LAKE OXYGEN ISOTOPES AS RECORDERS OF NORTH AMERICAN ROCKY MOUNTAIN HYDROCLIMATE

Oxygen isotope ratios (δ¹⁸O) of lake water contributes unique paleoclimatic information about the water cycle of terrestrial environments outside Polar Regions. Lake water isotopes, and corresponding sedimentary carbonate records, may reflect changes in effective moisture (precipitation minus evaporation, P-E), lake level (and volume), and surface-groundwater interactions. In unique cases, lake water δ¹⁸O most nearly reflect δ¹⁸O of precipitation, as ice cores, which is controlled by the temperature of condensation, vapor transport and source history, and in temperate environments with strong seasonal precipitation imbalances, precipitation seasonality. We use a compilation of ~600 lake water δ¹⁸O measurements within the greater western North American cordillera, from Alaska to Colorado, to provide a framework for understanding the δ¹⁸O relationships between sediment carbonate (endogenic calcite and aragonite), lake water, and precipitation. We identify eight regional groups from the lake water data and determine linear δ¹⁸O and δD correlations, or Regional Evaporation Lines (REL), to identify the REL intercept with the Global Meteoric Water Line (GMWL) which varies by latitude, continental position, and elevation similarly with precipitation-δ¹⁸O. Within this framework, we evaluate the 18 existing lake carbonate Holocene records and characterize each as an “isometer’ of the climate processes that control precipitation-δ¹⁸O, P-E, or a mixture of both. Of the 18 records, 14 are P-E or mixed isometers. Four lake records are precipitation isometers and two span the past 8 ka with decadal to centennial scale resolution. Understanding the sensitivity of lake water to evaporation-driven fractionation effects is critical for correctly assessing the paleoclimate information provided by lake carbonate-δ¹⁸O. Our evaluation further elucidates why comparisons of Holocene carbonate-δ¹⁸O variations between different lake systems must account for differences in sensitivity resulting from regional climate and extent of hydrologic closure.