Spatial and temporal variations in meteoric 10Be inventories and long-term deposition rates, Colorado Front Range

Abstract

Atmospherically-derived cosmogenic nuclides such as meteoric 10Be offer great potential in Quaternary science because they can be used as landform geochronometers and as sediment tracers. Use of meteoric 10Be inventories for determining landform age, soil residence time, or erosion rates requires knowing the long-term rate of deposition for this cosmogenic nuclide to the Earth's surface. Here, we present meteoric 10Be inventories measured at 6 dated sites near the Boulder Creek Critical Zone Observatory, Front Range, Colorado. Each site lies on a relatively stable landform (broad moraine or fluvial terrace) at slopes between <1 and 7° and supports well-developed soil horizons. Time-averaged rates of total meteoric 10Be deposition (precipitation, dryfall, and dust deposition) calculated from measured inventories at these sites range from 0.4 to 5.0 × 106 atoms cm−2 yr−1; none of the inventories match expected primary meteoric 10Be deposition rates that global, latitude-based models or precipitation-specific models predict for this region. Meteoric 10Be inventories and calculated deposition rates at the three older (>90 ka) sites fall below (0.3–0.7×) model predictions; inventories and rates at the three Pinedale-age sites (LGM; 21–15 ka) are higher (1.5–4×) than model predictions. Previous analysis of soils in the Front Range over the same timeframe as these sites indicate that long-term dust accumulation rates are low and associated secondary meteoric 10Be deposition is unlikely to account for the deviation from model predictions. At the Pinedale-age sites in particular, long-term dustfall would have to be 1–2 orders of magnitude higher to add the meteoric 10Be required to account for measured inventories. Low inventories at older sites are consistent with loss of meteoric 10Be through physical erosion or selective removal of near-surface soil enriched in meteoric 10Be; time-averaged rates of total meteoric 10Be deposition for these sites are minima. We speculate that higher inventories at Pinedale-age sites reflect the addition of meteoric 10Be due to snowdrifts and/or interflow associated with landform location, increasing the effective precipitation-related primary meteoric 10Be deposition at the sampled sites. Given these constraints, and the spatial variation observed around the Front Range, we suggest that 1.5 ± 0.2 × 106 atoms cm−2 yr−1 is an appropriate long-term meteoric 10Be deposition rate to use across much of the Front Range region that receives 45–55 cm of precipitation a year. This long-term total meteoric 10Be deposition rate is ∼30–50% higher than that predicted by global or annual precipitation-specific models, emphasizing that local calibration of deposition rates is essential for geomorphic studies around the globe.