Paper No. 8
Presentation Time: 10:30 AM
SOIL GEOMORPHOLOGICAL STUDIES OF HILLSLOPES IN SEMIARID REGIONS IN ARIZONA AND NEW MEXICO: INFLUENCES OF CLIMATE CHANGE AND ROCK TYPE
Soil-geomorphic research in study areas in the southwestern USA elucidates the nature of the impacts of climate change and rock type on soils and hillslope-forming processes in semiarid regions. In the NE Arizona study area, rapid weathering of Jurassic sandstone, rapid downslope diffusive transport and minimal dust accumulation favor formation of weakly developed soils (A-C-Cr-R) on mainly north-facing, vegetation-mantled transport-limited hillslopes. Hillslopes with mostly southern aspects and a more xeric topoclimate are generally steeper and detachment-limited, with thinner, discontinuous soils and/or exposed bedrock. Hillslopes with intermediate aspects have properties transitional between these end-member hillslope forms; spatial contrasts in vegetation communities define these transitional areas as ecotones. Spatial changes in vegetation, soils, and hillslope form in the ecotone show that the ecotone is not static, but instead is dynamically evolving. We hypothesize that these changes are primarily a response to Holocene climate change and represent the past and likely ongoing transformation of transport- to detachment-limited hillslopes in this region. Studies of hillslopes associated with Miocene sandstones in northern New Mexico also show similar aspect-related hillslope forms, but the contrasts in hillslope form also reflect geologically recent changes in base level that amplify aspect-related differences in a soil-slope-vegetation system close to threshold conditions.
Detachment-limited granite corestone hillslopes dominate the semiarid footslope areas of the Sandia Mountains in central NM. Transport-limited hillslopes associated with resistant aplitic colluvium, however, favor sustained dust entrapment and the formation of thick well developed soils (A-Bt-Btk-Crt-R) by accretion- and inflation-generated profile development. The transport-limited hillslopes in all of the study areas are similar with respect to their continuous soil mantles and vegetation, but the processes responsible for their formation profoundly differ. These studies emphasize the critical role rock type plays with respect to rates, processes and magnitude of soil development on hillslopes with otherwise generally similar local relief and vegetation in a semiarid climate.