DUST DEPOSITED ON COLORADO SNOW COVER: EFFECTS, COMPOSITIONS, AND CONNECTIONS TO DUST-SOURCE AREAS

Dust deposited on snow (DOS) in Colorado mountains has gained increased attention because of its dramatic effects on altering snow albedo and increasing absorption of sunlight, resulting in early melting of snow cover and earlier runoff. DOS frequency and intensity has been increasing in the Colorado mountains over the last decade. A large DOS event took place April, 2013 depositing about 138 metric tons of dust to Senator Beck Basin Study Area (291 hectares) in the San Juan Mountains (SJM, southwest Colorado). More dust was deposited during this single event than was deposited in total during the prior two years combined.

Compositions of DOS and their connections to dust-source areas can provide an understanding of dust-optical properties and associated impacts on snow cover. Attributing deposited dust to dust-source areas can be achieved through remote sensing and back-trajectory analyses. These analyses indicate that many DOS layers in the SJM are derived from the Four Corners region of the Western United States. Sources within this region can be more finely discriminated through comparisons of physical, chemical, isotopic, and mineralogical properties. Diagnostic and unique mineralogical associations were found from scanning electron microscopy, reflectance spectroscopy, Mössbauer spectroscopy, and magnetic property measurements. For example, identification of types and sizes of iron oxide minerals shows great promise for associating SJM DOS to source areas. The dominant iron oxide in different dust-source sediments across the Four Corners region is either hematite or goethite. Each of these minerals has exceptionally strong capacity to absorb solar radiation. Variable proportions of hematite and goethite in SJM DOS reflect relative contributions from different sources. Among other diagnostic components of DOS are particles resulting from coal mining, processing, and combustion, as well as minerals diagnostic of uncommon rock types in source areas. These results provide compositional linkage between DOS and dust sources, improve understanding of the effects of dust on melting snow cover, and bear broadly on land and water management in the southwest United States.