Talk by Prof. Edwin D. Waddington

(Dept of Earth and Space Sciences, University of Washington, Seattle)

Title: Straining to achieve a physically based description of transient firn compaction and permeability

Ed Waddington, Max Stevens, Brita Horlings, Annika Horlings, Knut Christianson, & Michelle Koutnik Earth and Space Sciences, University of Washington, Seattle, Kaitlin Keegan, Zoe Courville, & Mary Albert, Thayer School of Engineering, Dartmouth College

Abstract: Most models of firn compaction are based on a steady-state assumption (the Robin hypothesis), and empirically tuned coefficients relate depth-density profiles to site accumulation rate and mean annual temperature. With a few exceptions, the physical processes driving compaction are unspecified and are buried in the empirical coefficients.  Furthermore, transient models are needed for the most interesting times, when climate is changing; unfortunately, any steady-state model can be generalized to transient behavior in an infinite number of ways.
To move beyond the Robin hypothesis, we are instead starting to measure microphysical properties such as grain size, bond area, and coordination number along firn cores. Evolution equations for these properties will be calibrated using dated firn cores that have been micro-CT scanned.   New compaction equations for bulk firn then depend on the local evolving microstructural state, in addition to temperature and overburden load.
Because the pore spaces are just the negative image of the ice-grain structure, the micro-CT data also reveal the analogous pore-space properties, i.e. pore volumes, throat cross-sectional areas, and tortuosity, enabling development of microstructural-evolution models for permeability and gas transport.
Developing firn-compaction equations with this new approach requires micro-CT measurements of well-dated firn cores, together with real-time measurements of firn compaction rate and transient seasonal temperature profiles at the sites of the scanned firn cores. Micro-CT scans on a 130-m core from USP50 Camp near South Pole are underway, and we have been continuously measuring firn strain rates at USP50 in boreholes ranging from 5 m to 120 m in depth for the past 2 years. We are also continuously measuring firn temperatures down to 40 m depth.
Accumulation rates and site temperatures on the polar ice sheets are often highly correlated through saturation vapor pressure, introducing uncertainty or nonuniqueness into calibrations based on only temperature and accumulation rate. To separate this dependency, we have also proposed to revisit Taylor Dome, where accumulation varies by more than an order of magnitude along a 25km transect, but temperature varies by less than 2C. If funded, we will recover firn cores for micro-CT scans at 4 sites spanning the accumulation gradient, and we will continuously measure firn compaction with coffee-can strain meters, and firn temperatures with thermistor strings at each of those sites over a 12-month period.
We will also validate and calibrate pRES (phase-sensitive radar) measurements against the strain rates measured in the boreholes over that wide range of accumulation rates and vertical velocities. These measurements will allow us to formally establish the abilities and limitations of pRES as a tool to measure firn compaction in campaign mode, without needing multiple drill holes and cores.