photochemistry of paleoenvironments.

To investigate the utility of Hg as a paloenvironmental proxy, we present a record of Hg concentrations and isotopic compositions in mid-Proterozoic marine shales. The mid-Proterozoic (~1.8-0.8 Ga) was a critical time for both eukaryotic diversification and the evolution of multi-cellular life and there is considerable debate about the redox history of the oceans and atmosphere during this period. Due to the affinity of Hg for organic matter (OM) and for sulfide, the accumulation of Hg in marine sediments should respond to changes in primary productivity, OM preservation, and the extent of marine euxinia. Furthermore, Hg isotopes with odd atomic mass numbers (¹⁹⁹Hg and ²⁰¹Hg) undergo MIF through multiple pathways in nature, including the photoreduction of aqueous Hg²⁺. This Hg-MIF signal is sensitive to environmental conditions, including the amount and type of solar radiation, the complexing organic ligands present, and the Hg/dissolved organic carbon ratio. Because MIF signals are not affected by Hg-loss in ancient sediments, it is hoped they may provide robust records of changes in atmospheric composition or seawater chemistry. Preliminary results show low (<100 ppb) but increasing Hg concentrations with time in mid-Proterozoic shales and reveal spatial and temporal variations in the sign, magnitude and ratio (Δ¹⁹⁹Hg/Δ²⁰¹Hg) of the Hg-MIF signal. Although explanations for these observations remain to be explored, variations in Hg content and isotopic composition suggest that paleoenvironmental insights are possible through the study of Hg cycling on the early Earth.