Redox conditions, solute sources and biogeochemical cycling in the Orosirian ocean: Geochemical and isotopic evidence from stromatolitic carbonates of ∼1.9 Ga Tadpatri Formation, Cuddapah Basin, India

The Orosirian Period of the middle Paleoproterozoic Era was marked by a decline in atmospheric oxygen and a reducing ocean after a brief oxidized interval of the preceding Great Oxidation Event. Here, we present CO, Nd-Sr isotopic and trace element geochemistry of ∼1.9 Ga stromatolitic Tadpatri Carbonates (TC) to understand the redox condition, solute sources and carbon cycling in the Orosirian ocean. The stromatolite morphology shows deposition under inter-tidal to sub-tidal shallow marine conditions. Varying degrees of dolomitization of low-Mg calcites by an Mg-rich and ¹³C-depleted saline marine fluid at elevated temperature (140–200ºC) and high fluid-to-rock ratios (50–200) significantly modified the depositional CO isotopic composition, and δ¹³C values (–1.5‰ - –2‰) recorded in low-Mg calcite are likely closest to the depositional values. These carbon isotopic values are significantly lower compared to the global Orosirian records. Low U/Th, V/Sc, and Mo/Sc in TC suggest a small seawater inventory of redox-sensitive elements, and in conjunction with high Fe-, Mn- concentrations and absence of Ce-anomaly indicates an anoxic- ferruginous ocean. High radiogenic εNd_1.9Ga values (–5.6 - –3.9) of TC compared to the older Dharwar Craton (–9.6) suggest solute sources from a juvenile crust or mantle-derived hydrothermal fluids. Very high U/Th and V/Sc in the Lower Vempalle Formation suggest oxidized conditions at ca. 1999 Ma, and progressive upward decrease of U/Th and V/Sc in the Middle-, Upper-Vempalle and Tadpatri Formations to much lower levels indicate a drawdown in atmospheric pO₂ level.