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 remains a challenge. Using new measurements from the modern South Atlantic, we propose that the δ15N of organic matter preserved in the shells of fossil planktic foraminifera could be used to infer past changes in Agulhas leakage. We sampled a transect of the Cape Basin in winter 2017 that intersected a mature Agulhas eddy and found that mixed-layer nitrate-, zooplankton-, and foraminifer-δ15N (tissue and shells) was 2-3‰ lower in the eddy than the background Atlantic even though the δ15N of the underlying thermocline nitrate was indistinguishable. We suggest that the δ15N of eddy-N reflects 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 further lowered 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 convective mixing of thermocline and mixed layer waters at the Agulhas Current Retroflection where eddies form, and the subsequent consumption of Agulhas nitrate by phytoplankton in the mixed layer, which raises 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 may record variations in past Agulhas leakage.