13C enrichment in the Palaeoproterozoic carbonate rocks of the Aravalli Supergroup, northwest India: Influence of depositional environment

The dolomitic carbonates from the Jhamarkotra Formation of the Palaeoproterozoic Aravalli Supergroup are characterized by widely variable carbon isotope ratios (δ13C) ranging from near zero to as high as + 12‰ V-PDB. The estimated maximum age (ca. 2150 Ma) of the Aravalli carbonates help bracketing these with the coeval carbonate bodies of the world that show high positive carbon isotope values. The intriguing existence of normal marine δ13C values in some pockets suggest influence of local scale depositional conditions prevailing in different sub-basins. Amongst these two sub-basins which showed high δ13C values, a hypersaline evaporative condition is considered responsible for the necessary enrichment in one, while methanogenesis (possibly in conjunction with sulphate reduction processes) might have caused such enrichment in the other. From the empirical association of the sub-basins with profuse stromatolitic phosphorite we infer that the depositional setting that favoured cyanobacterial growth (leading to formation of stromatolites) prevented growth of methanogenetic archaea in such anoxic environments. Our study therefore highlights the fact that the early Palaeoproterozoic 13C excursion in the Aravalli Supergroup is not essentially a time-specific event but is greatly dependent on the variation in the depositional palaeoenvironment prevailing in different sub-basins.