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

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Journal Article
Biogeochemical processes within the Atlantic basin alter the nitrogen (N) and oxygen (O) isotopic compositions (δ 15 N and δ 18 O) 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 δ 15 N and δ 18 O measurements along the 2013 CLIVAR/GO-SHIP Atlantic section A16N from 6°S to 61°N are used to calculate the δ 15 N and δ 18 O of regenerated nitrate added to the interior at different latitudes and depths along the section. The δ 15 N of nitrate being regenerated is similar among depths and covaries with δ 15 N 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 δ 15 N of regenerated nitrate never declines below the δ 15 N of the nitrate supply, consistent with the dominance of subsurface nitrate over N 2 fixation and atmospheric N deposition in fueling net community production. In the low latitudes (<15°N), shallow and mid-depth nitrate δ 15 N 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 δ 18 O is similar to that measured in deep water. Its low δ 18 O indicates that nitrate at shallow and mid-depths is mostly regenerated, the low δ 18 O of which overprints the high δ 18 O of unused (“preformed”) nitrate from the Southern Ocean. Regenerated nitrate δ 18 O is calculated to be 1.1–1.2‰ higher than ambient seawater, consistent with previous work. At the southern end of the section, the δ 15 N of the sinking flux is as high as that of SAMW ( 6.2‰) or higher, maintaining the high δ 15 N of shallow/mid-depth waters and raising the δ 15 N of nitrate on deeper isopycnals, including those within NADW. SAMW ultimately supplies water for NADW formation, but the high nitrate δ 15 N of SAMW observed in the southern end of the section does not survive into newly formed NADW, which has a nitrate δ 15 N of 4.83‰ vs. air. This indicates that the high nitrate δ 15 N of northward-flowing shallow/mid-depth waters is overprinted by the input of low-δ 15 N fixed N to the Atlantic, largely by N 2 fixation. Our reconstruction of the δ 15 N 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 δ 15 N 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. © 2019 Elsevier Ltd
Deep-Sea Research Part I: Oceanographic Research Papers