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

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 ¹⁵N/¹⁴N ratio (δ¹⁵N) 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 δ¹⁵N of the underlying thermocline nitrate was indistinguishable between the two settings. We suggest that the δ¹⁵N 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 N₂ fixation that occurs in the Indian Ocean. Foraminifera δ¹⁵N may have been lowered further during eddy migration by in situ N₂ fixation and/or recycling of low-δ¹⁵N ammonium. The absence of low-δ¹⁵N 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 δ¹⁵N. The low δ¹⁵N of eddy foraminifera, apparent even after several months of eddy migration across the Cape Basin, suggests that fossil foraminifer-bound δ¹⁵N from the region could record variations in past Agulhas leakage.