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Water-filled ditches: Surface expressions of dead crevasses that are not connected to the bed
  • Kristin Poinar
Kristin Poinar
University at Buffalo

Corresponding Author:kpoinar@buffalo.edu

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The increasing ubiquity of high-resolution imagery has yielded many observations of water-filled crevasses across the surfaces of glaciers and ice sheets (e.g., Figure 1a). The subsurface character of these features, however, is not apparent in imagery, nor can it be fully elucidated even through field geophysics. Thus, what visible surface water in crevasses indicates about englacial hydrology, including whether there is a surface-to-bed connection, is currently subjective and interpreted differently by different scientists. Application of a physically based crevasse model to this problem shows that if a crevasse visibly holds water, it likely does not connect to the bed. The crevasse model incorporates depth-dependent visco-elastic deformation and refreezing to evolve the size and shape of a water-filled crevasse over hourly to decadal timescales (Figure 1b). Seasonally, visco-elastic closure tends to form a neck at the water line of most crevasses. Over a year or more, this neck can pinch off, isolating a pod of water that can extend hundreds of meters beneath the surface. The area above the neck persists as a 1–5 meter wide, 10–40 meter deep “ditch”: the surface expression of a dead crevasse that no longer receives surface melt. Accumulation of meltwater in these ditches is consistent with observations; the model results show that the ditches are not hydrologically connected to the crevasse or to the bed. These findings are consistent with recent observational work by Chudley et al. (2020), who concluded that visible water in crevasses sited in compressive stress settings was not connected to the bed. Observations of sudden drainage of these ditches show that reconnection to the englacial system, and potentially the bed, must be possible. The smooth bathymetry of the ditches, however, discourages formation of the starter crack needed to reactivate these hydrofractures. Thus, an external forcing, such as advection into a more-extensional stress setting, may be required to drain them. Overall, model results suggest that these water-filled ditches are shallow (<40 meters), overlie an englacial pod of liquid water that is in the process of refreezing, and are not connected to the bed.