UCLA

iiABSTRACT OF THE THESIS Influence of Bedrock Weathering on Soil Production Rates in the San Gabriel Mountains, California by Joshua Anthony Lee Master of Science in Geology University of California, Los Angeles, 2024 Professor Seul Gi Moon, Chair Quantifying the controls of soil production rates is essential for understanding hillslope evolution and morphology in soil-mantled landscapes. Previous studies have identified a negative exponential relationship between soil production rates, measured by cosmogenic 10Be, and soil depth. This trend is interpreted as the influence of surface disturbances, whose magnitudes and frequencies are reduced in deeper soils, on soil production rates. However, the properties of underlying weathered bedrock may also influence soil production rates through the supply of rock material with varying mechanical properties to the soil. Few studies have examined the spatial variations in the extent of weathered bedrock at depths and their influence on soil production rates. In this study, I investigate the impact of bedrock weathering on soil production rates in the San Gabriel Mountains, California, USA. I conducted four P-wave iii seismic velocity profiles, totaling 1176 meters, to image the deep critical zone structures to a depth of approximately 60 meters. I determined a total of sixteen soil production rates using cosmogenic 10Be concentrations in quartz minerals from saprolite and soil. I find that the depth to the base of saprolite is generally greater under ridges and shallower under channels at our sites. There are two distinctive trends between soil production rates and the depths to the base of saprolite, which likely represent two different processes. In areas with gentle slopes, soil production rates are generally low and increase with the increasing depths to the base of saprolite. This suggests that the supply of weak rock materials from below may facilitate easier breakdown, thereby increasing the rate of soil production. Conversely, in areas with steep slopes, soil production rates are generally high and increase with decreasing depths to the base of saprolite. This trend may reflect the effect of decreasing residence time of rock material near the surface, which occurs in response to increasing erosion rates. High erosion rates lead to increased soil production and a reduction in the extent of physical and chemical weathering of bedrock. Our work highlights the underappreciated, bottom-up controls of underlying bedrock on soil production rates and the interplay between bedrock weathering, soil production, transport processes, and hillslope morphology in soil-mantled landscapes.