HYDROGEN AND OXYGEN ISOTOPIC COMPOSITION OF LAKES IN THE WESTERN UNITED STATES RELATED TO HYDROLOGY AND SEASONALITY OF PRECIPITATION

Lake water was sampled from 100 lakes in close proximity to 45 SNOTEL stations across the western U.S. in the summer of 2006. Sampled lakes include a diversity of basin characteristics and sizes, and span different seasonal precipitation regimes. Analyses of hydrogen and oxygen isotopes in the water show coherent regional values of lake source water, which evolved locally along well-defined local evaporation lines. The isotopic composition of source water differs from the weighted average isotopic value of precipitation by only 0.07 - 0.59 in most areas, reflecting seasonal precipitation regimes. However, in northern New Mexico and Southern Colorado source water differs by 2.10 – 2.28 per mil from the weighted average isotopic value of precipitation due to a skew towards summer monsoon precipitation. Differences among lakes within a given climate region reflect higher evaporation rates at lower elevations in some areas, but the influence of elevation is overridden by evaporative evolution either within lakes or their watersheds in other areas. 68% of the variance in intra-regional isotope values is explained by hydrologic residence time, elevation, latitude, and seasonality of precipitation. From these results, we confirm that changes in seasonal energy and moisture balance could change average values of source composition, while changes in lake characteristics could change the way that a particular lake's hydrogen or oxygen isotope values track climate conditions. Paleoclimate studies may track these influences by comparing records of lakes in the same regions with different evaporative sensitivities, and thus improve interpretations of the isotope values of lake sediment components and reconstruct climate parameters over time.