Release of iron (Fe) from continental shelves is a major source of this limiting nutrient for phytoplankton in the open ocean, including productive Eastern Boundary Upwelling Systems. The mechanisms governing the transport and fate of Fe along continental margins remain poorly understood, reflecting interaction of physical and biogeochemical processes that are crudely represented by global ocean biogeochemical models. Here, we use a submesoscale-permitting physical-biogeochemical model to investigate processes governing the delivery of shelf-derived Fe to the open ocean along the northern U.S. West Coast. We find that a significant fraction (∼20%) of the Fe released by sediments on the shelf is transported offshore, fertilizing the broader Northeast Pacific Ocean. This transport is governed by two main pathways that reflect interaction between the wind-driven ocean circulation and Fe release by low-oxygen sediments: the first in the surface boundary layer during upwelling events; the second in the bottom boundary layer, associated with pervasive interactions of the poleward California Undercurrent with bottom topography. In the water column interior, transient and standing eddies strengthen offshore transport, counteracting the onshore pull of the mean upwelling circulation. Several hot-spots of intense Fe delivery to the open ocean are maintained by standing meanders in the mean current and enhanced by transient eddies and seasonal oxygen depletion. Our results highlight the importance of fine-scale dynamics for the transport of Fe and shelf-derived elements from continental margins to the open ocean, and the need to improve representation of these processes in biogeochemical models used for climate studies.

A document by Natalya Evans. Click on the document to view its contents.

Marine Oxygen Deficient Zones serve as hotspots for the loss of fixed nitrogen for the world’s oceans, and fixed nitrogen limits primary productivity in large expanses of the ocean. This fixed nitrogen loss occurs primarily through denitrification, where the stepwise reduction of nitrate to nitrite and ultimately to dinitrogen gas is coupled to organic matter oxidation. Nitrite, the first intermediate in denitrification, can also be re-oxidized back to nitrate in a reaction by chemoautotrophic microbes. Nitrite’s partitioning between reduction and oxidation determines if marine fixed nitrogen is lost or recycled. Nitrite oxidation in anoxic waters has been previously studied through stable and tracer isotope experiments, but the difficulty of these measurements has limited their geographical distribution and therefore requires extrapolation to understand their impact on the nitrogen cycling. Using basin-scale data, we analyze the progression of nutrients within the three water masses that feed the Eastern Tropical North Pacific Oxygen Deficient Zone. Significant deviations from the expected stoichiometry for denitrification demonstrate that 79% of the nitrite produced in the upper region of the Oxygen Deficient Zone is re-oxidized, whereas only 54% of the nitrite produced in the lower region of the Oxygen Deficient Zone is re-oxidized. These large estimates for nitrite re-oxidation reveal significant fixed nitrogen recycling across the Eastern Tropical North Pacific.