Dust layers deposited to snow cover of the Wasatch Range (northern Utah) in 2009 and 2010 provide rare samples to determine the relations between their compositions and radiative properties. These studies are required to comprehend and model how such dust-on-snow (DOS) layers affect rates of snow melt through changes in the albedo of snow surfaces. We evaluated several constituents as potential contributors to the absorption of solar radiation indicated by values of absolute reflectance determined from bi-conical reflectance spectroscopy. Ferric oxide minerals and carbonaceous matter appear to be the primary influences on lowering snow-cover albedo. Techniques of reflectance and Mössbauer spectroscopy as well as rock magnetism provide information about the types, amounts, and grain sizes of ferric oxide minerals. Relatively high amounts of ferric oxide, indicated by hard isothermal remanent magnetization (HIRM), are associated with relatively low average reflectance (<0.25) across the visible wavelengths of the electromagnetic spectrum. Mössbauer spectroscopy indicates roughly equal amounts of hematite and goethite, representing about 35% of the total Fe-bearing phases. Nevertheless, goethite (α-FeOOH) is the dominant ferric oxide found by reflectance spectroscopy and thus appears to be the main iron oxide control on absorption of solar radiation. At least some goethite occurs as nano-phase grain coatings less than about 50. nm thick. Relatively high amounts of organic carbon, indicating as much as about 10% organic matter, are also associated with lower reflectance values. The organic matter, although not fully characterized by type, correlates strongly with metals (e.g., Cu, Pb, As, Cd, Mo, Zn) derived from distal urban and industrial settings, probably including mining and smelting sites. This relation suggests anthropogenic sources for at least some of the carbonaceous matter, such as emissions from transportation and industrial activities. The composition of the DOS samples can be compared with sediments in a likely dust-source setting at the Milford Flat Fire (MFF) area about 225. km southwest of Salt Lake City. The MFF area represents geologically and physiographically similar and widespread dust sources west-southwest of the Wasatch Range and heavily populated Wasatch Front. The DOS layers and MFF sediments are similar in some textural, chemical, and magnetic properties, as well as in the common presence of goethite, hematite, magnetite-bearing basalt fragments, quartz, plagioclase, illite, and kaolinite. Textural and some chemical differences among these deposits can be explained by atmospheric sorting as well as by inputs from other settings, such as salt-crusted playas and contaminant sources.
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We are grateful to Jeff Deems and two anonymous reviewers for improving this manuscript. For motivating discussions and assistance in the field, we thank Marith Reheis and Jim Yount, and for laboratory analyses, we thank Jiang Xiao, Gary Skipp, and Eric Fisher. We are also grateful to Roger Clark and Todd Hoeffen for use of the USGS Spectroscopy Laboratory, Denver, and to Subir Banerjee for critical discussion of the magnetic and Mössbauer results. Collection of the dust-on-snow samples was part of a joint University of Utah-U.S. Forest Service project. Support was provided by U.S. Geological Survey’s Climate and Land Use Change Program and (for MEK) by NSF CHE-0118604 and the Arizona Technology Research and Innovation Fund for the ICPMS lab facilities. Part of this work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. The Institute for Rock Magnetism is supported by grants from the Instruments and Facilities Program of the National Science Foundation Division of Earth Science. This is IRM Contribution 1302.
© 2013 Published by Elsevier B.V.
- Atmospheric dust
- Black carbon
- Magnetic properties