Hillslope lowering rates and mobile-regolith residence times from in situ and meteoric 10Be analysis: Boulder Creek Critical Zone Observatory, Colorado

Abstract:

Mobile regolith is produced as weathered saprolite is entrained into the mobile layer. The rate of mobile-regolith production and its residence time on hillslopes shapes the topography and evolution of hillslopes. We calculate the production rate of mobile regolith and the mobile-regolith residence times on active hillslopes in Gordon Gulch, within the Boulder Creek Critical Zone Observatory (CZO), Colorado. We find mobile-regolith production rates (average 3.1 cm/ka) and residence times (average 10-20 ka) derived from both in situand meteoric methods agree. Lowering-rates derived from our study are also comparable to basin-averaged denudation rates for small basins in the Colorado Front Range (Dethier and Lazarus, 2006).

In this study, we have measured both in situ and meteoric ¹⁰Be in saprolite and mobile regolith separately. We find that, on average, two-thirds of in situ ¹⁰Be is produced within saprolite, and that at least one-tenth of the meteoric ¹⁰Be inventories are stored in saprolite. In the case of in situ ¹⁰Be, this simply reflects the exponential fall-off in production rates through a thin mobile-regolith cover. In the case of meteoric ¹⁰Be, our calculations suggest that >40% of the meteoric ¹⁰Be deposition occurs within the saprolite. Most studies that utilize ¹⁰Be report residence times and soil-production rates based on concentrations in either the mobile regolith or saprolite; therefore, our ¹⁰Be data highlight the importance of clearly identifying mobile and immobile portions of the regolith, constraining its ¹⁰Be inventory, and use of consistent terminology for the mobile-layer.