Reconstructing South Atlantic Ocean Circulation changes across the Eocene-Palaeocene

大西洋子午线翻转环流(AMOC)是一个由表层和深层洋流组成的系统，构成了现代全球海洋环流的重要组成部分。AMOC在全球海洋周围分配热量、盐和生物必需的营养物质，并对全球和区域气候产生根本性影响。然而，我们不知道目前的AMOC有多稳定，以及它可能会因持续的气候变化而发生什么变化。为了帮助我们更好地理解AMOC在气候变暖时可能会发生什么变化，我们可以调查过去的地质时间间隔，以确定海洋环流在比现代更温暖的气候中可能是如何运行的。始新世和古新世(3400万至6500万年前)是气候比现代温暖得多的时期，与对我们中长期未来的温暖水平的预测相似。由于始新世-古新世对于我们未来的气候是有用的近似，我们建议重建这些时期的海洋环流，以提供宝贵的见解，以了解现代AMOC可能如何随着持续的全球变暖而改变其运作。为了重建过去的海洋环流，我们使用了地球化学技术，例如测量海底有孔虫(微小海底生物)化石外壳的碳同位素(D13C)。海底有孔虫化石的碳同位素记录了它们生长贝壳时周围海水的溶解碳。在海洋中，海洋最上层的微小浮游生物吸收营养物质和碳，优先去除轻碳同位素(12C)。当这些浮游生物死亡并在海洋中定居时，它们的有机残骸在更深的水域中的再矿化释放出营养物质和富含12C的碳，在深海给予相对较低的d13C信号。沿着主要的深海流动路径，逐渐的有机碳再矿化导致d13C的逐渐下降，这被称为水团“老化”。碳同位素因此可以告诉我们深水最后一次出现在海洋表面的时间有多近，也就是说，大致相当于水团“相对”年龄的近似值。同一海底有孔虫化石的氧同位素(D18O)将被用来提供始新世-古新世期间深海温度和盐度的记录。第三种技术利用在鱼化石牙齿上测量到的钕同位素(完)来识别和追踪深水块的流动方向。海洋盆地通过来自周围大陆的同位素不同物质的风化而获得独特的末端特征，沐浴在这些海洋盆地中的水团反映了这些独特的末端特征。最终数据还可以确定水团之间的任何混合。这三种方法将应用于南大西洋国际海洋发现计划(IODP)390/393考察期间获得的样本。这将使下列目标得以实现：1.评估始新世-古新世期间南大西洋海洋环流的空间结构。390/393次远征的新记录将与东南大西洋(208次远征)产生的类似数据集进行比较。重建始新世-古新世的AMOC结构的演化，目的是研究北方来源的水域第一次延伸到南大西洋的时间，类似于今天的AMOC。这将通过将本项目产生的新记录与以前从北大西洋和热带大西洋产生的始新世-古新世末期和d13C记录结合起来实现。确定大西洋环流，特别是南大西洋环流在从古新世和始新世早期温室温度极高的气候过渡到最晚始新世较冷的冰库的气候过渡期间，在调节全球气候变化方面的作用。

The Atlantic meridional overturning circulation (AMOC) is a system of surface and deep ocean currents, which constitutes a major component of modern global ocean circulation. The AMOC distributes heat, salt, and bio-essential nutrients around the global oceans and has a fundamental influence on global and regional climates. Yet, we do not know how stable the current AMOC is and how it may change owing to ongoing climate change. To help us improve our understanding of how the AMOC might change in a warmer climate, we can investigate intervals in the geological past to determine how ocean circulation may have operated during warmer-than-modern climates. The Eocene and Palaeocene (34 to 65 million years ago) epochs were intervals of time when the climate was considerably warmer than modern, and similar to projections for the level of warmth for our medium- to long-term future. Because the Eocene-Paleocene are useful approximations for our future climate, we propose to reconstruct the ocean circulation during these epochs to provide invaluable insights into how the modern AMOC might change its operation with ongoing global warming. To reconstruct past ocean circulation, we use geochemical techniques such as measurements of the carbon isotopes (d13C) of fossil shells of benthic foraminifera (microscopic sea-floor dwelling organisms). The carbon isotopes of fossil benthic foraminifera record the dissolved carbon of the ambient seawater at the time they grew their shells. In the ocean, microscopic plankton in the uppermost ocean take up nutrients and carbon, preferentially removing the light carbon isotope (12C). As these plankton die and settle through the ocean, remineralization of their organic remains in deeper waters releases nutrients and 12C-enriched carbon, imparting a relatively low d13C signature at depth. Along the main deep ocean flow-path, progressive organic carbon remineralisation gives rise to a progressive decrease in d13C, which is termed water mass 'ageing'. Carbon isotopes can thereby tell us how recently the deep waters were last at the ocean surface i.e. a rough approximation of the water-mass' relative 'age'. Oxygen isotopes (d18O) of the same fossil benthic foraminifera will be used to provide a record of deep ocean temperature and salinity during the Eocene-Palaeocene. A third technique utilises neodymium isotopes (eNd) measured on fossil fish teeth to identify and trace the flow directions of deep water masses. Ocean basins acquire distinctive eNd signatures from the weathering of isotopically distinct material from the surrounding continents, with the water masses bathing these ocean basins reflecting these unique eNd signatures. eNd data also allow the determination of any mixing between water masses. These three methods will be applied to samples obtained during the International Ocean Discovery Program (IODP) Expedition 390/393 in the South Atlantic. This will allow the following objectives to be accomplished:1. Assess the spatial structure of ocean circulation across the South Atlantic during the Eocene-Palaeocene. New records from Expedition 390/393 will be compared to similar datasets generated from the south-east Atlantic (Expedition 208).2. Reconstruct the evolution of the AMOC structure through the Eocene-Palaeocene, with the objective to investigate the timing of when northern sourced waters first extended into the South Atlantic, similar to today's AMOC. This will be achieved by combining new records generated in this project with previously generated Eocene-Paleocene eNd and d13C records from the North Atlantic and Tropical Atlantic.3. Determine the role of Atlantic ocean circulation, particularly the South Atlantic, in regulating global climate change during the climatic transition from the extreme greenhouse warmth of the Paleocene and early Eocene to the cooler icehouse of the latest Eocene.