Shallow seismic surveys and ice thickness estimates of the Mullins Valley debris-covered glacier, McMurdo Dry Valleys, Antarctica DAVID E. SHEAN 1,2 *, JAMES W. HEAD III 1 and DAVID R. MARCHANT 2 1 Department of Geological Sciences, Brown University, Box 1846, Providence, RI 02912, USA 2 Department of Earth Sciences, Boston University, 675 Commonwealth Avenue, Boston, MA 02215, USA *dshean@bu.edu, David_Shean@alumni.brown.edu Abstract: Several debris-covered glaciers occupy tributaries of upper Beacon Valley, Antarctica. Understanding their flow dynamics and ice thickness is important for palaeoclimate studies and for understanding the origins of ancient ice elsewhere in the McMurdo Dry Valleys region. We present the results of several shallow seismic surveys in Mullins Valley, where the largest of these debris-covered glaciers is located. Our results suggest that beneath a thin sublimation till and near-surface horizon of dirty glacier ice, lies relatively pure glacier ice (P-wave velocity 3700 – 3800 m s -1 ), with total thickness estimates of 90–95 m towards the valley head, and 40– 65 m near the entrance to Beacon Valley, 2.5 km downglacier. P-wave velocities decrease downvalley, suggesting that the material properties of the ice change with increasing distance from the ice-accumulation zone. These new data are used to calibrate an ice thickness profile for the active portion of the Mullins Valley debris-covered glacier (upper 3.5 km) and to shed light on the origin and spatial distribution of enclosed debris. Received 25 August 2006, accepted 15 March 2007, first published online 16 August 2007 Key words: Beacon Valley, ice flow model, permafrost, rock glacier, shallow seismic reflection Introduction The McMurdo Dry Valleys of southern Victoria Land comprise a predominantly ice-free region within the Transantarctic Mountains (Fig. 1). On average, the region receives , 10 cm of annual precipitation (Schwerdtfeger 1984) and mean annual temperatures range from -308 to -158C (Doran et al. 2002). On the basis of mapped geomorphic features and spatial variations in modern soil moisture and atmospheric temperature, the region can be divided into a series of microclimate zones. Beacon Valley (77851’S, 160835’E, Fig. 1) is the largest valley within the stable upland micro- climate zone, the coldest and driest zone as mapped by Marchant & Denton (1996) and Marchant & Head (in press). The valley has received significant attention since the documentation of massive subsurface ice (Linkletter et al. 1973, Potter & Wilson 1984), some of which is related to southward incursions of an ancestral Taylor Glacier and may be of Miocene age (Sugden et al. 1995). An additional source for some of the buried ice in upper and central Beacon Valley is debris-covered glaciers that originate from cirques in tributary valleys of upper Beacon Valley (Fig. 1). The Mullins Valley debris-covered glacier (Figs 1 & 2) and the smaller Friedman Valley debris- covered glacier (Fig. 1) both grade from small, exposed alpine glaciers, covered only by a scattering of dolerite cobbles and boulders, to buried glacial ice that extends several kilometres downvalley. These features contain a demonstrable core of glacier ice, which we feel distinguishes them from most “rock glaciers” that typically, in whole or in part, consist of debris mobilized by flow of interstitial ice of secondary origin. The glacier ice in Mullins and Friedman valleys is capped by sublimation till, produced primarily as englacial debris is brought to the surface via sublimation of overlying ice (Schaefer et al. 2000, Marchant et al. 2002). The surface topography of these debris-covered glaciers is marked by a series of 1– 6 m high arcuate ridges and furrows (Figs 1 & 2). The ridges are cored by glacier ice and there is little change in the thickness of overlying till, suggesting that ridge morphology is related to compression or thrusting of subsurface ice rather than to localized variations in the sublimation till. Solar radiation during summer months warms low albedo (0.07) dolerite rocks above 08C. Where these isolated rocks occur scattered across a relatively clean ice surface, such as near valley heads, meltwater forms and flows down local slopes. Although most of this meltwater evaporates, some flows tens of metres before refreezing as superposed ice just inside the first major topographic ridge on both the Friedman and Mullins valleys debris-covered glaciers (Figs 1 & 2). Beyond the first few ridges, the sublimation tills are sufficiently thick ( . 10–50 cm) to prevent melting at the buried ice surface; ice loss in these regions is entirely by sublimation, with maximum rates likely reaching 0.1 mm yr -1 (Kowalewski et al. 2006). As reported in Levy et al. (2006), thermal-contraction polygons mark the surface of 485 Antarctic Science 19 (4), 485–496 (2007) & Antarctic Science Ltd 2007 Printed in the UK DOI: 10.1017/S0954102007000624