23. LATE EOCENE TO OLIGOCENE BENTHIC FORAMINIFERAL ISOTOPIC RECORD,

Oxygen and carbon isotopic data from mixed species of the benthic foraminifer Cibicidoides at Site 574 in the equa-torial Pacific are compared with benthic foraminiferal isotopic data from equatorial Pacific Site 77 (Keigwin and Keller, 1984) and western North Atlantic Site 563 (Miller and Fairbanks, 1983, in press). Cibicidoides within the age range of ~ 33 to 35 Ma (within the early Oligocene) at the Pacific sites were lower in δ 1 3C than those of the same age from the North Atlantic site. For those from ~26 to 33 Ma (late early to late Oligocene), the equatorial Pacific and western North Atlantic δ 1 3C values were similar, whereas for those from ~26 to 12 Ma (latest Oligocene to Miocene) (see Miller and Fairbanks, 1983), Pacific values were lower. We suggest that this reflects bottom-water production analogous to modern North Atlantic Deep Water (NADW) in the early Oligocene and latest Oligocene to Miocene and reduced pro-duction of "NADW " in the late early to late Oligocene. High δ 1 8 θ values (approximately 2.0‰) were recorded at Site 574 for Cibicidoides at 36 Ma (earliest Oligocene); high, oscillating δ 1 8 θ values occurred from 30 to 29 Ma (near the early/late Oligocene boundary). These intervals of high δ 1 8 θ values apparently represent either bottom waters colder than at present or the presence of significant continental ice sheets. We suggest that major periods of continental glacia-tion occurred at approximately 36 and from 29 to 30 Ma