Following the disintegration of the lower terminus in 2007, water–depth at the glacier front increased significantly as Tasman Glacier receded into the glacially–excavated trough (Figure 4.7), driving the glacier further from equilibrium. As a result, the glacier has entered a period of increased retreat distinct from the retreat seen prior to 2006, although not to the same extent as found between 2006 and 2007 (Figure 4.2). This is not inherently evident in changes in surface area (Figure 4.2), as the rate of growth has remained at similar levels to that prior to 2006 (Figure 4.5). However, given the dramatic shortening of the terminus in contact with Tasman Lake (L), and consistently increased retreat rate recorded between image acquisition dates (Table 4.1), it is clear that retreat has increased. This increase in retreat is suggested to have been driven by a transition in the style and impact of calving at Tasman Glacier. A period of high–magnitude, low frequency, buoyancy–driven calving events has commenced that is being superimposed on top of the high–frequency, low–magnitude, calving events that have continued to occur. This has been triggered by the increase in water–depth at the terminus due to disintegration of the terminus in early 2007 increasing the potential for flotation of the terminus (Figure 4.7C), changes in basin morphology and the continued presence of supraglacial ponds in the terminus region.
Ice loss and retreat between 2007 and 2011, shown in Figure 4.1D–F, appears to follow a cyclical sequence characterised by the calving of large ice peninsulas which developed as a result of formation of supraglacial ponds along the centre–line of the lower terminus. Formation of such peninsulas effectively isolates large sections of the central terminus removing support from surrounding ice and valley sides and increasing sensitivity to buoyant forces (van der Veen, 2002; Boyce et al., 2007). Calving retreat to the up–valley edge of supraglacial ponds takes place via a series of large calving events, with this cycle of retreat then repeated. Two examples of this progression are shown in Figure 4.8 and Figure 4.12. Figure 4.12 shows a series of icebergs that calved from a peninsula that developed during late 2008 giving rise to 378 m of retreat of the eastern peninsular between 2007 and 2009 (Figure 4.1). This process was again repeated during August 2010, when a section of the terminus approached flotation and became uplifted due to isolation from surrounding lateral support (Figure 4.9). This peninsula calved on the 22 August in a single calving event, leading to the rapid up–glacier retreat of the terminus by c. 260 m. This sequence is also evident for several other calving events that took place between 2007 and 2012 (see chapter 8) and highlights the continued importance of supraglacial ponds in ice loss from the terminus of Tasman Glacier.
Figure 4.12: Photographs of large calving events at Tasman Glacier that was initiated by flotation and buoyancy of the terminus in February 2009. Note that the icebergs have rotated as they calved from the terminus.
A secondary effect of this increase in ice loss from the terminus has been an apparent increase in ice velocity at the terminus (Redpath, 2011). For example, feature–tracking applied to ASTER images between 2000 and 2010 by Redpath (2011) suggests that there is an increase in surface ice velocity (to between 30 and 50 m a-1) within the terminus region, compared with lower velocities further up–glacier of the terminus. Several authors (Kirkbride, 1993; Kirkbride and Warren, 1999; Redpath, 2011) have attributed this glacier speed–up with extensional flow towards the terminus due to the presence of Tasman Lake. This extensive flow is evident in Figure 4.13, indicated by the development of transverse crevasses in the terminus zone, and has been identified during previous periods of retreat (Figure 6 in Kirkbride and Warren (1999)). The effect of this extensive flow is the potential thinning of the lower glacier, further increasing
its susceptibility to buoyancy–driven calving (Benn et al., 2007b). However, the extent to which this has affected calving and Tasman Glacier’s retreat between 2007 and 2011 is currently still unclear due to the large changes in terminus geometry and a lack of complete records of glacier thinning and detailed velocity measurements at the terminus.
Figure 4.13: Photograph showing the terminus of Tasman Glacier in May 2012. Note that no supraglacial ponds are present on the lower glacier tongue as well as the increase in glacier slope present from c. 700 m (white dotted line) up glacier from the terminus. It was along this line that the calving occurred in February 2013. Also note the depression present running along the centre of the glacier (black dotted line) and the presence of transverse crevasses at the terminus (arrowed).
Analysis of the January 2012 and January 2013 ASTER images indicates that there are no supraglacial ponds within the terminus region, in contrast to 2007 and 2011 (e.g., Figure 4.12). This is further corroborated by Figure 4.13, which shows that not only are there no supraglacial ponds, but that the slope of the glacier increases c. 700 m up–glacier from the terminus. In general, the surface slope of the glacier past this point increases from ~2° to between 3° and 4°. This increase in surface slope has the effect of limiting the ability of water to pool and for supraglacial ponds to form (Sakai et al., 2000). As a result, the development and isolation of peninsulas that have initiated large buoyancy–driven calving events identified between 2007 and 2011 may become even less frequent. Given that Tasman Glacier still terminates into deep water (>300 m), the terminus is still being subjected to increased torque from buoyant forces (see the following section for description), albeit potentially over a larger area. For example, the development of the western embayment since 2009 has had the effect of isolating the entire glacier front from the lateral margins. The result of this was the calving of the entire lower section of the glacier in February 2013 (see section 8.4.5 for description).