HR: 11:40h
AN: H52B-06    [Abstracts]
TI: Processes Influencing Regolith Development on Bodmin Moor, UK
AU: * Riggins, S G
EM: Susan.Riggins@colorado.edu
AF: Institute of Arctic and Alpine Research (INSTAAR), 1560 30th Street, Boulder, CO 80303, United States
AU: * Riggins, S G
EM: Susan.Riggins@colorado.edu
AF: Department of Geography, University of Colorado, Boulder, CO 80309-0260, United States
AU: Tye, A M
EM: atye@bgs.ac.uk
AF: British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham, NG12 5GG, United Kingdom
AU: Anderson, S P
EM: Suzanne.Anderson@colorado.edu
AF: Institute of Arctic and Alpine Research (INSTAAR), 1560 30th Street, Boulder, CO 80303, United States
AU: Anderson, S P
EM: Suzanne.Anderson@colorado.edu
AF: Department of Geography, University of Colorado, Boulder, CO 80309-0260, United States
AU: Smith, B
EM: bsmi@bgs.ac.uk
AF: British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham, NG12 5GG, United Kingdom
AB: Regolith is disaggregated, transportable material above bedrock created by physical processes such as fracture generation, frost cracking, tree throw, and animal burrowing and chemical processes such as dissolution and mineral transformation. Models typically parameterize the rate of regolith generation as a function of its thickness, without specifying the physical and chemical processes involved. This study examines a non-glaciated, granitic hillslope on Bodmin Moor, southwest England, UK, to identify processes creating regolith. The site has a temperate climate with relatively high mean annual temperature (MAT,~10°C) and rainfall (1381- 2580mm). We dug five soil pits ~80cm deep along a 120 m transect to examine the saprolite-regolith interface and extract solid phase and soil pore water samples. We find no evidence of physical processes such as frost cracking or bioturbation creating regolith. The site is treeless with vegetation dominated by grass. Although the site experiences ground frost and has sufficient water content to support frost cracking, a model of freezing front penetration indicates that the saprolite-regolith boundary does not fall within the frost cracking window of -3 to -10°C. A decrease in MAT of ~3°C would lower the depth of frost penetration to the current saprolite-regolith boundary (80cm). We speculate that frost cracking may have operated during the periglacial conditions of the Quaternary and would have pre-conditioned the bedrock for chemical processes currently operating. We use soil pore water and solid phase chemistry to determine the chemical processes at work. Pore water concentrations of K (4.5 mg/L), Na (14.2 mg/L), SO₄^2- (24.9 mg/L), Mg (1.7 mg/L), and Ca (1.7 mg/L) are highest in O-horizons and decrease as a function of distance from the surface, indicating that biological processes have a strong effect on geochemical cycling in the upper part of the soil. Soil water at greater depths within the regolith are likely under-saturated and able to transform saprolite into regolith via dissolution. A mass balance approach is used on the solid phase chemical data to quantify mass gains and losses. We calculate τ, the percent mass change of an element relative to parent material, using Zr as an immobile element. Na shows high mass losses (-0.84) regardless of landscape position. The τ values for K, Al, Mg, and Mn are dependent on landscape position with higher mass losses upslope (K -0.78, Al -0.67, Mg -0.35, and Mn -0.56) and lower mass losses or mass gains downslope (K -0.18, Al 0.24, Mg 0.43, and Mg 0.70). Quantitative mineralogy shows that mineral transformations occur across the regolith-saprolite boundary. Feldspar percentages decrease as a function of distance above the boundary while clay percentages increase. Chemical processes currently dominate the transformation of saprolite into disaggregated and transportable regolith at Bodmin Moor. We speculate that loss of feldspar and formation of clay weakens the material enough to render it susceptible to transport by soil creep. We cannot presently eliminate the possibility, however, that frost cracking was important in the evolution of the regolith in the Quaternary.
DE: 1039 Alteration and weathering processes (3617)
DE: 1065 Major and trace element geochemistry
DE: 1826 Geomorphology: hillslope (1625)
DE: 1886 Weathering (0790, 1625)
SC: Hydrology [H]
MN: 2007 Fall Meeting