QUANTIFYING SEDIMENT ACCUMULATION RATES WITH COSMOGENIC NE-21 ON GLACIER MORAINES IN BLOODY CANYON, CA

The concepts of local and nonlocal sediment transport processes on hillslopes predict varying rates of sediment disentrainment and travel distances, which result in different rates of landscape evolution and, consequently, different morphologies of landforms. The differences between these concepts have a strong physical and mathematical basis, but field observations and measurements have lagged behind our understanding of the theoretical differences. Here we present an initial effort to measure sediment accumulation rates on a glacier moraine in the Sierra Nevada, CA, in order to demonstrate whether local or nonlocal transport processes are dominant at this field site. In Bloody Canyon, CA, on the east side of the Sierra Nevada Mountains, we dug three soil pits at the toe of a Tahoe-aged moraine (c. 24-40 ka) along a topographic profile. Each pit is approximately 8-10 m farther downslope from the next pit. A fourth soil pit was dug along the topographic contour from the middle pit as a secondary test site. From each pit, we collected a vertical series of bulk sediment samples. From the sieved 250-500 um sized fraction of these bulk sediment samples, we isolated quartz minerals following standard lab procedures, and measured cosmogenic Ne-21 using noble gas mass spectrometry. The concentration of Ne-21 with depth in the soil pits depends on the amount of any Ne-21 inherited prior to deposition of the moraine, that which was accumulated during erosion and transport down the moraine, and the accumulation rate of sediment at the pit site. The model most commonly used to determine sediment accumulation rates is Lal et al. (1987), which solves for a constant rate of sediment accumulation, and is most akin to a local transport model. Nonlocal sediment transport models predict accumulation of sediment from various sources upslope, and thus cannot be solved analytically. We created a 1-D sediment transport model that tracks concentrations of Ne-21 for each node in the model space at each time step. With this, we can predict concentrations of cosmogenic Ne-21 in soil profiles, which are demonstrably different than the predictions for the constant accumulation model of Lal et al. Our measurements of Ne-21 will be compared to these model predictions to determine which sediment transport process is dominant on this landscape.