JIRP 2015 Student Project: Stable Water Isotopes to Examine Moisture Transport and Snowpack Evolution on the Juneau Icefield.
Project leader: J. Kavanaugh
This study will use measurements of the stable water isotopic ratios δ18O and δD (see Footnote #1) to examine several aspects of the Icefield’s hydrology and snowpack. These isotopic ratios are influenced by a range of important environmental parameters, including temperature, relative humidity, phase transitions, and transport path characteristics, and can thus be used to examine the movement of water through the hydrological cycle. The proposed research project will examine isotopic signatures of both freshly-fallen snow (to examine lateral and vertical gradients in isotopic values) and the upper several meters of the snow and firn pack. An additional potential project will track the change in isotopic content of one or several JIRP participants as they cross the icefield. Although not confirmed at this time, it is possible that a portion of the isotopic analyses will be performed on the icefield using a Los Gatos Water Isotope Analyzer, which can determine δ18O and δD values from samples. The remaining samples (and duplicates of some or all samples analyzed on the icefield) will be analyzed at the University of Alaska Anchorage.
Students participating in this project will read papers selected to demonstrate the use of water isotopic techniques to both cryospheric research in particular and Earth system science in general. Students involved in this project will have the option to either complete their contributions at or near the end of the summer field expedition (“Level 1”) or to extend their involvement through the Fall semester (“Level 2”).
1. Examining changes in isotopic ratios along lateral and vertical gradients. As moisture is transported from its source region inland, its isotopic signature changes as the result of (a) Rayleigh distillation (whereby moisture becomes progressively more depleted in heavy isotopes as less and less of the original moisture remains) and (b) the temperature dependence of isotopic fractionation upon phase change (e.g., condensation from the vapor phase). Snow samples will be collected along both lateral (i.e., along moisture path) and vertical (i.e., elevational) transects in order to tease out horizontal and vertical isotopic gradients. Ideally, these transects will be sampled in as short a time period as practical, and at least twice: the first during or shortly after a fresh snowfall (if conditions are deemed safe to do so) to capture unmodified isotopic values and the second after the snowpack has been exposed to several freeze/thaw cycles and other aging effects that could modify the isotopic signature. (Level 1 and 2) Following completion of JIRP, Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) models will be used to determine the air mass trajectory for the sampled precipitation events to determine along-path distances and moisture source characteristics. (Level 2)
2. Examining isotopic variations within the snowpack. A 2014 student study of isotopic signatures in snowpits indicated that water contained in ice lenses was generally isotopically lighter (i.e., more depleted in heavy isotopes) than was water contained in the surrounding snow. This difference is of interest because it can be used to examine whether ice lenses form from rainfall events, from the refreezing of melted snow, or from a combination of these two mechanisms. Students in 2015 will examine the isotopic signature of ice lenses, and the snow immediately above and below them, in much greater detail than was done in 2014, in order to address this question.
Additional work will be performed to examine the evolution of isotopic signatures with aging of the snow and firn. First, one or more snow pits will be dug to reveal two years’ worth of accumulated snow and firn (i.e., one year’s greater accumulation than typical). Firn samples in the layer dating from 1-2 years (i.e., corresponding to the snow sampled during JIRP 2014) will be analyzed, and isotopic values will be compared to those obtained in 2014 to determine the magnitude of change. Second, snow and firn will be sampled from the exposed faces of several crevasses and analyzed to determine whether isotopic values vary significantly (due to atmospheric exposure and possible meltwater contamination) from those obtained from snow and firn samples in nearby snow pits. Ideally, the multi-year snow/firn pits will be dug in locations that (a) were sampled for isotopic analysis in 2014 and (b) are near crevasses suitable for study. (Levels 1 and 2)
1These so called “delta values” are measures of the ratio of “heavy” vs “light” water molecules (e.g. those with 18O vs 16O isotopes, respectively) in any sample compared to a global standard.
Dansgaard, Willi. "Stable isotopes in precipitation." Tellus 16.4 (1964): 436-468.
Merlivat, Liliane, and Jean Jouzel. "Global climatic interpretation of the deuterium‐oxygen 18 relationship for precipitation." Journal of Geophysical Research: Oceans (1978–2012) 84.C8 (1979): 5029-5033.
Jouzel, Jean, and Liliane Merlivat. "Deuterium and oxygen 18 in precipitation: modeling of the isotopic effects during snow formation." Journal of Geophysical Research: Atmospheres (1984–2012) 89.D7 (1984): 11749-11757.
Kavanaugh, J. L., and Kurt M. Cuffey. "Space and time variation of δ18O and δD in Antarctic precipitation revisited." Global Biogeochemical Cycles 17.1 (2003).
Dansgaard, Willi, et al. "A new Greenland deep ice core." Science 218.4579 (1982): 1273-1277.