Showing papers by "Robert Bindschadler published in 2005"

Evidence for subglacial water transport in the West Antarctic Ice Sheet through three‐dimensional satellite radar interferometry

Abstract: [1] RADARSAT data from the 1997 Antarctic Mapping Mission are used interferometrically to solve for the 3-dimensional surface ice motion in the interior of the West Antarctic Ice Sheet (WAIS). An area of ∼125 km2 in a tributary of the Kamb Ice Stream slumped vertically downwards by up to ∼50 cm between September 26 and October 18, 1997. Areas in the Bindschadler Ice Stream also exhibited comparable upward and downward surface displacements. As the uplift and subsidence features correspond to sites at which the basal water apparently experiences a hydraulic potential well, we suggest transient movement of pockets of subglacial water as the most likely cause for the vertical surface displacements. These results, and related lidar observations, imply that imaging the change in ice surface elevation can help reveal the key role of water in the difficult-to-observe subglacial environment, and its important influence on ice dynamics.

Continued deceleration of Whillans Ice Stream, West Antarctica

Abstract: [1] Earlier observations indicated that Whillans Ice Stream slowed from 1973 to 1997. We collected new GPS observations of the ice stream's speed in 2003 and 2004. These data show that the ice stream is continuing to decelerate at rates of about 0.6%/yr2, with faster rates near the grounding line. Our data also indicate that the deceleration extends over the full width of the ice plain. Extrapolation of the deceleration trend suggests the ice stream could stagnate sometime between the middle of the 21st and 22nd Centuries.

Changes in the ice plain of Whillans Ice Stream, West Antarctica

Abstract: Data from the mouth of the decelerating Whillans Ice Stream (WIS), West Antarctica, spanning 42 years are reviewed. Deceleration has continued, with local areas of both thinning and thickening occurring. The mean thinning rate is 0.48 ± 0.77m a -1 . No consistent overall pattern is observed. Ice thickens immediately upstream of Crary Ice Rise where deceleration and divergence are strongest, suggesting expanded upstream influence of the ice rise. Thinning is prevalent on the Ross Ice Shelf. Grounding-line advance at a rate of 0.3 km a -1 is detected in a few locations. Basal stresses vary across an ice-stream transect with a zone of enhanced flow at the margin. Marginal shear is felt at the ice-stream center. Mass-balance values are less negative, but larger errors of earlier measurements mask any possible temporal pattern. Comparisons of the recent flow field with flow stripes suggest WIS contributes less ice to the deep subglacial channel carved by Mercer Ice Stream and now flows straighter. The general lack of geometric changes suggests that the regional velocity decrease is due to changing basal conditions.

Guiding the South Pole Traverse with ASTER imagery

Abstract: On 9 December 2004, one of us (R.B.) received a phone call requesting assistance in identifying crevasses in Antarctica. The US South Pole Traverse (SPT) party heading from McMurdo Station (77.908 S, 166.658 E) to South Pole Station via Leverett Glacier in the Transantarctic Mountains had stopped due to the discovery of numerous crevasses along their planned route. These crevasses had been detected by ground-based ice-sounding radar moving ahead of the heavy traverse vehicles. Investigation of numerous alternative routes also identified many crevasses. A quick assessment was required, forcing us to limit our investigation of the region between the field party and the mouth of Leverett Glacier to satellite remote sensing imagery already on hand. Two types were examined. The first was the RADARSAT mosaic produced from synthetic aperture radar (SAR) collected over the entire Antarctic continent in 1997 (Jezek and others, 2002). The highest-resolution data available are those of the 25m mosaic distributed by the Alaska SAR Facility. The second image source used was 15m multispectral optical imagery from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). To our surprise, the RADARSAT imagery failed to show many crevasses that ASTER showed clearly. Two examples are shown for comparison. Crevasse orientation can affect the backscatter signal that a SAR detects, making crevasses aligned with the SAR look direction nearly invisible. This does not explain the poor performance of RADARSAT in our case because the crevasses have various orientations. Some resampling of the SAR data is required to create the RADARSAT mosaic, but as time was pressing, we did not examine the original swath data. The crevasses visible in the cloud-free ASTER images but not visible in the RADARSAT imagery are of various types. At the margins of a slow-moving area (84.918 S, 157.308W) near the bend made by Scott Glacier as it enters the Ross Ice Shelf, are arcuate sets of crevasses on either side formed by shear between the slow-moving ice and Scott Glacier on the south side and Mercer Ice Stream/Reedy Glacier on the north side (Fig. 1). The upstream portion of the north set is visible in the RADARSAT image, but the continuation of these crevasses downstream is not. Even farther downstream on the ice shelf, in the ice that once flowed in Scott Glacier, there are many series of crevasses that cannot be seen in the RADARSAT mosaic (Fig. 2). These are up to 30m across (i.e. generally 1–2 pixels wide) and many are > 1 km long. We had two ASTER images (each 60 km 60 km) of the area yet to be crossed by the traverse party and were able to quickly order a third scene that included the current position of the field party. These three images covered the next 80 km of the traverse and came within a few kilometers of the mouth of Leverett Glacier. We defined what we believed to be a crevasse-free route by specifying the coordinates of seven waypoints. Geographic registration was accomplished by ingesting the level 1B ASTER data into ENVI image analysis software. We are happy to report that by following our new route, the field party was able to travel the 80 km to the end of our ASTER coverage in just 2 days and encountered no crevasses along the way. Our experience calls attention to the potential danger of assuming the 25m resolution of the RADARSAT mosaic is sufficient to identify all crevassed areas of the ice sheet. This is particularly important for assessing the safety of surface traverse routes.