During the Eocene‐Oligocene Transition (EOT; 34–33.5 Ma), 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 (~35.7 Ma). 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 35.7 Ma. 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.

Paleoclimate studies suggest that increased global warmth during the Eocene epoch was greatly amplified at high latitudes, a state that climate models cannot fully reproduce. However, proxy estimates of Eocene near-Antarctic sea surface temperatures (SSTs) have produced widely divergent results at similar latitudes, with SSTs above 20°C in the southwest Pacific contrasting with SSTs between 5 and 15°C in the South Atlantic. Validation of this zonal temperature difference has been impeded by uncertainties inherent to the individual paleotemperature proxies applied at these sites. Here, we present multiproxy data from Seymour Island, near the Antarctic Peninsula, that provides well-constrained evidence for annual SSTs of 10-17°C (1σ SD) during the middle and late Eocene. Comparison of the same paleotemperature proxy at Seymour Island and at the East Tasman Plateau indicate the presence of a large and consistent middle-to-late Eocene SST gradient of ∼7°C between these two sites located at similar paleolatitudes. Intermediate-complexity climate model simulations suggest that enhanced oceanic heat transport in the South Pacific, driven by deep-water formation in the Ross Sea, was largely responsible for the observed SST gradient. These results indicate that very warm SSTs, in excess of 18°C, did not extend uniformly across the Eocene southern high latitudes, and suggest that thermohaline circulation may partially control the distribution of high-latitude ocean temperatures in greenhouse climates. The pronounced zonal SST heterogeneity evident in the Eocene cautions against inferring past meridional temperature gradients using spatially limited data within given latitudinal bands.paleooceanography | clumped isotopes | organic geochemistry | climate modeling | high-latitude climate

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Dry deposition of organic trace gases addresses a poorly quantified process in the atmosphere (3, 10). We estimate a lower and upper bound for the annual deposition flux of gas phase oVOCs between 37 and 56% relative to the annual NMVOC emission flux on a carbon basis (table S4). It is conceivable that oVOC deposition fluxes to vegetation could increase as a consequence of acute or chronic exposure to high O 3 concentrations in polluted regions (16).

[1] Despite warm polar climates and low meridional temperature gradients, a number of different high-latitude plankton assemblages were, to varying extents, dominated by endemic species during most of the Paleogene. To better understand the evolution of Paleogene plankton endemism in the high southern latitudes, we investigate the spatiotemporal distribution of the fossil remains of dinoflagellates, i.e., organic-walled cysts (dinocysts), and their response to changes in regional sea surface temperature (SST). We show that Paleocene and early Eocene (∼65-50 Ma) Southern Ocean dinocyst assemblages were largely cosmopolitan in nature but that a distinct switch from cosmopolitan-dominated to endemic-dominated assemblages (the so-called "transantarctic flora") occurred around the early-middle Eocene boundary (∼50 Ma). The spatial distribution and relative abundance patterns of this transantarctic flora correspond well with surface water circulation patterns as reconstructed through general circulation model experiments throughout the Eocene. We quantitatively compare dinocyst assemblages with previously published TEX 86 -based SST reconstructions through the early and middle Eocene from a key locality in the southwest Pacific Ocean, ODP Leg 189 Site 1172 on the East Tasman Plateau. We conclude that the middle Eocene onset of the proliferation of the transantarctic flora is not linearly correlated with regional SST records and that only after the transantarctic flora became fully established later in the middle Eocene, possibly triggered by large-scale changes in surface-ocean nutrient availability, were abundances of endemic dinocysts modulated by regional SST variations.