Paper No. 6
Presentation Time: 3:40 PM
CHARACTERIZATION OF IRON MINERALS IN ATMOSPHERIC DUST
REYNOLDS, Richard1, GOLDSTEIN, Harland L.
2, MOSKOWITZ, Bruce
3, BERQUÓ, Thelma
4, LANDRY, Chris
5, FLAGG, Cody
2, KOKALY, Raymond F.
6, REDSTEER, Margaret
7, MUNSON, S.
1 and PAINTER, Thomas H.
8, (1)U.S. Geological Survey, Federal Center, Denver, CO 80225, (2)United States Geological Survey, Denver Federal Center, MS-980, Denver, CO 80225, (3)Institute for Rock Magnetism, Univ. of Minnesota, Minneapolis, MN 55455, (4)Concordia College, Moorhead, MN 56562, (5)Center for Snow and Avalanche Studies, Silverton, CO 81433, (6)United States Geological Survey, Denver Federal Center, MS-964, Denver, CO 80225, (7)U.S. Geological Survey, 2255 N. Gemini Drive, Flagstaff, AZ 86001, (8)Jet Propulsion Laboratory/Caltech, 4800 Oak Grove Drive, Pasadena, CA 91109, rreynolds@usgs.gov
Ferric oxide minerals are important constituents in atmospheric dust because of their capacity to absorb solar radiation, stimulate marine phytoplankton productivity, and affect human health. Such capacity varies depending on iron mineral type, size, surface area, and solubility. Despite their global importance, these iron minerals remain poorly described in atmospheric dusts, downwind aeolian sediments, and their source areas with respect to specific mineralogy, chemistry, particle size and surface area, or presence in far-traveled aerosol compounds. Our studies analyze both dust and source-area samples using techniques of rock magnetism (measurements at 5-300 K), Mössbauer and high-resolution visible and near-infrared reflectance spectroscopy, chemistry, and electron microscopy to observe directly the ferric oxide coatings and particles.
Dust deposited to snow cover of the San Juan Mountains (Colorado) and Wasatch Mountains (Utah) was used to characterize dust composition compared with properties of sediments exposed in source-areas that were identified from satellite retrievals. Preliminary results from multiple methods indicate that nanohematite is the dominant ferric oxide in most dust-on-snow (DOS) samples (collected 2005-2010) from the San Juan Mountains and in many redbed-derived dust sources in upwind regions. Goethite in some San Juan Mountain DOS is probably derived from source-area sediments from Cretaceous marine deposits. In contrast, goethite appears to be the dominant ferric oxide in Wasatch Mountains DOS collected in 2010. Goethite-bearing Late Pleistocene lake sediments in west-central Utah have been identified as the dominant dust sources for these DOS layers from satellite retrievals. The distinction between hematite and goethite is useful for modeling radiative forcing by dust in mountain snow and ice and in the atmosphere. In addition, the relative abundance of ferric oxides, with varied surface area and solubility values, may influence the overall reactivity of dust particles during atmospheric transport and after deposition. For example, chemisorption of metal ions and oxyanions with ferric oxides has been widely documented in soils and such reactions could explain metal enrichments often observed in dusts.
