Meteoric beryllium-10 fluxes from soil inventory measurements in the East River watershed, Colorado, USA

Abstract. Meteoric beryllium-10 (¹⁰Be_met) has a wide range of applications as a geochronometer and tracer of geological processes. ¹⁰Be_met is produced in the atmosphere by cosmic rays and delivered to Earth’s surface primarily via precipitation. ¹⁰Be_met is particularly suitable for quantifying surface process rates where use of in situ-produced ¹⁰Be is challenging, such as landscapes with quartz-poor bedrock. However, using ¹⁰Be_met for dating and quantifying surface process rates requires constraining depositional fluxes across space and time. Although empirical and physical models for predicting fluxes exist, the predictions can deviate substantially from measured values. Here we quantify ¹⁰Be_met flux in the East River watershed in Colorado, USA where precipitation is dominated by snowfall. We measured the ¹⁰Be_met inventory in soils on five glacial moraines 13–18 ka in age that span 700 m of elevation and calculated ¹⁰Be_met fluxes by dividing each inventory by moraine age. Inheritance-corrected fluxes range from 1.12 x10⁶–3.79x10⁶ atoms cm^-2 yr^-1, and are well correlated with elevation, mean annual precipitation, mean snow depth, and snow water equivalent (R² = 0.84 to 0.99). Regression models based on elevation, precipitation, snow depth and snow water equivalent predict watershed-averaged fluxes of 1.23x10⁶–3.62x10⁶ atoms cm^-2 yr^-1. Predicted fluxes from a published empirical model that estimates fluxes as a function of precipitation were within a factor of 1.1–1.6 of measured values at each site. Fluxes predicted by physically-based general circulation models (GCM) are generally within a factor of three of our estimated watershed-averaged values, but the GCM predictions are too coarse to capture the intra-watershed spatial variability in fluxes. Our results highlight both the importance of factors that drive variability in ¹⁰Be_met delivery to soils and how local calibration can improve estimates of ¹⁰Be_met flux in mountain watersheds.