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Thermodynamics of basal freeze-on beneath glaciers and ice sheet

Quantitative models of basal freeze-on and its effects on basal ice and subglacial sediments are needed because of the important role that basal freeze- on plays in controlling spatial and temporal patterns of fast ice flow in polar ice sheets. For instance, the location of interstream ridges bordering the trunks of West Antarctic ice streams appears to be controlled by the basal thermal regime. In addition, stoppage of an ice stream is expected to be associated with a switch from basal melting to basal freezing. In the latter case, basal freeze-on can be either a trigger of the stoppage or it can occur due to a decrease in basal shear heating accompanying the stoppage. Whereas basal melting is a destructive process, basal freezing produces distinct basal ice facies, which can be sampled and analyzed to infer the history of sub-ice-stream and sub-ice-sheet hydrology.

While being a part of the glaciology group at University of California, Santa Cruz, Dr Christoffersen constructed a high-resolution (subcentimeter) numerical model of coupled water, heat, and solute flow beneath a freezing ice base overlying a till layer. Using a complete form of the Clapeyron equation, the model incorporates the dependence of ice-water phase-change temperature on pressure, solute concentration, and capillary forces. Dr Poul Christoffersen used the model to make testable predictions and in general compliance with existing observations, the model predicted formation of basal ice with characteristic inter-layering of clean ice and debris-rich ice. Moreover, the model showed that the till layer underlying a freezing ice base experiences characteristic changes in vertical distribution of porosity and pore-water composition. The modelling results created a quantitative framework for the development of criteria needed to interpret the cause of spatial and temporal changes in the physical properties of basal ice layers and subglacial tills.

Image as described adjacent
Image as described adjacent

Figure 1. Depth-time diagrams showing modelled changes in the porosity (%) of a 5-m-thick till layer and formation of segregated ice lenses from downward progression of a freezing front.

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