The Gulf of Mexico receives considerable riverine nutrient inputs that contribute to eutrophication of northern shelf waters. However, it is unclear whether a significant fraction of these riverine nutrients delivered to the Gulf of Mexico are subsequently exported from the Gulf, potentially influencing the chemistry of the North Atlantic. Here, we report the first dual isotopic composition (“δ¹⁵N” and “δ¹⁸O”) measurements of nitrate (NO₃⁻) from three profiles collected in different years in the Gulf of Mexico, as well as a new profile from the Florida Straits. We compare these new measurements with prior measurements from the North and South Atlantic, as well as with the magnitude of riverine and other nitrogen fluxes to the Gulf of Mexico. We find that in the upper thermocline, NO₃⁻ δ¹⁵N measurements from the Gulf of Mexico are intermediate between prior measurements in the Sargasso Sea (lower in NO₃⁻ δ¹⁵N), and those from the South Atlantic (higher in NO₃⁻ δ¹⁵N), but are indistinguishable from Florida Straits and Sargasso Sea profiles on deeper isopycnal surfaces. There is little evidence for riverine NO₃⁻ influencing the isotopic composition of NO₃⁻ exported from the Gulf of Mexico, which, based on its δ¹⁵N, would be expected to raise the δ¹⁵N of shallow NO₃⁻. Lesser nitrogen fluxes are too small to significantly influence the isotopic composition of water column NO₃⁻ in the Gulf of Mexico. These results are consistent with recent modeling work suggesting that the majority of riverine nutrients are retained in the nearshore region.

中文翻译：

---

墨西哥湾和佛罗里达海峡硝酸盐的双重同位素组成

墨西哥湾收到大量河流养分，这有助于北部陆架水域富营养化。然而，目前尚不清楚这些运往墨西哥湾的河流养分中是否有很大一部分随后从海湾出口，这有可能影响北大西洋的化学性质。这里，我们报告第一双同位素组合物（“δ ¹⁵ N”和“δ ¹⁸ O”）硝酸盐的测量（NO ₃ ^-），分别来自墨西哥湾不同年份收集的三个剖面以及佛罗里达海峡的新剖面。我们将这些新测量值与北大西洋和南大西洋的先前测量值以及到墨西哥湾的河流和其他氮通量的大小进行了比较。我们发现，在上跃层，NO ₃ ^- δ ^15个从墨西哥湾N次测量是在马尾藻海在先测量之间的中间（降低NO ₃ ^- δ ¹⁵ N），以及那些从南大西洋（较高的NO ₃ ^- δ ¹⁵N），但与深海等深面的佛罗里达海峡和Sargasso Sea剖面没有区别。很少有证据表明河流NO ₃ ^-影响NO的同位素组成₃ ^-来自墨西哥，其中，基于其δ湾出口¹⁵ N，预计将提高δ ¹⁵浅NO N个₃ ^- 。较小的氮气通量太小显著影响水柱NO的同位素组成₃ ^-在墨西哥湾。这些结果与最近的模型研究结果一致，表明大多数河流营养素保留在近岸地区。