The Dual Isotopic Composition of Nitrate in the Gulf of Mexico and Florida Straits

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 (“δ 15 N” and “δ 18 O”) measurements of nitrate (NO 3 − ) 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, NO 3 − δ 15 N measurements from the Gulf of Mexico are intermediate between prior measurements in the Sargasso Sea (lower in NO 3 − δ 15 N), and those from the South Atlantic (higher in NO 3 − δ 15 N), 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 δ 15 N, would be expected to raise the δ 15 N of shallow NO 3 − . 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.