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Geochemical and modeling studies suggest that the transition from the "greenhouse" state of the Late Eocene to the "icehouse" conditions of the Oligocene 34-33.5 Ma was triggered by a reduction of atmospheric pCO 2 that enabled the rapid buildup of a permanent ice sheet on the Antarctic continent. Marine records show that the drop in pCO 2 during this interval was accompanied by a significant decline in high-latitude sea surface and deep ocean temperature and enhanced seasonality in middle and high latitudes. However, terrestrial records of this climate transition show heterogeneous responses to changing pCO 2 and ocean temperatures, with some records showing a significant time lag in the temperature response to declining pCO 2 . We measured the Δ 47 of aragonite shells of the freshwater gastropod Viviparus lentus from the Solent Group, Hampshire Basin, United Kingdom, to reconstruct terrestrial temperature and hydrologic change in the North Atlantic region during the Eocene-Oligocene transition. Our data show a decrease in growing-season surface water temperatures (∼10°C) during the Eocene-Oligocene transition, corresponding to an average decrease in mean annual air temperature of ∼4-6°C from the Late Eocene to Early Oligocene. The magnitude of cooling is similar to observed decreases in North Atlantic sea surface temperature over this interval and occurs during major glacial expansion. This suggests a close linkage between atmospheric carbon dioxide concentrations, Northern Hemisphere temperature, and expansion of the Antarctic ice sheets.clumped isotopes | paleoclimate T he Eocene-Oligocene transition 34-33.5 Ma represents one of the most dramatic climatic changes of the past 65 My (1-3). Studies suggest that by 34 Ma, pCO 2 reached a critical threshold where favorable orbital parameters and ocean circulation patterns allowed the rapid buildup of Antarctic ice, triggering widespread reduction in atmospheric pCO 2 and decreases in sea surface and deep ocean temperature (4-8). This event is marked by a +1.5‰ shift in the oxygen isotope ratios of carbonate from deep-sea benthic foraminifera, which reflects both the glaciation of Antarctica and rapid cooling of the surface and deep ocean (1, 3).Marine sediments provide high-resolution records of surface and deep ocean temperature responses to the Late Eocene decreases in pCO 2 and Antarctic glaciation (7,8). These show that cooling was amplified in high-latitude regions, with a decrease in sea surface temperature of >5°C from the Late Eocene to Early Oligocene (7). Tropical sea surface temperature (SST) and deep ocean records show mixed responses to global cooling across the Eocene-Oligocene transition (EOT), with some indicating only modest declines in temperature in the tropics (8) and others showing a 3-4°C decrease in SST during the first stage of the cooling event (EOT-1) (9). One recent multiproxy study suggests that cooling during the EOT was specifically linked to increased seasonality, with the majority of cooling occurring during the coolseason months (...

A new high-resolution, c. 1 Ma mammalian record in continental Eocene-Oligocene strata in the Hampshire Basin is used to investigate the nature and timing of the major Europe-wide mammalian faunal turnover termed the Grande Coupure. Whether this was caused by climate change or by competition with taxa dispersing from Asia is controversial. The mammalian faunas in this record, after rarefaction analysis, show a sharp reduction in diversity only after the Grande Coupure. Improved correlation of NW European successions to global events confirms the Grande Coupure as earliest Oligocene. It shows that a c. 350 ka hiatus interrupts the Hampshire and Paris Basin sequences prior to the first record of post-Grande Coupure Asian taxa. Hiatusbridging faunas from elsewhere in Europe record mainly post-Grande Coupure taxa, suggesting that the turnover occurred early in the hiatus, minimizing bias to the turnover pattern. A previously unrecorded, second, smaller turnover, involving European mammals only, immediately precedes the Grande Coupure in the Hampshire Basin, coinciding with vegetational change. This turnover is judged not to represent cooling. It is concluded that the Grande Coupure coincides with the earliest Oligocene Oi-1 glaciation and that climate change combined with competition to produce the turnover.

The Palaeocene-Eocene thermal maximum (PETM), a period of intense, global warming about 55 million years ago, has been attributed to a rapid rise in greenhouse gas levels, with dissociation of methane hydrates being the most commonly invoked explanation. It has been suggested previously that high-latitude methane emissions from terrestrial environments could have enhanced the warming effect, but direct evidence for an increased methane flux from wetlands is lacking. The Cobham Lignite, a recently characterized expanded lacustrine/mire deposit in England, spans the onset of the PETM and therefore provides an opportunity to examine the biogeochemical response of wetland-type ecosystems at that time. Here we report the occurrence of hopanoids, biomarkers derived from bacteria, in the mire sediments from Cobham. We measure a decrease in the carbon isotope values of the hopanoids at the onset of the PETM interval, which suggests an increase in the methanotroph population. We propose that this reflects an increase in methane production potentially driven by changes to a warmer and wetter climate. Our data suggest that the release of methane from the terrestrial biosphere increased and possibly acted as a positive feedback mechanism to global warming.

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