A 9,000-YEAR LACUSTRINE RECORD OF LONG-TERM CLIMATE CHANGE AND ABRUPT CLIMATE EVENTS FROM DRY LAKE, SOUTHERN CALIFORNIA, USA

Lacustrine sediment cores from Dry Lake (2763 m), located in the San Bernardino Mountains of southern California, provide the one of the first long-term, high-resolution records of terrestrial Holocene climate change from coastal southwestern North America. Twenty-seven AMS ¹⁴C dates, in conjunction with multi-proxy analyses, indicate a dynamic Holocene climate in response to long-term changes in seasonal insolation and non-linear responses to ocean-atmosphere forcing. The early Holocene (9000 to 7500 cal yr B.P.) is characterized by high sedimentation rates, sand content, and organic matter, and low CHI, indicating that the climate was significantly wetter than today in response to an insolationally strengthened North American Monsoon (NAM). At 7500 cal yr B.P., an abrupt shift to lower sedimentation rates, decreased sand and organic content, and increased CHI indicate a climate shift to drier, less stormy conditions, as waning insolation reduced effective NAM precipitation. This drying trend persisted through the middle Holocene (7500 to 4000 cal yr B.P.) and intensified into the late Holocene (4000 cal yr B.P. to present) as indicated by continued decreases in sedimentation rates, sand content, and CHI. In addition to long-term climate trends, the Dry Lake record also documents higher-frequency climate events, such as the 8.2 ka event. The 8.2 ka event registers as a 300-year cool period characterized by reduced monsoonal precipitation, depressed basin productivity, and increased erosion. Additional abrupt events also occur throughout the Holocene, and they display quasi-centennial periodicity. A comparison of these events to regional climate records from western North America show some spatial correlations between abrupt events, suggesting a regionally coherent climate response at short time scales. An attempt to link these abrupt events to solar variability, North Atlantic ice rafting events, and the GISP ice core record, however, did not reveal any conclusive correlations. We speculate, therefore, that these quasi-centennial storm events may be a response to stochastic variability in the Pacific ocean/atmosphere climate system.