The isotopic composition of water column nitrate in the Gulf of Mexico and Florida Straits

Angela N Knapp1, Samantha Howe1, Carlos Miranda2, Chris T Hayes3 and Robert T Letscher4, (1)Florida State University, Earth, Ocean, and Atmospheric Sciences, Tallahassee, FL, United States, (2)Florida State University, FL, United States, (3)University of Southern Mississippi, Stennis Space Center, MS, United States, (4)University of New Hampshire, Earth Sciences, Durham, United States
The Gulf of Mexico (GoM) is recognized for receiving significant riverine nutrient inputs that contribute to eutrophication of northern shelf waters. However, it is unclear whether these and/or other nutrients delivered to the GoM are exported from the Gulf, potentially influencing the water column chemistry of the North Atlantic. Here, we report the first nitrate dual isotopic composition (d15N and d18O) measurements from the GoM as well as in profiles from the Florida Straits. Samples were collected twice in the northern Gulf in June 2017 and April 2018, as well as off the west Florida Shelf in June 2015 and in the Florida Straits in September 2011. The nitrate d15N and d18O observed in the upper thermocline of the GoM and Florida Straits are intermediate between that previously measured in the South Atlantic and the Sargasso Sea. In the GoM and Florida Straits there is a minimum in nitrate d15N of 3.5 per mil on potential density surface 26.5, which is somewhat higher than that previously measured in the Sargasso Sea, 3.0 per mil, on the same isopycnal surface. Deeper water masses in the Gulf, largely Antarctic Intermediate Water and North Atlantic Deep Water, have isotopic compositions very similar to those previously described in the Atlantic Ocean. There is little evidence for nitrate from the Mississippi and Atchafalaya Rivers, which prior work has shown has an elevated nitrate d15N of 6 to 8 per mil, directly influencing the isotopic composition of nitrate in the GoM or Florida Straits on shallow (less than 150 m) isopycnal surfaces. These results are consistent with recent modeling work suggesting that the majority of nutrients delivered to the GoM from the Mississippi and Atchafalaya Rivers are retained in the near-shore region. Instead the isotopic composition of nitrate in the GoM appears to reflect the role of circulation, which introduces both southern and northern Atlantic waters into the Caribbean and then into the GoM.