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，它不间断地环绕南极洲，自东向西流动，使大陆与温暖的亚热带水域南下隔绝。ACC是海洋翻转(即热盐环流)的主要驱动力，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。在3400万至5000万年前的某个时候，随着南极、澳大利亚和南美洲大陆的分离，在南半球形成了一条开放的海上通道，ACC也随之发展起来。这些门户的开放与推动地球气候从大气二氧化碳水平高、南极洲冰盖很少或没有冰盖的温室状态转变为更凉爽、冰盖较大、大气二氧化碳水平较低的“冰屋”状态。ACC的启动被提议作为对南极洲进行热隔离的机制，使其更容易在大陆上生长大冰盖，并使全球海洋降温。然而，最近的研究表明，大气二氧化碳水平的下降是全球变冷的主要驱动力。我们目前缺乏高度分辨率的气候和海洋环流记录，这些记录靠近跨越关键过渡的开放门户，特别是恢复较差的始新世早期(约4500万至5000万年前)，这使得区分这些情景具有挑战性。2017年9月至11月，国际大洋钻探项目369号将在塔斯曼门户(ACC开发的最后一个夹点)以西发现全新的深海沉积物，这些沉积物横跨门户开放和ACC开发之间的时间间隔。我将测量深海沉积物中发现的海洋微化石有孔虫(单细胞生物)中的钙质贝壳的地球化学成分。有孔虫从它们生活的海水中沉淀出它们的壳，从而记录了海底温度、盐度和生产力的变化。有孔虫贝壳化学可用于追踪底层水，因为每种有孔虫都有一个独特的化学特征，取决于它们形成的区域和它们的相对“年龄”，即它们从地表水中分离出来的时间有多长。可以检测到这一特征，绘制海洋盆地内每个水团的范围图，并确定来源区域。这一既定工具将限制整个震源间隔内深水形成的来源和强度，以及与ACC发生的关系。最终，这项工作将提供对ACC如何以及何时进化的更多了解，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.