Where Three Oceans Meet: Nitrate Isotope Measurements from the South Atlantic Along 34.5°S

Abstract

The South Atlantic is an important conduit for water of Southern Ocean, North Atlantic, and Indian Ocean origin. Paired nitrate isotope measurements of \(\gamma\)\(^{15}\)N and \(\gamma\)\(^{18}\)O reveal signals of distant denitrification, N\(_{2}\) fixation, and remineralization of organic matter, as well as local processes. Lowered \(\delta\)(15–18) ( \(\gamma\)\(^{15}\)N- \(\gamma\)\(^{18}\)O) within Agulhas leakage observed across the Cape Basin indicate the transport of distinct signatures of Indian Ocean N\(_{2}\) fixation into central and intermediate South Atlantic waters. This represents a potential novel paleoproxy of Indian-Atlantic exchange within sediment and microfossil records of \(\gamma\)\(^{15}\)N. Additionally, the hydrographic and isotopic properties of Lower Circumpolar Deep Water (LCDW), North Atlantic Deep Water (NADW), and Upper Circumpolar Deep Water (UCDW) are compared with observations of these water masses from the Southern Ocean, Pacific Ocean, and North Atlantic. Mass balance calculations estimate the regeneration of 0.7 μM of nitrate with an average \(\gamma\)\(^{15}\)N of 4.7h within NADW flowing through the low-latitude Atlantic. Near-surface profiles show isotopic enrichment from nitrate assimilation to be consistent with a steady-state model of concurrent nutrient consumption and resupply. The N and O isotope effects of nitrate assimilation (15e and 18e) are estimated at 5.1h and 7.1h, respectively. Lastly, anomalously low values of \(\gamma\)\(^{15}\)N and \(\gamma\)\(^{18}\)O at the base of the mixed layer constitute new evidence of in-situ nitrification within a local ‘particle maximum.’