VARIATIONS OF SOIL-BEDROCK GEOCHEMISTRY ALONG A SAN GABRIEL VALLEY CATENA: IMPLICATIONS FOR LANDSCAPE EVOLUTION IN A CHANGING SOUTHERN CALIFORNIA XERIC CLIMATE

This study examines soil properties, especially soil-bedrock geochemistry, along a complex, soil toposequence (catena) in granitic parent material under chaparral vegetation in the San Gabriel Mountains and San Gabriel Valley of southern California. This catena is representative of complex land use histories across southern California and many xeric ecosystems, with key disturbances including habitat type conversion, (sub)urban development, and agricultural land use. Along with horizon morphology, mineralogy, pH, and texture, bulk and water soluble geochemistry were measured via XRF and ICP-OES on 44 soil and parent material samples from 14 profiles ranging from ~1050 m to 280 m in elevation. The loss of major elements relative to immobile elements (τ) (Muir and Logan, 1982), chemical depletion fraction (CDF) (Riebe at al., 2001), and chemical index of alteration (CIA) (Kirkwood and Nesbitt, 1991), were calculated from bulk geochemical data. Differences within and between profiles illustrate spatial variations in weathering intensity across the topographic gradient and highlight effects of geomorphic and anthropogenic processes on soil resources and ecosystems. Elemental concentrations (bulk and water soluble) were found to reflect soil age and meter-scale variations in local bedrock lithology. Despite some variability in texture and pH, the simpler lowland profiles all reflect young soil ages with limited chemical weathering. These geochemical data help underscore the fact that some of the most sensitive soils under threatened, native sage scrub communities are the most heavily impacted by agriculture and urban development, while some of the oldest, rarest, and most geochemically unique soils are increasingly threatened by residential encroachment. This study shows the utility of geochemical data in assessing the impacts of land use and ecosystem change across a topographic gradient, and underscores the challenges and complexity of the toposequence/catena concept.