Does Mineral Dust Fertilize the Amazon Basin and the Atlantic Ocean? (Invited Presentation)

The deposition of phosphorus (P) from African dust is believed to increase primary productivity in the Amazon Basin and Tropical Atlantic Ocean and, consequently, impact atmospheric carbon dioxide levels on a global scale. However, there are few measurements of African dust in South America that can robustly test this hypothesis and even fewer measurements of soluble P, which is readily available for stimulating primary production in the ocean. To test this hypothesis, we measured total and soluble P in long-range transported aerosols collected in Cayenne, French Guiana, a site located on the coast of the Tropical Atlantic Ocean and at the northeastern edge of the Amazon Basin. Our measurements confirm that in boreal Spring, when African dust transport is greatest, the majority of P is supplied by dust and 5% of this P is soluble. In boreal Fall, when dust transport is at an annual minimum, we measured unexpectedly high concentrations of 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 show that African biomass burning supplies up to half of the P deposited annually to the Amazon from transported African aerosol. This is the first observational study that links P-rich biomass burning aerosols from Africa to enhanced P deposition in the Amazon. Contrary to current thought, our results suggest that African biomass burning is more important than dust for alleviating P limitations in parts of the Tropical Atlantic Ocean, Indian Ocean, and Southern Ocean, particularly in the Summer and Fall, and may be more important for marine productivity in these ocean basins.