2003 Fall Meeting          
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Cite abstracts as Eos Trans. AGU, 84(46),
Fall Meet. Suppl., Abstract xxxxx-xx, 2003
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marchant

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HR: 17:15h
AN: C12C-06
TI: Origin of Sublimation Polygons in the Antarctic Western Dry Valleys: Implications for Patterned Ground Development on Mars
AU: * Marchant, D R
EM: marchant@bu.edu
AF: Boston Universiy, Department of Earth Sciences, Boston, MA 02215 United States
AU: Head, J W
EM: James_Head_III@Brown.Edu
AF: Brown University, Department of Geological Sciences, Providence, RI 02215 United States
AB: Two hypotheses have been developed in order to address the age, origin, and evolution of surface polygons in the western Dry Valleys region of southern Victoria Land. Resolution of this debate has direct relevance and implications for patterned ground in ice-rich terrain on Mars. One hypothesis, the dynamic hypothesis, states that growth of sand-wedges pervasively deforms sediment across polygonal terrain, recycling sediment from troughs to polygon centers and back again over time scales of thousands of years. A second hypothesis, the stability hypothesis, states that deformation associated with sand-wedge polygons, particularly those that form over buried ice, is restricted to polygon troughs; the implication is that polygon centers may contain undisturbed soils $>$1 million years old. Evidence comes from field data that show that the age, origin, and morphology of polygons that form over buried ice in the western Dry Valleys region is tied collectively to the location and rate of sublimation of underlying ice. In Beacon Valley, sublimation of debris-rich ice produces a dry surface lag that insulates and slows loss of remaining ice. Sub-zero temperature cycling of near-surface ice and soil creates tensile stresses that result in a network of hexagonal cracks, extending upward from buried ice toward the ground surface. Where fines sift downward into open thermal-contraction cracks, a coarse-grained lag deposit forms on top of the ice. Owing to spatial variations in till texture, rates of sublimation vary across the ice surface. High rates occur below coarse-grained lags that cap contraction cracks; low rates are found at polygon centers beneath fine-grained low porosity/permeability debris. Measured concentrations of in-situ produced cosmogenic 3He in two depth profiles through sublimation till show a steady decrease with depth, indicating negligible recycling of surface materials on million-year time scales. These data suggest that once polygon troughs deepen sufficiently, they trap windblown snow and initiate a negative feedback that slows further sublimation of the underlying ice. This negative feedback involves the development of secondary-ice lenses at the base of polygon troughs. In-situ ashfall deposits in Beacon Valley provide evidence that polygons are stable landforms that persist for $>$1 million years and that underlying ice there is $>$8.1 million years old. The high-centered sublimation polygons of Beacon Valley may be analogous to the latitude-dependent polygons and basketball-textured terrain on Mars.
DE: 1823 Frozen ground
DE: 5462 Polar regions
DE: 9310 Antarctica
SC: Cryosphere [C]
MN: 2003 Fall Meeting


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