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

Soil catenas model soil profile development across a topographic gradient considering climate, biology, geology, and landscape age as constants. While these factors in reality are all inseparably linked, and given that “true” catenas are rare, the catena concept is nonetheless highly useful for illustrating soil landscape complexity across topographic gradients, yielding a better understanding of landscape evolution and often providing invaluable paleoenvironmental insights. This study in the San Gabriel Mountains and San Gabriel Valley of southern California examines the variation of soil properties, especially soil-bedrock geochemistry, along a complex, chaparral toposequence in (mostly) granitic parent material. This catena is representative of the complex land use histories of southern California and many xeric ecosystems, with anthropogenic influences including habitat type conversion, (sub)urban development, and agricultural land use. Differences within and between profiles illustrate spatial variations in weathering intensity across the topographic gradient and the effects of geomorphic and anthropogenic processes on soil resources and ecosystems.

Geochemistry was measured on 44 soil and parent material samples from 14 profiles ranging in elevation from ~1050 m to 280 m a.m.s.l. along with mineralogy, horizon morphology, pH, and texture. The loss of major elements relative to immobile element concentrations (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 in elemental concentrations reflect soil age and meter-scale variations in local bedrock lithology. Despite some variations in texture and pH, the simple lowland profiles all reflect young soil ages with limited chemical weathering. The results demonstrate that some of the most fragile, poorest soils are the most heavily impacted and selected for agriculture and urban development, replacing native sage scrub communities, while some of the oldest and most geochemically unique soils are threatened by residential development. This study shows the utility of geochemical data in describing the impacts of land use and ecosystem changes across a vast soil catena.