The impact of microstructure and physical properties on pore close-off at WAIS Divide and Megadunes

by Gregory, Stephanie Ann, M.S., DARTMOUTH COLLEGE, 2013, 106 pages; 1553169

Abstract:

The depth at which atmospheric air becomes entrapped into bubbles at the bottom of the firn column plays an important role in creating an accurate age difference (Δage) between the age of the gas and the age of the ice at a given depth within an ice sheet. The top 50–120m of an ice sheet consists of polar firn, a porous medium, acting as a filter smoothing atmospheric signals as air rapidly diffuses to the base of the firn column on a decadal time scale. The density, permeability and microstructure of two firn cores retrieved from the Antarctic Ice Sheet, from different local climates, WAIS Divide and Megadunes, were examined. The microstructure of the firn was determined to have a greater impact on permeability and pore close-off than density had.

Comparison between sites shows that, at a given density, the pore structure of Megadunes firn was less complex and tortuous with greater permeability than WAIS Divide firn, characterized by smaller grains and a more complex pore structure. For both WAIS Divide and Megadunes, fine grained layers experience close-off shallower in the firn column than do coarse grained layers, regardless of which grain sized layer is the more dense layer at depth. Pore close-off occurs at an open porosity that is accumulation rate dependent. Low accumulation sites, with coarser grains, close-off at lower open porosities (<10%) than the open porosity (>10%) of high accumulation sites with finer grains. The greater the variability between fine grained and coarse grain firn layers at depth, the longer the lock-in zone will be. At Megadunes, the fine grained layers are remnants of past accumulation sites on the ice sheet surface. These layers reach pore close-off significantly shallower depths than densification models predict for polar sites with similar local climate. A shallow lock-in depth, the point where air no longer exchanges with the atmosphere, would decrease Δage to values similar to those predicted through δ15N reconstructions of firn thickness. Application of this work in future studies could reduce the data-model mismatch, improve modeling of the lock-in zone through firn densification models, and reduce the need for inverse firn air modeling.

AdviserMary Albert
SchoolDARTMOUTH COLLEGE
Source TypeThesis
SubjectsPaleoclimate science; Materials science
Publication Number1553169

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