Isotopes in North American Rocky Mountain snowpack 1993–2014

We present ∼1300 new isotopic measurements (δ¹⁸O and δ²H) from a network of snowpack sites in the Rocky Mountains that have been sampled since 1993. The network includes 177 locations where depth-integrated snow samples are collected each spring near peak accumulation. At 57 of these locations snowpack samples were obtained for 10–21 years and their isotopic measurements provide unprecedented spatial and temporal documentation of snowpack isotope values at mid-latitudes. For environments where snowfall accounts for the majority of annual precipitation, snowmelt is likely to have the strongest influence on isotope values retained in proxy archives. In this first presentation of the dataset we (1) describe the basic features of the isotope values in relation to the Global Meteoric Water Line (GMWL), (2) evaluate space for time substitutions traditionally used to establish δ¹⁸O-temperature relations, (3) evaluate site-to-site similarities across the network and identify those that are the most regionally representative, (4) examine atmospheric circulation patterns for several years with spatially coherent isotope patterns, and (5) provide examples of the implications this new dataset has for interpreting paleoclimate records (Bison Lake, Colorado and Minnetonka Cave, Idaho). Results indicate that snowpack δ¹⁸O is rarely a simple proxy of temperature. Instead, it exhibits a high degree of spatial heterogeneity and temporal variance that reflect additional processes such as vapor transport and post-depositional modification. Despite these complexities we identify consistent climate-isotope patterns and regionally representative locations that serve to better define Holocene hydroclimate estimates and their uncertainty. Climate change has and will affect western U.S. snowpack and we suggest these changes can be better understood and anticipated by oxygen and hydrogen isotope-based reconstructions of Holocene hydroclimate using a process-based understanding of the controls on snowpack isotope ratios.