Reconstructing South Atlantic Ocean Circulation changes across the Eocene-Palaeocene

大西洋经向翻转环流（AMOC）是一个由表层和深层洋流组成的系统，是现代全球海洋环流的重要组成部分。AMOC将热量、盐和生物必需营养素分布在全球海洋周围，对全球和区域气候具有根本影响。然而，我们不知道目前的AMOC有多稳定，也不知道它会因持续的气候变化而如何变化。为了帮助我们更好地理解AMOC在更温暖的气候下是如何变化的，我们可以研究过去地质时期的间隔，以确定海洋环流在比现代气候更温暖的时期是如何运作的。始新世和古新世（距今3400万至6500万年前）是气候比现代要温暖得多的时期，与我们对中长期未来的温暖水平的预测相似。由于始新世-古新世是我们未来气候的有用近似，我们建议重建这些时期的海洋环流，以提供宝贵的见解，了解现代AMOC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，例如测量底栖有孔虫（微小的海底生物）化石壳的碳同位素（d13C）。底栖有孔虫化石的碳同位素记录了它们长出外壳时周围海水的溶解碳。在海洋中，最上层海洋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，它们在较深水域的有机残留物的再矿化释放出营养物质和富含12c的碳，从而在深海中形成相对较低的d13C特征。沿着主要的深海流动路径，渐进的有机碳再矿化导致d13C逐渐减少，这被称为水体“老化”。因此，碳同位素可以告诉我们深海最后出现在海洋表面的时间有多近，即对水质量“相对”年龄的粗略估计。同一底栖有孔虫化石的氧同位素（d18O）将用于提供始新世-古新世期间深海温度和盐度的记录。第三种技术利用在化石鱼牙齿上测量的钕同位素（eNd）来识别和追踪深水团块的流动方向。海洋盆地从周围大陆的同位素不同物质的风化过程中获得了独特的eNd特征，沐浴在这些海洋盆地中的水团反映了这些独特的eNd特征。eNd数据还允许确定水团之间的任何混合。这三种方法将应用于国际海洋发现计划（IODP）在南大西洋探险390/393期间获得的样品。这将使下列目标得以实现：评估始新世-古新世跨南大西洋海洋环流的空间结构。来自远征390/393的新记录将与东南大西洋（远征208）产生的类似数据集进行比较。重建始新世-古新世AMOC结构的演化，目的是研究北方水源首次延伸到南大西洋的时间，类似于今天的AMOC。这将通过将本项目生成的新记录与北大西洋和热带大西洋的始新世-古新世末和d13C记录相结合来实现。确定在从古新世和始新世早期的极端温室变暖到始新世晚期较冷的冰窖的气候过渡期间，大西洋环流，特别是南大西洋在调节全球气候变化中的作用。

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.