POST-FIRE STEEPLAND EROSION AND BIOTURBATIVE SOIL MIXING MEASURED THROUGH SHORT-LIVED RADIONUCLIDES

Wildfires on steep hillslopes can produce large fluxes of dry ravel that are poorly modeled by typical diffusive hillslope flux models. Here we examine the controls on steepland sediment fluxes at a site near San Jose, CA, where dry ravel following the 2020 SCU Lightning Complex Fire is driven primarily by bioturbation by ground squirrels. We use short-lived radionuclide concentration depth profiles from soil cores to assess soil mixing and spatial patterns of erosion and deposition at three cross-slope transects at a range of surface gradients along a ~60-m hillslope. We examine the influence of mobile grain size and vegetation recovery on particle exposure times and inferred travel distances using nuclide concentrations in recently excavated material collected from squirrel burrows and individual mobile soil granules collected along cross-slope transects at 10-m downslope intervals. Radionuclide measurements are corroborated with repeat field experiments, high resolution topographic data collection and physical measurements. Together, these data test the hypothesis that, under sufficiently steep, smooth and bouncy conditions—such as those found in newly burned steeplands—disturbance by burrowing animals can entrain long-distance sediment fluxes that escape representation by diffusive flux models.