The Dual Isotopic Composition of Nitrate in the Gulf of Mexico and Florida Straits
The Dual Isotopic Composition of Nitrate in the Gulf of Mexico and Florida Straits
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DOI：
10.1029/2020jc016047
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发表时间：
2020-09-01
影响因子：
3.6
通讯作者：
Knapp, Angela N.
中科院分区：
文献类型：
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作者：
Howe, Samantha;Miranda, Carlos;Knapp, Angela N.
墨西哥湾接受了大量的河流营养物质输入，这有助于北部大陆架水域的富营养化。然而，目前还不清楚这些输送到墨西哥湾的河流营养物质中的很大一部分是否随后从墨西哥湾出口，这可能会影响北大西洋的化学物质。在这里，我们首次报道了从墨西哥湾不同年份采集的三个剖面的硝酸盐(NO3-)的双同位素组成(“Delta N-15”和“Delta O-18”)测量，以及来自佛罗里达海峡的一个新剖面。我们将这些新的测量结果与北大西洋和南大西洋以前的测量结果进行了比较，并与流入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，在上温跃层中，墨西哥湾N3-Delta N-15的测量值介于马尾藻海(N3-Delta N-15较低)和南大西洋(N3-Delta N-15较高)的测量之间，但在更深的等同面上与佛罗里达海峡和马尾藻海的剖面没有区别。几乎没有证据表明河流NO(3)(-)影响从墨西哥湾输出的NO(3)(-)的同位素组成，根据其增量N-15，预计将抬高浅层N3-的增量N-15。较小的氮通量太小，不能显著影响墨西哥湾水柱NO(3)(-)的同位素组成。这些结果与最近的模拟工作一致，这些模拟工作表明，大部分河流营养物质被保留在近岸地区。
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 ("delta N-15" and "delta O-18") 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-delta N-15 measurements from the Gulf of Mexico are intermediate between prior measurements in the Sargasso Sea (lower in NO3-delta N-15), and those from the South Atlantic (higher in NO3-delta N-15), but are indistinguishable from Florida Straits and Sargasso Sea profiles on deeper isopycnal surfaces. There is little evidence for riverine NO(3)(-)influencing the isotopic composition of NO(3)(-)exported from the Gulf of Mexico, which, based on its delta N-15, would be expected to raise the delta N-15 of shallow NO3-. Lesser nitrogen fluxes are too small to significantly influence the isotopic composition of water column NO(3)(-)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.