Quantifying African dust deposition to the Gulf of Mexico

Saharan dust is transported across the Atlantic and deposited in the Gulf of Mexico at close to the northern and western extent of the dust plume. It is not known how the magnitude of this source of lithogenic metals compares to more local continental sources such as riverine outflow or continental shelf sediment dissolution. Using the long-lived thorium isotopes, one can quantify the total flux of dissolved ²³²Th to the Gulf of Mexico, which reflects the combination of North American and North African sources. Thorium sources can be used as indicators for the sources of bioactive metals, such as iron, important to the ecosystem of the Gulf of Mexico. When compared to sites in the remote northwestern Atlantic, whose only source of ²³²Th is from African dust deposition, one can estimate the percentage contribution of African dust to the inventory of dissolved iron in the Gulf. Here we present thorium isotope depth profiles from 3 sites in the deep Gulf of Mexico, including several occupations of the Deepwater Horizon spill site, to show that about 25% of the iron in the Gulf of Mexico is sourced from African dust rather than local sources. We also show that the residence time of dissolved iron in the Gulf of Mexico is similar to that in the North Atlantic, about 6 months in surface waters. This implies that inter-annual variations in trans-Atlantic dust input are likely to have an impact on the biota (e.g., nitrogen fixers) relying on iron sources in the oligotrophic Gulf of Mexico.