PRENSA DIGITAL

The terminus geometry of Tasman Glacier changed during the study period (October 2011 to November 2012), primarily as the result of iceberg calving. Changes in terminus position, determined from ASTER satellite images, are shown in Figure 5.13 and Table 5.3. Due to the intermittent nature of ASTER imagery acquisition, derived terminus positions did not match exactly with the study period and as a result changes in terminus position between March 2011 and January 2013 are shown to highlight the overall variability of retreat. From Figure 5.13 it is clear that over the entire study period the retreat of Tasman Glacier has been centred predominantly along the southern and western ice cliffs. Minor retreat along the eastern embayment ice cliff is also centred towards the southern end of the ice cliff, highlighting the fact that most major changes in terminus geometry have resulted in the up–glacier retreat of Tasman Glacier. The result of this has been the retreat of the glacier c. 223 m along the southern ice cliff (Table 5.3) at a rate of 0.57 m d-1 (206 m a-1).

The greatest change in terminus position occurred between March 2011 and March 2012 (Table 5.3). Figure 5.14 shows time–lapse images that correspond to the March 2011 and March 2012 ASTER images. From the images it is clear that prior to October 2011 a large section of the terminus had already been lost. This was primarily centred along the southern ice cliff, with retreat of the terminus to the up–glacier side of a supraglacial pond that had developed on the lower glacier (Figure 5.13). Between October 2011 and January 2012 ice loss from Tasman Glacier appears to involve relatively minor retreat along the southern ice cliff and the southern

section of the western ice cliff (Figure 5.14). Retreat subsequently accelerated between January 2012 and June 2012 as the peninsula (that had developed on the western side of the southern ice cliff) was lost.

Figure 5.13: Changes in the terminus of Tasman Glacier between March 2011 and June 2012 from ASTER satellite imagery. Dark grey area on the lower terminus in March 2011 is a supraglacial pond. WE, is western embayment. SI, is southern ice cliff. EE, is eastern embayment.

Table 5.3: Retreat data of Tasman Glacier from ASTER satellite imagery for the three ice cliff regions used in this study, indicating the regional variability of retreat between March 2011 and January 2013. Positive values represent up–valley retreat, whereas negative values indicate the down–valley movement of the terminus (i.e. advance of the glacier).

Date t WE SI EE Entire

Lmean LPD[ ur Lmean LPD[ ur Lmean LPD[ ur TL

d m m m d-1 m m m d-1 m m m d-1 km 03/02/11 3.88 01/01/12 305 35 ± 27 81 0.11 110 ± 40 165 0.33 24 ± 9 37 0.07 3.54 23/03/12 82 33 ± 48 113 0.40 36 ± 30 133 0.43 2 ± 19 17 0.03 3.16 22/04/12 30 26 ± 7 36 0.88 37 ± 8 50 1.24 20 ± 6 30 0.66 3.05 24/05/12 32 11 ± 8 30 0.36 34 ± 24 74 1.07 11 ± 9 23 0.36 2.97 11/06/12 18 -4 ± 17 20 -0.21 6 ± 14 27 0.31 1 ± 14 22 0.04 2.94 01/01/13 209 34 ± 15 62 0.16 57 ± 10 70 0.27 44 ± 14 68 0.21 3.29 Total 467 137 347 0.28 280 494 0.57 102 197 0.21

Note: t, is the interval between image dates; Lmean, is the mean retreat of the glacier between current and previous

image dates; LPD[, is the maximum retreat between image dates; ur, is rate of retreat (Lmean divided by t to determine

Retreat between March and June 2012 occurred almost entirely along the southern ice cliff, with comparably smaller losses centred along the southern margins of both the western and eastern ice cliffs (Table 5.3). Retreat (37 ± 8 m) along the length of the southern ice cliff was relatively uniform between March and April 2012 occurring at a rate of 1.24 m d-1, with the only major retreat taking place between April and May 2012. This retreat was centralised on the eastern section of the southern ice cliff (Figure 5.13), and was significantly affected by a large calving event in this region on 29 April that lead to 74 m of retreat (Table 5.3).

The terminus of Tasman Glacier was stable between May and June 2012 (Table 5.3), with glacial advance evident along the central and western sections of the southern ice cliff (Figure 5.13). This is not considered to be an artefact of the short time period between images and the error associated with image digitisation, as when compared to images taken on those respective days (Figure 5.14), little change in the terminus is visible. Tasman Glacier may, therefore, be able to temporarily sustain glacial advance during periods of decreased calving.

Between June 2012 and the end of the study period ur for the western embayment (0.16 m d

-1

), southern ice cliff (0.27 m d-1) and eastern embayment (0.21 m d-1) were similar, as shown in Figure 5.13. A secondary effect of this retreat was the shortening of all three ice cliffs either through up–glacier retreat of the terminus shortening the western and eastern embayments, or due to the narrowing of the southern ice cliff. This may indicate that towards the end of the study period the southern sections of the western and eastern embayments and the southern ice cliff were subjected to a similar set of conditions. In comparison to the retreat of the southern section of the terminus, the northern sections of both embayments indicate a more complex pattern of advance and retreat likely linked to limited variability in the external variables (i.e. uplift of sections of the glacier, and higher ice velocities).

Figure 5.14: Time–lapse images taken at the beginning and end of the study, as well as at approximately the same time as satellite imagery used in Figure 5.13. Note that the pointing angle and location of the camera was changed between some images.