HYDROLOGIC AND ISOTOPE MASS-BALANCE MODELING OF SMALL LAKE SYSTEMS AND IMPLICATIONS FOR RECONSTRUCTING HOLOCENE HYDROCLIMATE USING LACUSTRINE SEDIMENT OXYGEN ISOTOPE RECORDS

Computational models are essential for the quantitative interpretation of lacustrine sediment oxygen isotope (δ¹⁸O) records, which can be used to reconstruct hydroclimatic conditions on timescales of decades to millennia. Here we present a coupled hydrologic and isotope mass-balance model designed for small, surficially closed-basin lakes that provides insight on lake sensitivity to changes in climate (e.g. air temperature, relative humidity, precipitation). The model simulates lake volume/level and water and sediment δ¹⁸O signatures through time using a system of differential equations. Two lakes, Castor and Scanlon, located in eastern Washington (USA), were modeled in order to quantitatively interpret their sediment records through determination of the hydroclimate conditions that could produce the measured geochemical and sedimentological characteristics, and to anticipate the potential responses of these (and similar) lake systems to future climate change. Model validation was achieved by comparing observational data with model outputs over the same timeframe. Steady-state sensitivity experiments conducted using 13 years of daily on-site weather station data as model inputs indicate that lake level and water δ¹⁸O ratios vary strongly with changes in drought-related climate variables, especially the timing and amount of precipitation. Additional experiments were carried out on Lake Kail (western Guatemala highlands) with 40 years of continuous daily satellite weather data as model inputs. The results were compared to lake-level changes estimated using satellite imagery and ground observations, as well as measured water δ¹⁸O signatures, and indicate that the model is capable of approximating Lake Kail responses to hydroclimate change. This study demonstrates the applicability of the model to the interpretation of isotope records from small lakes that exhibit substantial changes in lake level and volume in response to hydroclimate variability.