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Nitrate supply routes and impact of internal cycling in the North Atlantic Ocean inferred From nitrate isotopic composition
Deman, F.; Fonseca-Batista, D.; Roukaerts, A.; García-Ibáñez, M.I.; Le Roy, E.; Thilakarathne, E.P.D.N.; Elskens, M.; Dehairs, F.; Fripiat, F. (2021). Nitrate supply routes and impact of internal cycling in the North Atlantic Ocean inferred From nitrate isotopic composition. Global Biogeochem. Cycles 35(4): e2020GB006887. https://dx.doi.org/10.1029/2020GB006887
In: Global Biogeochemical Cycles. American Geophysical Union: Washington, DC. ISSN 0886-6236; e-ISSN 1944-9224, more
Peer reviewed article  

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Author keywords
    Atlantic; isotopy; nitrate

Authors  Top 
  • Deman, F., more
  • Fonseca-Batista, D., more
  • Roukaerts, A., more
  • García-Ibáñez, M.I.
  • Le Roy, E.
  • Thilakarathne, E.P.D.N., more

Abstract
    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.

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