Near-Surface Processes Affecting Gas Exchange: West Antarctic Ice Sheet

Mary Albert

This project investigates the physical processes that affect the manner in which heat, vapor, and chemical species in air are incorporated into snow and firn. The research involves both field measurements at Siple Dome, Antarctica, and mathematical modeling. Field measurements of snow and firn stratigraphy, permeability, and density were taken in the top 20 m. Permeability measurements reveal values ranging from 10 to 80 x 10-10 m2, being highly dependent both on layer type and depth. The lowest permeability in the top 10 m was measured in the surface windpack. The permeability generally increased with depth down to a maximum between 3 to 4 m, and decreased below that. Firn density, which generally increases in the top 20 m, is not a good indicator of permeability. These results point to the possibility of increased subsurface mixing of gases in the top several meters of firn when the forces driving the mixing are present. Thick-section samples of snow and firn microstructure were obtained in the field for quantitative microscopy and grain scale characteristics. The microstructure imaging suggests that the permeability profile is due to grain growth and compaction.

Inert tracer gas experiments were conducted to investigate gas diffusion rates in near-surface snow and firn. Diffusion coefficients for SF6 varied according to microstructure and ranged between 0.02 to 0.06 cm2/s. For snow and polar firn, the diffusion coefficients can be related to permeability through the formation factor concept.

Numerical modeling of snow and firn ventilation, using the measured permeability was conducted. For surface roughness with characteristic wavelength 3m, the firn is sufficiently permeable that interstitial transport due to ventilation could occur within the top meter at rates higher than diffusion rates. Surface roughness with wavelength 10 m could increase the effective depth of ventilation to approximately 2 m. The impact of ventilation on various species may vary, due to differences in advective-diffusive balance between interstitial airflow and overall species gradients driving diffusion.

The broader impacts of this study included both educational aspects and impacts on other research disciplines. Undergraduate students were involved in the research via the NSF- REU program, Dartmouth's Women in Science (undergraduate) program, and DoD's High School Apprenticeship program. A high school science teacher participated in the research through the NSF-TEA program. An hour-long interview/discussion of this research was broadcast throughout New England by radio station WNTK, and presentations were made each year of the grant to local elementary school students. Through this project and other air- snow transfer projects funded in support of ice core interpretation, members of the international atmospheric chemistry community became aware that the snow might play a first-order role in changing atmospheric chemistry profiles. Starting from being an invited speaker at the international Polar Sunrise meeting (of atmospheric chemists), Dr. Albert is now working with atmospheric chemists from U.S., Canada, France, and Italy in experiments and modeling studies that are serving to change the paradigm in the field of atmospheric chemistry over snow covered areas.