EVIDENCE FOR A HEAVILY GLACIATED ANTARCTIC CONTINENT DURING THE EARLY OLIGOCENE RECOVERY (33-30 MA): ODP SITE 690

Stable isotope (δ¹⁸O and δ¹³C) records obtained from benthic foraminifers recovered from Ocean Drilling Site 690 suggest that the Early Oligocene Antarctic ice sheet volume was more heavily glaciated than previously thought. Site 690, situated on the southern flank of Maud Rise (65°9.63'S, 1°12.30'E) in a water depth of 2,914 m, is the most southerly site with excellent preservation of calcareous microfossils. In addition, this location is bathed by Antarctic Bottom Waters, resulting in colder and smaller fluctuations in bottom water temperatures. Taken together, δ¹⁸O records from Site 690 should provide the best indicator of Antarctic glacial ice volume during the Oligocene.

A robust age model was created for Hole 690B using Florindo and Roberts (2005) indicating that Lower Oligocene extends from 73.4 mbsf to 92.9 mbsf and is dated between 30.1 Ma and 33.5 Ma. Stable isotopic measurements were obtained from monospecific samples using Nautiloides, Orvidosalis and Cibicidoides spp. The d18O offsets for Nautiloides and Oridosalis spp. to Cibicidoides spp. were determined by obtaining isotopic measurements from each species from the same sample.

Previous isotopic studies show that after the glacial maxima at the base of the Oligocene, a recovery to lighter values occurred that have been interpreted as a significant retreat of the ice sheet. In contrast, δ¹⁸O_calcite values range from 2.7‰ to 3.1‰ at Site 690, which are consistent with a heavily glaciated Antarctic continent and cold bottom water temperatures (BWT) during the early Oligocene. An explanation for this mismatch can be ascribed to that isotopic data from previous studies are from lower latitude sites, where the bathing of bottom water masses other than Antarctic Bottom Water would result in higher BWT. This in turn could possibly result in the masking of the true size of the ice sheet, which during the early Oligocene appears to have been closer to near modern day size based on this study.