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.

译文

:过去8500万年来全球最温暖的温度发生在长时间的温室事件中,即所谓的始新世最佳气候(52-50 Ma)。始新世气候最优化以长期降温趋势终止,最终从34 Ma开始,南极大陆规模化冰川化。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通道的构造开放,而最近的研究则在降低大气温室气体浓度(例如CO2)方面发挥了主导作用。但是,现场数据的稀缺性阻止了对这些假设的实证评估。我们提供了从南极东部威尔克斯地缘开始,从始新世中期到中新世过渡的海洋微化石和有机地球化学记录。鞭毛藻的生物地理学和海面温度的古温度计表明,最早的南极逆流流经塔斯马尼亚海峡的南部开口开始流向〜49-50 Ma。这种早期开放与区域地表水和大陆降温(2-4°C)的同时发作同时发生,这通过基于生物标志物和花粉的古温度计进行了证明。我们解释说,塔斯马尼亚河道的西行电流导致南极地表水和海岸的冷却,并通过南部高纬度的活跃深对流将其输送到全球中间水域。尽管仅靠大气CO2强迫就能提供更均匀的中始新世冷却,但塔斯马尼亚海峡的开放更好地解释了在明显缺乏(亚)赤道冷却的情况下南大洋地表水和全球深海冷却。

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