Constitutive Modeling of Glacial Till
Background
Exposure of glacial till layers in bluffs above the Nelson River, northern Manitoba, from which specimens of the Sky Pilot Till, deposited by the Laurentide Ice Sheet, were obtained.
We have recently completed a set of geotechnical experiments (Sane et al., 2008) using the Disturbed State Concept (DSC) model (Desai, 2001) to evaluate the rheology of two regionally significant North American Pleistocene tills (Carlson et al., 2004). The results suggest that soft beds could, at certain stages of deformation–assuming sufficiently strong coupling at the ice-till interface–support shear stresses of about 20 kPa, substantially greater than the residual strengths measured in other tills by Kamb (1991) and Iverson et al (1997; Iverson et al., 1998) (<3 kPa). Having thus developed the DSC model for the fully-coupled case (in which there is assumed to be no “sliding” or “slippage” between the basal ice and the underlying bed of “soft” (i.e., water-saturated) till, the research team proposes now to extend the application of the DSC model to the mechanics of relative motion at the interface between the basal ice and underlying till. The proposed work will utilize the specialized Cyclic Multi-Degree-of-Freedom (CYMDOF) shear device at the University of Arizona´s Constitutive Modeling Laboratory, which has been developed and proven in previous advanced geotechnical research applications. The CYMDOF test box will be modified to accommodate ice-till interface specimens with controlled configurations guided by insights gained from continued field study by the research team and from previous observations published by others. The model will be validated by independent tests and then implemented in numerical simulations to predict the motion resulting from the combined effects of interface motion and deformation in the till and ice. The proposed work will take advantage of previously proven methodology and the mathematical and numerical models developed and tested in the previous project.
Applications of Research
Ice sheet modelers have long sought a reliable model for soft-bedded ice sheet movement. Despite the important role of slip in ice movement,however, no truly predictive models have yet been developed for slip of ice over a till surface. The results of this project will be an important contribution toward the ultimate development of sophisticated ice sheet models that could provide reliable predictions of growth, decay, and stability of glaciers and ice sheets–both Pleistocene and modern–and their interaction with climate. Such models may also yield new insights into landscape evolution in glacial terrains, such as whether soft-bed deformation could have mobilized sufficient sediment flux to account for the large volumes of till in the Midwestern United States or the large deposits of glacigenic material in the Barents Sea. Answers to such questions have applications to important economic activities in glaciated regions, including water resource management and mineral prospecting and mining.
- Carlson, A. E., J. W. Jenson, and P. U. Clark, 2004, Field observations from the Tiskilwa Till, IL, and Sky Pilot Till, MB of the Laurentide Ice Sheet: Geographie Physique et Quaternaire, v. 58, p. 229-239.
- Desai, C. S., 2001, Mechanics of Materials and Interfaces: The Disturbed State Concept: Boca Raton, Florida, CRC Press.
- Iverson, N. R., R. W. Baker, and T.S. Hooyer, 1997, A ring shear device for the study of till deformation: Tests on tills with contrasting clay contents: Quaternary Science Reviews, v. 16, p. 1057-1066.
- Iverson, N. R., T. S. Hooyer, and R. W. Baker, 1998, Ring-shear studies of till deformation: Coulomb-plastic behavior and distributed strain in glacier beds: Journal of Glaciology, v. 44, p. 634-642.
- Kamb, B., 1991, Rheological nonlinearity and flow instability in the deforming bed mechanism of ice stream motion: Journal of Geophysical Research, v. 96, p. 585-16,595.
- Sane, S. M., C. S. Desai, J. W. Jenson, D. S. Contractor, A. E. Carlson, and P. U. Clark, 2008, Disturbed state constitutive modeling of two Pleistocene tills: Quaternary Science Reviews, v. 27, p. 267-283.
Climate History of Guam Study
A stalagmite collected by WERI researcher, Dr. Jenson and his students, in April 2005 from a cave on northern Guam is providing clues as to how the climate of Guam and the surrounding region evolved over the past 28,000 years. Stable isotope (18O and 13C) and trace element (Mg/Ca and Sr/Ca) analysis on the stalagmite layers are being conducted by collaborators at the University of Texas-Austin School of Geosciences and provide chemical clues about climate history. The initial results from the stalagmite are consistent with results from other islands in the Pacific and suggest, among other things, that the regional climate was much drier some 5000-6000 years ago. The western Pacific Ocean plays an important, but still poorly understood, role in global climate.