The shelf-to-basin transport of iron from the Northern U.S West Coast to
the Pacific Ocean
Abstract
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.