Tracking the Mesoproterozoic Ocean anoxia: Inferences from Mo isotopic study of black shales from Cuddapah Basin, India

We report new chemical (trace elements and Fe-speciation) and Mo isotopic (δ⁹⁸Mo) data for a Mesoproterozoic black shale sequence from the Cumbum Formation, Cuddapah Supergroup, India. These datasets have been used to reconstruct the ocean redox state and its areal extent. This reconstruction has significance in addressing the existing diverging views on the linkage between limited eukaryotic diversification and ocean oxygenation during this time period. The shales are comprised of higher organic carbon (1.1 ± 0.2 %) and sulfur (1.0 ± 0.3 %) concentrations, compared to the average upper continental crust (UCC) compositions. Most of the major elements have near-UCC values, except the lower values observed for Na, Fe and Ca. Among the trace elements, high enrichment factors for selected redox-sensitive elements (Mo, U and V) have been observed. Average Mo/TOC values (~6.3 ppm/wt%) of these shales fall close to the Proterozoic mean (~6.4 ppm/wt%). The δ⁹⁸Mo values of these samples range from 0.43‰ to 1.02‰ with an average of ~0.68‰ which matches well with the Mesoproterozoic average of ~0.67‰. The Fe-speciation data indicates that the iron in these shales is mostly associated with sulfide-rich phases, with minor contribution from other phases. Average Fe_HR/Fe_T (0.61 ± 0.18) and Fe_py/Fe_HR (0.72 ± 0.14) values indicate a fluctuating (euxinic to ferruginous) bottom water redox state in the basin. Our preliminary mass balance modelling using the Mo isotopic composition of selected (those deposited in euxinic conditions) shale samples shows that most of the oceanic Mo sink during this time were scavenged through anoxic pathways, with very little removal via oxic phases. An expanded sulfidic environment with rapid removal of bio-essential elements (hence, their reduced availability in the oceans) might have hindered the oceanic productivity and biological expansion.