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 (across Oxygen isotope shift Oi-1) productivity showed major changes in the region around Australia (DSDP Site 592 off New Zealand, ODP Site 763 off NW Australia). We estimated paleoproductivity at these sites using benthic foraminiferal accumulation rates, as well as accumulation rates of siliceous and other calcareous microfossils, and carbon isotope data. In the Eocene productivity was generally higher at Site 763, where it reached 4 maxima, but dropped 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 fluctuted during the early Oligocene at higher levels than in the Eocene. We attribute the difference in Eocene productivity to differences in oceanographic settings: 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 flowing 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 strong increase in carbon isotopic values, indicating increased burial of organic matter on a global scale. This productivity increase as well as the increased burial indicates that the oceanic carbon cycle may have been part of the climate change in the earliest Oligocene.