Tracking Agulhas Leakage in the South Atlantic Using Modern Planktic Foraminifera Nitrogen Isotopes

Abstract Seawater transported into the South Atlantic from the Indian Ocean via “Agulhas leakage” modulates global ocean circulation and has been linked to glacial‐interglacial climate cycles. However, constraining past Agulhas leakage has been a challenge. We sampled a transect of the Cape Basin in winter 2017 that intersected a mature Agulhas eddy and found that the 15 N/ 14 N ratio (δ 15 N) of mixed‐layer nitrate, zooplankton, and foraminifera (tissue and shells) was 2‰–3‰ lower in the eddy than in the background Atlantic even though the δ 15 N of the underlying thermocline nitrate was indistinguishable between the two settings. We suggest that the δ 15 N of foraminifera and other zooplankton in the eddy reflects the original Agulhas Current thermocline nitrate, which is ∼2‰ lower than that of the South Atlantic due to N 2 fixation that occurs in the Indian Ocean. Foraminifera δ 15 N may have been lowered further during eddy migration by in situ N 2 fixation and/or recycling of low‐δ 15 N ammonium. The absence of low‐δ 15 N Agulhas nitrate in the eddy thermocline can be explained by partial assimilation of the nitrate as it was mixed into the euphotic zone during and after eddy formation, raising its δ 15 N. The low δ 15 N of eddy foraminifera, apparent even after several months of eddy migration across the Cape Basin, suggests that fossil foraminifer‐bound δ 15 N from the region could record variations in past Agulhas leakage.