Carbon cycling, redox condition and solute fluxes in the middle Paleoproterozoic Ocean: Constraints from elemental and C-O, Nd-Sr isotope geochemistry of stromatolitic carbonates from Vempalle Formation, Cuddapah Basin

Abstract

The early Orosirian Period (2.05–1.9 Ga) was marked by unprecedented changes in carbon cycling and the redox state of the global ocean, and marine carbonates are a useful archive to study these changes. Here, we present detailed C-O, Nd-Sr isotopic, and trace element chemostratigraphy of a well-preserved stromatolitic carbonate sequence (∼1.2 Km thick) from ∼ 2.0–1.9 Ga Vempalle Formation (VF), Cuddapah basin, India; and aim to elucidate redox conditions, carbon cycling, and solute sources to the post-Great Oxidation Event (GOE) ocean. The basal VF (0–200 m) records a prominent negative δ¹³C excursion of magnitude ∼ 3 ‰, which is followed by an invariant δ¹³C (0.46–1.35 ‰) in the rest of the upper section. Oxidative remineralisation of organic matter in a relatively low dissolved inorganic carbon (DIC) oceanic reservoir explains the negative δ¹³C excursion, whereas muted δ¹³C variations in the upper part would suggest environmental stability under high _pCO₂. The ¹³Δ (δ¹³C_carbonate- δ¹³C_org) values between 24.9 and 28.8 (avg. 27.4) show photoautotrophic carbon fixation by primary producers. The transition from relatively well-oxygenated to less-oxygenated surface conditions and contraction of oceanic redox-sensitive elements (RSE) reservoir is indicated by a marked decline of U and V abundances after an initial high concentration recorded in the basal VF (0–80 m). The Light Rare Earth Element (LREE) or Middle Rare Earth Element (MREE) enriched patterns, very high Fe and Mn concentrations in the upper VFs suggest a predominant anoxic-ferruginous ocean and active redox cycling of REE below the Mn- or Fe-redoxcline. These data, combined with the shallow depositional bathymetry, imply a very shallow redoxcline. High radiogenic εNd_{2 Ga} values in the VF compared to the average Dharwar crust indicate significant inputs of mantle-derived hydrothermal fluid to the ocean. Large-scale advection of hydrothermal fluid onto the shallow carbonate platform would indicate a tectonically active basin.