HR: 0830h
AN: B21F-0783    [PDF]
TI: Aerosol Deposition to Hyperarid Soils of the Atacama Desert
AU: * Ewing, S A
EM: saewing@nature.berkeley.edu
AF: University of California at Berkeley, Ecosystem Science Division, ESPM 151 Hilgard Hall #3110, Berkeley, CA 94720 United States
AU: Stewart, B W
EM: bstewart@pitt.edu
AF: University of Pittsburgh, Department of Geology and Planetary Science 200 SRCC, Pittsburgh, PA 15260 United States
AU: Kendall, C
EM: ckendall@usgs.gov
AF: United States Geological Survey, 345 Middlefield Road MS 434, Menlo Park, CA 94025 United States
AU: McKay, C P
EM: cmckay@arc.nasa.gov
AF: NASA-Ames, Space Science Division Mail Stop 345-2, Moffett Field, CA 94035 United States
AU: Amundson, R G
EM: earthy@nature.berkeley.edu
AF: University of California at Berkeley, Ecosystem Science Division, ESPM 151 Hilgard Hall #3110, Berkeley, CA 94720 United States
AB: The influence of atmospheric deposition in most soils is difficult to establish due to leaching and biogeochemical transformations. In contrast, the soils of the Atacama Desert in northern Chile reflect a long-term history of atmospheric deposition that is arguably not preserved elsewhere on Earth. In order to determine the mass and origin of salts accumulated, we present isotopic and chemical data for soils and aerosols along a climate gradient from the extreme hyperarid core of the Atacama ($<$ 5 mm y$^{-1}$), to sites that receive slightly more rain (up to 15 mm y$^{-1}$). In the driest region, accumulation of aerosol-derived salts has caused nearly 500$%$ volumetric expansion of the soil. Here we focus on the origin of two of the most important salts: gypsum (calcium sulfate) and sodium nitrate. Water-soluble $^{87}$Sr$/$$^{86}$Sr decreases with depth, indicating changing sources of sulfate-associated Ca. Sulfate $\delta$$^{34}$S also decreases with depth, indicating either isotopic fractionation with downward transport or changing sources of sulfate with time. Water-soluble $^{87}$Sr$/$$^{86}$Sr values are higher than those reported for local granitic rocks, as might be expected with very limited weathering, and lower than the value reported by Rech et al. (2003) for pedogenic gypsum at a nearby site. While near-surface sulfate $\delta$$^{34}$S values imply a marine component, our observed $^{87}$Sr$/$$^{86}$Sr values fall within the range observed for sulfate salts from salars to the east, suggesting that water-soluble soil Ca contains a significant continental component that may increase or vary in origin with depth in the profile (and thus distance of transport). Nitrate concentrations and nitrate $\delta$$^{15}$N values increase with depth, as would be predicted by a advection$/$reaction model of downward nitrate transport. In soils with increasing precipitation, overall nitrate concentrations are dramatically reduced and overall sulfate concentrations decrease, occurring at greater soil depths. In aerosol samples, nitrate is present at concentrations significantly greater than in sea water or in the remote marine boundary layer of the tropical South Pacific, suggesting formation of nitrate from NO$_{x}$ on sea-salt aerosol. Aerosol sulfate concentrations indicate significant non-sea-salt sulfate, thought to be produced by oxidation of dimethyl sulfide or SO$_{2}$. We consider marine upwelling as a possible source of N- and S- containing trace gases, and thus nitrate and sulfate in aerosols and soils, in this remote area.
DE: 0305 Aerosols and particles (0345, 4801)
DE: 0315 Biosphere/atmosphere interactions
DE: 0330 Geochemical cycles
DE: 1040 Isotopic composition/chemistry
DE: 1615 Biogeochemical processes (4805)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting