2.2 Subglacial Topography and Floatation Criteria
Crane Glacier’s bed elevation was refined by a marine geophysical survey in the Larsen B Inlet in 2006 (Rebesco et al., 2014), which captured the bed over which the Crane terminus retreated in 2004. Radar campaigns designed to capture ice-bottom elevations were flown as part of NASA’s Operation IceBridge from 2009-2017, but because of the narrow fjords and complex topography of the Antarctic Peninsula, those data suffer from significant off-axis clutter, resulting in an ambiguous basal reflector and an unknown ice thickness profile. We use the stack of all available radar data, compared with the multibeam swath bathymetry, to derive a range of reasonable bed elevations for the glacier interior (Figures S2, S3).
Monitoring the grounding line position and quantifying the stress imbalance at a marine ice cliff requires knowledge of the ice thickness relative to the floatation thickness (the minimum ice thickness required to ground the ice bottom on the sea floor). With the density of ice (\(\rho_{i}\)= 917 kg/m3), the density of sea water (\(\rho_{\text{sw}}\)= 1027 kg/m3), and the sea-floor elevation relative to the sea surface (\(b\)), the ice height at floatation (hf ) can be calculated by:
\begin{equation} h_{f}=b\left(1-\frac{\rho_{\text{sw}}}{\rho_{i}}\right)\nonumber \\ \end{equation}
Ultimately, we derive floatation heights from the radar data, multi-beam bathymetry, and estimates of the tidal state from sea-surface altimetry. This can be compared to the measured surface elevation to estimate the grounding line position. Variability in the bed topography across-flow affects our estimate of the grounding zone position. To account for this, we calculate the floatation thickness using a range of ocean bottom elevations, sampling +/- 100 m orthogonal to the centerline (Figure 1B).