Isotope variations in a Sierra Nevada snowpack and their relation to meltwater

Isotopic variations in melting snow are poorly understood. We made weekly measurements at the Central Sierra Snow Laboratory, California, of snow temperature. density, water equivalent and liquid water volume to examine how physical changes within the snowpack govern meltwater delta(18)O. Snow,pack samples were extracted at 0.1 m intervals from ground level to the top of the snowpack profile between December 1991 and April 1992. Approximately 800 mm of precipitation fell during the study period with delta(18)O values between -21.35 and -4.25parts per thousand. Corresponding snowpack delta(18)O ranged from -22.25 to -6.25parts per thousand. The coefficient of variation of delta(18)O in snowpack levels decreased from -0.37 to -0.07 from winter to spring, indicating isotopic snowpack homogenization. Meltwater delta(18)O ranged from -15.30 to -8.05parts per thousand, with variations of up to 2.95parts per thousand observed within a single snowmelt episode, highlighting the need for frequent sampling. Early snowmelt originated in the lower snowpack with higher delta(18)O through ground heat flux and rainfall. After the snowpack became isothermal, infiltrating snowmelt displaced the higher delta(18)O liquid in the lower snowpack through a piston flow process. Fractionation analysis using a two-component mixing model on the isothermal snowpack indicated that delta(18)O in the initial and final half of major snowmelt was 1.30parts per thousand lower and 1.45parts per thousand higher, respectively, than the value from simple mixing. Mean snowpack delta(18)O on individual profiling days showed a steady increase from -15.15 to -12.05parts per thousand due to removal of lower delta(18)O snowmelt and addition of higher delta(18)O rainfall. Results suggest that direct sampling of snowmelt and snow cores should be undertaken to quantify tracer input compositions adequately. The snowmelt sequence also suggests that regimes of early lower delta(18)O and later higher delta(18)O melt may be modeled and used in catchment tracing studies. (C) 2002 Elsevier Science B.V. All rights reserved.