THE INFLUENCE OF CLIMATE AND VOLCANIC ASH FALL ON LAKE LEVELS IN THE PACIFIC NORTHWEST DURING THE HOLOCENE (Invited Presentation)

Paleoproxy records of lake level change can provide valuable information on past variability in regional hydroclimate. The oxygen isotopic composition (δ¹⁸O) of authigenic carbonates in lake sediment is one such proxy, affording reconstructions of lake water δ¹⁸O values, which are controlled by changes in lake volume, the proportion of water lost through evaporation, and δ¹⁸O values of precipitation. Here we present multiple, continuous Holocene records of lake-level change based on sedimentological interpretations and oxygen isotope analyses of authigenic carbonates. The study sites are distributed across the interior Pacific Northwest from central British Columbia to Washington and Montana, providing a broad spatial perspective on regional climate change. Collectively the records indicate that lake levels varied substantially throughout the Holocene, and that the principal driving mechanism was climate change on multidecadal to millennial timescales. An additional and perhaps less well studied control on lake level is pedogenic responses to volcanic ash fall and the role of soil evolution in determining catchment water storage and runoff. We apply lake-catchment hydrologic and isotope mass balance models to investigate the responses of lakes to changes in precipitation-evaporation balance driven by externally forced changes in climate, and to presumed variations in catchment water retention in response to ash fall from the Mazama climactic eruption. The study lakes occur well within, near the edge, or outside the range of the ash fall, and are therefore well positioned for disentangling the climate signal from potential catchment hydrological responses to the sudden deposition of tephra and its subsequent weathering, erosion and incorporation into soils. We further assess the climate change signal afforded by these records in the context of proxy and climate model based evidence for Pacific basin ocean-atmosphere variability. This research has implications for the improvement of water resource policies in western North America that currently are based on a timeframe too short to fully characterize the magnitude and frequency of natural drought/pluvial cycles.