Intermediate water circulation changes in the western equatorial Atlantic over the past 20,000 years

南极中间水环流（AAIW）被认为对全球海洋-气候系统做出了重要贡献。在现代海洋中，AAIW为平衡北大西洋深水（NADW）的南向输出提供了很大一部分向北流动，因此在大西洋经向翻转环流（AMOC）中发挥了关键作用。尽管AAIW具有重要的作用，但由于缺乏高时间分辨率的气候档案，对过去2万年的AAIW及其如何参与AMOC的详细了解仍然是气候研究的一个重大挑战。与AMOC减少相关的AAIW向北渗透的突变已被假设为过去2万年的突发性冷事件。在这些寒冷事件中，AAIW向北的气流是增加还是减少，这一问题一直存在争议。因此，很明显，需要更好地了解AAIW变异性及其如何参与AMOC。来自西赤道大西洋（以下简称WEA）的高时间分辨率气候档案可以揭示AAIW对AMOC变化的潜在作用。从过去2万年的气候事件链中产生了两个关键问题：(1)AMOC在多大程度上受到过去AAIW翻转环流扰动的影响，从而有利于AAIW在突发性冷期的增减？(2)南半球气候变率对AAIW翻转环流中过去扰动的驱动作用是什么？为了解决这些问题，我们建议通过在WEA选定的6个沉积物岩心样带上应用互补代理来重建AAIW/NADW的变化，该样带覆盖了南北水团之间的整个界面。WEA在全球海洋传送带中占据关键位置，因此对了解全球海洋气候变化至关重要。首先，我们将分析底栖有孔虫的稳定同位素组成，以重建中间水体几何形状的过去变化。其次，我们将测量中深度WEA样带的钕同位素比值。这种方法也可以作为一种强有力的工具来约束过去由AMOC重组驱动的水质量混合变化。第三，我们将测量氧化还原敏感金属的浓度，这是由海底溶解氧的数量控制的。该方法可用于评估AAIW源的底层地层过程信息。由于预测未来几十年AMOC会减弱，因此本文提出的对WEA AMOC可能变化的适当评价成为一个主要问题。因此，全面了解WEA的不同可能行为是预测未来气候的关键因素。

The circulation of Antarctic Intermediate Water (AAIW) is thought to make an important contribution to the global ocean-climate system. In the modern ocean, AAIW supplies a large fraction of the northward flow required to balance the southward export of North Atlantic Deep Water (NADW), and thus represents a key player in the Atlantic meridional overturning circulation (AMOC). Despite of its critical role, a detailed understanding of AAIW over the past 20,000 years and how it participates in the AMOC remains a significant challenge in climate research due to the lack of high temporal resolution climate archives. Abrupt changes in northward penetration of AAIW associated with reductions of the AMOC have been hypothesized for abrupt cold events over the last 20,000 years. Controversy persists as to whether the northward flow of AAIW is waxing or waning during these cold events. Thus, it is clear that a better understanding of the AAIW variability and how it participates in the AMOC is required. High temporal resolution climate archives from the western equatorial Atlantic (referred to hereafter as WEA) can shed some light on the potential role the AAIW plays in contributing to AMOC changes. Two key questions arise from the chain of climate events of the past 20,000 years: (1) To what extent was the AMOC affected by past perturbation in the AAIW overturning circulation, thereby favoring a waxing or waning of AAIW during abrupt cold periods? (2) Which role Southern Hemisphere climate variability plays in driving past perturbation in the AAIW overturning circulation? To address these questions, we propose to reconstruct changes in AAIW/NADW by applying complementary proxies on a transect of 6 selected sediment cores in the WEA that covers the full range of the interface between southern- and northern water masses. The WEA forms a key location in the global ocean conveyor belt, and it thus critical in understanding global ocean-climate changes. First, we will analyze the stable isotopic composition of benthic foraminifera to reconstruct past changes in the intermediate water mass geometry. Second, we will measure the neodymium isotope ratios from the mid-depth WEA transect. This method can also be used as a powerful tool to constrain past changes in water-mass mixing, driven by reorganizations of the AMOC. Third, we will measure the concentrations of redox-sensitive metal which are controlled by the amount of dissolved oxygen at the sea floor. This method can be used to assess information about the underlying formation processes at the AAIW source. The appropriate evaluation of the possible changes in AMOC in the WEA as proposed herein became a major issue, since a weakening in AMOC is predicted for the coming decades. Accordingly, a comprehensive understanding of the different possible behaviors of the WEA is a key element in predicting future climate.