QUANTITATIVE RECONSTRUCTION OF LACUSTRINE SEDIMENT OXYGEN ISOTOPE RECORDS OF HYDROCLIMATE CHANGE IN THE PACIFIC NORTHWEST THROUGH THE APPLICATION OF A HYDROLOGIC AND ISOTOPE MASS-BALANCE MODEL

Although oxygen isotope records from lake sediments are widely used to reconstruct basin- and regional-scale hydroclimatic change on decadal to millennial timescales, interpretations of these records typically lack a quantitative basis. The application of modeling approaches allow not only the evaluation of uncertainties within the proxies and the records, but also a more detailed determination of how proxies respond within specific systems. In this study, we present the calibration and application of a hydrologic and mass-balance model to two small (<0.5 km² surface area), surficially closed-basin lakes: Castor and Scanlon in eastern Washington (USA), in order to investigate the controls on water balance and isotopic signatures of lake water and authigenic carbonate minerals (aragonite and calcite). The model was calibrated with 13 years of daily climate and lake-level data from an in-situ weather station and level logger at Castor lake, as well as discrete δ¹⁸O values from water samples from the same time period, and validated using a compilation of 100 years of weather data from nearby weather stations. Sensitivity tests were conducted in order to estimate lake responses to specific changes in hydroclimate variables, such as air temperature, precipitation and relative humidity, and thereby quantitatively interpret climate signals in the oxygen isotope data, as well as assess potential influences of future climate change on lake water balance. Model based reconstructions of hydroclimate variables for various time periods including the middle and early Holocene provide quantitative insight into the extent of variability and timing of drought events in the Pacific Northwest. Results show that winter precipitation is the strongest control on both the water balance and carbonate δ¹⁸O values of these systems and that soil water capacity changes due to the deposition of volcanic ash might also play a role in lake hydrologic balance on long timescales. This study demonstrates the importance of a computational modeling approach to developing a thorough understanding of lake system responses to climate change, as well as to the reconstruction of past changes in hydroclimate through the quantitative interpretation of oxygen isotope signals in lake sediment.