Thursday, April 24, 2014, 4:30PM - 5:30PM
RL-3 room 248
Hillslope lowering rates and mobile-regolith residence times from in situ and meteoric 10Be analysis: Boulder Creek Critical Zone Observatory, Colorado
Release of mobile regolith from underlying saprolite is a crucial process in the evolution of landscapes. 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 test the assumptions underlying these calculations by developing an analytical solution for the mean-concentrations of both in situ and meteoric 10Be in mobile regolith on an active hillslopes. We find that our analytical model works well as an interpretative framework. Landscape-lowering rates within Gordon Gulch are approximately 2-3.5 cm/ka for both surface-lowering and saprolite-lowering rates; mobile-regolith residence times are largely between 10-25 ka, with higher residence times along the north-facing toeslope (Dethier et al., 2012).
Both our measurements and model treat 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 40-90% of the meteoric 10Be deposition occurs in the saprolite. As most studies that use 10Be report residence times and soil-production rates that are based on concentrations in the mobile regolith on hillslopes, our 10Be data highlight the importance of clearly identifying mobile and immobile portions of the regolith and its 10Be inventory, and use of consistent terminology for the mobile layer.