Collaborative Research: Firn structure, interstitial processes and the composition of firn air at Summit, Greenland

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ABSTRACTAlbertOPP-0520445SeveringhausOPP-0520564BattleOPP-0520460Intellectual merit: An important issue of our time involves questions of how human activity has been impacted by the atmospheric composition of our planet. A major area as part of the International Polar Year (IPY) involves developing a better understanding of past climates and the impact of anthropogenic activity on the Earth's atmosphere. Because instrumental records of atmospheric chemistry are limited, natural archives of atmospheric composition must be made, such as polar firn. Its porous nature, tens of meters in depth, permits interstitial diffusion of gases over time with the oldest air at the bottom of the firn column which allows the sampling of large quantities of pre-industrial air to explore anthropogenic effects on the atmosphere. This project will investigate the underlying physics controlling firn's ability to store atmospheric samples from the past. The Principal Investigators will make high-resolution measurements of the diffusivity profile, permeability profile, and accompanying microstructure at Summit from the surface to pore close-off, and compare the results to the diffusivity profile inferred from measurements of firn air chemical composition. They will partner with Dr. Atsumu Ohmura, Swiss Federal Institute, and Dr. Christophe Ferrari of LGGE, France. This project has four goals: 1) Quantify the dependence of interstitial transport processes on firn microstructure, and determine the dependence of gas diffusivity on microstructure characteristics from the surface down to the pore close-off depth; 2) Quantify post-depositional changes in the physical properties of snow and firn and use measured properties of firn and meteorological data to evaluate and develop models of the physical transport processes which drive firnification where temperature gradients are large. 3) Conduct firn air chemical measurements as the firn characteristics are determined, and compare the co-registered diffusivity profile inferred from the firn air chemistry measurements to the high-resolution tracer gas measurements made on the firn core itself. 4) 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. Broader Impacts: This study will establish quantitative relationships that will enable a better understanding of the firn as a repository of past atmospheric composition, but will also enable us to understand mechanisms that may impact firn air composition at other sites. Results of the research will be published in journal articles and made widely available. This project will form one part of the PhD dissertation of a student from Dartmouth. Several undergraduates will be involved. They will interact with students from Switzerland and France to design and construct an IPY museum exhibit, at the Montshire Museum of Science in Norwich, Vermont. The exhibit will be interactive and will illustrate the ability of snow and firn to serve as an archive of important events of the past. It will allow the viewer to act as the "detective" to track down the meaning of different chemical composition profiles in the firn air.