RECONSTRUCTING SEA-SURFACE TEMPERATURE AND SALINITY USING DELTA-O-18 AND ALKENONE RECORDS

THE oxygen isotope (deltaO-18) composition of foraminiferal tests from deep-sea sediments is widely used as a palaeoclimate proxy, but it includes contributions from sea surface temperature, global ice volume and local salinity, which are difficult to separate. Recently a new technique for deriving palaeotemperatures has been developed which is based on the abundance ratios of unsaturated alkenones in phytoplankton algae1,2. Here we use a combination of oxygen isotope and alkenone records in a deep-sea core from the juncture of the Arabian Sea and the Bay of Bengal to extract the salinity signal from the former record. Variations in salinity are related to the balance between evaporation and precipitation3, and are thus a sensitive indicator of climate change. Our 170-kyr salinity record enables us to reconstruct changes in the Indian monsoon over this period, considerably extending earlier studies (which reached back to 18 kyr ago)4-8. Like these previous studies, we find that large variations in the monsoon occurred during the transition from the last glacial period to the present interglacial, but our results also provide a view of the monsoon throughout the last glacial and demonstrate the potential of this approach for reconstructing palaeosalinity.