Nitrate isotopic composition across a North-South transect in the Eastern Atlantic Ocean: Significance of nitrogen input through N2 fixation

Dual N and O isotopic composition of nitrate is a powerful tool of increasing use aiming at the study of marine N involved processes. It allows an integrative characterization of the main N fluxes including the source (N₂ fixation) and sink (denitrification) of the oceanic N pool, but also processes involved in internal cycling such as nitrate assimilation and regeneration.

We present depth profiles (0–700 m depth) of nitrate δ¹⁵N and δ¹⁸O from an N–S transect in the Eastern Atlantic (38°N–26°S, RV Polarstern, Nov. 2012) and from a transect across the Bay of Biscay into the Iberian Margin (48–36°N, RV Belgica, May 2014).

In the North Atlantic we found a decoupling between areas of high N₂ fixation activity and regions of prominent imprint of low δ¹⁵N nitrogen input to the nitrate pool: the Tropical North Atlantic (0–20°N) with δ¹⁵N-NO₃^- ranging from 4.6–5.5‰ in subsurface waters (100–300 m) as opposed to the Subtropical North Atlantic (20–40°N) indicating a subsurface minimum in δ¹⁵N-NO₃^- (2.5–4.5‰) concomitant with a Δ(15, 18) minimum ranging from -0.8 to 0.8‰, respectively. In fact, nitrate Δ(15, 18) computed as [δ¹⁵N – δ¹⁸O] is considered conservative during mixing, nitrate assimilation and denitrification, but does deviate from its deep stable value (~3‰) as a results of nitrogen and/or oxygen input through N₂ fixation and nitrification, respectively. In the South Atlantic, the nitrate pool seems to be affected by two major components, apart from surface nitrate assimilation: input from deeper layers (Antarctic Intermediate Waters) of high δ¹⁵N-NO₃^- originated from partial assimilation of nitrate in the photic layer of the Open Antarctic Zone. This resulted in an increase beyond the global average (~5‰) of the deepest δ¹⁵N from our profiles (700 m). Secondly, input of low δ¹⁵N-NO₃^- seemed to affect the subsurface waters of the region between 0–15°S, potentially N₂ fixation, considering that rates measured in-situ revealed high level of activity.