LA-ICPMS Ba/Ca analyses of planktic foraminifera from the Bay of Bengal: Implications for late Pleistocene orbital control on monsoon freshwater flux

Abstract

Indian Summer Monsoon (ISM) indices are characterized by large secular variation during both glacials and interglacials. Although much information about palaeomonsoon intensity is derived from such indicies, current data sets do not relate simply to precipitation. In order to directly constrain the variability of ISM freshwater flux to the Bay of Bengal, we report Ba/Ca LA-ICPMS data of the surface-dwelling foraminifera Globigerinoides ruber from core RC12-343 (central Bay of Bengal) between 68 and 47 ka. Planktic foraminifera Ba/Ca directly relates to seawater Ba/Ca, in turn principally controlled by freshwater flux. Our foraminifera-derived Ba/Casw record for the central Bay of Bengal is highly coherent with that derived from δ18O measurements of the same material, implying that these reconstructions are not significantly biased by potential shifts in δ18Ofreshwater. Validating this method allows us to produce a freshwater stack for the last 80 ka for the Bay of Bengal, enabling the orbital controls on ISM precipitation to be examined for the first time. The highest freshwater flux in the last 80 ka was 3X larger than present and occurred during the early-mid Holocene. We show that the orbital timing of this record is best explained by a combination of factors with a weighting of 45% given to the 30°N-equator mean summer insolation gradient and 55% given to the variability of May-July insolation at the equator and June-July insolation at 30°N. These processes are consistent with current mechanistic understanding of ISM forcings and demonstrate a dominant orbital control on monsoon precipitation amount on millennial timescales.