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

Abstract

We report and interpret the nitrate δ15N and δ18O distributions in the cross-basin transect of the US GEOTRACES North Atlantic expedition. Low nitrate δ15N observed in the shallow thermocline of the central western portion of the basin (reaching as low as 2.5‰ vs. air; ~2.3‰ lower than deep Atlantic nitrate) reflects the remineralization of nitrogen with a low δ15N, presumably mostly from nitrogen fixation. A strong correlation between the δ15N and concentration of nitrate argues that the focusing of the low δ15N 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 δ15N 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 δ15N is ~1‰ higher than in deep water while the δ18O is similar to that in deep water. The δ15N elevation derives from the partial assimilation of nitrate in Southern Ocean surface waters prior to Antarctic Intermediate and Subantarctic Mode Water formation. The δ18O 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 δ15N elevation. In deep water, the nitrate δ15N 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 δ15N in northward flowing intermediate waters is almost entirely erased by the input of low-δ15N 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.