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

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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.