Hillslope lowering rates and mobile-regolith residence times from in situ and meteoric10Be analysis, Boulder Creek Critical Zone Observatory, Colorado

Abstract

Cosmogenic radionuclides (CRNs) are commonly employed to quantify both the production rates and residence times of mobile regolith. Meteoric and in situ CRNs have different accumulation mechanisms and can independently constrain landscape evolution rates. Here we use both in situ and meteoric 10 Be to investigate where in the regolith 10 Be is stored, and to quantify production rates and residence times of mobile regolith on active hillslopes in Gordon Gulch, within the Boulder Creek Critical Zone Observatory (CZO), Colorado, USA. Our data reveal that two-thirds of in situ 10 Be in regolith is produced within saprolite, and at least one-tenth of the meteoric 10 Be inventories is stored in saprolite, highlighting the importance of consistent terminology and identification of the mobile regolith–saprolite boundary. We find that mobile-regolith production rates are on average 3.1 cm/k.y., and residence times are between 10 and 20 k.y. A notable exception exists at the depositional north-facing footslope, where residence times likely exceed 40 k.y. Close agreement between the meteoric and in situ results indicates that upper- and mid-slope positions are consistent with steady, uniform lowering of the landscape. In addition to comparing the two methods, we develop a one-dimensional analytical model for the 10 Be concentration fields on an active, steady-state catena with uniform erosion. We then compare model predictions with measurements to evaluate how well our sites adhere to the steady-state assumption underlying the calculations for mobile-regolith residence time and production rates. Such comparisons suggest that calculated residence times and lowering rates are likely no closer than ±25% of the geomorphic reality.