Figure 4 and figs. S2 to S6 and S8 to S12 used normalized surface elevations to reveal the continuous nature of troughs from grounded to floating ice in shear margins. These troughs are identifiable in surface elevation transects perpendicular to the shear margins (Figs. 3, C and D, and 4, B to Dand figs. S2 to S6 and S8 to S12), but they are difficult to represent in continuous, gridded views of shear margin elevations due to the strong elevation gradients present near grounding lines. Removing the background trend makes the troughs apparent.

To remove the background trend, we selected a grounding line region with high shear strain rates and drew a trace (blue dashed lines in Fig. 4A) along the approximate center of the shear margin, according to the shear strain rates. It is unlikely that troughs will follow the center of the shear margins precisely, due to complex shear margin flow as well as differential accumulation and firn densification (34). Therefore, we generated elevation profiles perpendicular to the trace (figs. S2 to S6A and S8 to S12A) that were long enough to span the entire shear margin, ensuring that the shear margin trough was included somewhere in each profile.

We extracted elevations along the perpendicular profiles and used these to recreate the original topography, which is shown in Fig. 4F and figs. S2 to S6E and S8 to S12E. There are two important advantages to using these perpendicular profiles to represent the topography: (i) Plotting the profiles on a regular grid effectively straightens the shear margins so that they are approximately parallel to the edges of the grid; and (ii) because troughs are expected to be approximately parallel to shear margins, topography normalization will most clearly reveal troughs if it is applied to slices that are perpendicular to the trough, so that none of the cross-trough trend is removed from the data.

We selected the point with the minimum elevation in the central half of each perpendicular transect. This point was assumed to be the bottom of a shear margin trough. Each perpendicular transect was then adjusted linearly up or down so that the new trough bottom points all aligned at the mean trough bottom elevation. The resulting gridded and normalized elevation plots are shown in Fig. 4G and figs. S2 to S6F and S8 to S12F. If the plot showed that the trough bottom lay outside the central half of each perpendicular transect, then we reran the code to select the minimum value in the first or second half of each perpendicular transect. At no time did we manually dictate the exact locations of the trough bottoms.

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