Nitrate Supply Routes And Impact Of Internal Cycling In The North Atlantic Ocean Inferred From Nitrate Isotopic Composition

In this study we report full-depth water column profiles for nitrogen and oxygen isotopic composition (delta N-15 and delta O-18) 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- delta N-15 and delta O-18 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- delta O-18 versus delta N-15 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- delta N-15 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- delta O-18 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- delta O-18 toward the delta O-18 of regenerated NO3-. The low NO3- delta N-15 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 NO3- delta N-15 signal observed between 600 and 1500 m depth close to the Iberian margin.