Snow and Firn Microstructure and Transport Properties: U.S. ITASE

Dr. Mary Albert doing permeability measurements on a firn sample at Siple Dome, Antarctica.

The goal of this project is to measure the properties of the snow and firn at the coring sites along the ITASE traverse and to model the processes that happen in the snow and firn. Firn is snow that is more than one year old. Because there is not any, or hardly any, melt at the traverse sites, the snow just keeps accumulating over the centuries. New snow accumulates on top of the old snow and firn and the weight of the snow and young firn on the underlying old firn becomes so great that the old firn becomes compacted more and more, eventually turning into ice. The reason that the ITASE cores are being drilled is to determine what the past climate was like, by looking at the concentrations of various chemical concentrations down through the core. Mostly it is assumed that the concentration at some depth in the ice reflects what the atmosphere contained, back in the time when those ice crystals were snow on the surface. For some chemical species, when the wind blows and the sun shines on the snow, it could change the nature of the chemical concentrations from the top down to a depth of about several meters. Below that, diffusion through the pore space in the snow can cause slower chemical changes. The purpose of this project is to measure the properties of the snow and firn in the top 50 feet (15 m) and to determine whether changes in the near-surface snow and firn properties could cause changes in the processes in the snow and firn there that affect the chemical concentrations.

Snow and firn are highly layered and the layering affects the transport processes.

The traverse team will measure near-surface snow and firn properties, which include permeability, density, stratigraphy, grain size, surface-to-volume ratio, and specific surface. Snow and firn are highly layered because of differences in the way that the snow accumulates and because of aging. The stratigraphy notes the snow layering. Snow density and grain size give an indication of the type of snow. Permeability is the parameter that controls how much air can flow through the pore space in snow. The surface-to-volume ratio and specific surface area measurements done on preserved sections of snow that tell us how much area is available on the surface of the snow for chemical interactions, for example. The stratigraphy, density, permeability, and preservation of snow samples will be done in snow pits dug along the traverse. Firn cores will also be drilled and shipped back to CRREL in New Hampshire, where we will conduct similar measurements on the deeper layers. Once we have measured the properties of the snow and firn, we will use the measurements in our computer model that simulates the effects of air flow and diffusion in the snow, and try to determine whether there are differences in the transport processes at the different sites along the traverse. We will work with chemists to combine our modeling with chemical modeling to explain the observed changes in chemical concentrations. We will also try to develop a model to predict the way that snow changes over time as it is buried, a process called firnification. There are other applications of this data: the snow and firn properties will also provide ground truth to other investigators who are using remote sensing to map the spatial variations of snow, firn, and ice properties.

Snow and firn are highly layered porous media. The image to the left depicts the difference in microstructure that exists in Antarctic snow between a sample taken at a depth of 6.5 feet (2 m) and one taken at 4 inches (10 cm) deep. You can see that the wind-packed snow on the surface has much finer grains (crystals) than the deeper snow. This affects the permeability and other transport parameters. Before looking at an image like this, would you think that a more dense snow would have higher permeability?

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