Origins of the Nitrate ¹⁵N Depletion in the Mediterranean Sea

Previous studies have reported a nitrate ¹⁵N depletion in the Mediterranean Sea compared to the global ocean, attributed to either N₂ fixation or atmospheric deposition of anthropogenic N. In this study, we report basin-wide full-depth profiles of nitrate δ¹⁵N (vs. Air) and δ¹⁸O (vs. Vienna Standard Mean Ocean Water, VSMOW) in the Mediterranean Sea. Our results confirm a consistent ¹⁵N depletion across the entire Mediterranean Sea, 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 a supply of low-δ¹⁵N 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 δ¹⁵N. 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 N₂ fixation, atmospheric deposition, or their combination—is adequate to produce the observed low δ¹⁵N of Mediterranean nitrate. Additionally, partial degradation of dissolved organic nitrogen imported from the Atlantic may add low-δ¹⁵N 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 δ¹⁵N through time using either time-series data of nitrate δ¹⁵N or calcareous fossil-bound organic nitrogen isotope ratios.