HOLOCENE ALLUVIAL HISTORY DERIVED FROM SOIL-STRATIGRAPHY, SANTA CATALINA ISLAND, CALIFORNIA

Much attention has been focused on variations in geomorphic processes over the last glacial-interglacial transition in arid to semi-arid landscapes in the southwest USA, but studies of the relations between Holocene climate change and geomorphic response are becoming increasingly important. Alluvial stratigraphy and soils were characterized along ephemeral axial and tributary streams across Santa Catalina Island (Channel Islands, California). Soil-stratigraphy associated with surficial deposits, ranging in thickness from 2 to 8 meters, provides an excellent record of Holocene periods of hillslope instability and associated aggradation along stream channels. The robust fluvial record contains multiple buried soils having strong A horizons, many with weakly developed Bw horizons, which help identify multiple episodes of notable incision and deposition. Radiocarbon dates indicate a major period of channel aggradation, consisting of pebble-gravel deposits overlying bedrock channels, began about 6940 to 5300 cal yr BP. This was followed by multiple discrete episodes of widespread deposition with the last two episodes occurring between about 1180 to 920 cal yr BP and between about 780 to 230 cal yr BP. Available radiocarbon dates and soil stratigraphy observed in the major watersheds on Santa Catalina Island consistently show episodic deposition that is linked to hillslope-derived sediments. Initial analysis of soil stratigraphy and records of Holocene climate change suggest that periods of fluvial aggradation are related to periods of intense storm activity, possibly associated with periods of increased wildfire activity. In comparison, more than one hundred years of intense grazing-caused vegetation removal (noted in historic documents and photographs) had limited geomorphic response (intact, well-developed hillslope soils and a sparse depositional record) despite record precipitation events during the period of grazing. This suggests that (1) Holocene climate perturbations affected the magnitude of geomorphic responses more profoundly than is observed historically and (2) there is a need to revisit conceptual models of geomorphic response to climate change in arid to semi-arid landscape systems.