MICRO-RELIEF AND DEBRIS COVER DEVELOPMENT IN POLYGONAL PATTERNED ICE IN ANTARCTICA: A MODELING PERSPECTIVE

Contraction cracks are ubiquitous in the Dry Valleys of Antarctica where they define extensive areas of polygonal patterned ground. Despite the hyper-arid conditions of the region, the contraction cracks reflect the presence of extensive subsurface ice as ice-cemented soil and, in places, massive ice beneath a debris mantle. The micro-relief provides useful clues about the nature of the subsurface, and because it can be detected remotely, it is likely to be of considerable interest in studies of permafrost regions not only on Earth but also on Mars.

Herein, we describe a modeling approach aimed at better understanding the micro-relief and debris cover that develop in the long term above actively sublimating, massive, debris-rich ice. A realistic quantitative model must account for the progressive growth of sand- or ice-wedges at sites of contraction cracking, extensive sublimation of the ice and surface accumulation of debris formerly contained within the massive ice, modulation of ice sublimation rate by the protective debris cover, and lateral motion of the debris driven by surface slope as the micro-relief develops. Our model unifies these processes to produce a slowly evolving micro-topography and a debris-layer thickness distribution that are comparable to those of polygons measured in the field. It also calculates deformation fields in the ice and debris that are consistent with the spatially varying sublimation rate, steeply dipping foliation commonly seen in the ice at the periphery of polygons, and exposure age histories obtained from vertical profiles of cosmogenic isotopes in the debris cover, as well as in the ice. The model results help define the time scale over which the ground surface would be completely reworked, which has rich implications for the age and interpretation of landscape surfaces in Antarctica.