Article révisé par les pairs
Résumé : Abstract Previous studies have reported a nitrate 15 N depletion in the Mediterranean Sea compared to the global ocean, attributed to either N 2 fixation or atmospheric deposition of anthropogenic N. In this study, we report basin‐wide full‐depth profiles of nitrate δ 15 N (vs. Air) and δ 18 O (vs. Vienna Standard Mean Ocean Water, VSMOW) in the Mediterranean Sea. Our results confirm a consistent 15 N depletion across the entire Mediterranean Sea, with significantly lower nitrate δ 15 N values in the eastern basin (2.2 ± 0.2‰) than in the western basin (2.9 ± 0.1‰). In contrast, there is no significant difference in nitrate δ 18 O between the two basins (2.2 ± 0.3‰ and 2.1 ± 0.2‰, respectively). These observations point to a supply of low‐ δ 15 N N to the Mediterranean Sea, accumulating as regenerated nitrate, which is diluted by the nitrate in the Atlantic inflow, creating an west‐to‐east gradient in nitrate δ 15 N. A four‐box model reveals that, given a water residence time of 120–170 years in the Mediterranean, a modest input rate of 1–3 Tg N yr −1 —originating from N 2 fixation, atmospheric deposition, or their combination—is adequate to produce the observed low δ 15 N of Mediterranean nitrate. Additionally, partial degradation of dissolved organic nitrogen imported from the Atlantic may add low‐ δ 15 N nitrate to the Mediterranean, but it alone cannot explain the full isotopic signal. Distinguishing among these sources will be aided by the reconstruction of Mediterranean nitrate δ 15 N through time using either time‐series data of nitrate δ 15 N or calcareous fossil‐bound organic nitrogen isotope ratios.

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