REPRODUCIBILITY OF 10BE CONCENTRATIONS IN STREAM SAND FOLLOWING THE 2018 WOOLSEY FIRE IN SOLSTICE CANYON, SANTA MONICA MOUNTAINS, CALIFORNIA

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.