PLIOPLEISTOCENE PALEOCEANOGRAPHY OF THE TROPICAL INDIAN-OCEAN (ODP-HOLE-709C) - RADIOLARIAN AND CACO3 EVIDENCE

Sediments spanning the Olduvai subchron (1.79-1.95 Ma) of ODP Hole 709 C in the tropical Indian Ocean were sampled at 5-cm intervals. High-re solution quantitative radiolarian and CaCO3% data have been used as pr oxies in interpreting palaeoceanographical conditions, and a compariso n made with previously obtained oxygen isotope data. Radiolarian data were subjected to principal components analysis, resulting in four var imax factors. Factor 1 accounted for the greatest variation and compri sed well-known warm-water and cold-water taxa, with high +ve and -ve s cores respectively, which were used in the construction of a temperatu re index. Results indicate a tripartite temporal division of Olduvai p alaeoceanography. In the early Olduvai, prior to 1.895 Ma, and the lat e Olduvai, approximating the Plio-Pleistocene boundary at 1.81 Ma, div ergence-driven equatorial upwelling and high-productivity coincide wit h isotope-defined 'interglacials', and with generally low CaCO3%. Howe ver, during the mid-Olduvai, upwelling, productivity and sedimentation rates were all reduced. These changes in palaecoceanography are thoug ht to be linked directly to eccentricity modulated variations in the p recession of the Earth's orbit. The early and late Olduvai coincide wi th periods of minimum eccentricity, when precessional reinforcement of the southwest monsoon is weakest, coastal upwelling is increased. The mid-Olduvai, however, corresponds with an eccentricity maximum, durin g which monsoonal activity is stronger and equatorial divergence upwel ling weak.