3.1 Period 1: Retreat (2002-2004)
From 2002-2004, terminus retreat of over 10 km occurred, with moderate
retreat rates from November 2002 to March 2004, and more rapid retreat
from March to November 2004 (Figure 2). During this two year window, the
glacier thinned by 60.9 m at the location of the November 2004 ice
front.
Satellite-derived, pre-LBIS collapse grounding line positions do not
agree with the sea-floor sedimentary record or the updated bathymetry
measured post-collapse (Rebesco et al., 2014). Satellite data placed the
pre-collapse grounding line near the opening of the fjord (Rack & Rott,
2004), implying that the full extent of retreat observed from 2002-2004
occurred by failure of a grounded ice cliff. Radiocarbon-dated sediment
cores show that ice was not grounded there in 2002, and suggest the
pre-collapse grounding line was located further inland of the fjord
opening (Rebesco et al., 2014). In addition, measured surface elevations
from 2002 at the purported grounding line fell far below the floatation
threshold (Fig. 1B). This means that a substantial portion of the
initial terminus retreat occurred in ice that was at or below the
floatation thickness.
Using our floatation criterion, we calculate an updated grounding line
position for November 2002. Surface elevation data show a break in slope
at ~9.5 km along our reference line, more consistent
with calculated grounding lines assuming 0 m of FAC. We calculate
retreat rates and cliff heights assuming this value.
While the majority of the Larsen B collapsed during February and March
of 2002, a floating ice shelf remained at Crane. Slow terminus retreat
began, but had not reached the 2002 grounding line by March of 2004,
after 5.30 km of retreat (Figure 2). Retreat continued, and during the
following 8 month period, the glacier experienced 4.75 km of additional
terminus retreat, beginning 1.47 ± 0.03 km down-flow of the updated 2002
grounding line and halting at the 2004 terminus. We quantify the
effective rate of ice loss during the periods preceding and following
March 26, 2004 using a calving rate, equal to the sum of the terminus
retreat rate and the ice flow speed at the terminus
(~1.5 km per year, the nearest available satellite
measurement in space and time, and consistent with estimates from
Scambos et al., 2004). This results in minimum calving rate estimates of
5.48 km/a for the period ending on March 26, 2004, and 8.50 km/a in the
period after, a 55% increase in calving rate over the 2 years following
LBIS collapse.
If grounded ice cliff failure occurred at Crane, it must have happened
during this final period of retreat. While there are no direct
observations of a grounded ice cliff during that period, the first cliff
that could have formed (at the 2002 grounding line position) would have
existed with a height of 111 m (as measured above the water line; Figure
1B).