A QUANTITATIVE ASSESSMENT OF SNOW SHIELDING EFFECTS ON SURFACE EXPOSURE DATING IN THE WESTERN UNITED STATES

Cosmogenic surface exposure dating is a widely applied tool in glaciated landscapes for determining the history of past glaciations. In the western United States (US) surface exposure dating primarily with ¹⁰Be has been applied on bedrock and moraine boulders to determine former glacier positions and infer past climate conditions. The ¹⁰Be production rate varies in space and time, so a spatiotemporal scaling is required in the age calibration. However, effects of many geological and climatological processes on ¹⁰Be ages, which also vary spatially, are not quantified and incorporated into the age calibration. This could lead to discrepancies amongst ages across relatively large spatial ranges. Here we compile a dataset of 1,524 ¹⁰Be ages from more than 40 alpine moraines across the western US (https://sparrow-cosmo.geoscience.wisc.edu). Steep climatologic gradients across this complex terrain can influence ¹⁰Be ages in several ways but most notably in snow shielding. To test potential effects of snow shielding, we first use remote sensing and snow reanalysis data (Brown and Brasnett 2020) to reconstruct the modern annual dynamics of snow coverage, depth, and density across the western US. We then calculate the snow shielding factor for each sample in our ¹⁰Be dataset. Finally, we use the TraCE-21ka (Liu et al. 2009) output over the past 22 ka to derive a time-integrated snow factor for each sample. We also compile 770 boulder heights from existing syntheses (Heyman et al. 2016) and other studies and find that sampled boulders are 97 cm above the reconstructed local snow depth on average, but approximately 10% of those boulders are smaller than the local snow depth. We partition the snow shielding as two end member states. If the snow accumulation is wind-swept on top of boulders and that boulders are only shielded when the total accumulation exceeds boulder height, the snow adjustment for all samples is 5.1% on average across the western US. If snow depths on top of boulders are equal to local snow depths, the adjustment is 10.2% on average. We demonstrate that boulder height considerations are important for snow shielding effects on ¹⁰Be dating. Our study provides a consistent framework for estimating potential snow shielding across the western US over the past 22 ka to provide better insight into past glacier and climate changes.