ASPECT, HILLSLOPE PEDOGENESIS, AND THE EMERGENCE OF TOPOGRAPHIC ASYMMETRY

Topographic asymmetry of catchments draining the western slope of the Sangre de Cristo Mountains, Colorado, in Great Sand Dunes National Park and Preserve provides a natural setting for studying the effects of microclimate from the critical zone to the landscape scale. Our paired study of landscape evolution and pedogenesis evaluates the emergence of topographic asymmetry at Mosca Creek, where climate is similar throughout the drainage (58 cm MAP, 4°C MAT) but aspect-related differences in microclimate are clearly displayed by dense fir/aspen forests on wetter north-facing slopes and pine woodland with brush understory on drier south-facing slopes. Hillslope soils were described and sampled from hand-dug pits at three sites along each of three north- and south-facing paired transects, at elevations ranging from 2930 m to 3075 m, at sites where underlying bedrock is metamorphosed granite. In the uppermost pit on each transect, a soil temperature/moisture monitoring station was installed to record in situ conditions, and samples were collected for optically stimulated luminescence (OSL) and cosmogenic radionuclide (CRN) analyses to help quantify relative soil mobility and hillslope denudation rates. Parent material at all sites is colluvium derived from bedrock and a smaller component of suspected eolian fines, but the colluvium is generally thicker on north-facing slopes. Soils are weakly developed Cryepts, suggesting the colluvial mantle has been relatively mobile through time on both slope aspects. Still, differences reflecting greater effective moisture on north-facing slopes are reflected in the soils, which characteristically have thick O horizons, thin A horizons with low color values and occasional, weakly developed E horizons. Comparatively, soils on south-facing slopes lack O and E horizons and have relatively thick A horizons with higher color values. The colluvial mantle overlies physically shattered and chemically altered bedrock at all sites but intact rock is notably closer to the surface on the south-facing slopes, suggesting the latter have higher rates of evapotranspiration compared to their north-facing counterparts, resulting in differences in vegetation type and density that promotes erosion of the soil mantle on the south-facing slopes.