AN OLIGOCENE MULTISPECIES FORAMINIFERAL OXYGEN AND CARBON-ISOTOPE RECORD FROM ODP HOLE 758A (INDIAN-OCEAN) - PALEOCEANOGRAPHIC AND PALEO-ECOLOGIC IMPLICATIONS

An Oligocene-basal Miocene (Zones P19-P22/N4) oxygen and carbon isotope stratigraphy is presented based on mixed benthic foraminiferal faunas and different species of planktic foraminifers from Eastern Indian Ocean ODP Hole 758A. Lack of covariance in planktic and benthic delta(18)O ratios indicates that many Oligocene sea level fluctuations, including the major fall at 30 Ma, are not of glacio-eustatic origin. delta(18)O values suggest that test morphology is a poor indicator of preferred depth habitat of Oligocene planktic foraminifers. Also, some groups show considerable shifts in preferred depth habitat. Reconstructed surface water temperature was nearly constant during the investigated period, but even assuming a large Antarctic icecap, values are unrealistically low, 18 degrees-20 degrees C. Most likely, water masses with markedly different oxygen isotopic composition existed, this poses great problems for quantitative temperature reconstructions. Planktic foraminiferal species show similar delta(13)C ratios, indicating well-mixed surface waters. This contrasts with the strong oxygen isotopic gradient, which suggests well-stratified surface waters. During the Oligocene the difference in delta(13)C ratio between the average ratio of planktic foraminiferal species and the benthic foraminifers ('Delta delta(13)C P-B') shows an inverse relation with the known circulation intensity record and thus with productivity. This Delta delta(13)C pattern is interpreted as being caused by the presence of younger bottom waters at this site during periods with lower temperatures and/or larger Antarctic ice sheets.