Carbon and Oxygen Isotopic Compositions of Carbonate Concretions of the Talchir formation and their Palaeoenvironmental Implications

Ubiquitous presence of carbonate rich nodules in glaciogene sediments of various Permo-Carboniferous basins (Talchir) in peninsular India offers promise for delineating the climatic condition and environment of deposition at that time. Isotopic composition of carbon and oxygen have been determined in carbonate nodules collected from the basal formation (Talchir) of three Gondwana basins of east-central India along with a few samples from contemporaneous Dwyka tillite of South Africa. Petrographic, cathodoluminescence and sedimentary evidences suggest that many of these nodules contain primary carbonate precipitates and therefore their geochemical signatures can be used for palaeoclimatic inference. The mean δ¹⁸0 and δ¹³C values of the calcites in the nodules are 10.8% w.r.t. SMOW and - 9.7%0 w.r.t. PDB respectively, suggesting a freshwater environment (probably lacustrine) for formation of these concretions. The mean oxygen isotopic composition of meteoric water at that time (Early Permian) and location (70° S palaeolatitude) estimated from δ¹⁸Oof calcite is -22.7%0 and is depleted by a few per mil from the expected isotopic composition of precipitation at this latitude. This depletion can be interpreted either in terms of an amount effect due to enhanced rainfall or an altitude effect if the precipitation occurred at high altitude (as expected for development of the Talchir glacier).