The Impact of Tasman Gateway Opening on Early Paleogene Oceans and Climate

南大洋的南极绕极流（ACC）是世界上最大的洋流;它环绕南极洲，不间断地从东向西流动，将南极洲与温暖的亚热带沃茨向南流动隔离开来。ACC是海洋倾覆的主要驱动力（即，海洋环流（包括温盐环流）在世界各地重新分配热量、盐度和营养物质，从而在调节全球气候方面发挥关键作用。南极洲、澳大利亚和南美洲大陆在3400万至5000万年前的某个时候分离，在南半球形成了一条开放的海洋通道。这些门户开口与推动地球气候从大气二氧化碳（CO2）水平高的温室状态和南极洲很少或没有冰盖到具有大冰盖和较低大气CO2水平的较冷的“冰库”状态有关。ACC的启动被提议作为南极洲热隔离的机制，使其更容易在大陆上生长大型冰盖并冷却全球海洋。然而，最近的研究表明，大气中二氧化碳水平的下降是全球变冷的主要驱动力。我们目前缺乏从接近开放门户的高度分辨率的气候和海洋环流记录，这些记录跨越了关键的过渡，特别是恢复不佳的始新世早期（约4500 - 5000万年前），使这些情景之间的区分具有挑战性。2017年9月至11月，国际大洋钻探项目第369次考察将从塔斯曼通道（ACC开发的最后一个关键点）西部恢复全新的深海沉积物，该沉积物跨越了通道打开和ACC开发的时间段。我将测量在深海沉积物中发现的海洋微化石有孔虫（单细胞生物）的钙质壳的地球化学成分。有孔虫从它们生活的海水中沉淀出它们的外壳，从而提供了海底温度、盐度和生产力变化的记录。有孔虫壳化学可用于追踪底层沃茨，因为每一种都有独特的化学特征，这取决于它们形成的区域和它们的相对“年龄”，即，它们与地表沃茨隔离了多久。这种特征可以被检测出来，海洋盆地内每个水团的范围可以被绘制出来，来源区域也可以被识别出来。这个既定的工具将限制整个震源间隔的深水形成的来源和强度以及与ACC发作的关系。最终，这项工作将使人们更好地了解ACC是如何以及何时进化的，以及它在塑造古代气候变化中的作用。该项目还将提供关键数据，以填补测试和配置数值古气候模型的空间空白。

The Antarctic Circumpolar Current (ACC) in the Southern Ocean is the largest ocean current in the world; it circles Antarctica uninterrupted flowing east to west isolating the continent from the southward flow of warm subtropical waters. The ACC is a major driver of ocean overturning (i.e., thermohaline circulation) redistributing heat, salinity and nutrients around the world, thus playing a critical role in regulating global climate. The ACC developed as Antarctic, Australia and South America continents separated creating an open marine passage in the Southern Hemisphere sometime between 34 and 50 million years ago. These gateway openings have been linked to driving the Earth's climate from a greenhouse state with high atmospheric carbon dioxide (CO2) levels and little or no ice sheets on Antarctica to a cooler 'icehouse' state with large ice sheets and lower atmospheric CO2 levels. The initiation of the ACC has been proposed as the mechanism for thermally isolating Antarctica, making it easier to grow large ice sheets on the continent and cooling the global ocean. However, more recent work suggests that declining atmospheric CO2 levels were the primary driver of global cooling. We currently lack highly resolved climatic and ocean circulation records from close to the opening gateways which spanning the critical transition, in particularly the poorly recovered early Eocene (~45-50 million years ago), making distinguishing between these scenarios challenging. In September-November 2017, International Ocean Drilling Project Expedition 369 will recover brand new deep-sea sediments from the west of the Tasman Gateway (the final pinch-point in the development of the ACC), that span the interval during which the gateway opened and the ACC developed. I will measure the geochemical composition of calcareous shells from the marine microfossils foraminifera (single-celled organisms) found in deep-sea sediments. Foraminifera precipitate their shell from the seawater in which they live, and thus provide a record of the changing temperature, salinity and productivity at the seafloor. Foraminiferal shell chemistry can be used to trace bottom waters as each has a distinctive chemical signature dependent on the area where they were formed and their relative 'age', i.e., how long they have been isolated from surface waters. This signature can be detected and the extent of each water mass within ocean basins mapped, and the source regions identified. This established tool will constrain the source and intensity of deep-water formation throughout the focal interval and the relationship to ACC onset. Ultimately, this work will provide a greater understanding of how and when the ACC evolved, and its role in shaping ancient climate change. This project will also provide key data to fill spatial gaps for testing and configuration of numerical palaeoclimate models.