Direct evidence for Antarctic Ice-Sheet variability across the Eocene-Oligocene boundary climate transition

About ~34 million years ago, at the Eocene-Oligocene (E-O) boundary, Earth's climate underwent a substantial change from relatively ice-free "greenhouse" conditions to a glacial state marked by the establishment of a permanent ice sheet on Antarctica. Geochemical data and modeling suggests that decline of atmospheric pCO2 to a threshold level around 2.5X pre-industrial levels concomitant with an optimal orbital configuration for cold southern hemisphere summers provided the precursory conditions for extensive glaciation of Antarctica. Previous geological drill core records from the Antarctic margin indicate that the ice sheet reached continental extent and was calving at the coastline by the Earliest Oligocene. Notwithstanding these, our understanding of Antarctic cryospheric evolution across the E-O climate transition, relies almost entirely on marine geochemical proxies and reconstruction of sea-level changes from continental margin sequences. Hitherto it has not been possible to reconcile the pattern of inferred ice-sheet growth from these "far-field" proxy records with direct physical evidence of ice-volume changes from the Antarctic continental margin. Here we correlate cyclical changes recorded in sediment cores from western Ross Sea, which are related to oscillations in the volume of a growing East Antarctic Ice Sheet, with well-dated lower latitude records of orbital forcing and climate change across the E-O transition. We show that the EAIS was relatively small and unstable during the first ~200 thousands of years (kyr) of the E-O transition and that fully glaciated conditions did not occur until ~32.8 Ma, more than a million years after the glacial maximum of the climatic transition as recorded by geochemical proxies.