Stable carbon and oxygen istotope ratios of Oligocene foraminifera from ODP Hole 121-758A of the northern Ninetyeast Ridge, Indian Ocean (Appendix 1)

Lack of covariance in planktic and benthic delta18O ratios indicates that many Oligocene sea level fluctuations, including the major fall at 30 Ma, are not of glacio-eustatic origin. Delta18O 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°-20°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 delta13C ratios, indicating well-mixed surface waters.