Reconstructing redox conditions of mesoproterozoic-early Cambrian seawater using δ98/95Mo

Full text: Mo isotope composition of seawater is determined by its relative removal in oxic and anoxic sinks as Mo isotope fractionates during its removal from seawater in oxic sediments whereas it remains invariable during its precipitation in reducing sediments. Hence, Mo isotope composition of reducing sediments can be used to track the temporal variation in redox condition of the global ocean. In this study an attempt is being made to reconstruct the redox conditions during Mesoproterozoic-Early Cambrian period using δ98/95∼Mo in black shales from Vindhyan and Outer belt of Lesser Himalaya. This time span witnessed the important evolutionary changes in the early earth such as rise in atmospheric oxygen and changes in the biosphere. Samples processed for this study were collected from Bijaigarh Shales of Kaimur formation of the Vindhyan and Lower Tal formation of the Lesser Himalaya. The depositional ages of these formations was estimated to be -1200 Ma and -540 Ma for Bijaigarh and Tal formations respectively. Mo isotope composition of black shales were determined by their complete acid digestion in presence of 97Mo-100Mo double spike followed by column chromatography separation. Mo isotope composition in purified samples were analysed using Finnigan Neptune MC-ICP-MS at Physical Research Laboratory. δ98/95% Mo measured in Bijaigarh Shales indicate variation from 0.4 to 1.35% with concentration ranging from 1.6-34 ppm whereas the Tal black shales display variability of 0.3-1.5%0 with concentration 16-80 ppm. In general, higher 09%sMo in Bijaigarh shales are associated with higher Mo concentration indicating the dominance of seawater Mo. Lower δ99/95%Mo in some samples could result from detrital component. Based on these data it can be inferred that Mesoproterozoic ocean was more anoxic compared to that of Early Cambrian and the contemporary ocean. More work is underway to track and further quantify the anoxicity of the ocean with time