Influence of Textural and Topographic Variability on Sublimation of Buried Ice: Implications for near Surface Ice Stability in Antarctica

Introduction: The McMurdo Dry Valleys of Antarctica have commonly been used as an analog for Mars; analogies are due in part to the regions hyperarid and cold climate as well as its abundance of similar-appearing microscale and macroscale morphological features such as gullies, lineated valley fill, and polygons [eg. 1,2,3]. With the recent direct observations of water ice on Mars from the NASA Phoenix lander and MRO Satellite images [4], a complete understanding of the physics of buried-ice preservation in Antarctica is becoming increasingly important in helping to model the stability and age of buried ice deposits on Mars. Geological Setting: Beacon Valley, Antarctica is an ideal site for investigating the preservation of ancient ice buried beneath a variety of surface deposits and morphologies [5]. The Mullins Valley alpine glacier, which occupies a tributary valley to Beacon Valley, is a slow-moving-to-stagnant cold-based glacier. Its capping sublimation till is composed of dolerite-and-sandstone clasts set within a sand-rich matrix (Dolerite Till). Collectively, the till is approximately 10 to 15 cm thick in the upper ablation zone and thickens to ~50 cm towards the terminus of the glacier. The top 10 cm is slightly oxidized [6]. Insitu overlying ashfall deposits at the terminus have been dated at ~7.9 Ma [7]. Alternative methods for determining the chronology of the glacier include: cosmogenic nuclide dating of surface rocks, integrated flow from synthetic aperture radar interferometry, numerical glacial modeling, and direct dating of atmospheric gases within the ice [eg. 7-9]. Polygons at the till surface mature with increasing distance down glacier. In some portions of central Beacon Valley, stagnant, buried glacier ice is derived from an ancient advance of Taylor Glacier, an outlet glacier from the East Antarctic Ice Sheet, up into Beacon Valley, rather than from expansion of Mullins Glacier. The remnant stagnant lobe ice is protected by a ~50-cm thick, silt rich sublimation till (Granite Drift) with clasts of dolerite, sandstone, and granites; the latter are not native in Beacon Valley. Here the polygons are fully mature and troughs can reach 2-3 meters in depth. Methods: Vapor flux in sublimation till is governed primarily by two mechanisms: molecular diffusion of vapor in pore spaces and advection of air Fig. 1. Till and surface textures in Beacon Valley. a) Dolerite Till of fresh dolerite clasts and minimal sands. b) Granite Drift with a silt and sand rich matrix. c) Desert Pavement displaying interwoven and highly weathered clasts.