Isotope and Trace Element Geochemistry of Eocene and Oligocene Foraminifers from Site 748, Kerguelen Plateau

Stable carbon and oxygen isotope analyses were conducted on well-preserved planktonic and benthic foraminifers from a continuous middle Eocene to Oligocene sequence at Ocean Drilling Program (ODP) Site 748 on the Kerguelen Plateau. Benthic foraminifer δ 1 θ values show a 1.0‰ increase through the middle and upper Eocene, followed by a rapid 1.2%c increase in the lowermost Oligocene (35.5 Ma). Surface-dwelling planktonic foraminifer δ 1 θ values increase in the lowermost Oligocene, but only by 0.6%e whereas intermediate-depth planktonic foraminifers show an increase of about l.O‰. Benthic foraminifer δC values increase by 0.9‰ in the lowermost Oligocene at precisely the same time as the large δ 1 θ increase, whereas planktonic foraminifer δC values show little or no change. Site 748 oxygen isotope and paleontological records suggest that southern Indian Ocean surface and intermediate waters underwent significant cooling from the early to late Eocene. The rapid 1.2%o oxygen isotope increase recorded by benthic foraminifers just above the Eocene/Oligocene boundary represents the ubiquitous early Oligocene δ 1 8 θ event. The shift here is unique, however, as it coincided with the sudden appearance of ice-rafted debris (IRD), providing the first direct link between Antarctic glacial activity and the earliest Oligocene δ 1 8 θ increase. The δ 1 θ increase caused by the ice-volume change in the early Oligocene is constrained by (1) related changes in the planktonic to benthic foraminifer δ 1 θ gradient at Site 748 and (2) comparisons of late Eocene and early Oligocene planktonic foraminifer δ 1 θ values from various latitudes. Both of these records indicate that 0.3%c to 0.4%0 of the early Oligocene δ 1 8 θ increase was ice-volume related.