The Agulhas Current Transports Signals of Local and Remote Indian Ocean Nitrogen Cycling

The greater Agulhas Current region is an important component of the climate system, yet its influence on carbon and nutrient cycling poorly understood. Here, we use nitrate isotopes (δ15N, δ18O, Δ(15–18) = δ15N–δ18O) to trace regional water mass circulation investigate nitrogen in adjacent recirculating waters. deep intermediate waters record processes occurring remotely, including partial assimilation Southern Ocean denitrification Arabian Sea. In thermocline surface, tropically sourced are biogeochemically distinct from subtropically waters, confirming inhibited lateral mixing across current core. (Sub)tropical δ15N lower (4.9–5.8‰) than sub-thermocline source, Subantarctic Mode Water (6.9‰); attribute this difference local N2 fixation. Using a one-box model simulate newly fixed flux, estimate fixation rate 7–25 Tg N.a−1, with upper limit likely biased high. mixed layer, δ18O rise unison, indicating that phytoplankton dominates surface nitrification restricted deeper Because vertically decoupled, plus can be used approximate export. Thermocline mixed-layer low, due both coupled nitrification. Similarly low-Δ(15–18) rings indicates leakage low-δ15N into South Atlantic, which should recorded organic matter sinking seafloor, providing potential tracer past leakage.