Fluctuations of the Indian Monsoon Regime During the Last 170,000 Years: Evidence from Sea Surface Temperature, Salinity and Organic Carbon Records

We studied the variations of the Indian Monsoon regime during the last glacial cycle recorded in deep-sea sediments of Core MD900963 recovered at the junction between the Arabian Sea and the Bay of Bengal. Sea surface temperature (SST) variations were established by means of the unsaturation ratio ofalkenones. The SST record allowed us to apply temperature correction to the δ18O curve of Core MD900963 which was established on the planktonic foraminifera Globigerinoides ruber (white). Using a global δ18O sea water curve and the δ18O /salinity ratios for the Arabian Sea and the Bay of Bengal we reconstructed the variability of sea surface salinity during the last 170,000 years. The salinity variations can probably be attributed to the changes in the evaporation-precipitation balance (E-P) and ultimately to the relative intensity between the monsoons over the last 170,000 years. Our results suggest that during stage 5.5 and the early Holocene, the SW monsoon was probably stronger which resulted in higher precipitation and lower surface salinity. In contrast, during the time interval between 90 and 15 kyr and in particular during the glacial stages 6, 2 and substage 5.2 the SW monsoon was probably reduced and the dry NE monsoon was the dominant feature of the seasonal climate.The organic carbon content of Core MD900963 is strongly correlated with the δ18O record: highest values are associated with glacial δ18O maxima and lowest values with interglacial minima. The correlation between the organic carbon, the total C37 alkenone concentration, the mass accumulation rates of these compounds, and a coccolith productivity index suggests the occurence of important variations in carbon accumulation rate on Core site MD900963 during the last 170,000 years. The carbon and biomarker contents and accumulation rates can reflect both, changes in productivity and/or changes in preservation. In contrast, the coccolith productivity index suggests that within the glacial stages 6, 4, 2 and substages 5.4 and 5.2 the oceanic productivity was increased, probably due to the injection of nutrient-rich subsurface water into the euphotic zone in response to a stronger and predominating NE monsoon. Consequently, strengthened preservation due to a oxygen minimum or anoxic conditions is not the dominant cause for high organic carbon contents.