HR: 15:25h
AN: V13F-08    [Abstracts]
TI: The Fingerprint of Present and Past Rainfall on Soil Geochemistry
AU: * Amundson, R
EM: earthy@nature.berkeley.edu
AF: Ecosystem Sciences, 137 Mulford Hall, University of California, Berkeley, CA 94720,
AU: Owen, J
AF: Ecosystem Sciences, 137 Mulford Hall, University of California, Berkeley, CA 94720,
AU: Ewing, S
AF: Ecosystem Sciences, 137 Mulford Hall, University of California, Berkeley, CA 94720,
AU: Ewing, S
AF: Earth and Planetary Science, McCone Hall, University of California, Berkeley, CA 94720,
AU: Nishiizumi, K
AF: Space Sciences Laboratory, University of California, Berkeley, CA 94720,
AU: Finkel, R
AF: Earth and Planetary Science, McCone Hall, University of California, Berkeley, CA 94720,
AU: Chadwick, O
AF: Department of Geography, University of California, Santa Barbara, CA 93106,
AU: Dietrich, W
AF: Earth and Planetary Science, McCone Hall, University of California, Berkeley, CA 94720,
AB: Research conducted in many locations show that soil weathering rates vary with time and environmental conditions. Here, we assemble long-term (10⁵ to 10⁶ y) chronosequence studies of soil chemistry in sites varying in MAP from ~1 to ~4000 mm y^-1 to examine how the rate and magnitude of chemical weathering varies with climate, particularly at the dry end of the spectrum. In humid, vegetated landscapes, soil chemical weathering generally releases an array of rock forming elements that are removed via leaching, causing a subsequent mass loss and volumetric collapse. The rate of this process is non-linear, with instantaneous weathering rates declining greatly with time. In general, the rates and magnitude (for a soil of a given age) of chemical weathering decline greatly with decreasing rainfall. At the arid/hyperarid boundary, where rainfall decreases to the point that biota are essentially absent, chemical weathering nearly ceases, retention of atmospheric solutes and dust increases, and soil chemistry becomes mainly inorganic. The rates of mass gain and volumetric expansion in extremely hyperarid regions is hypothesized to be linear, differing from the non- linearity of processes in more humid regions. While the "fingerprint" of rainfall on soil properties is greatly magnified with increasing soil age, ancient soils commonly bear the imprint of multiple climate changes. In humid regions, detecting these changes in soil chemistry is difficult, whereas an increase in aridity, and a shift from net mass loss to net mass gain, produces a unique geochemical signal, and additionally preserves the weathering signal that occurred during the earlier pluvial episode. In two ancient (Miocene) well-preserved landscapes along a modern (and ancient) rainfall gradient in the Atacama Desert, the geochemical signal of climate change is clearly evident. Paleoclimate reconstructions suggest that northern Chile was under a permanent El Nino-like condition until the late Pliocene, and since that time has generally experienced its present condition of cold upwelling offshore waters and corresponding aridity. Presently, the soils are in arid to hyperarid regions where the main geochemical process is sulfate (north) and carbonate (south) retention, and the soils show enormous gains of these solutes. In stark contrast, the silicate matrix of the soils shows elemental losses of Si, Al, and Fe (and clay formation) that increase with southerly distance (and rainfall), illustrating that past (pre-late Pliocene) geochemical processes differed greatly from modern conditions. These starkly different geochemical signatures (salts over a weathered silicate matrix) reflect soil polygenesis, and have significant implications for Mars soils which contain both weathered silicates and secondary phyllosilicates, and a later stage overlay of sulfates and chlorides.
DE: 0486 Soils/pedology (1865)
DE: 1030 Geochemical cycles (0330)
DE: 6225 Mars
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting