This summer I investigated the physical properties of partially dense ice, or firn, in the near surface regions of ice caps and glaciers. We explored the influence of physical variables present in nature by compacting samples of synthetically fabricated firn in a controlled lab environment. To date we have primarily studied grain size influences and to a lesser extent impurity content influence. Measuring the permeability evolution of gas flow through the sample is the main additional feature yet to be incorporated in our experiment.
The two processes of focus in our experiments are densification and the closure of air channels and pockets between ice grains. A key problem in establishing a history of the CO2 record in deep ice cores is the unknown lag between the (older) age of the firn at the bubble close off depth and the (younger) age of the gas trapped within the bubbles. Establishing accurate dates for the climate record in ice cores thus requires fundamental knowledge of the densification of porous ice. Such knowledge is also critical for converting satellite measurements of changes in the surface heights of ice sheets and glaciers into changes in ice mass to assess the response of these ice bodies to a warming climate.
Layers of firn both at any given site and between different sites exhibit co-varying material and environmental properties. This co-variation results in drastic changes between different near surface regions of ice caps. An influential material property is inter-layer grain size variation, a result of past surface accumulation rates. Impurity inputs from sea salt and soil dust, which are heightened during glacial periods, have also been suggested as a strong physical parameter. We thus conducted constant-load compaction experiments on lab-fabricated ice samples to explore the influence of these material properties on densification. Our preliminary results suggest the expected grain sensitivity in the two densification mechanism regimes of interest: frictional, mechanical packing and plastic creep. An exciting feature of our experiments is the compaction of very fine-grained samples (10 m) which allows us to explore grain sensitive creep mechanisms under lab timescales. Furthermore, we have begun to demonstrate impurity content’s acceleration of densification.
Using both empirical data and theoretical, constitutive equations we aim to build upon our findings this summer to fine tune our understanding of compaction mechanisms in ice. Applying myself in experimental geophysics, a field I have always been fascinated by, this summer was an educational and rewarding experience. I expanded both my technical skills and developed a foundation of experimental thinking, while adding an uniquely applied space within my education here at Penn.