When glaciers grind against underlying bedrock, they produce a
silty powder with grains finer than sand. Geologists call it “glacial flour” or
“rock flour.” This iron- and feldspar-rich substance often finds its ways into
rivers and lakes, coloring the water brown, grey, or aqua. When rivers or lake
levels are low, the flour accumulates on drying riverbanks and deltas, leaving
raw material for winds to lift into the air and create plumes of dust.
That’s what was happening on October 23, 2012, when the Moderate
Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite acquired this image of
northerly winds blowing a plume of dust from the Copper River delta out over
the Gulf of Alaska. Large dust storms like this are most common in Alaska in
the fall, when river levels are at their lowest. Satellites have observed similar events in recent years.
This dust storm was first noticeable in MODIS imagery on October
21 and continued through October 25. Though the plume extended for more than a
hundred miles, it did not appear thick in comparison to some produced during
previous events. A two-week event in 2006, for instance, which
researchers described in detail in a paper inGeophysical Research
Letters, produced
large plumes that lofted between 30 and 80 kilotons of glacial flour into the
atmosphere.
Since 2011, as part of a project to improve understanding of
arctic dust storms and validate satellite observations, a team of NASA-funded scientists have been monitoring filtered air
samples from a site on Middleton Island, a small island in the Gulf of Alaska.
Scientists monitor Arctic dust for a number of reasons. The storms can reduce
visibility enough to disrupt air travel, and they can also pose health hazards
to people on the ground when they occur inland. The dust is also a key source
of iron for phytoplankton, whose growth is limited by the
availability of minerals and nutrients. Finally, there is the possibility that
dust events are becoming more frequent and severe due to ongoingrecession of glaciers in coastal Alaska.
· References
Crusius, J. (2011, July 10) 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.
· Gasso, S. (n.d.) It’s
not all from Asia: Satellite observations of dust transport of Alaskan dust
into the North Pacific (pdf).
هیچ نظری موجود نیست:
ارسال یک نظر