Collaborative Research: Examining linkages between the Agulhas Leakage and ocean overturning in the last glacial cycle and through the mid-Pleistocene transition

对地球过去的气候和海洋的研究对于了解气候系统中可能的行为范围是很重要的。该项目旨在重建从印度洋到大西洋的“阿古拉斯泄漏”的历史，并了解这种泄漏的变化与过去气候和海洋环流的关系。阿古拉斯流是一股沿非洲东南部边缘向南流动的大型边界流（类似于墨西哥湾流）。当它到达非洲南端时，大部分洋流转向东方，但有些环会旋转进入大西洋。这种泄漏将比大西洋水更咸的水带入大西洋，这些盐被包括墨西哥湾流在内的表层洋流带到北大西洋，在那里它有助于北大西洋深水的形成。因此，阿古拉斯洋流系统是海洋大规模翻转环流的关键部分。过去气候因素的变化可能导致了这一系统的变化，本项目的目标是通过结合国际海洋发现计划（IODP）第361次远征队收集的沉积物岩心中表层和深层海洋环流的证据来研究这种变化。U1479站点的位置非常适合制作地表和深海的代理记录，因此可以使用一系列广泛的地球化学工具同时检查Agulhas泄漏和NADW生产之间的变化和联系。该项目涉及与探险队科学家的广泛国际合作，并将包括培训和指导一个多元化的研究团队，该团队包括本科生、研究生、博士后和早期职业教师。该项目旨在详细说明阿古拉斯泄漏和北大西洋深水（NADW）在末次冰期（0-150 kyr）和中更新世过渡（MPT, 0.7-1.3 Ma）期间的相对变化时间，这是在没有潜在轨道强迫变化的情况下发生的气候系统重组的重要时期。利用详细的陆源粘土物源分析研究Agulhas渗漏的变异性，同时利用鱼牙齿和有孔虫涂层中的nd同位素研究NADW的变异性。建议将深层和地表信号结合的主要地点是IODP站点U1479，该站点目前处于NADW，位于Agulhas环流入南大西洋的地方，包括Agulhas渗漏。此外，来自U1474遗址的粘土物源代用物，位于完整的阿古拉斯流开始附近，将限制阿古拉斯端元的陆源成分的任何端元变化。由于这两组代理数据将从相同的样品中收集，并且还将从正在进行底栖同位素地层学处理的相同样品中收集，因此将共同记录Agulhas端元组成变化的相对时间、Agulhas泄漏量和NADW变化。IODP远征361收集了来自更广泛的阿古拉斯洋流系统的沉积记录，提供了一个无与伦比的机会来详细研究阿古拉斯泄漏与大西洋经向翻转环流（AMOC）之间的联系，以及阿古拉斯洋流系统与南部非洲气候之间的联系。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Studies of Earth's past climate and oceans are important for developing an understanding of the range of possible behaviors in the climate system. This project seeks to reconstruct the history of the "Agulhas Leakage" from the Indian Ocean into the Atlantic Ocean, and to understand how changes in this leakage are related to past climate and the ocean's circulation. The Agulhas Current is a large boundary current (similar to the Gulf Stream) that flows southward along the southeast African margin. When it gets to the southern tip of Africa, most of the current turns back to the east, but some rings spin off into the Atlantic. This leakage brings water that is saltier than Atlantic water into the Atlantic, and this salt is carried by surface currents, including the Gulf Stream, to the far North Atlantic where it contributes to the formation of North Atlantic Deep Water. Thus, the Agulhas Current system is a key part of the ocean's large-scale overturning circulation. Changes in climate factors in the past may have led to changes in this system, and the goal of this project is to investigate such changes by combining evidence for the surface and deep ocean circulation from sediment cores collected on International Ocean Discovery Program (IODP) Expedition 361. The location of Site U1479 is ideal for making proxy records for both the surface and deep ocean, so it will be possible to simultaneously examine the changes and linkages between Agulhas Leakage and NADW production using a broad suite of geochemical tools. The project involves extensive international collaboration with the scientists from the expedition and will include training and mentorship of a diverse research team that includes undergraduates, graduate students, a postdoctoral fellow and an early career faculty member.The project seeks to detail the relative timing of variability in Agulhas Leakage and North Atlantic Deep Water (NADW) through the last glacial cycle (0-150 kyr) and over the mid-Pleistocene transition (MPT, 0.7-1.3 Ma), an important time of reorganization of the climate system that took place in the absence of changes in the underlying orbital forcing. The variability of Agulhas Leakage will be studied using detailed terrigenous clay provenance analyses, while the NADW variations will be studied with Nd-isotopes in fish teeth and foraminifera coatings. The primary site proposed for the combined deep and surface signal is IODP Site U1479, which is currently bathed in NADW and located where the Agulhas rings are shed into the South Atlantic, comprising the Agulhas Leakage. Additionally, clay provenance proxies from Site U1474, which is located near the beginning of the fully-constituted Agulhas Current, will constrain any end-member changes to the terrigenous composition of the Agulhas endmember. Because the data for both sets of proxies will be collected from the same samples, and also from the same samples that are being processed for benthic isotope stratigraphy, the relative timing of changes in the composition of the Agulhas endmember, the amount of Agulhas leakage, and the changes in NADW will be co-registered. IODP Expedition 361 collected sedimentary records from the broader Agulhas Current System that provide an unparalleled opportunity to examine in detail linkages between the Agulhas Leakage and Atlantic Meridional Overturning Circulation (AMOC) as well as linkages between the Agulhas Current System and southern African climates.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.