Abstract

In high-relief environments, soil geochemistry and morphology reflect the weathering of both parent material and materials added to weathering profiles by downslope transport and dustfall. In the Colorado Front Range, transport is limited in alpine soils developed on stable surfaces. In the montane zone, soils and regolith on hillslopes are mobile and mix during downslope transport. In this study, measurement of soil texture, citrate-buffered dithioniteextractable iron (Fed), and bulk geochemistry permitted evaluation of weathering, downslope transport, and eolian deposition in the Critical Zone of the Boulder Creek catchment. The accumulated mass of Fe and clay are positively correlated with deposit age in the catchment. Stable alpine soils form from Pinedale age till and the cool, moist climate generates sufficient acidity to develop strong horizonation. Soil morphology in the upper montane Gordon Gulch is controlled mainly by downslope transport; soils on the north-facing slope thicken downslope and have complex morphology. Soils on the south-facing slope are thin and overlie saprolite at a maximum depth of 53 cm. Soils on the north-facing slope have higher clay and Fed contents than those that face south, indicating more rapid erosion of the south-facing slope and greater weathering on the north-facing slope. On older surfaces of low relief, soils with thick Bt horizons develop from deeply weathered saprolite and regolith, and locally contain buried sequences and features that suggest periglacial mass movement and slope instability in pre-Holocene time.

In soils throughout the catchment, enrichment of fine particles and low concentrations of Fe_d in surface horizons suggests eolian sedimentation. Enrichment of fines is most apparent at stable sites, but soils on lower positions on slopes are also enriched. The amount of clay and fine silt produced in situ and the amount added from dustfall is poorly constrained, but the dustfall rate is less than 60 g cm^-2 100kyr^-1. Immobile element geochemistry indicates that surface enrichment in high field strength elements (HFSE) is not uniform throughout the study area, suggesting that the dust deposition may vary spatially and/or temporally. Ratios of the immobile elements Ti, Zr, and Nb, suggest that surface fines are geochemically distinct from the dominant parent materials in the catchment; however, elemental ratios may also reflect increased fines in the surface horizons released by weathering, because Zr and Nb are preferentially enriched in fine fractions of parent rocks in the study area. The composition of fine sediments in surface horizons are similar to silt mantles in other basins in the Indian Peaks Wilderness Area and to silt-sized alluvium in North Park and Middle Park to the west of the study area. Surface enrichment of fines with low Fed and distinct immobile element ratios of low compositional variability suggest that a substantial portion of the fine fraction of soils examined in this study originate as dustfall, potentially derived from North Park and Middle Park.

Citation

MacCarthy, James A. (2011): Assessing eolian contributions to soils in the Boulder Creek Catchment, Colorado,. Thesis submitted in partial fulfillment of the requirements for the Degree of Bachelor of Arts With Honors in Geosciences(Williams College).