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150万年来非洲南部南大洋Agulhas地区副热带锋（STF）位置的变化。洋流在全球海洋的热量和盐的分布中起着重要作用。最终，这些洋流的变化会影响海洋营养物的分布、海洋对大气CO2的吸收以及全球气候。在非洲南部，阿古拉斯洋流将温暖的咸水从南印度洋带入南大西洋。然而，在接近全球变冷的时期，人们认为非洲周围的暖水逃逸量减少，甚至停止，被STF向北迁移所切断。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的物理运动如何影响区域和全球气候的理解。在过去的150万年里，STF的迁移相对较难解决，部分原因是由于在前线迁移的关键区域缺乏沉积物岩心。这项研究结合了多个代理记录的海洋变化在一个敏感的位置STF的位置，研究如何STF运动可能会影响海洋环流和CO2吸收。该项目将支持两名女专业督察（一名早期职业）以及两名研究生和几名本科生。更广泛的公众宣传计划在K-12课堂和基于网络的材料的背景下，调查人员将产生一个综合重建的上层海洋动力学沿着副热带锋在过去150万年的沉积物在一个新的国际海洋发现计划核心（站点U1475）恢复从南印度洋在一个位置非常适合监测STF的位置。他们将使用多个沉积物代理生成一个综合重建的上层海洋动态沿着STF使用重建的水温，输出生产和营养利用。STF的南北迁移被假设为通过印度-大西洋门户控制水的运动，这种交换对CO2的海气平衡和大西洋纬向翻转环流的强度具有重要影响。由此产生的数据将被用来记录的STF的位置相对于核心网站和营养状况的亚南极洲地区的扩张事件期间。具体来说，调查人员将制作新的高分辨率水温记录（使用UK 37、TEX 86和使用现代模拟技术的有孔虫普查计数），出口生产（使用总有机碳、生物二氧化硅和总烯酮（C37 tot）含量），和营养动态（使用有孔虫结合的有机物d15 N）在过去的150万年来限制的历史STF及其对气候变化的反应。重点将放在中更新世过渡期，即125万至70万年前的这段时间，当时气候周期的主要周期性从41千年变为100千年，而轨道强迫没有实质性变化。重建整个MPT的STF的位置是理解STF迁移在改变南大洋生物泵（随之而来的是大气CO2的变化）中可能发挥的作用的关键。

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).