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anisotropy in the deep ice at siple dome

anisotropy in the deep ice at siple dome. erin pettit Ed Waddington Paul Jacobson Throstur Thorsteinsson Greg Lamorey. Thanks to NSF and all people who have worked on Siple Dome, especially… Will Harrison, Mark Zumberge, John Morack, Dan Elsberg. image courtesy of USGS AVHRR.

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anisotropy in the deep ice at siple dome

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  1. anisotropy in the deep ice at siple dome erin pettit Ed Waddington Paul Jacobson Throstur Thorsteinsson Greg Lamorey Thanks to NSF and all people who have worked on Siple Dome, especially… Will Harrison, Mark Zumberge, John Morack, Dan Elsberg image courtesy of USGS AVHRR

  2. goals and questions… stress pattern Siple Dome has answered many questions about climate history and ice sheet dynamics, it has also forced us to ask many new questions. How does anisotropy affect the ice divide flow pattern? Is Siple Dome’s flow pattern typical or unusual? How has this flow pattern changed over time, with changing climate?

  3. about Crystal Anisotropy Ice Crystals have a preferred deformation direction c-axis Under stress, they tend to rotate until they can deform in this easy glide way Over time crystals in an ice sheet align, Divide Fabric – c-axes clustered vertically Cone Angle o o o Isotropic 90 30 Anisotropic 5

  4. (Engelhardt) (Lamorey,Thorsteinsson) (DiPrinzio, Wilen, Alley) Cone angle can be measured through the borehole sonic velocity log Primary factors influencing fabric development Time Temperature (and Temp History) Strain-Rate History Grain Size Impurities Small grains, tight fabric

  5. about ice divides Soft Hard Hard Soft Hard Soft • on the flank… • pure shear • + simple shear • higher deviatoric stress • near the divide… • pure shear • lower • deviatoric stress

  6. the combined flow law finite element model Glen: the Crossover Stress: k Linear v. Nonlinear Part . Anisotropic Part the Softness Parameters E1 – Holocene Ice E2 – Ice Age Ice E3 – Recrystallized Ice

  7. Flank flow: the Shear Zone (Engelhardt) (Lamorey,Thorsteinsson) (DiPrinzio, Wilen, Alley) This shear zone acts as a “false bed” – the ice below it experiences little shear stress

  8. Divide flow: the Bump ice flow law the Glen Flow Law Ice is non-linear: e =At t effective viscosityh= 2 . ij ij . h eis strain rate tis deviatoric stress 1 2At 2 t Non-linear flow producesstiff ice in low stress region at divide. This creates arch in the isochrones: Raymond Bump 600m Kamb Ice Stream -800m -80km South 0 image courtesy of Nadine Nereson, Tony Gades Raymond Bump

  9. Anisotropy and Isochrones Ice with a small cone angle is hard under compression (divide) and soft under shear (flank)

  10. Anisotropy and Surface Profiles

  11. Three effects… The False Bed Effect Anisotropy Bump Surfaces

  12. is the anisotropy important? (Engelhardt) (Lamorey,Thorsteinsson) (DiPrinzio, Wilen, Alley) since the end of the ice age the shear band has moved downward and strengthened Horizontal Velocity (approximation) The flow pattern within the ice sheet has been evolving over time

  13. is the anisotropy important? yes! histories of accumulation rate, thickness, temperature… depend on the history of the flow pattern (strain rate). Taylor et al depth-age scale image courtesy of USGS AVHRR

  14. is Siple Dome Unique? converted from sonic logs Maybe? Maybe not? What will WAIS Divide look like?

  15. local Ice Dynamics a Climate Signal? abrupt climate change? 680m Argon isotope (Severinghaus et al 2003) 722m Deuterium excess Argon isotope (Taylor et al 2004)

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