HR: 0800h
AN: H41D-0758    [Abstracts]
TI: Mining soil survey databases to explore lithologic, climatic and topographic controls on hillslope production of bedload-size rock fragments
AU: * Marshall, J A
EM: jillgeo@sfsu.edu
AF: Department of Geosciences, San Francisco State University, 1600 Holloway Ave., San Francisco, CA 94132, United States
AU: Sklar, L S
EM: leonard@sfsu.edu
AF: Department of Geosciences, San Francisco State University, 1600 Holloway Ave., San Francisco, CA 94132, United States
AB: The grain size distribution of sediments supplied by hillslopes to channel networks may strongly influence landscape dynamics at both the long time scale of landscape evolution and the short time scale of channel response to changes in land use. Little is known, however, about how lithology, climate, and the processes and rates of sediment production and transport on hillslopes, control the grain size distribution supplied to channels. A wealth of soil size distribution measurements have been collected and archived by state and local agencies, which to our knowledge have not been systematically examined to uncover patterns in the mass fraction and size distribution of rock fragments large enough to move as bedload in rivers. Here we report results obtained from data mining soil survey archives in the western United States. To focus on hillslopes with active sediment production, we exclude data from soil pits dug in valley bottoms, terraces and other topographic settings where long-term net accumulation is likely. Among the many questions that can be explored with these data, we are interested in how the mass fraction of rock fragments (>2mm) and their size distribution, averaged over the soil column, varies with factors such as depth-to-bedrock, hillslope gradient, lithology, and mean annual temperature and precipitation. We are also looking for variations with depth within the soil column, that might provide insight into soil disturbance, transport, and weathering regimes. Several general patterns emerge from our work to-date. A large fraction of hillslope soil pedons do not contain any rock fragments, with lithology possibly the dominant control. Where the rock fragment mass fraction is greater than about 0.3, there is commonly an increase in rock fragment concentration with depth within the soil column. Pedons with lower depth-averaged rock fragment concentrations more commonly show no gradient in concentration with depth, suggesting that the disturbance processes that produce well-mixed soils are effective at mechanically weathering rock fragments. Steeper hillslope gradients correlate with greater rock fragment concentrations, but surprisingly we see no significant correlation with depth-to-bedrock. In general, rock fragments constitute a mode distinct from the sand-sized and finer portion of the bulk soil size distribution, supporting the use of bi-modal distributions in modeling of sediment supply to channels.
DE: 1625 Geomorphology and weathering (0790, 1824, 1825, 1826, 1886)
DE: 1815 Erosion
DE: 1826 Geomorphology: hillslope (1625)
DE: 1865 Soils (0486)
DE: 1886 Weathering (0790, 1625)
SC: Hydrology [H]
MN: 2007 Fall Meeting