Late Quaternary Surface Circulation of the Southern Indian Ocean and its Relationship to Orbital Variations

Authors

  • William R. Howard,

  • Warren L. Prell


Abstract

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 foraminiferal faunas in surface sediments of the southern Indian Ocean. Using a transect of piston cores between 42°S and 48°S at about 90°E, we have reconstructed the latitudinal distribution of planktonic foraminiferal faunas over the past 500,000 years. These faunal variations imply changes in the paleolatitudes of the STC and APF and the surface isotherms associated with the fronts. Stratigraphic and chronologic control is provided by δ18O, %CaCO3, and biostratigraphy. Our reconstruction indicates that the STC has been equatorward of its present position (∼40°S) for most of the past 500,000 years and has been poleward of that position for only four relatively brief (∼10,000-year) intervals during that time. We estimate six equatorward excursions of the APF over the same period, with a maximum total range of about six degrees of latitude. The average paleo-position of the APF is about 46°S, about 4°north of its present position (∼50°S). Ice-rafted debris, another indicator of APF position, occurs as far north as 45°S during glacial intervals. Latitudinal gradients in SST show little glacial-interglacial change between 42° and 48°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 δ18O. SST in the subantarctic Indian Ocean slightly precedes changes in δ18O occurring in the 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. The 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.

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