Award: OCE-2023708

James Moffett (USC) and Daniele Bianchi (UCLA) investigated the distribution of iron (Fe) along the Oregon Coast on two research cruises in 2021. The composition of seawater in coastal waters is much different than the open oceans. Nutrients are much higher in coastal waters, stimulating high productivity. Indeed, Oregon coastal waters are among the most productive fisheries in North America. There are big gradients in ocean chemistry in the boundaries between the coastal waters and blue waters of the open ocean. No gradients are larger than those of iron, a critical micronutrient. But what controls where these boundaries are? Our research revealed that the extent of high iron penetration into the offshore ocean is determined by the interaction of coastal ocean physics and chemistry at the seafloor of the continental shelf. Strong currents flow northwards along the outer edge of the continental shelf. They interact with submarine canyons and other features to generate turbulent eddies that move vast amounts of water offshore. Our models showed that there are three hotspots of high transport along the Oregon coast that could account for half of the offshore transport. The other requirement is hypoxia (absence of oxygen) on the adjacent shelf. Previous workers showed that the amount of iron coming out of shelf sediments is much higher under hypoxic conditions. Our findings confirmed this prediction. Moreover, we found that at least two of the hotspots are near hypoxic zones, with high standing stocks of iron in coastal waters with strong offshore eddies. Hypoxia in Oregon is an increasing problem that is greatest in the summer. This is also the time when subsurface eddy generation is highest. Therefore, transport of iron (and other materials on the shelf) is greatest during this period. Broader impacts of the project are both global and local. The results help oceanographers to determine the amount of iron and other elements (including organic and inorganic carbon) from the continental margin to offshore waters. The data can be used to develop budgets, enabling us to determine how these elements are cycled globally. The results have an important local impact as well, enabling us to predict how future changes in hypoxia and inputs of iron from the continent will affect the Oregon coastal ecosystem. Last Modified: 01/15/2024 Submitted by: JamesWMoffett