Mediterranean pluvial periods and sapropel formation over the last 200 000 years

Three Mediterranean deep sea cores have been studied to reconstruct the hydrological conditions at the time of deposition of interglacial and glacial sapropels during the time interval 200–60 kyr BP. The isotopic and sea surface temperature records of planktonic foraminifera were used to estimate the oxygen isotopic composition of surface water (δw) in the Tyrrhenian Sea and Levantine basin. Our results show a strong δw/salinity decrease in both basins associated with each sapropel. These δw decreases reflect a drastic change in the local freshwater budget and were comparable to that observed during the last sapropel (S1) centred at ca. 8 kyr BP. A strong precipitation increase transformed the whole Mediterranean Sea into a non-concentration basin. The low surface salinity was responsible for the water column stratification, while nutrients brought in by the runoff enhanced productivity. It resulted in a significant reduction in the rate of deep water oxygenation and sapropel formation in the eastern basin. In addition, the Mediterranean Sea δ18O record is very similar to that of the Bay of Bengal. As the latter is very sensitive to the summer monsoon rain, this similarity suggests that both glacial and interglacial sapropels result from enhanced monsoon-like precipitation due to an increase in summer insolation driven by precession variations.