Positive delta C-13 excursion in carbonate and organic fractions from the Paleoproterozoic Aravalli Supergroup, Northwestern India

Carbon and oxygen isotopic investigations have been carried out on the Arch ean and Paleoproterozoic carbonate rocks of the Udaipur region in the Arava lli Mountain Belt. northwestern India. The study has led to the interesting finding of C-13 enrichment in the carbonate rocks of the Jhamarkotra Forma tion (delta C-13(carb) up to 11.1 parts per thousand V-PDB) belonging to th e similar to 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 C-13 (delta C-13(org) up to - 11.1 parts per thousand). However. it must be emphasized that the C-13 enriched carbonates and the organic fractions are from different strat igraphic levels although in the same formation. the former being at a lower stratigraphic level. A critical analysis of the field geological, petrolog ical as well as the isotopic data indicates that the observed delta C-13 ex cursion in the carbonate carbon is not due to local causes such as methanog enesis or evaporitic conditions. The shallow water environment of depositio n of these carbonates as well as the immediately overlying formations rules out the stratified ocean model for the observed excursion. The model invol ving high sedimentation rates and organic carbon burial has been preferred to explain the delta C-13 excursion in the carbonates of the Jhamarkotra Fo rmation. The C-13 enrichment in organic fraction occurred in localized regi ons of the Aravalli sedimentary basin, where high productivity was supporte d by phosphorous supply indicating that the diffusion-limited pathways of o rganic carbon fixation were operative in the Paleoproterozoic itself. The C -13 enriched carbonate rocks of the Jhamarkotra Formation serve as the Indi an example for the Paleoproterozoic global delta C-13 excursion. (C) 2001 E lsevier Science B.V. All rights reserved.