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