Oxygen isotopic studies of a species of Pitar (Hyphantosoma) from Quilon Formation, Kerala, Southwest India; inferences on seasonality during the Miocene (late Burdigalian)

The carbonaceous shale of the Miocene (late Burdigalian) Quilon Formation (Kerala Basin, southwest India) were collected and processed to retrieve the microfaunal component. In the present study, the oxygen isotope analysis of the growth bands of the recovered bivalves Pitar (Hyphantosoma) simonnei and Ostrea sp. (both retrieved from the Quilon Formation) is emphasized to help reconstruct the palaeomonsoonal seasonality. The oxygen-isotope record for the specimens of P. (Hyphantosoma) simonnei and Ostrea sp. exhibits regular, high amplitude delta (super 18) O cycles caused by the yearly seasonal temperature and monsoonal variations. Based on the number of delta (super 18) O cycles, P. (Hyphantosoma) simonnei and Ostrea sp. have been considered to be approximately 2- and 4-year-old, respectively, at the time of deposition. The calculated palaeo-seawater delta (super 18) O values between -0.6 to -1.1 ppm [for P. (Hyphantosoma) simonnei] allow us to distinguish between the wet and dry seasons. In addition, the two-component mixing model is suggestive of seasonal freshwater discharge of approximately 70-80% during wet season and approximately 20-40% during the dry season into the depositional system. This reconstructed palaeo-seasonality clearly indicates presence of monsoon system in the south west of India during the Miocene (late Burdigalian).