A rheology for temperate ice in glaciers (RTIG) based on borehole deformation experiments

Martin Peter Lüthi , Martin Funk, Andreas Bauder

The rheology of temperate glacier ice, which contains small amounts of liquid water, largely controls the fast dynamics of glaciers and polar ice streams. Recent laboratory experiments suggest a constant-viscosity ice rheology when the water content between ice grains exceeds 0.6%. Such a linear flow law, where the deformation rate is proportional to the applied stress, is in contrast to the power-law relations usually assumed for glacier ice. Here, we use in-situ deformation measurements from 16 boreholes to derive a new rheology for temperate ice in glaciers (RTIG). Our analysis of borehole tilt data from Greenland, the Alps and Canada suggests two distinct deformation regimes depending on stress magnitude. At intermediate stresses, a linear rheology fits the measurements, with a viscosity that is 40 times higher than in the laboratory experiments. At stresses exceeding 0.157 MPa the data fits a pseudo-plastic rheology with a power n = 22. The combination of both deformation regimes
can be described with the Ellis flow rule. Applying this rheology for temperate glacier ice has profound consequences for our understanding of the flow of large ice masses, calling for a radical reassessment of how such ice is modeled.