Workreport 2019-19



Greenland ICE Project, Final Report


Harper, J., Maierbachtol, T., Humphrey, N.



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Safety assessments of potential underground repository locations prone to future glaciations must
consider the impact of ice sheet advance on site conditions. This requires detailed knowledge of
the processes and conditions at the boundary coupling ice sheet and earth systems. The ICE Project
employed the Greenland ice sheet as a field laboratory to advance understanding of the physical

processes governing ice/water/earth interactions.

Detailed instrumentation, data collection, and analysis of a portion of the Greenland ice sheet were
performed to address four science objectives focused on: 1) interpreting the physical framework of
the ice sheet bed, 2) quantifying the magnitude of, and spatial gradients in, basal water pressure,
3) determining the presence of short duration water pressure transients in the ice sheet basal drainage
system, and 4) advancing understanding of the distribution of water along the ice sheet bed. Access
to the full ice thickness was created through nine boreholes drilled to the bed by a hot water drill.
Sensors installed at the surface, within the ice, and at the basal boundary of a ~700 × 700 × 700-meter
block of ice generated a comprehensive dataset spanning three years. These data were interpreted
with respect to the ~50 km long transect of observations previously collected by the Greenland
Analogue Project (GAP) in order to accomplish the project objectives.

Findings from the ICE Project demonstrate that the bed of the study site and the GAP borehole study
transect is bedrock with a relatively thin cover of sediment, indicating that the subglacial drainage
system is of a ‘hard-bed’ type rather than porous-till type. The basal water pressure was confined
to a limited range between 0.8–1.1 as a fraction of ice overburden pressure. Two types of spatial
gradients were identified: 1) a primary gradient driven by the large-scale longitudinal change in
ice overburden pressure, and 2) secondary local gradients related to the dynamics of the subglacial
drainage system. Measurements showed that drainage system pressure changes arise from mechanical
adjustments to drainage system volume, in addition to basal water flow dynamics.

Very rare pulses in pressure occurred within the subglacial drainage system, reaching magnitudes
of 0.72 MPa, but were spatially localized. Ice deformation inhibits the reliable estimation of water
volume changes at the ice/bed interface utilizing elevation changes of the ice surface. The large-scale
geometry of the ice sheet reflects enhanced basal sliding, most likely related to the occurrence of
water at the ice/bed interface.


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