Nitrate isotope distributions on the US GEOTRACES North Atlantic cross-basin section: Signals of polar nitrate sources and low latitude nitrogen cycling

We report and interpret the nitrate delta N-15 and delta O-18 distributions in the cross-basin transect of the US GEOTRACES North Atlantic expedition. Low nitrate delta N-15 observed in the shallow thermocline of the central western portion of the basin (reaching as low as 2.5 parts per thousand vs. air; similar to 2.3 parts per thousand lower than deep Atlantic nitrate) reflects the remineralization of nitrogen with a low 815N, presumably mostly from nitrogen fixation. A strong correlation between the delta N-15 and concentration of nitrate argues that the focusing of the low delta N-15 signal in the upper thermocline of the North Atlantic subtropical gyre is at least partly due to the low concentration of nitrate in those waters, such that the east-to-west delta N-15 decrease in shallow thermocline nitrate does not require an east-to-west increase in nitrogen fixation rate. In underlying intermediate-depth water, which enters the section largely from the South, the nitrate delta N-15 is similar to 1 parts per thousand. higher than in deep water while the delta O-18 is similar to that in deep water. The delta N-15 elevation derives from the partial assimilation of nitrate in Southern Ocean surface waters prior to Antarctic Intermediate and Subantarctic Mode Water formation. The delta O-18 elevation of preformed nitrate in southern-sourced intermediate waters has been diluted by the addition of nitrate from regeneration/nitrification in the low latitudes, which does not similarly weaken the delta N-15 elevation. In deep water, the nitrate delta N-15 in samples with a high relative contribution of North Atlantic Deep Water is almost indistinguishable from that of samples with a higher contribution of Antarctic Bottom Water (4.8-4.9% vs. air). Thus, the high nitrate delta N-15 in northward flowing intermediate waters is almost entirely erased by the input of low-delta N-15 newly fixed nitrogen to the Atlantic (including that from the Mediterranean) prior to the incorporation of those intermediate depth waters into southward flowing North Atlantic Deep Water. The data appear consistent with a nitrogen isotopic balance between northward and southward nitrate transports across the GEOTRACES section, suggesting that most Atlantic nitrogen fixation occurs south of the section. (C) 2015 Elsevier B.V. All rights reserved.