HR: 0800h
AN: H51C-0374 [Abstracts]
TI: Resisting Erosion: Quantifying Controls on Soil Production
AU: * Byersdorfer, J P
EM: joel.byersdorfer@dartmouth.edu
AF: Dartmouth College, 6105 Fairchild Hall, Hanover, NH 03755
AU: Burke, B C
EM: Benjamin.C.Burke@Dartmouth.EDU
AF: Dartmouth College, 6105 Fairchild Hall, Hanover, NH 03755
AU: Heimsath, A
EM: Arjun.Heimsath@Dartmouth.EDU
AF: Dartmouth College, 6105 Fairchild Hall, Hanover, NH 03755
AU: Dade, B
EM: William.B.Dade@Dartmouth.edu
AF: Dartmouth College, 6105 Fairchild Hall, Hanover, NH 03755
AB:
Soil production from a bedrock surface is influenced by climate, mineralogy, seismic activity, and glacial history. This
study examines the relationships between chemical weathering, soil production rates, and new observations of physical
strength and, microscopic weathering textures of saprolite in granitic terrain. Our study areas are soil-mantled hillslopes
on two well-studied geographically disparate sites: Points Reyes National Seashore, CA and Nunnock River, Australia. Soil
production rates at both sites were determined with in situ produced cosmogenic nuclides and decrease exponentially with
increasing soil thickness. We hypothesize that saprolite strength should i) constrain soil production rates, and ii)
diminish with degree of alumino-silicate weathering. Until now, these ideas have not been systematically tested.
To measure the physical competency of the saprolite we use conventional shear vane and dynamic cone penetrometers tests. We
find that soil production rates and resistance to penetration each exhibit maximal values at the hillcrest and diminish with
increasing soil cover and distance downslope as the soil thickness increases. Conversely, saprolite shear strength increases
under thicker soil profiles that accompany lower soil production rates and decreasing values of Kr ratio (SiO2/Al2O3 +
Fe2O3). We speculate that the contrasting decrease in penetration resistance and increasing shear strength of saprolite is
due to accumulation of clays with increasing residence time at the base of the soil column. The mechanistic significance of
saprolite strength as a constraint on soil production depends on whether the soil producing processes are dominated by
penetration (e.g. tree roots) or shearing forces (e.g. burrowing mammals).
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
DE: 1625 Geomorphology and weathering (0790, 1824, 1825, 1826, 1886)
DE: 1815 Erosion
SC: Hydrology [H]
MN: Fall Meeting 2005