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 is poorly understood. Here, we use nitrate isotopes (delta N-15, delta O-18, delta(15-18) = delta N-15-delta O-18) to trace regional water mass circulation and investigate nitrogen cycling in the Agulhas Current and adjacent recirculating waters. The deep and intermediate waters record processes occurring remotely, including partial nitrate assimilation in the Southern Ocean and denitrification in the Arabian Sea. In the thermocline and surface, tropically sourced waters are biogeochemically distinct from adjacent subtropically sourced waters, confirming inhibited lateral mixing across the current core. (Sub)tropical thermocline nitrate delta N-15 is lower (4.9-5.8 parts per thousand) than the sub-thermocline source, Subantarctic Mode Water (6.9 parts per thousand); we attribute this difference to local N-2 fixation. Using a one-box model to simulate the newly fixed nitrate flux, we estimate a local N-2 fixation rate of 7-25 Tg N.a(-1), with the upper limit likely biased high. In the mixed layer, nitrate delta N-15 and delta O-18 rise in unison, indicating that phytoplankton nitrate assimilation dominates in surface waters, with nitrification restricted to deeper waters. Because nitrate assimilation and nitrification are vertically decoupled, the rate of nitrate assimilation plus N-2 fixation can be used to approximate carbon export. Thermocline and mixed-layer nitrate delta(15-18) is low, due to both N-2 fixation and coupled partial nitrate assimilation and nitrification. Similarly low-delta(15-18) nitrate in Agulhas rings indicates leakage of low-delta N-15 nitrogen into the South Atlantic, which should be recorded in the organic matter sinking to the seafloor, providing a potential tracer of past Agulhas leakage.