Isotopic evolution of a seasonal snowpack and its melt

The study of isotopic variation in snowmelt from seasonal snowpacks is usef ul for understanding snowmelt processes and is important for accurate hydro graph separation of spring runoff. However, the complex and variable nature of processes within a snowpack has precluded a quantitative link between t he isotopic composition of the original snow and its melt. This work studie s the isotopic composition of new snow and its modification by snow metamor phism and melting. To distinguish individual snowstorms, we applied solutio ns of rare earth elements to the snow surface between storms. The snowmelt was isotopically less variable than the snowpack, which in turn was less va riable than the new snow, reflecting isotopic redistribution during metamor phism and melting. The snowmelt had low delta O-18 values early in the seas on and became progressively enriched in O-18 as the pack continued to melt. On a given day, meltwater delta O-18 was systematically lower whenever mel t rates were low than when melt rates were high. The progressive enrichment in delta O-18 of snowmelt and the dependence of delta O-18 on melt rates c an be explained by isotopic exchange between liquid water and ice. A one-di mensional (l-D) model of the melting process, including advection and water -ice isotopic exchange kinetics, reproduces the observed progressive O-18 e nrichment of snowmelt.