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Strasser, J.C., 1996

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Bibliographic Reference

Strasser, J.C., 1996, Subglacial ice growth, basal accretion, and debris entrainment at the Matanuska Glacier, Alaska: Bethlehem, Pennsylvania, Lehigh University, Ph.D. dissertation, 137 p.

Abstract

The debris-rich basal ice exposed at the terminus of the Matanuska Glacier varies typically between 1 and 4 m in thickness. This ice is generally stratified in appearance, with bulk debris concentrations of 30-40% by mass. Moraines are constructed at the glacier terminus dominantly of material transported in this basal zone. The basal ice of this glacier forms dominantly by accretion of ice from glaciohydraulically supercooled waters flowing in the subglacial drainage system. Supercooling of subglacially-flowing water occurs as water flows uphill out of an overdeepening. Ice accretes to the glacier sole as discontinuous layers and lenses in a distributed drainage system, most likely during the periods of highest subglacial discharge, and debris is presumably entrained concomitant by processes of filtering from the discharge water and regelation. Frazil ice and other ice forms grow in the waters discharging from subglacially-fed vents at the terminus of the Matanuska Glacier, forming rims and terraces that surround subglacially-fed vents. Debris is entrapped in the initially open framework of platy ice accretions, and continued ice growth leads to indurated masses of debris-rich ice. The physical similarities and the spatial continuity between debris-rich basal ice and ice accretions at discharge vents suggest a similar genesis. Moreover, the occurrence of frazil crystals in vent discharge waters is evidence of supercooled conditions in the subglacial drainage system, where similar processes of ice accretion and sediment entrapment are thus likely. The debris-rich basal ice of this glacier exhibits elevated concentrations of bomb-produced tritium, indicating net freeze-on of several meters of ice since 1952. A detailed vertical profile through the basal ice zone reveals a peak in tritium content near the top of the stratified facies. Correlation of this peak to the 1963 peak in regional atmospheric tritium indicates a net accretion rates up to 7 cm y-1. Analyses of δ18O and δD also demonstrate that basal ice forms from water flowing in the subglacial drainage system.

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