Remote control of tropical Indian Ocean paleohydrography

Appropriate integration of the Indian Ocean in climate predictions requires that we understand what controls the properties of its tropical waters. The Indonesian Throughflow as the main upper ocean pathway from the western Pacific into the tropical Indian Ocean may exert substantial control on regional oceanography. We have reconstructed the surface paleo-hydrography of the South Equatorial Current for the last 65,000 years using sediment core WIND 28K from the western Indian Ocean (10°S, 52°E, 4157m water depth). The ice-volume corrected planktonic δ18O records show a remarkable similarity with temperature records from Antarctica, in particular during the well-dated end of the last ice age and during the semi-glacial period 30-60 ka. Mg/Ca temperature reconstructions show that tropical Indian Ocean surface waters warmed by 1.5-3°C in phase with Antarctic warm events A1-A4 and by up to 5°C during glacial terminations. At least two mechanisms appear plausible for this Antarctic-Indian Ocean thermal connection. One transports the temperature signal via Subantarctic Mode Water (SAMW), which forms north of the subpolar frontal zone, spreads northward into the Indian Ocean, and upwells in the equatorial region. The other, less direct connection, involves variable transport of heat and freshwater from the western Pacific through the Indonesian Passage into the Indian Ocean. This latter mechanism is more related to changes in global sea-level or regional atmospheric circulation over the Indonesian archipelago rather than directly to Antarctic temperature. The options are discussed in the context of evidence for changes in paleo-throughflow and sea level.