MOLYBDENUM ISOTOPE AND TRACE METAL SIGNALS IN AN IRON-RICH MESOPROTEROZOIC OCEAN: A SNAPSHOT FROM THE VINDHYAN BASIN, INDIA

Fundamental questions persist regarding the redox structure and trace metal content of the Mesoproterozoic oceans. Multiple lines of evidence suggest expanded anoxia in the deep oceans compared to today, and iron speciation indicates that anoxia was largely accompanied by dissolved ferrous iron (i.e., ferruginous conditions). Still, exceptions exist—euxinic conditions have been reported from some ocean margin and epeiric sea settings, and oxic conditions were reported in one deeper water environment. Constraining the temporal evolution of Mesoproterozoic marine redox structure is critical because it likely governed redox-sensitive trace metal availability, and in turn, played a role in marine diazotrophy and the evolution of early eukaryotes.

Here, we present a new, multi-proxy geochemical dataset from the ~1.2 Ga Bijaygarh Shale (Kaimur Group, Vindhyan Basin, India), including total organic carbon, iron speciation, and trace metals, as well as sulfur, nitrogen, and molybdenum isotopes. This unit was deposited in an open shelf setting near or just below storm wave base. Taken together, our data provide a unique snapshot of a biologically important shallow shelf setting during the Mesoproterozoic Era, including: 1) ferruginous waters below the zone of wave mixing; 2) muted enrichment of trace metals sensitive to general anoxia (e.g., chromium) and variable enrichment of trace metals sensitive to euxinia (e.g., molybdenum and, to a lesser extent, vanadium); 3) general sulfate limitation; and 4) nitrogen fixation by molybdenum-nitrogenase and a dominantly anaerobic nitrogen cycle in offshore settings. Differential patterns of trace metal enrichment are consistent with data from other basins, and suggest a largely anoxic ocean with limited euxinia during the Mesoproterozoic Era. We also provide the first reported molybdenum isotope data from unambiguously marine shale deposited between 1.4 and 0.75 Ga, with recorded values up to +1.18 ± 0.12 ‰ that are analogous to data from other Mesoproterozoic shale units. Ultimately, this study provides a broad, multi-proxy perspective on the redox conditions that accompanied early eukaryotic evolution.