COMPARISON OF METEORIC AND IN SITU-PRODUCED 10BE DEPTH PROFILES: EVALUATING EROSION RATES, METEORIC 10BE FLUX, SOIL MIXING, AND STEADY STATE

In this study, we compare the results of previously published [1] in situ-produced ¹⁰Be depth profiles with new meteoric ¹⁰Be concentrations ([¹⁰Be_met]) in depth profiles from the same samples to evaluate erosion rates and soil dynamics between both methods, each capturing a different perspective in terms of processes (e.g. rate of production, delivery mechanism) and timescales involved. The Pinedale (~21 ky) and Bull Lake (~140 ky) glacial moraines at Fremont Lake, Wyoming provide an ideal study area to resolve differences between in situ and meteoric [¹⁰Be] in tracking soil erosion at the crest of a hillslope. The age, grain sizes, weathering indices, and soil properties of the moraines are known, as are erosion/denudation rates calculated from multiple methods. Erosion rate calculations from [¹⁰Be_met] in depth profiles require knowledge of the flux of this nuclide to the site. These moraines provide a unique opportunity to independently determine the flux to this location, given known denudation rates from [1]. A back-calculated flux of 0.74 x 10⁶ atoms cm^-2 y^-1 yields erosion rates of 22 mm ky^-1 and 9 mm ky^-1 for the Pinedale and Bull Lake moraines, respectively, which are in excellent agreement (±7%) with the average denudation rates of [1] after accounting for weathering. This calculated flux is half that of the ¹⁰Be_met flux map of [2], calling into question the suitability of using either a general precipitation- and latitude-dependent scaling method [3] or a model-derived regional ¹⁰Be_met flux [2] for a local field site. Incorporation of the temporally varying parameters in our calculations that effect delivery (paleomagnetic field intensity) and beryllium retention (pH, fluid flow) bring the best-fit flux closer to the value reported in [2]. The erosion rates calculated from the full ¹⁰Be_met inventory and from the surface [¹⁰Be_met] are virtually identical, indicating that both moraine crests have achieved steady state, as previously suggested. We also explore differences in degree and depth of soil mixing resolved from each depth profile method.

[1] Schaller et al. 2009. doi:10.1029/2007JF000921

[2] Heikkilä U, von Blanckenburg F. 2015. doi:10.5880/GFZ.3.4.2015.001

[3] Graly et al. 2011. doi:10.1016/j.epsl.2010.12.020