Thermodynamics of basal freeze-on:Subglacial properties and ice sheet response

• Main Author: Christoffersen, Poul
• Format: Book
• Language: English
• Published: Technical University of Denmark 2003
• Subjects: Antarc*; Antarctica; Ice; Ice Sheet; permafrost; West Antarctica
• Online Access: https://orbit.dtu.dk/en/publications/0e0805a8-faac-4430-87b7-86a1f680517d; https://backend.orbit.dtu.dk/ws/files/5277063/byg-r049.pdf

Ice sheets are drained by fast flowing ice streams and outlet glaciers that discharge large quantities of ice into circumpolar oceans. Changes in the flow pattern of these fast glaciers can affect the mass balance of ice sheets. Even small property changes in the till layer beneath ice streams may have a large effect upon ice dynamics. Cryostatic dewatering can trigger ice stream stoppage because basal freezing beneath fast flowing glaciers can become a run-away process. This thesis provides the first quantitative explanation for ice stream stoppage as observed in West Antarctica. Basal freeze-on is modelled analogously to frost heave, which is a much-studied permafrost process. Ice-water surface tension and osmotic pressure are included into a subglacial model in which the flows of water, heat and solutes are coupled. It is shown that a till porosity reduction of a few percent is sufficient to impede fast flow and ice streams may shut down after less than 100 years of basal freezing. Ice stream shut-down by basal freeze-on is not restricted to extremely cold environments such as Antarctica. A combination of numerical modelling and geotechnical observation show that palaeo-ice streams along the southern rim of Pleistocene ice sheets may have stopped because horizontal advection of cold ice caused a thermal switch into basal freezing. The freezing rate associated with ice stream stoppage is high because latent heat of fusion must replace a large amount of frictional heat. Basal freeze-on can entrain subglacial sediment and produce a debris-bearing accretion ice layer. Basal accretion ice layers have recently been observed in West Antarctica using a borehole camera system. A model set up to simulate subglacial accretion is able to reproduce several of the most prominent types of accretion ice seen in the borehole videos. It is important to identify the facets of subglacial accretion because the composition of debris-bearing basal ice contains information about pre-existing ice sheet conditions. Such information ...