Dust contributes an important flux of iron to Gulf of Alaska surface waters beyond the shelf break, while shelf sediments and glacial meltwater drive Fe supply over the northern Gulf of Alaska shelf
Abstract
For more than twenty years we have known that iron (Fe) limits phytoplankton growth in large areas of the ocean. Yet we still have a poor quantitative understanding of the sources of Fe and of the controls on dissolved Fe concentrations. Dust was long thought to be the primary source of Fe, but recent work has suggested that other Fe sources are more important in certain regions of the ocean, including the Southern (S.) Ocean. In this work we examine several possible Fe sources to the subarctic N. Pacific, presenting both water column Fe data and insights from published dust flux estimates. Late winter, spring and summer water-column sampling and numerical modeling suggest that shelf sediments and meltwater, together, can contribute all the dissolved Fe (DFe) required by the highly productive coastal waters. However, scavenging confines the vast majority of the particulate Fe and DFe to within 100 km of the shelf break. This is consistent with other Fe datasets from similar high-latitude settings, including the coastal S. Ocean and waters off Greenland. The dust flux to large regions of the subarctic Pacific, estimated via multiple independent methods, is close to 1 g/m2/yr. Assuming this dust is supplied in brief storms either in autumn from the Alaskan coastline (Crusius et al, 2011) or in spring from the Gobi desert (Zdanowicz et al, 2006), this dust flux is sufficient to drive an initial surface-water DFe concentration throughout much of the Gulf of Alaska of 1 nmol kg-1 or higher (assuming mixed layer depth=100 m and Fe dissolution = 10%). This suggests that the dust flux to the subarctic N. Pacific is an important Fe source beyond the shelf break. This claim is in contrast to recent work in the S. Ocean that suggests dust is unimportant as an Fe source there. One important reason for the difference is that the temperate climate in southern Alaska promotes considerable melting of glaciers on land, making a large amount of dust source material available for transport by storms to the ocean. In contrast, temperatures in Antarctica are far lower, cause far less glacier melting on land, and generate limited local dust source material. Recent estimates of dust flux to the S. Ocean are thirty or more times lower than to the subarctic N. Pacific, making dust a far less important source of Fe in the S. Ocean than in in the subarctic N. Pacific.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.A24B..04C
- Keywords:
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- 0305 Aerosols and particles;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 0312 Air/sea constituent fluxes;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 0315 Biosphere/atmosphere interactions;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES