LATE QUATERNARY SURFACE CIRCULATION OF THE SOUTHERN INDIAN OCEAN AND ITS RELATIONSHIP TO ORBITAL VARIATIONS

The paleoceanographic history of the Southern Indian Ocean is reflected by the movement of two prominent dynamical features of the Southern Ocean: the Subtropical Convergence (STC) and the Antarctic Polar Front (APF). These fronts, and their associated sea surface temperature (SST) signatures, are well delineated by planktonic foramimeferal faunas in surface sediments of the southern Indian Ocean. Using a transect of piston cores between 42 degrees S and 48 degrees S at about 90 degrees E, we have reconstructed the latitudinal distribution of planktonic foraminiferal faunas over the past 5 00,000 years. These faunal variations imply changes in the palcolatitudes of the STC and APF and the surface isotherms associated with the fronts. Stratigraphic and chronologic control is provided by delta O-18, %CaCO3, and biostratigraphy. Our reconstruction indicates that the STC has been equatorward of its present position (similar to 40 degrees S) for most of the past 500,000 years and has been poleward of that position for only four relatively brief (similar to 10,000-year) intervals during that time. W e estimate six equatorward excursions of the APF over the same period, with a maximum total range of about six degrees of latitude. The average palco-position of the APF is about 46 degrees S, about 4 degrees north of its present position (similar to 50 degrees S). Ice-rafted debris, another indicator of APF position, occurs as far north as 45 degrees S during glacial intervals. Latitudinal gradients in SST show little glacial-interglacial changes between 42 degrees and 48 degrees S, suggesting that surface isotherms were displaced uniformly or that compression of SST gradients occurred outside the transect. Time series analyses of the SST records in this transect reveal statistically significant concentrations of variance in the primary orbital frequency bands. SST variations in these bands are coherent with orbital variations and with changes in delta O-18. S SST in the subantarctic Indian Ocean slightly precedes changes in delta O-18 occurring in thee eccentricity, obliquity, and precession bands, and lag orbital variations in the obliquity and precession bands. These results place important constraints on possible mechanisms of interhemispheric climatic timing. T he similarities in temporal patterns and timing among Southern Ocean SST, atmospheric CO2, and relative flux of North Atlantic Deep Water implicate atmospheric carbon dioxide and deep water circulation as possible interhemispheric pacing mechanisms.