GRANITIC BOULDER EROSION CAUSED BY CHAPARRAL WILDFIRE; IMPLICATIONS FOR COSMOGENIC RADIONUCLIDE DATING OF BEDROCK SURFACES

Rock surface erosion by wildfire is significant and widespread but has not been quantified in southern California, or for chaparral ecosystems. Quantifying rates and processes of surface erosion of bedrock outcrops and boulders is critical for interpreting age relations using cosmogenic radionuclide techniques, as even modest surface erosion removes the accumulation of the cosmogenic radionuclides leading to significant underestimating of age. This study documents the effects on three large granitic boulders following the Esperanza fire of 2006 in southern California. The volume of spalled rock fragments was quantified by measuring the removed rock volume from each measured boulder. The dark-colored char, ubiquitously deposited on the rock surfaces, made the spalled volumes easy to discern and measure. Between 8 – 46% of the total surface area of the boulders spalled in this single fire. The volume of spalled material, when normalized across the entire surface area, represents a mean surface lowering of 0.9 – 10 mm. Spalled material was thicker on the flanks of the boulders, and the height of the fire effects significantly exceeded the height of the vegetation prior to the wildfire. In this study area, the height of the fire effect was approximately 4 m, and the height of the vegetation was approximately 2 m. Surface erosion of boulders and bedrock outcrops as a result of wildfire-induced spalling results in fresh surfaces that appear essentially unaffected by chemical weathering. Such surfaces may be preferentially selected by researchers sampling for cosmogenic surface dating because of their fresh appearance, leading to an underestimation of age. Researchers should be mindful of wildfire spalling effects when selecting dating sites.