Collaborative Research: Tracking the Subtropical Front across the Mid Pleistocene Transition Using Sea Surface Temperature and Nutrients

该项目旨在重建过去150万年来非洲南部大洋阿古拉斯地区副热带锋(STF)位置的变化。洋流对全球海洋中的热和盐的分布起着重要的作用。最终，这些洋流的变化会影响海洋营养物质的分布、海洋对大气二氧化碳的吸收，以及全球气候。在非洲南部，阿古拉斯流将温暖的咸水从南印度洋输送到南大西洋。然而，在接近全球变冷的间隔期间，人们认为，由于STF向北迁移，逃逸到非洲各地的温水数量减少，甚至被阻止。由于STF的北移有可能扩大南大洋的表面积，并限制印度洋和大西洋之间的水交换，因此非洲南部地区是一个重要的气候目标，以促进我们对STF的物理移动可能如何影响区域和全球气候的理解。在过去的150万年里，STF的迁移问题解决得相对较差，部分原因是在前沿迁移的关键区域以前缺乏沉积岩芯。这项研究结合了对STF位置敏感的位置的海洋变化的多个替代记录，以研究STF移动可能如何影响海洋环流和二氧化碳吸收。该项目将支持两名女性个人投资指导员(一名早期职业)以及两名研究生和几名本科生。在K-12课堂和基于网络的材料方面，计划进行更广泛的公众宣传。调查人员将从新的国际海洋发现方案核心(U1475站点)的沉积物中综合重建过去150万年来副热带锋沿线的上层海洋动力学，该核心是在一个非常适合监测STF位置的位置从南印度洋发现的。他们将使用多个沉积物替代物，通过重建水温、出口生产和养分利用，生成STF沿线上层海洋动力学的综合重建。据推测，STF的南北迁移是为了控制水通过印度-大西洋门户的移动，这种交换对二氧化碳的海-气平衡和大西洋经向翻转环流的强度具有重要影响。所产生的数据将被用来记录STF相对于核心地点的位置和南大洋亚南极区域在扩张期间的营养状况。具体地说，研究人员将制作过去150万年的水温(使用Uk37、TEX86和使用现代模拟技术的有孔虫普查计数)、出口产量(使用总有机碳、生物硅和总烯酮(C37tot)含量)和营养动态(使用与有孔虫结合的有机物d15N)的新的高分辨率记录，以限制STF的历史及其对气候变化的反应。重点将放在中更新世过渡(MPT)，即125万至70万年前之间的时间间隔，当时在轨道强迫没有实质性变化的情况下，气候周期的主要周期从41000年变化到100000年。重建跨MPT的STF位置是理解STF迁移可能在改变南大洋生物泵(从而改变大气二氧化碳)方面发挥的作用的关键。

This project seeks to reconstruct changes in the position of the Subtropical Front (STF) in the Agulhas region of the Southern Ocean south of Africa over the last 1.5 million years. Ocean currents play a significant role in the distribution of heat and salt in the global ocean. Ultimately, changes in these currents influence marine nutrient distribution, ocean uptake of atmospheric CO2, and global climate. South of Africa, the Agulhas Current moves warm, salty water from the southern Indian Ocean into the South Atlantic Ocean. However, during intervals of near-global cooling, it is thought that the amount of this warm water escaping around Africa decreases or is even stopped, cutoff by a northward migration of the STF. Because a northward shift of the STF has the potential to both expand the surface area of the Southern Ocean and limit water exchange between the Indian and Atlantic Oceans, the region south of Africa is an important climate target to advance our understanding of how the physical movements of the STF may impact regional and global climate. Migrations of the STF are relatively poorly resolved for the past 1.5 million years, in part due to a prior lack of sediment cores in the critical region of front migration. This research combines multiple proxy records of ocean changes at a location sensitive to STF position to study how STF movement may have influenced ocean circulation and CO2 uptake. The project will support two female PIs (one early career) as well as two graduate students and several undergraduates. Broader public outreach is planned in the context of K-12 classroom and web-based materials.The investigators will generate an integrated reconstruction of upper ocean dynamics along the Subtropical Front over the last 1.5 million years from sediments in a new International Ocean Discovery Program core (Site U1475) recovered from the southern Indian Ocean at a location well suited to monitor STF position. They will use multiple sediment proxies to generate an integrated reconstruction of upper ocean dynamics along the STF using reconstructions of water temperature, export production, and nutrient utilization. North-south migrations of the STF have been hypothesized to control movement of water through the Indo-Atlantic Gateway, an exchange that has important implications for the air-sea balance of CO2 and the strength of the meridional overturning circulation of the Atlantic. The resulting data will be used to document the location of the STF with respect to the core site and the nutrient status of the subantarctic region of the Southern Ocean during episodes of expansion. Specifically, the investigators will produce new high-resolution records of water temperature (using Uk37, TEX86, and foraminiferal census counts using a Modern Analog Technique), export production (using total organic carbon, biogenic silica, and total alkenone (C37tot) content), and nutrient dynamics (using foraminiferal-bound organic matter d15N) for the last 1.5 million years to constrain the history of the STF and its response to climate change. Emphasis will be placed on the Mid Pleistocene Transition (MPT), the time interval between 1.25 and 0.7 million years ago when the dominant periodicity of climate cycles changed from 41 kiloyears to 100 kiloyears in the absence of substantial change in orbital forcing. Reconstructing the location of the STF across the MPT is key to understanding the role that STF migration may have played in altering the Southern Ocean biological pump (with consequent changes in atmospheric CO2).