Nitrate isotopic gradients in the North Atlantic Ocean and the nitrogen isotopic composition of sinking organic matter

Abstract

Biogeochemical processes within the Atlantic basin alter the nitrogen (N) and oxygen (O) isotopic compositions (δ15N and δ18O) of nitrate before this nutrient is carried from the upper water column into the deep ocean by the formation of North Atlantic Deep Water (NADW). Here, nitrate δ15N and δ18O measurements along the 2013 CLIVAR/GO-SHIP Atlantic section A16N from 6°S to 61°N are used to calculate the δ15N and δ18O of regenerated nitrate added to the interior at different latitudes and depths along the section. The δ15N of nitrate being regenerated is similar among depths and covaries with δ15N of the nitrate supply to overlying surface waters. These observations are consistent with regenerated nitrate deriving dominantly from sinking N, rather than from dissolved organic or suspended particulate organic N that is circulated through the ocean interior. The δ15N of regenerated nitrate never declines below the δ15N of the nitrate supply, consistent with the dominance of subsurface nitrate over N2 fixation and atmospheric N deposition in fueling net community production. In the low latitudes (<15°N), shallow and mid-depth nitrate δ15N entering from the South in Subantarctic Mode Water (SAMW) is as high as 6.2‰ vs. air due to partial nitrate assimilation in the Southern Ocean surface, while the nitrate δ18O is similar to that measured in deep water. Its low δ18O indicates that nitrate at shallow and mid-depths is mostly regenerated, the low δ18O of which overprints the high δ18O of unused (“preformed”) nitrate from the Southern Ocean. Regenerated nitrate δ18O is calculated to be 1.1–1.2‰ higher than ambient seawater, consistent with previous work. At the southern end of the section, the δ15N of the sinking flux is as high as that of SAMW (~6.2‰) or higher, maintaining the high δ15N of shallow/mid-depth waters and raising the δ15N of nitrate on deeper isopycnals, including those within NADW. SAMW ultimately supplies water for NADW formation, but the high nitrate δ15N of SAMW observed in the southern end of the section does not survive into newly formed NADW, which has a nitrate δ15N of 4.83‰ vs. air. This indicates that the high nitrate δ15N of northward-flowing shallow/mid-depth waters is overprinted by the input of low-δ15N fixed N to the Atlantic, largely by N2 fixation. Our reconstruction of the δ15N of sinking N indicates that most newly fixed N is added south of the North Atlantic subtropical gyre. In the subpolar North Atlantic, the A16N data indicate that the δ15N of the nitrate in NADW does not differ from that of its regional upper ocean sources, for two reasons. First, nitrate is completely consumed in much of the subpolar North Atlantic surface, minimizing the isotopic signal from partial nitrate assimilation. Second, much of the sinking N from the North Atlantic surface is regenerated in newly formed NADW, reuniting the nitrate assimilated in the surface with the preformed nitrate carried down in NADW.