SOIL TRANSPORT AND WEATHERING ACROSS AN EROSION-RATE GRADIENT, PENINSULAR RANGES, CALIFORNIA

Chemical weathering and soil transport are defining processes that shape hillslopes and produce boulder and tor-studded landscapes characteristic of semi-arid regions. Here, we investigate the relationship between weathering and soil transport across nine hillslopes categorized by different rates of erosion and extent bedrock exposure in southern California. We studied 1 km² catchments within the McCain Valley region of the eastern Peninsular Ranges. This area, situated on the rift flank of the Salton Trough, is dominated by outcrops of the La Posta Pluton. Catchment average erosion rates determined from cosmogenic ¹⁰Be range from 0.04 to 0.10 mm/yr (Longinotti, 2012). XRF analysis was performed on sediment samples from nine catchments spanning this erosion-rate gradient. Mineralogical compositions were determined and compared against the parent La Posta Pluton, revealing an enrichment in quartz from the host rock inversely proportional to the rate of erosion. Regolith thickness was measured along hillslope transects in a subset of these catchments. We find that areas of higher erosion rate are characterized patchier, but not necessarily thinner soil cover. On the slowest eroding catchment sampled, with erosion rate < 0.05 mm/yr, soil thickness increased monotonically down slope, supporting depth dependent transport and implying downslope trade-off between physical and chemical weathering. In contrast, catchments with erosion rates ≥ 0.05 and bedrock exposure ≥ 10% exhibit less predictable relationships. We hypothesize that as erosion rate increases, soil-cover enhanced chemical weathering alternating with physical weathering of bare bedrock outcrops produces boulder and tor-studded landscapes. These findings suggest the importance of analyzing catchments with high erosion rate and bedrock exposure to further understand the relationship between bedrock exposure, chemical weathering, and soil transport.