Iron sources to the Fe-poor northern Gulf of Alaska: insights from water column and atmospheric dust time series

Waters of the northern Gulf of Alaska continental shelf and slope host a productive ecosystem, yet the mechanisms that supply nutrients in the region remain poorly understood. The continental shelf break defines a transition between continental sources of nutrients and open ocean sources. The nutrient iron limits productivity in the Gulf of Alaska, but the relative importance of the various Fe fluxes to iron-limited waters is poorly characterized. A dataset that includes both continuously collected (weekly) atmospheric dust concentrations from Middleton Island at the shelf break and water-column dissolved and particulate Fe concentrations collected in the winter, spring and summer extending from the Copper River plume across the continental shelf and into waters over the continental slope, makes it clear that glacial melt-dominated rivers and sediment resuspension are among the important sources of Fe to shelf waters. However, the processes that dominate transport of Fe to sites beyond the shelf break are not clear. Atmospheric dust is transported infrequently, but over long distance, through the atmosphere. Many water column processes transport far more iron to coastal waters, but much of that material (particulate material) settles out fairly close to shore, such that it is not clear what process dominates transport of iron beyond the shelf break. These data collectively can shed light on this question. Important physical mechanisms that have been suggested by others to drive offshore transport beyond the shelf break include cycles of wind-driven upwelling and downwelling, tidal mixing in canyons, and eddies. We will discuss Fe supply beyond the shelf break from these processes at this key transition from Fe-rich coastal waters to the Fe-poor Gulf of Alaska from our rich dataset of both water-column dissolved and particulate Fe, as well as atmospheric dust concentrations.