Basal Shear Stress of the Ross Ice Streams from Control Method Inversions Ian Joughin Jet Propulsion Lab, California Institute of Technology Slawek Tulaczyk University of California, Santa Cruz Douglas R. MacAyeal Dept. of Geophysical Sciences, University of Chicago

Mercer Whillans Kamb Bindschadler MacAyeal A B C D E

Ice Stream Motion Over a Till Bed Ice ~1000 m Till ~6 m Depth Velocity Depth Velocity Viscous Deforming Bed ( Alley,Blankenship et al ) Weak Plastic Bed ( Kamb,Tulaczyk,Engelhardt et al )  A weak bed implies inability to support high driving stress

Ice Stream Equations Viscous Plastic  Control-method techniques can be used to invert equations for basal shear stress (MacAyeal, 1992).

Similar solutions for both plastic and viscous models (force balance). Inversions are robust with respect to errors in velocity and bed topography. Inversions are sensitive to surface elevation noise Inversion Result Forward Model Noise Inversion Procedure Inversion Simulations

Bed Elevation - BEDMAP Data Sets Surface Elevation – RAMP/SOAR B – Temperature Model

Ice Streams A&B

Ice Stream C

Ice Streams D&E

Weak Margins ?

Summary  Fairly uniform weak (few kPa) bed beneath ice streams (E has significant sticky spots).  Tributaries a mixture of strong and weak bedded regions.  Some ambiguity in basal shear stress because flow law not well constrained.  Neither model nor data appear to support weak margins.