Meteoric 10Be, clay, and extractable iron depth profiles in the Colorado Front Range: Implications for understanding soil mixing and erosion

Abstract

Analyzing meteoric 10Be in soil profiles along with soil measurements such as pedogenic Fe and clay content permits better understanding of meteoric 10Be transport in soils, soil formation, and hillslope geomorphology. This study presents meteoric 10Be depth profiles from saprolite-derived soil catenas sampled in the Gordon Gulch catchment, Colorado Front Range and from nearby c. 130 and 15ka glacial moraines. We compare meteoric 10Be data with more classically used soil analyses of clay and extractable iron (Fed) content. Meteoric 10Be and Fed do not show consistent trends in the hillslope profiles. Meteoric 10Be and clay concentrations, however, generally decrease with depth and highest concentrations of 10Be in horizons coincide with highest clay concentrations. Soils at moraine sites are better developed than those on hillslopes, and correlations of 10Be, Fed, and clay are stronger. In Gordon Gulch, south-facing hillslopes display meteoric 10Be bulges at depth and lower near-surface concentrations compared to the declining profiles and higher near-surface concentrations of north-facing hillslopes. The aspect differences imply that south-facing hillslopes experience greater vertical mixing than north-facing hillslopes, and that greater lateral transport and erosion has occurred over the last 15–20ka on south-facing hillslopes relative to north-facing hillslopes. Meteoric 10Be depth profiles at moraine sites display bulge profiles with highest meteoric 10Be concentrations in B-horizons, reflecting landform stability and pedogenic processes. Overall, our data demonstrate that signatures of chemical weathering in soils (Fed and clay) may or may not correlate to the meteoric 10Be added through atmospheric deposition, and that profile shape of meteoric 10Be can provide insight into the relative contribution of vertical mixing and surface erosion in relation to catena location and aspect during hillslope evolution.