HR: 1340h
AN: B23D-1596    [Abstracts]
TI: Small Degree of Trace Element Enrichment in Soils not Surprising, in View of Measured Deposition by Dusts
AU: * Hinkley, T
EM: thinkley@usgs.gov
AF: U.S. Geological Survey, Denver Federal Center, Denver, CO 80225, United States
AU: Lamothe, P J
EM: plamothe@usgs.gov
AF: U.S. Geological Survey, Denver Federal Center, Denver, CO 80225, United States
AU: Meeker, G P
EM: gmeeker@usgs.gov
AF: U.S. Geological Survey, Denver Federal Center, Denver, CO 80225, United States
AU: Xiao, J
EM: xiaoj@usgs.gov
AF: U.S. Geological Survey, Denver Federal Center, Denver, CO 80225, United States
AB: Atmospheric dust deposited onto the surface of the land is enriched in trace elements (e.g., Zn, Cu, Pb, Cd, others). That is, trace elements in the dust have greater abundances than can be accounted for by the expected amounts in the crystals of the minerals of which the dust is composed. This has been true since pre-industrial times. Substantial excess masses of trace elements are currently deposited to the land surface in the U.S. Southwest (e.g., in ug/m2/yr: Cu, 160; Zn, 500; As, 15; Sb, 5; Pb, 60; Bi, 2). If these annual excess masses were newly arriving from some distant source and accumulating in the local soils, times on the order of 50Ky would be required to double the trace element concentrations in a 10 cm thickness of surficial material (soil) of crustal composition. But much of the trace element mass coming with dust is recycled, through both local and distant soils, and some is exported (to oceans) during recycling. Therefore times required for doubling of concentrations are longer, and any trace element enrichments in soils (elevations of concentration above crustal levels) must have begun to accumulate long before modern industrial times. It is therefore not surprising that the few available data on trace elements in soils show that enrichments are small ("enrichment factors" in non-quartz fraction of some Utah soils (Goldstein et al, 2005) are: As, 6; Pb, 3; Sb, 6, Tl, 1.2; Zn, 1.6; Cd, 1.6, Cu. 1.5). Several natural sources of supply of excess of trace elements to the atmospheric dusts (and hence to soils) have been proposed: volcano emissions, plants, sea surface slimes, preferential weathering of rocks. But only the volcanic source has been documented and quantified (Hinkley et al., 1999; Matsumoto and Hinkley, 2001). We note that the present rate of apparent excess deposition in the Southwest is about 10-100 times the present rate of worldwide emission to the atmosphere through degassing by volcanoes. Considering the long times available for accumulation, and that modern measured deposition is recycled through the soil reservoir and the atmosphere, the volcanic source may have sufficient strength to be a main ultimate source of the trace element enrichment observed in atmospheric dust.
DE: 0330 Geochemical cycles (1030)
DE: 0370 Volcanic effects (8409)
DE: 0486 Soils/pedology (1865)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting