Publication Date:
2022-10-26
Description:
Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 35(4), (2021): e2020GB006887, https://doi.org/10.1029/2020GB006887.
Description:
In this study we report full-depth water column profiles for nitrogen and oxygen isotopic composition (δ15N and δ18O) of nitrate (NO3−) 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 NO3− δ15N and δ18O shows that assimilation by phytoplankton is the main process controlling the NO3− isotopic composition in the upper 150 m, with values increasing in a NO3− δ18O versus δ15N space along a line with a slope of one toward the surface. In the subpolar gyre, a single relationship between the degree of NO3− consumption and residual NO3− δ15N supports the view that NO3− 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 NO3− assimilation and nitrification in the deep mixed layer of the subpolar gyre elevates subsurface NO3− δ18O 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 NO3− δ18O toward the δ18O of regenerated NO3−. The low NO3− δ15N observed south of the Subarctic Front is mostly explained by N2 fixation, although a contribution from the Mediterranean outflow is required to explain the lower NO3− δ15N signal observed between 600 and 1500 m depth close to the Iberian margin.
Description:
The GEOVIDE project was co-funded by the French national program LEFE/INSU (GEOVIDE), ANR Blanc (GEOVIDE) and RPDOC, LabEX MER and IFREMER. F. Deman was supported by the Belgian Federal Science Policy Office (Belspo contract BL/12/C63) while writing the manuscript. This work was financed by Flanders Research Foundation (FWO contract G0715.12N) and Vrije Universiteit Brussel, R&D, Strategic Research Plan “Tracers of Past & Present Global Changes”. During the preparation of the manuscript, Debany Fonseca-Batista was supported by funding from the Canada First Research Excellence Fund, through an International Postdoctoral Fellowship of the Ocean Frontier Institute (OFI) at Dalhousie University.
Description:
2021-10-02
Keywords:
Atlantic
;
Isotopy
;
Nitrate
Repository Name:
Woods Hole Open Access Server
Type:
Article
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