Late Eocene Southern Ocean Cooling and Invigoration of Circulation Preconditioned Antarctica for Full-Scale Glaciation

During the Eocene-Oligocene Transition (EOT; 34-33.5Ma), Antarctic ice sheets relatively rapidly expanded, leading to the first continent-scale glaciation of the Cenozoic. Declining atmospheric CO2 concentrations and associated feedbacks have been invoked as underlying mechanisms, but the role of the quasi-coeval opening of Southern Ocean gateways (Tasman Gateway and Drake Passage) and resulting changes in ocean circulation is as yet poorly understood. Definitive field evidence from EOT sedimentary successions from the Antarctic margin and the Southern Ocean is lacking, also because the few available sequences are often incomplete and poorly dated, hampering detailed paleoceanographic and paleoclimatic analysis. Here we use organic dinoflagellate cysts (dinocysts) to date and correlate critical Southern Ocean EOT successions. We demonstrate that widespread winnowed glauconite-rich lithological units were deposited ubiquitously and simultaneously in relatively shallow-marine environments at various Southern Ocean localities, starting in the late Eocene (similar to 35.7Ma). Based on organic biomarker paleothermometry and quantitative dinocyst distribution patterns, we analyze Southern Ocean paleoceanographic change across the EOT. We obtain strong indications for invigorated surface and bottom water circulation at sites affected by polar westward-flowing wind-driven currents, including a westward-flowing Antarctic Countercurrent, starting at about 35.7Ma. The mechanism for this oceanographic invigoration remains poorly understood. The circum-Antarctic expression of the phenomenon suggests that, rather than triggered by tectonic deepening of the Tasman Gateway, progressive pre-EOT atmospheric cooling played an important role. At localities affected by the Antarctic Countercurrent, sea surface productivity increased and simultaneously circum-Antarctic surface waters cooled. We surmise that combined, these processes contributed to preconditioning the Antarctic continent for glaciation. Plain Language Summary The ice sheets of Antarctica are geologically a relatively recent phenomenon. Only by the end of the Eocene Epoch (34 million years ago), major ice sheets began to develop, likely related to declining greenhouse gas concentrations. We still do not understand what the roleif anyof the tectonic openings of key land bridges (i.e., the present-day ocean conduits between Antarctica and Tasmania and the southern tip of South America) was in cooling the Antarctic continent and stimulating it to become glaciated. In this study we use organic marine microfossils to date and correlate several marginal marine sediment successions, dispersed throughout the Southern Ocean. We then show that the sediment composition at these sites changed abruptly throughout the Southern Ocean by about 35.7 million years ago, roughly two million years before the ice sheets rapidly expanded. We interpret this change in sediment composition to reflect enhanced surface ocean circulation. We furthermore analyzed chemical fossils to derive changes in past sea-water temperatures. By combining these data with counts of the marine organic microfossil species, we reconstructed past environmental change across the periods prior, during and after the growth of the Antarctic ice sheets. The results indicate that from about 35.7 million years ago onward, enhanced surface ocean circulation led to sediment winnowing, higher biological productivity in- and cooling of the surface waters around Antarctica. Irrespective of deepening of the Tasman Conduit, progressive intensification of ocean currents, probably as a result of stronger atmospheric circulation need to be considered in understanding the conditions that allowed rapid Antarctic ice sheet to expansion.