African biomass burning is a substantial source of soluble phosphorus deposition to the Amazon, Tropical Atlantic Ocean, and Southern Ocean with potential impacts on carbon sequestration

Aerosols affect Earth's climate by modulating biogeochemical cycles leading to changes in primary production and atmospheric carbon dioxide levels. The long-range transport of African dust is thought to deposit fertilizing nutrients, such as phosphorus (P), that promote primary productivity and the subsequent sequestration of atmospheric carbon dioxide in the P-limited Amazon and Tropical Atlantic Ocean. The solubility of P present in aerosols is critically important as soluble forms of P are readily available for stimulating primary production in the oceanand have a larger impact on biogeochemical cycles in these ecosystems and, consequently, global atmospheric carbon dioxide levels. However, there are few measurements of P in aerosols and even fewer measurements of soluble P that can be used to test whether African dust is the most important source of P to the oceans and the Amazon Basin. In this study, we tested the hypothesis that African dust supplies P to the Amazon and Tropical Atlantic Ocean by measuring total and soluble P in long-range transported aerosols collected in Cayenne, French Guiana, a site located along the Tropical Atlantic Ocean coast on the northeastern edge of the Amazon. Our measurements support the popular hypothesis that African dust supplies P to the Amazon and Tropical Atlantic Ocean when African dust transport is greatest in boreal Winter and Spring; however, the P in dust is mostly insoluble. In boreal Fall, however, when dust transport is at an annual minimum, we measured unexpectedly high concentrations of P and soluble P, which we show is associated with the transport of biomass burning from southern Africa. Integrating our results into a chemical transport model, we found that African biomass burning supplies up to half of the P deposited annually to the Amazon from transported African aerosol. Contrary to current thought, we also show that African biomass burning is a more important source of soluble P than dust in parts of Tropical Atlantic Ocean, Indian Ocean, and Southern Ocean, particularly in Summer and Fall, and may be more important for stimulating marine productivity in these ocean basins, thereby increasing their ability to act as global carbon sinks.