Inland thinning of Pine Island Glacier, West Antarctica

A.Shepherd, D.J. Wingham, J.A.D. Mansley, H.F.J. Corr,
Science, February 2nd, 2001

Figure 1. Map of West Antarctica, showing the location of the Pine Island Glacier drainage basin (gray), and the downstream region shown in Figure 3 (black).

Figure 2. The change in surface elevation 13 km upstream of the grounding line of Pine Island Glacier between 1992 and 1999, observed by the ERS-1 (stars) and ERS-2 (squares) satellite altimeters. The 6-month period of simultaneous operation permitted cross-calibration of the altimeters. Data gaps are the result of instrument operations. The location of this time-series is highlighted with a black perimeter in Fig. 3.

Figure 3. The rate of elevation change of the lower 200 km of the Pine Island Glacier between 1992 and 1999 (colored dots) registered with a map of the ice surface speed (gray scale). The dots are located at crossing points of the ERS orbit ground-tracks, and have an area equal to the radar altimeter footprint. The velocity field is restricted to regions of overlap between ascending and descending ERS SAR coverage. Also shown is the trajectory of a recent radar echo flight (red dashed line), and the boundary (black dashed line) we chose to delimit the glacier trunk from the remainder of the drainage basin. The trunk is bounded laterally by the 200 m yr-1 velocity contour, and streamwise by the grounding line (lower black line) located by Rignot (13) and the intersection of the easternmost tributaries, which coincides roughly with a deep bedrock trough (see Fig. 4A). The greatest elevation change is adjacent to the grounding line, and the thinning is concentrated over fast-flowing ice. Changes beyond the region of fast flow were much smaller, although there is some evidence that the tributary joining the glacier at the southern edge of the grounding line is also thinning. We calculate the mean rate of elevation change of the glacier trunk to be -0.75 ± 0.07 m yr-1 and that of the remainder of the basin (including that not shown here) to be -0.11 ± 0.01 m yr-1.

Figure 4. (A) Bedrock and surface elevation along Pine Island Glacier (PIG) from airborne ice penetrating radar (24). The grounding line (GL) lies within a (~ 30 km) zone of near-buoyancy, where small changes in ice thickness result in large GL migration. Upstream of this region there is a deep subglacial trough, which holds the central trunk of the PIG. The trunk boundary is coincident with the intersection of the flight trajectory and the locus of fast flow described in Fig. 3. The easternmost edge of Fig.3 (E) is also marked. The glacier develops a driving stress in excess of 100 kPa in surmounting the bedrock trough, and the associated upstream thickening appears to determine the location of the present grounding line. (B) Rate of elevation change (1992 - 1999) measured by ERS radar altimeter at discrete locations along the same transect. Also shown is the value determined by Rignot (13) for the thinning (1992 - 1996) at the grounding line. The thinning is largely confined to the region of high driving stress, suggesting that the progress of retreat may be controlled by the dynamics of this section of PIG.