Stable isotopic studies of microbial carbonates from Talchir sediments of east-central India

Laminated microbial carbonate (LMC) bodies with micro-morphological feature s resembling algal stromatolites were found in two Talchir basins of east-c entral India. Samples of these bodies along with carbonate nodules obtained from the Talchir sediments were analysed geochemically. delta O-18 and del ta C-13 values of the carbonate phase in the LMC range from 8.8 to 14.3 par ts per thousand (w.r.t. SMOW) and -8.2 to -14.3 parts per thousand (w.r.t. PDB) respectively, indicating precipitation in freshwater environment, The carbon isotopic composition of carbonate free residual matter in the LMC ra nges between -20.6 and -25.5 parts per thousand. These values are similar t o those of bacterial mats of modern lakes and confirm the biogenic origin o f the LMC. The carbon to nitrogen ratio also matches with the composition o f modern cyanobacterial matter. Presence of microbial mat on top of glacial debris suggests climatic amelioration subsequent to the sterile environmen t of Talchir glaciation.