Toward Constraining Sources of Lithogenic Metals in the Northern Gulf of Mexico

North African dust is known to be deposited in the Gulf of Mexico, but its deposition rate and associated supply of lithogenic dissolved metals, such as the abiotic metal thorium or the micronutrient metal iron, have not been well-quantified. Th-232 is an isotope with similar sources as iron and its input can be quantified using radiogenic Th-230. By comparing dissolved Th-232 fluxes at three sites in the northern Gulf of Mexico with upwind sites in the North Atlantic, we place an upper bound on North African dust contributions to Th-232 and Fe in the Gulf of Mexico, which is about 30% of the total input. Precision on this bound is hindered by uncertainty in the relative rates of dust deposition in the North Atlantic and the northern Gulf of Mexico. Based on available radium data, shelf sources, including rivers, submarine groundwater discharge, and benthic sedimentary releases are likely as important if not more important than dust in the budget of lithogenic metals in the Gulf of Mexico. In other words, it is likely there is no one dominant source of Th and Fe in the Gulf of Mexico. Finally, our estimated Fe input in the northern Gulf of Mexico implies an Fe residence time of less than 6 months, similar to that in the North Atlantic despite significantly higher supply rates in the Gulf of Mexico.

Automated summary

This study compares the dissolved Th-232 fluxes in the Gulf of Mexico and the North Atlantic and places an upper limit on the contribution of North African dust to Th-232 and Fe in the Gulf of Mexico, which is about 30% of the total input. The study also suggests that shelf sources in the Gulf of Mexico, including rivers, submarine groundwater discharge, and benthic sedimentary releases, may be as important as or even more important than dust in the budget of lithogenic metals. Additionally, the estimated Fe input in the Gulf of Mexico implies a residence time of less than 6 months, similar to that in the North Atlantic.