4.1 Evaluating the Retreat Behavior of Crane Glacier
Without observations during the rapid 8 month retreat window, there is
no direct evidence of ice cliff failure at Crane Glacier. But there is
evidence for a geometric instability in the system, and indirect
evidence that cliff failure may have occurred. We examine these two
ideas separately.
As we observed at Crane, fracture processes dominated following LBIS
break-up, where a terminal ice cliff was observed from 2002-2004. But
immediately following the forcing event (LBIS collapse), retreat by
fracture was slow. As the terminus retreated into thicker ice, calving
and retreat rates increased despite no new climatic forcing, consistent
with a geometric instability. Retreat continued, with a rapid final
phase occurring where the ice had been clearly grounded in 2002. The
system eventually reached a configuration with a terminal stress state
that inhibited further retreat, and the system restabilized.
Many conditions are known to inhibit cliff failure and stabilize glacier
systems, including narrowing of the fjord (Schlemm & Levermann, 2021),
development of sea-ice or mélange in the fjord which can apply
sufficient backstress to stabilize the cliff (Bassis et al., 2021;
Crawford et al., 2021; Schlemm & Levermann, 2021), retreat into thinner
ice along a prograde slope (Bassis et al., 2021), or retreat into ice
that is not preconditioned for failure by crevassing or other processes
that induce damage (Clerc et al., 2019). As Crane’s terminus retreated,
the fjord narrowed to a point where calving and ice flow were in
balance. Then, viscous processes dominated, and the system thinned. In
summer 2011, perennial land-fast sea ice formed in Crane Glacier’s
embayment (Figure S4), which likely enabled the growth of an ice shelf,
consistent with models of sea ice backstress (Robel, 2017) and
documented sea-ice circulation patterns in the region (Christie et al.,
2022). The reestablishment of a floating ice shelf facilitated Crane
Glacier’s recent thickening and velocity decrease.
Cliff failure is thought to be one of the fastest calving processes.
Crane’s final minimum calving rate of 8.50 km/a represented an increase
of both its pre-collapse ice shelf calving rate (Alley et al., 2008) and
its calving rate during the initial period following collapse, which
could indicate a change in the mechanism of calving over that period.
But rates greater than 11 km/a have been recently observed elsewhere on
the WAIS (Milillo et al., 2022), and at Jakobshavn and Helheim Glaciers
in Greenland, whose calving rates exceed 17 km/a (Joughin et al., 2014)
and 11 km/a (Howat et al., 2005), respectively. The calving rates at
Jakobshavn and Helheim, in part, accommodate their high terminus
velocities (9.5 to 15.5 km/a faster than the terminus of Crane), which
is an important qualitative difference between these systems and Crane.
As such, the calving rate observed at Crane does not fall outside of the
range experienced by floating systems elsewhere in Antarctica and
Greenland, but is anamolous for a glacier flowing only 1.5 km/a at its
terminus.
Ultimately, we see 10.05 km of terminus retreat, 3.28 ± 0.03 km of which
could have occurred in grounded ice, with calving rates that accelerated
over the course of retreat.