Tuesday, 13 November 2012

Dust over Southwestern Alaska

“Dust in the wind” may mean insignificance, but dust has consequence. Severe dust storms can clog freeway traffic and send cars to the repair shop, or clog lungs and send people to the emergency room. Regions rich in sand and waterborne sediments experience frequent dust storms, but dust can easily travel far from its source. Soils in the Amazon rainforest, for example, owe their existence to the Bodele Depression on the other side of the Atlantic, while Asian dust migrates across the Pacific Ocean to North America. And dust can occasionally confound the scientists who study it. Its ability to travel vast distances, or linger in the same region where it originated, can raise questions about where dust plumes might have originated.
Dust plumes hovered over the ocean off southwestern Alaska in early November 2012. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite observed dust over Bristol Bay on November 6, and dust over the Gulf of Alaska on November 7. But although the dust over the Gulf of Alaska could easily be traced to sediments along the Copper River, the source of the dust over Bristol Bay wasn’t so obvious. Researchers looking at this dust plume offered different explanations for its source.
John Crusius of the U.S. Geological Survey, Woods Hole Field Center, attributed the dust to glacial flour from the Alaskan coastline. Glaciers are common in this region, and as they slowly grind over local rocks, they wear away sediment. “Something like 75 percent of the freshwater to the Gulf of Alaska comes from small, coastal rivers, and all of these rivers draining ‘glacierized’ landscapes would have deposits of glacial flour at this time of year,” he explained. River levels rise and fall depending on the season, and river discharge generally reaches its minimum in in the Northern Hemisphere autumn. Lower river levels mean more exposed sediment available for dust storms, which are common between late October and mid-November. So the dust plume over Bristol Bay could have arisen from a local source, namely glacial flour.
Meanwhile, Santiago Gassó of NASA Goddard Space Flight Center, and Colin Seftor of NASA’s Suomi-NPP Ozone Science Team, proposed a different source for the dust over Bristol Bay on November 6. Citing the plume’s width—as opposed to the relatively narrow dust plume blowing from the Copper River over the Gulf of Alaska—Gassó doubted that the plume had a local source. ”It might be pollution and dust from China,” he said. Thick smog frequently fills the skies over eastern China at this time of year.
Seftor suggested a specific source for the plume over the Gulf of Alaska. In early November 2012, severe dust storms struck the Taklimakan Desert of western China. For several days, thick dust blew out of that desert toward the east. The Ozone Mapping Profiler Suite (OMPS) tracks aerosols worldwide, and OMPS observations from November 2 through 8 indicated that dust traveled from the Taklimakan Desert across China, across the Pacific Ocean to southwestern Alaska.
So it is possible that, while glacial flour blew southward off the Alaskan coast, more exotic dust from Asia also hovered overhead. But the issue wasn’t yet resolved. Crusius remarked, ”Other clues are needed (above and beyond images) to tell the whole story.”
  1. References

  2. Crusius, J., Schroth, A.W., Gassó, S., Moy, C.M., Levy, R.C., Gatica, M. (2011) Glacial flour dust storms in the Gulf of Alaska: Hydrologic and meteorological controls and their importance as a source of bioavailable iron. Geophysical Research Letters, 8, L06602.
  3. Hansen, K. (2012, August 2) Dust dominates foreign aerosol imports to North America. NASA. Accessed November 12, 2012.
  4. Holmes, H. (2001) The Secret Life of Dust. John Wiley and Sons, Inc., Hoboken, New Jersey.
  5. Wang, J., Jin, M., Musgrave, D.L., Ikeda, M. (2004) A hydrological digital elevation model for freshwater discharge into the Gulf of Alaska. Journal of Geophysical Research, 109, C07009.
NASA image courtesy Jeff Schmaltz, LANCE MODIS Rapid Response Team at NASA GSFC. Caption by Michon Scott.

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