Wintertime nitrate isotope dynamics in the Atlantic sector of the Southern Ocean

Includes bibliographical references.We provide the first data on wintertime patterns of the nitrogen (N) and oxygen (O) isotopes of seawater nitrate for the region south of Africa. Water column profile and underway surface samples collected in July 2012 span a range of latitudes from the subtropics to 57.8°S, just beyond the Antarctic winter sea-ice edge (56.7°S). The data are used in the context of simple models of nitrate consumption (including the Rayleigh model) to estimate the isotope effect (the degree of isotope discrimination) associated with the assimilation of nitrate by phytoplankton. We focus on the Antarctic region (south of 50.3°S), where application of the Rayleigh model to depth profile N isotope data yields considerably lower isotope effect estimates (1.6-3.3‰) than commonly observed in the summertime Antarcti