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
In this study we report full-depth water column profiles for nitrogen and oxygen isotopic composition (d15N and d18O) 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- d15N and d18O 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- d18O versus d15N 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- d15N 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- d18O 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- d18O toward the d18O of regenerated NO3-. The low NO3- d15N 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- d15N signal observed between 600 and 1500 m depth close to the Iberian margin.
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