Plio‐Pleistocene palaeoceanography of the tropical Indian Ocean (ODP Hole 709C): Radiolarian and CaCO 3 evidence

Sediments spanning the Olduvai subchron (1.79–1.95 Ma) of ODP Hole 709C in the tropical Indian Ocean were sampled at 5‐cm intervals. High‐resolution quantitative radiolarian and CaCO 3 % data have been used as proxies in interpreting palaeoceanographical conditions, an a comparison made with previously obtained oxygen isotope data. Radiolarian data were subjected to principal components analysis, resulting in four varimax factors. Factor 1 accounted for the greatest variation and comprised well‐know warm‐water and cold‐water taxa, with high +ve and ‐ ve scores respectively, which were used in the construction of a temperature index. Results indicate a tripartite temporal division of Olduvai palaeoceanography. In the early Olduvai, prior to 1.895 Ma, and the late Olduvai, approximating the Plio‐Pleistocene boundary at 1.81 Ma, divergence‐driven equatorial upwelling and high‐productivity coincide with isotope‐defined ‘interglacials’, and with generally low CaCO 3 %. However, during the mid‐Olduvai, upwelling, productivity and sedimentation rates were all reduced. These changes in palaeoceanography are thought to be linked directly to eccentricity modulated variations in the precession of the Earth's orbit. The early and late Olduvai coincide with periods of minimum eccentricity, when precessional reinforcemnet of the southwest monsoon is weakest, coastal upwelling along the Oman margin is reduced, and zonal wind‐driven equatorial divergence upwelling is increased. The mid‐Olduvai, however, corresponds with an eccentricity maximum, during which monsoonal activity is stronger and equatorial divergence upwelling weak.