The basal roughness of Pine Island Glacier, West Antarctica D.M. RIPPIN, 1 D.G. VAUGHAN, 2 H.F.J. CORR 2 1 Environment Department, University of York, Heslington, York YO10 5DD, UK E-mail: david.rippin@york.ac.uk 2 British Antarctic Survey, Natural Environment Research Council, Madingley Road, Cambridge CB3 0ET, UK ABSTRACT. We assess basal roughness beneath Pine Island Glacier (PIG), West Antarctica, based on a recent airborne radio-echo sounding dataset. We identify a clear relationship between faster ice flow and decreased basal roughness in significant parts of PIG. The central portion and two of its tributaries are particularly smooth, but the majority of the tributaries feeding the main trunk are rougher. We interpret the presence of a smooth bed as being a consequence of the deposition of marine sediments following disappearance of the West Antarctic ice sheet in the Pliocene or Pleistocene, and, conversely, a lack of marine sedimentation where the bed is rough. Importantly, we also identify a patchy distribution of marine sediments, and thus a bed over which the controls on flow vary. While there is a notable correspondence between ice velocity and bed roughness, we do not assume a direct causal relationship, but find that an indirect one is likely. Where low basal roughness results in low basal resistance to flow, a lower driving stress is required to produce the flux required to achieve mass balance. This, in turn, means that the surface in that area will be lower than surrounding areas with a rougher bed, and this will tend to draw flow into the area with low bed roughness. Since our studies shows that bed roughness beneath the tributaries of the trunk varies substantially, there is a strong likelihood that these tributaries will differ in the rate at which they transmit current velocity changes on the main trunk into the interior of the glacier basin. INTRODUCTION Basal roughness is the vertical variation in the ice/sub- glacial-bed interface with distance in the horizontal plane (Siegert and others, 2004, 2005; Taylor and others, 2004; Rippin and others, 2006a; Bingham and Siegert, 2007, 2009; Bingham and others, 2007). In attempting to under- stand the flow dynamics of ice sheets, and predict future changes, it is critical to understand how roughness variations impact upon ice dynamics. In this paper, we assess the roughness beneath Pine Island Glacier (PIG), which drains ~175 000 km 2 of the West Antarctic ice sheet (WAIS; Vaughan and others, 2001). It consists of a series of tributaries and a main trunk (Fig. 1; Stenoien and Bentley, 2000). Recent thinning and retreat (Rignot, 1998; Shepherd and others, 2004) suggests it may be particularly vulnerable to the rate at which the ocean melts ice close to the grounding line. Close to the grounding line, PIG is particu- larly fast-flowing (>2.5 km a –1 ) and shares many features (such as occupying a deep trough) with rapidly changing glaciers in Greenland, such as Jakobshavn Isbræ (Stenoien and Bentley, 2000; Truffer and Echelmeyer, 2003; Vaughan and others, 2006). As a consequence of recent periods of acceleration, PIG is currently no longer in balance, but it is uncertain whether this imbalance indicates the onset of a deglaciation of the basin, or is simply a short-term (multi- decadal to century) fluctuation (Vaughan and others, 2006). We have only a limited understanding of the mechanisms behind recent changes, in part because of a lack of data and knowledge of conditions at the bed. We are interested in the roughness of the PIG bed because, in general, the resistance that roughness exerts upon ice flow is regionally the most significant stress opposing the glacier driving stress. Since a glacier, or ice sheet, will tend to thicken until the driving stress produces ice flow to maintain the ‘balance flux’, a bed that has a low roughness, and which provides little restraint on ice flow, will achieve balance flux with relatively less thickness and less surface slope. The magnitude of the basal stress exerted by the bed is controlled by roughness and subglacial water pressure (Iken, 1981; Bindschadler, 1983; Lliboutry, 1987; Jansson, 1995; Willis, 1995; Harper and others, 2007). While many studies have sought to relate subglacial water pressures to basal motion (e.g. Iken, 1981; Bindschadler, 1983; Jansson, 1995; Harper and others, 2007), only a few concern the relationship between bed roughness and basal motion; this may in part be due to the difficulty of measuring subglacial roughness (Bennett, 2003; Taylor and others, 2004). Bed echoes from radio-echo sounding (RES) across the Antarctic ice sheet (and other ice bodies) provide data from which bed roughness can be assessed. To date, however, only a few researchers have tackled this issue, using a range of methods to extract roughness from bed echoes across a range of scales (e.g. Hubbard and others, 2000; Taylor and others, 2004; Siegert and others, 2005; Rippin and others, 2006a; Bingham and Siegert, 2007, 2009; Bingham and others, 2007). Our objective is to assess basal roughness beneath the drainage basin of PIG, and the possible future behaviour of the glacier in response to future changes. METHODOLOGY We used unmigrated data acquired from airborne radio- echo sounding (RES) profiles, collected as part of an aerogeophysical survey of the Amundsen Sea sector of West Antarctica, in the austral summer of 2004/05 (Vaughan and others, 2006). Due to resolution considerations, discussed below, migration was not necessary. Thirty airborne RES sorties were flown over the PIG basin from a field camp and fuel depot located at 77834 0 S, 95856 0 W (Fig. 1). The RES system was fitted in a Twin Otter aircraft, also equipped with Journal of Glaciology, Vol. 57, No. 201, 2011 67