Surface-circulation change in the southwest Pacific Ocean across the Middle Eocene Climatic Optimum: inferences from dinoflagellate cysts and biomarker paleothermometry
Surface-circulation change in the southwest Pacific Ocean across the Middle Eocene Climatic Optimum: inferences from dinoflagellate cysts and biomarker paleothermometry
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DOI：
10.5194/cp-16-1667-2020
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发表时间：
2020-09-01
影响因子：
4.3
通讯作者：
Bijl, Peter K.
中科院分区：
文献类型：
--
作者：
Cramwinckel, Margot J.;Woelders, Lineke;Bijl, Peter K.
始新世早期（52-50 Ma）至始新世晚期（34 Ma）全球气候变冷。与此同时，南大洋的构造演变的特点是环南极门户的打开和加深，这影响到表层和深海环流。塔斯马尼亚门户在调节澳大利亚和南极洲之间的海洋流通方面发挥了关键作用。南大洋表面电流通过和周围的塔斯马尼亚门户留下了可识别的示踪剂的时空分布的浮游生物化石，包括有机壁甲藻孢囊。这种时空分布取决于水团的物理化学性质和表面洋流的路径。然而，气候和构造在多大程度上影响了表层洋流的分布和组成，从而影响了化石组合，这一点仍然不清楚。特别是，气候变化对海洋变化的贡献，叠加在长期和逐渐变化引起的构造，仍然知之甚少。为了解开在西南太平洋的构造和气候的影响，我们的目标是长期始新世冷却趋势的气候偏差：中始新世气候最佳（MECO;类似于40 Ma）。这500 kyr阶段的全球变暖与区域构造无关，因此提供了一个测试案例，以调查海洋的物理化学响应气候变化。我们重建的地表水循环和温度的变化和周围的塔斯马尼亚门户在MECO通过新的孢粉和有机地球化学记录从中央塔斯马尼亚门户（大洋钻探计划网站1170），奥特威盆地（澳大利亚东南部），和汉普顿海滩部分（新西兰）。我们的研究结果证实，dinocyst社区跟踪特定的表面洋流，但社区内的变化可以由叠加的温度变化驱动。再加上塔斯马尼亚门户以东公布的结果，我们的新结果表明，在MECO温暖的高峰期，表层海洋环流发生了变化。同时在塔斯马尼亚门户地区的高海面温度，花粉组合表明暖温带雨林与paratrophic元素沿着澳大利亚东南缘。最后，根据新的年龄限制，我们建议，区域东南澳大利亚海侵可能是一致的MECO。
Global climate cooled from the early Eocene hothouse (similar to 52-50 Ma) to the latest Eocene (similar to 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; similar to 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.