Microstructure of West Antarctic Firn and its Effect on Air Permeability
ENGINEER RESEARCH AND DEVELOPMENT CENTER HANOVER NH COLD REGIONS RESEARCH AND ENGINEERING LAB
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The microstructure of snow and firn has a great impact on the transport of chemical species from the atmosphere to the underlying firn. For improved ice core interpretation, it is important to understand air-snow interactions within the firs layers and how they are affected by snow microstructure. Permeability and thick-section microstructure measurements have been made from snowpit and firn core samples retrieved during the U.S.-International Trans-Antarctic Science Expedition ITASE 1999-2001 field seasons. Our measurements have shown that the permeability of the snow at all of the sites generally increases with depth into the firn to about 3 m, then decreases due to microstructure changes, although at several sites there were areas of increased permeability at depth because of local changes in weather and climate. Thick-section microstructure measurements show that the grain size generally increases with depth, and the specific surface decreases with depth. Rapid grain growth is caused by diurnal and seasonal temperature gradients near the surface. Deeper in the core, the grain growth slows as the firn temperature gradients become small. The grain growth and specific surface trends do not follow those of the permeability. Pore size correlates well with the permeability of the snow samples a formula was developed that predicts the firs permeability from pore characteristics. in addition to variation with depth in the core, the permeability and microstructure vary greatly from site to site, revealing that meteorological-effects, such as accumulation rate and mean annual temperature, are important factors in shaping the firn micro structure. High accumulation rates or low mean annual temperatures will result in low permeability due to little or no metamorphism in the firn, although accumulation rate seems to be the dominant factor.
- Snow, Ice and Permafrost