10BE REPRODUCIBILITY IN THE SANTA MONICA MOUNTAINS FROM 2016–2021: A LONGITUDINAL 10BE REPLICATE STUDY THROUGH FIRE, ATMOSPHERIC RIVERS, AND MASS MOVEMENTS

The 2018 Woolsey Fire and post-fire rainfall from eleven atmospheric rivers triggered widespread erosion and mass movements in the Santa Monica Mountains, California. Pre-fire samples for ¹⁰Be collected in 2016 (n = 7) enable us to use the fire and post-fire erosion as a means of assessing how measurements of the cosmogenic nuclide, ¹⁰Be, in stream sand (300–710 μm) changes following a severe wildfire in a steep, tectonically active, semi-arid mountain range. Four of these stream basins were burned entirely by the fire, two were not, and one was partially burned; all basins were affected by similar post-fire rainfall.

Pre-fire ¹⁰Be concentrations ranged from 1.04–4.37 x 10⁴ atoms g^-1. Post-fire temporal replicate samples were collected four times in the three years after the fire, and they remained largely consistent with pre-fire ¹⁰Be data in the two unburned basins (1.04–1.43 x 10⁴ atoms g^-1); the standard deviation of the mean in each unburned basin provides an estimate of natural ¹⁰Be variability in the Santa Monica Mountains (±9–13%). ¹⁰Be concentrations in three burned catchments increased above pre-fire concentrations in the year following the fire (1.53–4.27 x 10⁴ atoms g^-1); in the fourth burned basin ¹⁰Be concentrations decreased below pre-fire concentrations (7.55 x 10³ atoms g^-1). ¹⁰Be concentrations returned to pre-fire concentrations within three years in only two of the burned basins.

We also tested the reproducibility of ¹⁰Be concentrations in five grain-size fractions collected before and after the fire from a burned and an unburned basin. We find that ¹⁰Be concentrations of all grain sizes reproduced well in both basins (within a factor of 2), though ¹⁰Be concentrations increased in the one year following the fire in the burned basin, but not the unburned basin. Results from these analyses will be used to guide interpretations of spatial distributions of erosion in the three years following the fire and to guide interpretations of pre-fire ¹⁰Be concentrations from stream sand as landscape erosion rates, which inform us about the geological evolution of the Santa Monica Mountains over millennial timescales.