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Abstract:
This study presents basin-wide, full-depth profiles of nitrate δ15N (vs. Air) and δ18O (vs. Vienna Standard Mean Ocean Water, VSMOW) in the Mediterranean Sea, based on seawater samples
collected during three separate cruises: the TalPro cruise in summer 2016, the MSM72 cruise in spring 2018, and the HaiSec45 cruise in spring 2021. Our results reveal a consistent 15N
depletion across the entire Mediterranean Sea in comparison to the global ocean, with significantly lower nitrate δ15N 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 δ18O between the two basins (2.2 ± 0.3‰ and 2.1 ± 0.2‰, respectively). These observations point to an external supply of
low-δ15N N (ultimately, nitrate) to the Mediterranean Sea. This supply is gradually diluted by the Atlantic nitrate inflow, thus creating an east-to-west gradient in nitrate δ15N. Earlier studies have
attributed this external low-δ15N N supply to either N2 fixation or atmospheric deposition of anthropogenic N. A prognostic four-box model reveals that given the 120 – 170-year residence
time of water in the Mediterranean Sea, modest rates of these sources, individually or in combination, can account for the observed low δ15N values. Additionally, we identify partial
degradation of dissolved organic nitrogen, introduced into the Mediterranean Sea from the Atlantic Ocean, as another possible source of low-δ15N nitrate. Distinguishing among these
sources will require reconstruction of Mediterranean nitrate δ15N through time, using paleoceanographic proxies.
