Positive δ¹³C excursion in carbonate and organic fractions from the Paleoproterozoic Aravalli Supergroup, Northwestern India

Abstract

Carbon and oxygen isotopic investigations have been carried out on the Archean and Paleoproterozoic carbonate rocks of the Udaipur region in the Aravalli Mountain Belt, northwestern India. The study has led to the interesting finding of ¹³C enrichment in the carbonate rocks of the Jhamarkotra Formation (δ¹³C_carb up to 11.1%. V-PDB) belonging to the ~2200-1900 Ma Paleoproterozoic Aravalli Supergroup. Further, it is observed that the organic carbon from phosphorite bearing stromatolitic dolomites of the same formation are also enriched in ¹³C (δ¹³C_org up to −11.1%.). However, it must be emphasized that the ¹³C enriched carbonates and the organic fractions are from different stratigraphic levels although in the same formation, the former being at a lower stratigraphic level. A critical analysis of the field geological, petrological as well as the isotopic data indicates that the observed δ¹³C excursion in the carbonate carbon is not due to local causes such as methanogenesis or evaporitic conditions. The shallow water environment of deposition of these carbonates as well as the immediately overlying formations rules out the stratified ocean model for the observed excursion. The model involving high sedimentation rates and organic carbon burial has been preferred to explain the δ¹³C excursion in the carbonates of the Jhamarkotra Formation. The ¹³C enrichment in organic fraction occurred in localized regions of the Aravalli sedimentary basin, where high productivity was supported by phosphorous supply indicating that the diffusion-limited pathways of organic carbon fixation were operative in the Paleoproterozoic itself. The ¹³C enriched carbonate rocks of the Jhamarkotra Formation serve as the Indian example for the Paleoproterozoic global δ¹³C excursion.