Bidirectional permeability measurements of polar firn Gina L. LUCIANO, 1 Mary R. ALBERT 2 1 Department of Earth Sciences, Dartmouth College, Hanover, NH 03755, U.S.A. E-mail: gina.luciano@alum.dartmouth.org 2 Geophysical Sciences Division, U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, NH 03755-1290, U.S.A. ABSTRACT . Ice cores provide a valuable archive of climate history. For a complete understanding of this archive, it is important to understand air^snow exchange processes through the snow and firn in order to fully decode the ice-core record. Transport processes through the snow and firn are dependent upon their physical properties. In this paper, bidirectional permeabilities from selected sections of a 13 m core from Summit, Greenland, are presented. Differences between lateral and vertical permeabilities are evident through- out the core in permeameter data and in microstructure statistics. Both lateral and vertical permeabilities are consistent with overall patterns of previous polar permeability data with depth.The differences between lateral and vertical permeability measurements for some samples can be attributed to equivalent sphere radius. Further studies examining mean free-path length may be helpful in chemical modeling and in deriving an equation relating permeability to microstructure. INTRODUCTION Snow^air transfer processes are important in a variety of fields. Ice-core interpretation requires knowledge of the trans- fer of mass and chemical species through the snow and firn before they become embedded in the ice-core record. Recent findings in atmospheric chemistry suggest that some of the key processes for boundary-layer ozone depletion may in fact not be occurring in the atmosphere, but instead in the under- lying snowpack (Sumner and Shepson, 1999). The air^snow transfer processes affect the concentrations of the chemical species in the snow (Waddington and others, 1996; Gjessing, 1997), and are dependent on the physical properties of the snow. The primary material parameter for advective pro- cesses is permeability, which, for snow, reflects the nature of the interconnected pore space and controls the degree to which a fluid or gas may move through the snow. Most of the modeling studies of air advection through snow (ventilation) have assumed that the snow is uniform (e.g. Colbeck, 1989; CunninghamandWaddington,1993), and although a few have included varying permeability with depth (Albert, 1996; McConnell and others,1998), all have assumed that the per- meability is a scalar. Albert (1996) presented measurements of firn permeabilities at Summit, Greenland, that showed that firn permeability is directional for some samples; in particu- lar, samples that contained hoar layers had a much larger lat- eral permeability than a vertical permeability. The current work contains additional measurements and microstructure analysis on samples of uniform firn in order to assess the impact of crystal structure on the directional nature of perme- ability. These measurements will contribute to a better under- standing of air^snow transfer processes and will be useful in modeling for chemical transport through snow. METHODS Selected 10cm samples from a 13m firn core from Summit were obtained approximately 1m apart down through the firn. The layers analyzed were at least 10cm thick, so the same samples of snow could be measured for permeability in both lateral and vertical directions. In addition, they were layers that visually appeared to contain a uniform snow microstructure pattern (e.g. each sample was composed of only one layer of snow and had no visible stratigraphic hori- zons within the sample). The permeability of each sample was measured using an airflow permeameter (Albert and others, 2000). The airflow and pressure drop through the sample were measured at a variety of user-set flow rates. For flow rates in the linear regime, Darcy’s law was used to calculate the permeability given the measured flow rates and pressure drops across each sample. For each sample, 6^8 pairs of flow rates and pressure drops were measured, all of which almost perfectly fit upon a linear regression line ensuring the suitability of Darcy’s law to describe the flow. After the vertical perme- ability was measured for each sample, a core from the sample was drilled laterally and the permeability measured. Stratigraphic ``up’’ relative to the sample’s prior polar sur- roundings was noted because the microstructure analysis is directionally dependent. The density of each sample was measured using a balance. Thick sections representing vertical and lateral sections of the sample were cut and preserved using dimethyl phthal- ate pore filler for processing as described by Perla (1982). The vertical thick section was cut downwards through the core, and lateral sections were cut laterally through the core (Fig.1). Each section was then halved lengthwise and micro- Annals of Glaciology 35 2002 # International Glaciological Society 63 https://doi.org/10.3189/172756402781817095 Published online by Cambridge University Press