STABLE ISOTOPES OF ESTUARINE WATER AND OYSTER SHELLS FROM GANGES DELTA, INDIA: PALAEOENVIRONMENTAL IMPLICATIONS

_{Stable isotope analyses of oyster species Crassostrea cuttackensis (exhibiting easily identifiable seasonal growth bands) vis-à-vis modern isotope hydrology of Ganges delta estuary indicate significant change in palaeoenvironment. 180/160 and 13C/12C ratio in oyster are controlled by the local salinity, temperature and d13C DIC of water where they inhabit. d180 water and d13C DIC of the estuarine water show positive correlation with salinity, indicating that both are controlled by the seasonal monsoon induced freshwater discharge into this estuarine system. Predicted d180c values exhibit a seasonal spread of ~5.5 ‰ while that of d13Cc show a spread of ~16 ‰. The results show that the strong monsoon periods are characterised by depleted oxygen and carbon isotope values owing to the high river discharge and CO2 from decomposed terrestrial organic matter. Conversely the periods of bad monsoon exhibit enriched oxygen and carbon values. d180d}13_C data of both oyster and predicted equilibrium carbonates show positive correlation of similar slope indicating that salinity cum river discharge controlled the isotopic compositions of carbonates the same way during the oyster growth as it does today. Compared to predicted values of ~5.5 ‰ the total range in oyster is less, about ~4.1 ‰. Also the oyster calcite shows enriched values both in maximum and minimum d18_0d18₀d13_C indicates that riverine fresh water discharge changes salinity and d13_CDIC on seasonal scale. Predicted highly depleted d13_C values during the peak river discharge are not recorded in the oyster as their growth are stunted or even stopped during this extreme low salinity condition. The results indicate that systematic stable isotope analyses of fossil oyster beds from this area, deposited during the historical period, can be a promising tool for studying the eustatic and palaeomonsoon change over Indian subcontinent.