Premium
Nitrate Supply Routes and Impact of Internal Cycling in the North Atlantic Ocean Inferred From Nitrate Isotopic Composition
Author(s) -
Deman F.,
FonsecaBatista D.,
Roukaerts A.,
GarcíaIbáñez M. I.,
Le Roy E.,
Thilakarathne E. P. D. N.,
Elskens M.,
Dehairs F.,
Fripiat F.
Publication year - 2021
Publication title -
global biogeochemical cycles
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.512
H-Index - 187
eISSN - 1944-9224
pISSN - 0886-6236
DOI - 10.1029/2020gb006887
Subject(s) - ocean gyre , oceanography , upwelling , geology , nitrate , geotraces , north atlantic deep water , water mass , mode water , subtropics , transect , thermohaline circulation , chemistry , ecology , organic chemistry , biology
In this study we report full‐depth water column profiles for nitrogen and oxygen isotopic composition (δ 15 N and δ 18 O) of nitrate (NO 3 − ) during the GEOTRACES GA01 cruise (2014). This transect intersects the double gyre system of the subtropical and subpolar regions of the North Atlantic separated by a strong transition zone, the North Atlantic Current. The distribution of NO 3 − δ 15 N and δ 18 O shows that assimilation by phytoplankton is the main process controlling the NO 3 − isotopic composition in the upper 150 m, with values increasing in a NO 3 − δ 18 O versus δ 15 N space along a line with a slope of one toward the surface. In the subpolar gyre, a single relationship between the degree of NO 3 − consumption and residual NO 3 − δ 15 N supports the view that NO 3 − is supplied via Ekman upwelling and deep winter convection, and progressively consumed during the Ekman transport of surface water southward. The co‐occurrence of partial NO 3 − assimilation and nitrification in the deep mixed layer of the subpolar gyre elevates subsurface NO 3 − δ 18 O in comparison to deep oceanic values. This signal propagates through isopycnal exchanges to greater depths at lower latitudes. With recirculation in the subtropical gyre, cycles of quantitative consumption‐nitrification progressively decrease subsurface NO 3 − δ 18 O toward the δ 18 O of regenerated NO 3 − . The low NO 3 − δ 15 N observed south of the Subarctic Front is mostly explained by N 2 fixation, although a contribution from the Mediterranean outflow is required to explain the lower NO 3 − δ 15 N signal observed between 600 and 1500 m depth close to the Iberian margin.