HR: 09:00h
AN: H41H-05    [Abstracts]
TI: The Topographic Control of Chemical Weathering in Hillslope Soils
AU: * Yoo, K
EM: kyoo@nature.berkeley.edu
AF: Division of Ecosystem Sciences, University of California, Berkeley, 151 Hilgard Hall, Berkeley, CA 94720 United States
AU: Amundson, R
EM: earthy@nature.berkeley.edu
AF: Division of Ecosystem Sciences, University of California, Berkeley, 151 Hilgard Hall, Berkeley, CA 94720 United States
AU: Heimsath, A M
EM: Arjun.Heimsath@Dartmouth.EDU
AF: Department of Earth Sciences, Dartmouth College, 6105 Fairchild Hall, Hanover, NH 08755 United States
AU: Dietrich, W E
EM: bill@geomorph.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, McCone Hall, Berkeley, CA 94720 United States
AU: Brimhall, G H
EM: brimhall@eps.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, McCone Hall, Berkeley, CA 94720 United States
AB: Chemical weathering drives biogeochemical cycles from local to global scales, and has the power to regulate the earth's climate on geological time scales. However, little is known of the spatial variation in weathering on hillslopes, and the mechanisms behind those variations. This study addresses the topographic control on soil chemical weathering on convex uplands. We developed a process-based mass balance model that integrates chemical mass losses with physical sediment transport. We applied the model along a $\sim$60 meter long, semi-arid eucalyptus-grassland savanna hillslope underlain by granodiorite, in the southwestern Australian Highlands. Measurements of soil elemental chemistry, cosmogenic isotope-based saprolite-to-soil conversion rates, and a fine scale topographic survey provided model data. The soil weathering rates varied from the losses of $\sim$35 g m$^{-2}$ yr$^{-1}$ on the ridge to the net gains of $\sim$28 g m$^{-2}$ yr$^{-1}$ at the lowest portion of the slope. A net chemical mass loss occurred in all soils along the entire slope, decreasing from $\sim$65 % near the convex ridge to $\sim$35 % at the base of the slope. The mechanism for the apparent discrepancy between spatially constrained weathering rates and net weathering losses (relative to saprolite) is that as sediment moves faster with an increasing slope gradient in the downslope direction, soils eroded from upslope positions pass quickly through the downslope zones of chemical gains, which are able to only partially replenish the pre-weathered soil material. Differences in chemical mobility of elements, and biological nutrient demand, significantly modified the spatial redistribution of elements released by weathering: P and Ca, relative to Si, Al, and Fe, were preferentially retained, particularly within an apron of relatively high fertility that mantled the hillslope base. Finally, when chemical weathering losses were subtracted from the overall sediment mass balance, the slope-dependent physical soil transport rate was reduced by half, and the resulting physical transport was found to increase nonlinearly with increasing slope gradients. In conclusion, this study integrates chemical weathering and physical transport on convex uplands, complementing recent watershed scale analyses and providing a spatial perspective of both weathering and elemental redistribution on hillslopes that have applications to ecology as well as geochemistry.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1886 Weathering (1625)
DE: 1615 Biogeochemical processes (4805)
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting