HR: 1340h
AN: H43C-1507    [Abstracts]
TI: Soil and Solution Based Assessments of Weathering along a Hillslope Transect in Coastal California
AU: * Yoo, K
EM: kyoo@udel.edu
AF: University of Delaware Plant and Soil Sciences Department, 531 S. College Av. 152 Townsend Hall, Newark, DE 19716-2170, United States
AU: Sanderman, J
EM: jsandman@nature.berkeley.edu
AF: Ecosystem Sciences Division Department of Environmental Science, Policy and Management University of California, Berkeley, 137 Mulford Hall – MC3114, Berkeley, CA 94720, United States
AU: Mudd, S M
EM: simon.m.mudd@ed.ac.uk
AF: School of GeoSciences University of Edinburgh, The King's Buildings, Edinburgh, EH9 3JW, United Kingdom
AU: Amundson, R
EM: earthy@nature.berkeley.edu
AF: Ecosystem Sciences Division Department of Environmental Science, Policy and Management University of California, Berkeley, 137 Mulford Hall – MC3114, Berkeley, CA 94720, United States
AB: Understanding the genesis of hillslope soils is challenging. They are the products of geomorphic, hydrologic, and geochemical processes that are interacting among themselves and are affected by the soils they shape. Our goal is to mechanically and quantitatively integrate the soil production and transport, chemical weathering of minerals, and solute fluxes with the observed topographic variation of soil elemental compositions. We studied a grass covered hillslope in coastal California where geomorphic processes of soil production and transport are well characterized. The parent material is clay-rich sandstone and bioturbation drives the slope- dependent soil transport. 10 sites were excavated to the depth of 10-20 cm beyond the soil-saprolite boundary, and soil and saprolite samples were collected for total chemical analyses of major elements. Soil solution was sampled throughout the year at multiple depths at 6 hillslope locations, along with outflow stream samples. We report that long term weathering rates, determined by the enrichment of weathering resistant element (Zr), are approximately proportional to soil production rates, whereas solute measurements indicate fastest chemical weathering rate near the toeslope where the soil production rate is lowest due to the thick soils. In saprolite, the Zr enrichment increased with increasing soil thickness, a trend consistent with the fact that soil production rate declines with soil thickness. If we assume mineral residence time in the saprolite increases with a decreasing conversion rate of saprolite to soil, the saprolite residence time may explain the greater degrees of saprolite chemical alteration in the depositional slope. Comparisons of elemental compositions of the soils and saprolite suggest that less than 10 % of the original saprolite mass has been lost via dissolution and leaching during the soil formation. Despite the mass losses of most elements, phosphorous and calcium appear to be selectively retained within the soils. Along the hillslope, the fractional mass loss only slightly increases in the downslope direction, indicating the balance between the rates of dissolution mass loss and soil production as the soil material moves downslope. This finding contrasts with the solute flux measurements. Greater mineral-water contact times and water flux in the lower slope positions combined to result in short-term chemical weathering loss rates (mass per volume of soil per time) that are greater than at the upper slope positions. This discrepancy may occur because colluvial flux increases in the downslope direction; the soils in the downslope direction quickly pass through the zone of highest chemical weathering rate such that the fractional mass losses of the soils increase only slightly. Additionally, at any given slope position, short-term weathering rates from solute fluxes increased with both soil temperature and soil CO2 concentrations. Because the colluvial flux increases in the downslope direction, and data for both long term and annual to storm timescale weathering fluxes are available, this study site allows us to test whether chemical weathering rates are controlled primarily by hydrologic processes or mineral residence time (see abstract by Mudd, Yoo, and Sanderman, this session).
DE: 0330 Geochemical cycles (1030)
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0486 Soils/pedology (1865)
DE: 1826 Geomorphology: hillslope (1625)
DE: 1886 Weathering (0790, 1625)
SC: Hydrology [H]
MN: 2007 Fall Meeting