Bed properties and hydrological conditions underneath

During three summer field seasons (2003, 2005 and 2006) we carried out radio-echo sounding measurements with a 5MHz (central frequency) ice-penetrating radar on McCall Glacier, Arctic Alaska, USA, along the central flowline and 17 cross-profiles. Two-way travel time was, after migration, converted to ice thickness, which, in combination with a recent digital elevation model of the surface of the glaciated area, resulted in a detailed map of the bed topography. This reveals a complex basal topography in the confluence area of the different glacial cirques. The pattern of subglacial water flow following the hydraulic potential gradient was calculated for the whole glacier area and shows a confluence of subglacial water downstream from the confluence of the glacier cirques. From the icethickness map the total ice volume was estimated as slightly less than 0.5 km3. Bed reflection power (BRP) was determined for the glacier after correction for ice-thickness dependence. Results reveal a clear relationship between the BRP pattern and basal sliding anomalies along the central flowline. INTRODUCTION McCall Glacier is situated at 69◦18′N, 143◦48′W, in the northeastern part of the Brooks Range, Alaska, USA. The glacier is ∼7.6 km long and covers an area of 6 km2 (Fig. 1). It extends from an altitude of 2500m down to 1365m at the terminus, with an equilibrium-line altitude (ELA) between 2000 and 2400m (Nolan and others, 2005). McCall Glacier is known to be a polythermal glacier, with a temperate basal layer along a section of the lower glacier (Rabus and Echelmeyer, 1997; Pattyn and others, 2005). The glacier has been studied extensively, from the International Geophysical Year (1957/58) onwards through to the International Hydrological Decade (1969–72) and from the mid-1990s to the present. These studies have shown that McCall Glacier has been losing mass for decades, and that the rate of mass loss has increased over the past 10–20 years (Rabus and others, 1995; Nolan and others, 2005). Flowline model estimates have shown that accelerated glacier retreat since the early 1990s is related to the recent increase in ELA and the sudden reduction in accumulation-area ratio (AAR) (Delcourt and others, 2008). To further investigate the basal hydrological controls on the dynamical behavior of McCall Glacier, we reconstructed the basal topography of the glacier, based on an extensive radio-echo sounding (RES)field survey, carried out during the summer seasons of 2003, 2005 and 2006. From the basal topography, the pattern of basal water flow is investigated. Finally, an analysis of bed reflection power (BRP) from the radar signals is presented in relation to ice dynamics. RADIO-ECHO SOUNDING (RES) RES measurements were carried out with a 5MHz (central frequency) ice-penetrating radar (Narod and Clarke, 1994). The system consists of a monopulse transmitter generating 1600V pulses across a resistively loaded 10m dipole antenna, and an airwave-triggered oscilloscope receiver connected to a palmtop computer for digital data recording. Pulses are generated at 512Hz and have a bandwidth of 1–200MHz. The system has already been used successfully on several glaciers in Arctic and sub-Arctic regions (Pattyn and others, 2003, 2005). Two longitudinal and seventeen cross profiles were surveyed in August 2003, 2005 and 2006 (Fig. 1). The horizontal distance between successive measurements along each track varied between 19 and 38m. The network of profiles was concentrated in the confluence area of the branches coming from the upper (UC), middle (MC) and lower (LC) cirques. On all measurements, static corrections were performed for the trigger delay caused by the 38m separation between transmitter and receiver. A propagation velocity of 168mμs−1 for glacier ice was used, a value obtained from a common-midpoint experiment in the confluence area. Migration was carried out by considering, for a given bedrock echo, the locus of possible reflectors as a semi-ellipse, with transmitter and receiver points at the focal points (Rabus and Echelmeyer, 1997). The bed along a given profile is then the envelope of these intersecting ellipses, as shown in Figure 2 for one of the cross-profiles in the confluence area. The quality of the ice-thickness measurements is estimated from the intersection points of the different radar profiles, leading to root-mean-square errors of 8.38 and 12.04m for migrated and non-migrated ice thickness, respectively (n = 22). The two-dimensional basal topography was obtained after interpolation based on a Delaunay triangulation of the measured bedrock profiles combined with a digital elevation model (DEM) of the surrounding area (Fig. 3a), which also resulted in an ice-thickness map (Fig. 3b). The total ice volume of McCall Glacier is estimated as 0.5 km3, with a mean ice thickness H = 82.39m for a surface of 6.02 km2. Pattyn and others: Conditions underneath McCall Glacier 81 7688 7689 7690 7691 7692 7693 7694 7695