The Concentration and Isotopic Composition of Nitrate in the Water Column Along a Cross-Basin Transect of the North Atlantic Ocean

Abstract

This study examines the concentration and isotopic composition of nitrate from a cross-basin transect of the North Atlantic Ocean, as part of the DIMENSIONS study. Goals of this study include using a combined analysis of nitrate isotope distribution and hydrographic data to study the marine nitrogen cycle and enhance our understanding of major sources and sinks of nitrate in the ocean. Nitrate δ\(^{15}\)N and δ\(^{18}\)O values were measured experimentally through the ‘denitrifier’ method and subsequent mass spectrometry analysis of N\(_{2}\)O gas. Major features included coupled elevation of δ\(^{15}\)N and δ\(^{18}\)O in surface waters, indicating in situ nitrate assimilation. In the sub-polar North Atlantic surface waters, the calculated fractionation effect of partial nitrate assimilation was 5.82‰ for nitrogen and 6.09‰ for oxygen. At sampling stations in the western portion of the basin, low δ\(^{15}\)N excursions in the water column reflected the mineralization of low δ15N organic matter, most likely introduced by N fixation in nutrient deprived surface waters. Western stations had an additional subsurface anomaly characterized by elevated [NO\(_{3}\)-] and elevated δ1\(^{15}\)N, which is likely to be a signal of partial assimilation in the Southern Ocean. Water masses at intermediate depths travel from the Southern Ocean to the North Atlantic, and on the way they trap nutrients and preserve δ\(^{15}\)N elevation through complete assimilation of nitrate in surface waters and the regeneration of nitrate from organic matter. Across the transect, paired excursions of δ\(^{15}\)N and δ\(^{18}\)O made it possible to distinguish between the occurrence of in situ processes and the transport of signals. Near the sub-polar region, subsurface water mass movement transported nitrate with δ\(^{18}\)O elevation but no major δ\(^{15}\)N excursion. The signal was most defined at the easternmost stations, however there was evidence of this dynamic across several stations.