RECONSTRUCTING LIMNOLOGICAL AND HYDROLOGICAL CHANGE DURING THE LATE GLACIAL TO EARLY HOLOCENE AND TRACKING THE WESTERN UNITED STATES PRECIPITATION DIPOLE USING SEDIMENTS FROM BARLEY LAKE

A sediment core from Barley Lake (Mendocino County, CA) was studied to describe and explain past limnologic and hydrologic variability across the late Glacial to Holocene transition (12,850 – 8,130 calendar years before present (cal yr BP)). Barley Lake is located just south of the modern boundary of the western United States precipitation dipole (~40° N latitude), and thus it represents a key location for understanding dipole variability over time. To describe past limnologic and hydrologic variability, a combination of physical and chemical analyses, including magnetic susceptibility, total organic matter, total carbonate, total organic carbon, bulk d¹³C_{(bulk organic carbon)}, and grain size were used. An age model was constructed using Bacon version 2.2 based on twelve radiocarbon dates on discrete organic materials. The data reveal two distinct lake states: 1) a variable to deep lake with low productivity during the late Glacial (12,850 – 11,500 cal yr BP), and 2) a shallow to transgressive lake with high productivity during the early Holocene (11,500-8,150 cal yr BP). The former encapsulating the Younger Dryas chronozone, suggesting wet conditions in Northern Coast Range during the Younger Dryas. Correlative to the Younger Dryas chronozone, paleo-archives in the Pacific northwest, above 35° N latitude, indicated wet and/or variable dry/wet conditions; whereas, paleo-archives in the Pacific southwest United States, below 35° N latitude, indicated dry and/or variably dry/wet conditions. This analysis indicates a southerly shift in the average position of the western United States precipitation dipole during the Younger Dryas. The large-scale changes in lake conditions at Barley Lake during the late-Glacial/Younger Dryas to early Holocene is attributed to complex ocean-atmosphere restructuring coupled with changing summer-winter insolation forcing.