A Holocene hydroclimatic investigation of Lake Elsinore, Southern California, using seismic reflection and sediment core data

Lake Elsinore, located 120 kilometers southeast of Los Angeles, California, is Southern California's largest natural lake. In December 2008, we collected over 75 km of high-resolution seismic reflection data using a CHIRP single-channel seismic-reflection Edgetech Geostar and the Geopulse(TM) Boomer systems. Interpretations reveal several distinct seismic sequences over the past 10,000 calendar years before present (cy BP- age control is based on dated sediment cores tied to the seismic lines). Four distinct stratigraphic sequences were identified from their erosional boundaries and internal reflection characteristics. Sequence IV (6,320 to 3,830 cy BP) is interpreted to represent short-lived, low amplitude lake-level regressions and transgressions that generated variable basin sedimentation. Sequence III shows evidence for a sustained lake level regression; this interval is interpreted as a lake level lowstand bracketed between ~ 3,830 and 1,940 cy BP. Sediment core data in the form of mudcracks, decreased average grain size, and high delta 18O(calcite) values from profundal and littoral environments corroborates the interpretation that Sequence III represents a significant lowstand. The timing of this late-Holocene lowstand is contemporaneous with a wide variety of data from the western and coastal southwest United States that indicates synchronous hydroclimatic change. Sequence II (1,940 to 40 cy BP) is interpreted as a rise in base level and a relatively sustained high lake level, which is adduced from a maximum flooding horizon in the seismic data. Finally, the upper seismic sequence (I: 40 to -53 cy BP) encompasses the 1950's drought during which Lake Elsinore desiccated. These millennial scale changes in the region's hydroclimatology are likely forced by complex ocean-atmosphere dynamics in the North Pacific that influence the frequency and/or magnitude of winter storms crossing Lake Elsinore's drainage basin.