Global climate cooled from the early Eocene hothouse (∼52–50 Ma) to the latest Eocene (∼34 Ma). At the same time, the tectonic evolution of the Southern Ocean was characterized by the opening and deepening of circum-Antarctic gateways, which affected both surface- and deep-ocean circulation. The Tasmanian Gateway played a key role in regulating ocean throughflow between Australia and Antarctica. Southern Ocean surface currents through and around the Tasmanian Gateway have left recognizable tracers in the spatiotemporal distribution of plankton fossils, including organic-walled dinoflagellate cysts. This spatiotemporal distribution depends on both the physicochemical properties of the water masses and the path of surface-ocean currents. The extent to which climate and tectonics have influenced the distribution and composition of surface currents and thus fossil assemblages has, however, remained unclear. In particular, the contribution of climate change to oceanographic changes, superimposed on long-term and gradual changes induced by tectonics, is still poorly understood. To disentangle the effects of tectonism and climate in the southwest Pacific Ocean, we target a climatic deviation from the long-term Eocene cooling trend: the Middle Eocene Climatic Optimum (MECO; ∼40 Ma). This 500 kyr phase of global warming was unrelated to regional tectonism, and thus provides a test case to investigate the ocean's physicochemical response to climate change alone. We reconstruct changes in surface-water circulation and temperature in and around the Tasmanian Gateway during the MECO through new palynological and organic geochemical records from the central Tasmanian Gateway (Ocean Drilling Program Site 1170), the Otway Basin (southeastern Australia), and the Hampden Beach section (New Zealand). Our results confirm that dinocyst communities track specific surface-ocean currents, yet the variability within the communities can be driven by superimposed temperature change. Together with published results from the east of the Tasmanian Gateway, our new results suggest a shift in surface-ocean circulation during the peak of MECO warmth. Simultaneous with high sea-surface temperatures in the Tasmanian Gateway area, pollen assemblages indicate warm temperate rainforests with paratropical elements along the southeastern margin of Australia. Finally, based on new age constraints, we suggest that a regional southeast Australian transgression might have been coincident with the MECO.

中文翻译：

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整个始新世中期最佳气候期间，西南太平洋的表面环流变化：鞭毛藻囊肿和生物标志物古温度计的推论

全球气候从始新世初期的温室（〜52 – 50  Ma）降温到最新的始新世（〜34  Ma））。同时，南大洋构造演化的特征是南极环围通道的开放和加深，影响了地表和深海环流。塔斯马尼亚门户在调节澳大利亚和南极之间的海洋通流中发挥了关键作用。通过塔斯马尼亚海峡及其周围的南洋海流在浮游生物化石的时空分布中留下了可辨认的示踪物，包括有机壁的鞭毛藻囊肿。这种时空分布既取决于水团的物理化学性质，又取决于表面洋流的路径。然而，气候和构造学在多大程度上影响了地表电流的分布和组成，从而影响了化石的集合，目前尚不清楚。尤其是，人们仍然很少了解气候变化对海洋变化的贡献，而这种变化叠加在构造学引起的长期和逐步变化上。为了消除构造和气候对西南太平洋的影响，我们的目标是从始新世长期冷却趋势中得出气候偏差：中始新世最佳气候（MECO； 2005年）。〜40  Ma）。这500 吉尔全球变暖的阶段与区域构造无关，因此提供了一个测试案例来研究海洋对气候变化的单独理化反应。我们通过塔斯马尼亚中部海峡（海洋钻探计划工地1170），奥特韦盆地（澳大利亚东南部）和汉普登的新的孢粉学和有机地球化学记录，重建了MECO期间塔斯马尼亚海中及其附近地表水循环和温度的变化。海滩部分（新西兰）。我们的结果证实，藻囊群落追踪特定的表面海洋流，但是群落中的变异性可以由叠加的温度变化来驱动。结合塔斯马尼亚门户东部的已公布结果，我们的新结果表明，在MECO暖峰期间，表层海洋环流发生了变化。组合表明在澳大利亚东南边缘有温带温带雨林和亚热带元素。最后，根据新的年龄限制，我们建议澳大利亚东南部的一次海侵可能与MECO一致。