Paleoproductivity increase at the Eocene-Oligocene climatic transition: ODP/DSDP sites 763 and 592

During the late Eocene-early Oligocene period of rapid climate change (acro ss Oxygen isotope shift Oi-1) productivity showed major changes in the regi on around Australia (DSDP Site 592 off New Zealand, ODP Site 763 off NW Aus tralia). We estimated paleoproductivity at these sites using benthic forami niferal accumulation rates, as well as accumulation rates of siliceous and other calcareous microfossils, and carbon isotope data. In the Eocene produ ctivity was generally higher at Site 763, where it reached 4 maxima, but dr opped at about 34.3 Ma. At both sites productivity increased strongly at Oi -1 (as recognized in the oxygen isotopic record at the sites), and fluctute d during the early Oligocene at higher levels than in the Eocene. We attrib ute the difference in Eocene productivity to differences in oceanographic s ettings: Site 592 was probably under a N-S flowing western boundary current of the Pacific gyre, whereas Site 763 showed periodic high productivity as a result of local upwelling. This upwelling occurred when a warm, N-S, flo wing current (proto-Leeuwin current) replaced the cold, S-N flowing eastern boundary current of the Indian Ocean gyre. Upwelling ended at the opening of the Tasman Sea. The earliest Oligocene increase in productivity occurred at many other locations in the Southern Oceans, and is accompanied by a st rong increase in carbon isotopic values, indicating increased burial of org anic matter on a global scale. This productivity increase as well as the in creased burial indicates that the oceanic carbon cycle may have been part o f the climate change in the earliest Oligocene. (C) 2001 Elsevier Science B .V. All rights reserved.