This award supports a project to investigate the transformations from snow to firn to ice and the underlying physics controlling firn's ability to store atmospheric samples from the past. Senior researchers, a graduate student, and several undergraduates will make high-resolution measurements of both the diffusivity and permeability profiles of firn cores from several sites in Antarctica and correlate the results with their microstructures quantified using advanced materials characterization techniques (scanning electron microscopy and x-ray computed tomography). The use of cores from different sites will enable us to examine the influence of different local climate conditions on the firn structure. We will use the results to help interpret existing measurements of firn air chemical composition at several sites where firn air measurements exist. There are three closely-linked goals of this project: to quantify the dependence of interstitial transport properties on firn microstructure from the surface down to the pore close-off depth, to determine at what depths bubbles form and entrap air, and investigate the extent to which these features exhibit site-to-site differences, and to use the measurements of firn air composition and firn structure to better quantify the differences between atmospheric composition (present and past), and the air trapped in both the firn and in air bubbles within ice by comparing the results of the proposed work with firn air measurements that have been made at the WAIS Divide and Megadunes sites. The broader impacts of this project are that the study will this study will enable us to elucidate the fundamental controls on the metamorphism of firn microstructure and its impact on processes of gas entrapment that are important to understanding ice core evidence of past atmospheric composition and climate change. The project will form the basis for the graduate research of a PhD student at Dartmouth, with numerous opportunities for undergraduate involvement in cold room measurements and outreach. The investigators have a track record of successfully mentoring women students, and will build on this experience. In conjunction with local earth science teachers, and graduate and undergraduate students will design a teacher-training module on the role of the Polar Regions in climate change. Once developed and tested, this module will be made available to the broader polar research community for their use with teachers in their communities.
Investigations into the physical characteristics of deep firn near the lock-in zone through pore close-off improve understanding the process of natural archival of past atmospheric composition in the polar ice sheets. The permeability and microstructure profiles of the firn through the diffusive column influence the entrapment of air into bubbles. The purpose of this study is to examine the nature of pore closure processes at two polar sites with very different local temperatures and accumulation rates. Density, permeability, and microstructure measurements were made on firn cores from the West Antarctic Ice Sheet (WAIS) Divide, a site that has moderate accumulation rates with a seasonal climate archive, and Megadunes in East Antarctica, a site that is a natural laboratory for accumulation rate effects in the cold low-accumulation desert. We found that the open pore structure plays a more important role than density in predicting gas transport properties, throughout the porous firn matrix. For firn below 50 m depth at both WAIS Divide and Megadunes, finer-grained layers experience close-off shallower in the firn column than do coarser-grained layers, regardless of which grain size layer is the denser layer at depth. Pore close-off occurs at a critical open porosity that is accumulation rate dependent. The microstructure and permeability even near the bottom of the firn column are relic indicators of the nature of accumulation when that firn was at the surface. The physical structure and layering are the primary controlling factors on pore close-off. In contrast to current assumptions for polar firn, the depth and length of the lock-in zone is primarily dependent upon accumulation rate and microstructural variability due to differences in grain size and pore structure, rather than the density variability of the layers. This project formed the basis for the Master’s Thesis of Stephanie Gregory at the Thayer School of Engineering at Dartmouth, and the Undergraduate Honors Thesis of Alden Adolph at Dartmouth. In addition, five female undergraduate Dartmouth students participated in the lab studies and decided to remain in science and engineering majors at Dartmouth. The following journal papers resulted from this work: Gregory, S.A., M.R. Albert, and I. Baker, 2014. Impact of physical properties and accumulation rate on pore close-off in layered firn. The Cryosphere 8, 91-105. doi:10.5194/tc-8-91-2014 Adolph, A., and M.R. Albert, 2013. An Improved Technique to Measure Firn Diffusivity. International Journal of Heat and Mass Transfer, v. 61, p. 598-604. Adolph, A.C. and M.R. Albert, 2014. Gas diffusivity and permeability through the firn column at Summit, Greenland: measurements and comparison to microstructural properties. The Cryosphere, 8, 319-328, doi:10.5194/tc-8-319-2014.
