Atmospheric Dust Inputs, Iron Cycling, and Biogeochemical Connections in the South Pacific Gyre

Geographic patterns in biogeochemical processes such as primary production and nitrogen fixation that require micronutrients like iron (Fe) are modulated in part by the spatial distribution of dust supply. The South Pacific Ocean is a particularly important region for understanding dust and Fe inputs, given its extreme oligotrophy, vanishingly low surface nutrient concentrations, and potential Fe-limitation of nitrogen fixation. We present new estimates of dust flux, Fe supply rates, and Fe residence times derived from measurements of long-lived thorium isotopes (²³⁰Th, ²³²Th) from the UltraPac transect in the South Pacific Gyre.

We find dust fluxes in the center of the gyre that are the lowest of any mean annual dust input rates measured in the global oceans, but that are consistently an order of magnitude higher than those estimated by global climate models. From these estimates, we determine dust-borne Fe fluxes and re-assess the importance of individual Fe sources to the euphotic zone in the South Pacific Gyre, finding that dust dissolution, not vertical or lateral eddy diffusion, is the primary Fe supply mechanism. We combine our estimates of Fe flux in dust with previously-published cellular and enzymatic quotas to determine theoretical upper limits on annual average nitrogen fixation rates for a given Fe deposition rate, compare these upper limits with measured N₂-fixation rates in the South Pacific, and discuss the biogeochemical consequences of our results.