Department of Earth Sciences, Faculty of Geosciences, Utrecht University, 3584 CD Utrecht, The Netherlands.
Proc Natl Acad Sci U S A. 2013 Jun 11;110(24):9645-50. doi: 10.1073/pnas.1220872110. Epub 2013 May 29.
The warmest global temperatures of the past 85 million years occurred during a prolonged greenhouse episode known as the Early Eocene Climatic Optimum (52-50 Ma). The Early Eocene Climatic Optimum terminated with a long-term cooling trend that culminated in continental-scale glaciation of Antarctica from 34 Ma onward. Whereas early studies attributed the Eocene transition from greenhouse to icehouse climates to the tectonic opening of Southern Ocean gateways, more recent investigations invoked a dominant role of declining atmospheric greenhouse gas concentrations (e.g., CO2). However, the scarcity of field data has prevented empirical evaluation of these hypotheses. We present marine microfossil and organic geochemical records spanning the early-to-middle Eocene transition from the Wilkes Land Margin, East Antarctica. Dinoflagellate biogeography and sea surface temperature paleothermometry reveal that the earliest throughflow of a westbound Antarctic Counter Current began ~49-50 Ma through a southern opening of the Tasmanian Gateway. This early opening occurs in conjunction with the simultaneous onset of regional surface water and continental cooling (2-4 °C), evidenced by biomarker- and pollen-based paleothermometry. We interpret that the westbound flowing current flow across the Tasmanian Gateway resulted in cooling of Antarctic surface waters and coasts, which was conveyed to global intermediate waters through invigorated deep convection in southern high latitudes. Although atmospheric CO2 forcing alone would provide a more uniform middle Eocene cooling, the opening of the Tasmanian Gateway better explains Southern Ocean surface water and global deep ocean cooling in the apparent absence of (sub-) equatorial cooling.
过去 8500 万年中最温暖的全球气温出现在一个被称为早始新世气候最优期（52-50 百万年前）的漫长温室期。早始新世气候最优期以长期冷却趋势结束，最终导致从 3400 万年前开始南极大陆范围的冰川作用。虽然早期研究将始新世从温室气候向冰室气候的转变归因于南大洋门户的构造开启，但最近的研究则认为大气温室气体浓度（例如 CO2）的下降起主导作用。然而，野外数据的缺乏阻止了对这些假说的经验评估。我们展示了来自东南极威尔克斯地边缘的早始新世过渡期的海洋微体古生物和有机地球化学记录。沟鞭藻生物地理学和海面温度古温度计显示，最早的西行南极对流转流始于 49-50 百万年前，通过塔斯马尼亚门户的南部开口。这种早期的开启与区域地表水和大陆冷却（2-4°C）同时发生，这一点得到了基于生物标志物和花粉的古温度计的证据。我们推断，穿越塔斯马尼亚门户的西行流导致南极表层水和海岸冷却，这通过在南半球高纬度地区增强的深层对流传递到全球中层水。尽管大气 CO2 强迫本身将提供更均匀的中始新世冷却，但塔斯马尼亚门户的开启更好地解释了南大洋表层水和全球深层海洋的冷却，而在赤道附近冷却（亚赤道冷却）似乎不存在。
