Basal processes at subfreezing temperatures:
Meserve Glacier revisited

Meserve Glacier (77o31'S, 162o17'E) is a cold-based alpine glacier on the south side of Wright Valley, in Victoria Land, Antarctica. It is located between Bartley Glacier and Hart Glacier. It was named by Dr. Robert Nichols for William Meserve, who was a geological field assistant to Dr. Nichols during the 1959-60 Antarctic field season.

Basal layers whose composition and mechanical properties differ markedly from that of overlying ice exist beneath many cold-based ice sheets and glaciers. Such layers have been observed at the bottom of deep ice cores from the Arctic and Antarctic, and at the margins and in tunnels beneath polar and sub-polar glaciers. The origin and rheological properties of these basal layers are poorly understood. Yet accurate ice sheet flow modelling requires knowledge of the viscosity and spatial extent of these layers because deformations are strongest near the bed where shear stresses are highest. Further, understanding the origins of these layers could help in extracting paleoclimate records from ice cores. For example, entrainment processes are thought to be slow or non-existent at subfreezing temperatures and it is not clear how a debris-rich basal layer can form under these conditions. This raises the possibility that the layers form by regelation or freeze-on under temperate conditions which implies radically different conditions at the bed in the past. Alternatively, if basal processes are active at subfreezing temperatures, this would place a limit on how close to the bed one could obtain an undisturbed stratigraphic sequence for paleoclimate analyses.

During the 1995-96 field season we revisited the basal layers of Meserve Glacier (Antarctica), a site first investigated by Gerald Holdsworth and colleagues in the late 1960's. Meserve Glacier has a silt-rich basal layer with a distinctive amber color. The basal temperature is about -17oC. We used electric chainsaws, picks and shovels to excavate a 30-m long tunnel into the tongue of the lower glacier. Image: Kurt Cuffey (with pick), Tony Gades (with chainsaw) and Howard Conway at tunnel entrance.

More pictures from the field here

Although not a perfect analogue for conditions under a large ice sheet, the Meserve basal layer provides an excellent natural laboratory for examining processes at subfreezing temperatures. We measured diagnostic properties (stable isotopes, gases, solid and chemical impurities, and crystal fabrics), as well as deformation. Results showed that the chemical and solid impurity content of the basal ice is 3-4x greater than the overlying glacier ice. Measurements from strain grids installed on the tunnel walls indicate variations in shear strain rate are at least partly correlated with impurity content.

An exciting early discovery was that Meserve Glacier is sliding (at least locally) even though the basal temperature is -17^oC. Cavities in the lee of boulders imply active sliding, and most convincing, we measured sliding directly (5-15 mm/yr) using displacement markers and transducers. This and a related observation of ice segregation are best explained as manifestations of unfrozen water films at ice-rock interfaces. Using Shreve's theory for sub-freezing sliding, we estimated the film thickness to be 20-40 nanometers (Cuffey et al, 1999).

In addition, our measurements of gas content and stable isotopes indicate that entrainment processes are currently active beneath Meserve glacier despite the cold temperatures. Subfreezing entrainment is facilitated by the existence of interfacial water films, and associated sliding (Cuffey et al, 2000a). The measurements were used to develop a simple model describing the dependence of strain rate on ice crystal size and impurity content. Results show a strong dependence on ice crystal size, suggesting a grain-size sensitive deformation mechanism such as grain boundary sliding is important. Chemical impurities act through a grain-size sensitive mechanism, but do not dominate this component of deformation (Cuffey et al, 2000b & 2000c).

Radio-echo sounding measurements on the lower glacier tongue and in the upper accumulation basin reveal interesting bed morphologies. In the upper reaches, both an over-deepening and a classical U-shaped trough are indicative of glacial erosion, while the lower portion of the glacier overlies a planar bed. It is still not clear whether the upper reaches were eroded under cold conditions or whether conditions at the bed were warmer in the past.

People involved in the project:,
Howard Conway, Kurt Cuffey, Tony Gades, Bernard Hallet, Nadine Nereson, Charlie Raymond and Ron Sletten.
We also thank Bob Hawley, Living Nightingale, Kevin Whilden and John Wright for their assistance in the field.

Publications and thesis

• Conway, H., K. Cuffey, A. Gades, B. Hallet, C.F. Raymond and R. Sletten, 1996. Observations of basal sliding at sub-freezing temperatures: Report on studies at Meserve Glacier, Antarctica. Antarctic Journal of the US, 31(2), 67-68
• Cuffey, K.M., 1999. Glaciological investigations beneath an active polar glacier. PhD thesis, University of Washington, 113pp.
• Cuffey, K.M., H. Conway, B. Hallet, A.M. Gades and C.F. Raymond, 1999. Interfacial water in polar glaciers and glacier sliding at -17^oC. Geophys. Res. Letters, 26(6), 751-754 (pdf)
• Cuffey, K.M., H. Conway, A.M. Gades, B. Hallet, R. Lorrain, J.P. Severinghaus, E.J. Steig, B. Vaughn and J.W.C. White, 2000a. Entrainment at cold glacier beds. Geology, 28(4), 351-354 (pdf)
• Cuffey, K.M., H. Conway, A.M. Gades, B. Hallet, C.F. Raymond and S. Whitlow, 2000b. Deformation properties of subfreezing glacier ice: Role of crystal size, chemical impurities and rock particles, inferred from in situ measurements. J. Geophys. Res.105(B12), 27895-27915 (pdf)
• Cuffey, K.M., T. Thorsteinsson and E.D. Waddington, 2000c. A renewed argument for crystal size control of ice sheet strain rates. J. Geophys. Res. 105(B12), 27889-27894 (pdf)