The biogeochemical cycling of Fe and Fe isotopes in the Gulf of Mexico and the Gulf Stream system

Tim M Conway1, Brent A Summers2, Salvatore Caprara3, Travis Mellett4, Ryan Schlaiss3, Kristen N Buck5, Phoebe Dreux Chappell6 and Angela N Knapp7, (1)University of South Florida, St. Petersburg, FL, United States, (2)University of South Florida, College of Marine Science, St Petersburg, United States, (3)University of South Florida, College of Marine Science, St Petersburg, FL, United States, (4)University of South Florida, College of Marine Science, St. Petersburg, United States, (5)Oregon State University, College of Earth, Ocean, and Atmospheric Sciences, Corvallis, OR, United States, (6)Old Dominion University, Ocean, Earth and Atmospheric Sciences, Norfolk, United States, (7)Florida State University, Earth, Ocean, and Atmospheric Sciences, Tallahassee, FL, United States
Via the Loop Current, the Gulf Stream provides a teleconnection from the tropical North Atlantic and warm Gulf of Mexico & Caribbean Sea with the subtropical North Atlantic Gyre. As such, the Gulf Stream system plays a vital role in transporting heat and salt between the tropics and high latitudes, with the potential to also influence regional biogeochemistry. In the North, Gulf Stream eddies have been shown to facilitate the transfer of phosphate and Fe into the subtropical gyre [1-2], potentially fueling primary productivity and nitrogen fixation. However, despite the obvious potential for the Gulf Stream to influence North Atlantic biogeochemical cycling, trace metal data from the Gulf of Mexico and the southern constituent currents of the Gulf Stream remain scarce. Here, we present dissolved Fe concentrations and isotopes (δ56Fe) from two cruises: Spring 2018 near the West Florida Shelf, and a Spring 2019 surface transect across the Florida Current of the Gulf Stream. Using these data, we provide the first characterization of Fe isotope cycling off the West Florida Shelf and within the Loop and Florida Currents. In the Gulf of Mexico, we observe a clear signal of Fe release along the West Florida Shelf from sediments or groundwater, characterized by elevated Fe concentrations (3-10 nM) and light δ56Fe (-0.4 to +0.2‰). Further into the Gulf, and associated with the Loop Current, we observe much lower Fe concentrations (<0.5 nM) that are sometimes associated with very heavy δ56Fe (up to +1.25‰). We interpret these as the net result of dust dissolution in concert with organic ligands, potentially influenced by Trichodesmium activity. By comparison, the Florida current was characterized by ~0.5 nM Fe and heavy δ56Fe (+0.9‰), reflecting the importance of Gulf of Mexico biogeochemical processes in forming the Florida Current and for influencing the North Atlantic subtropical gyre.

[1] Conway et al. (2018). Nature Geoscience 11, 594-598.

[2] Palter et al. (2011). Global Biogeochemical Cycles 25(4), GB4007.