THE IMPACT OF THE 2018 WOOLSEY FIRE ON 10BE CONCENTRATIONS MEASURED IN STREAM SAND IN THE SANTA MONICA MOUNTAINS

Measurements of ¹⁰Be in stream sand collected after wildfires is often used to set benchmark geological erosion rates against which post-fire sediment yields may be compared, but it is not known how measurements of ¹⁰Be in stream sand collected after a wildfire compare to measurements of ¹⁰Be in stream sand prior to the fire. The Woolsey Fire burned through >380 km² of the Santa Monica Mountains, California, in November 2018. We use the Woolsey Fire to assess the degree to which ¹⁰Be measured from quartz stream sand is changed by widespread post-fire erosion in a semi-arid, mountainous landscape. Post-fire ¹⁰Be measurements were made on stream sand from seven stream basins (>10 km²) across the Santa Monica Mountains in December 2018 and January and March 2019 and compared to ¹⁰Be measurements made on alluvium from the same streams in March and November 2016, three years earlier. Analysis of initial data show that post-fire ¹⁰Be measurements from unburned and partially burned basins replicated pre-fire ¹⁰Be measurements within measurement uncertainties. ¹⁰Be data from the burned catchments were more variable, increasing by ~60–70% in two basins, remaining unchanged within uncertainties in the third, and decreasing by ~55% in December 2018 but returning to pre-fire concentrations by March 2019 in the fourth. Additionally, pre- and post-fire measurements of ¹⁰Be from different grain size fractions were made on sediment from a burned catchment in November 2016 and January and November 2019. These data demonstrate a consistent trend of measured ¹⁰Be concentrations nominally increasing above pre-fire measurements over the eleven months following the extinction of the Woolsey Fire. Presently, we suggest that shallow soils and the inferred occurrence of shallow widespread erosion, rather than deep mass movements, ultimately prevented significant ¹⁰Be dilution in stream sand and that widespread intense burning in high-elevation tributaries, which have higher ¹⁰Be production rates, explains the observed increase in ¹⁰Be in the first two burned catchments and possibly the increase in ¹⁰Be from the grain size data. These data will, in part, provide important insight to the long-term reproducibility of ¹⁰Be data measured in stream sand from steep, wildfire-prone landscapes with relatively thin soils.