GSA Connects 2021 in Portland, Oregon

Paper No. 159-3
Presentation Time: 9:00 AM-1:00 PM


BOGOSKI, Collin J.1, PORTENGA, Eric1, CORBETT, Lee B.2, BIERMAN, Paul R.3 and CAFFEE, M.W.4, (1)Geography & Geology, Eastern Michigan University, 140 Strong Hall, Ypsilanti, MI 48197, (2)Department of Geology, University of Vermont, Delehanty Hall, 180 Colchester Ave, Burlington, VT 05405, (3)Department of Geology, University of Vermont, Burlington, VT 05405, (4)Department of Physics, Purdue University, 1396 Physics Bldg, West Lafayette, IN 47907

Measuring the cosmogenic isotope 10Be in stream sand is widely used to infer erosion rates at watershed scales. Generally, one assumes that 10Be integrates through rare, large-magnitude events like floods or mass movements, thus providing geologically-meaningful average rates of landscape change. This assumption has been assessed for watersheds subjected to landslides, earthquakes, hurricanes, and floods, generally demonstrating that deep erosion during or following these rare, large-magnitude events lowers 10Be concentrations, which then results in overestimated erosion rates. It is not known whether 10Be concentrations in stream sediment are biased due to erosion following large wildfires.

The Woolsey Fire burned ~97,000 acres of the Santa Monica Mountains, California from November 8–21, 2018, after which rainfall triggered widespread mass movements. Here, we test the hypothesis that 10Be concentrations measured in stream sand after the Woolsey Fire was extinguished were lower relative to pre-fire concentrations. We speculate that lowered 10Be concentrations would be caused by post-fire erosion that removed a large volume of hillslope material containing low concentrations of 10Be from deep below the surface. Previous studies have shown that 10Be measured from different grain-size fractions may be affected by landslide dilution to varying degrees; thus, we also test the hypothesis that 10Be measured in the coarse grain-size fraction in the Santa Monica Mountains are lower following the fire compared to fine to medium fractions.

We measured 10Be from stream sand collected in November 2016 (before the Woolsey Fire) and in January and March 2019 (after the fire) in the fully-burned Solstice Canyon catchment. Post-fire 10Be concentrations measured from Solstice Canyon replicate within 2σ uncertainties of pre-fire concentrations from all grain-size fractions, suggesting that post-fire mass movements in Solstice Canyon were not deep enough to effectively dilute the measured concentration of 10Be in stream sand. These findings demonstrate that 10Be-based erosion rates at Solstice Canyon, and potentially in similar landscapes with shallow soils, accurately reflect the long-term rate of landscape change, even after a rare, large-magnitude wildfire-triggered erosion.