[S-MP27] Evolution of the early continents and the oceans understood by multi-geological proxies

[SMP27-P06] Carbon and Strontium Isotope Stratigraphy in Proterozoic Sedimentary Sequences of India

*Bivin Geo George¹ (1.Indian Institute of Technology Bombay, Mumbai, India)

キーワード：Proterozoic, India, Chemostratigraphy, Seawater evolution

The Indian subcontinent hosts a number of Proterozoic sedimentary sequences, some of which continue well into the Cambrian. The facts that most of these largely undeformed and unmetamorphosed sequences contain thick marine carbonate deposits, and that the subcontinent was part of the supercontinents of Columbia and Rodinia, it is natural to expect that rocks in these basins would have recorded physical and/or chemical evidence for the important global events of the Proterozoic. Unfortunately, however, there have been very limited attempts to unravel such signatures from these rocks. In an attempt to fill gaps in the record, we carried out carbon, oxygen and strontium isotope analyses of carbonate formations in the Paleo-Neoproterozoic Kaladgi Supergroup, Mesoproterozoic Chhattisgarh Supergroup, Neoproterozoic Vindhyan Supergroup and Neoproterozoic-early Cambrian Marwar Supergroup. Least altered nature of microspar carbonate matrices in the limestones from all the supergroups except Balwan Formation of Upper Vindhyan and dolostone formations of Kaladgi, as inferred from their low Mg/Ca (< 0.1), Mn/Sr (< 10) and δ¹⁸O (< -10‰), suggests that these components most likely have preserved their original δ¹³C and ⁸⁷Sr/⁸⁶Sr compositions. The Balwan and the dolostone formations of Kaladgi has a range of components from micrites to early formed dolospars with variable Mg/Ca and Mn/Sr, but possess δ¹⁸O <-10‰. Their δ¹³C and ⁸⁷Sr/⁸⁶Sr however appears to be primary and fits within the existing geochronological data. Overall, the stagnant nature of δ¹³C (0 ± 2‰) is reflected in the Paleoproterozoic carbonates of lower Kaladgi whereas it increases steadily with time from +2.6‰ to +3.6‰ from bottom to top in the Raipur Group in the Chhattisgarh. We interpret this increasing trend to be a result of enhanced primary productivity at the basin margins; a condition that was prevalent in the Mesoproterozoic global oceans at a time when only the surface ocean remained oxygenated. The continued rise is also observed in the early Neoproterozoic Balwan Limestone before it falls to ~-5‰ in the middle of the formation and recovers. This dynamic change is reflected across the basin and can be correlated with the globally seen Bitter Springs anomaly. Similar fluctuations can be observed also in the early Ediacaran Konkankoppa Limestone of upper Kaladgi. In the Neoproterozoic – early Cambrian Marwar Supergroup, the δ¹³C varies from -7‰ to +2.1‰. There are two basin wide anomalies of magnitude 7‰ and 5‰ observed, which are correlated to BACE and SHICE of early Cambrian respectively. The Sr isotopic composition largely follows the global trend. ⁸⁷Sr/⁸⁶Sr remains at 0.705 in the Paleoproterozoic before moving to 0.707 in Mesoproterozoic and decreases to 0.7063 in the early Neoproterozoic before rising to the early Cambrian values of 0.7081. Overall, Indian successions capture the global changes in the carbon and strontium isotopic compositions during Proterozoic; however, their variations during some of the major Proterozoic events like snowball Earth and Gaskiers glaciations and Shuram remains elusive due to hiatuses in the depositional records.