Field evidence for the recognition of glaciohydrologic supercooling

Abstract

In a series of recent papers we have demonstrated the occurrence of glaciohydrologic supercooling and basal ice accretion (freeze-on) at the Matanuska Glacier in Alaska (Strasser et al., 1992, 1994a, b, 1996Lawson et al., 1998) and developed the theoretical basis for the process in general (Alley et al., 1998, this volume). In these papers we demonstrated, using tritium and isotopic analysis, that the debris-laden basal ice at the Matanuska Glacier is young and accreted in an open hydrologic system. The interested reader is referred to these papers for details of the glaciohydrologic supercooling process and our interpretation of the origin of stratified basal ice. ABSTRACTGlaciohydrologic supercooling at Matanuska Glacier results in abundant and conspicuous summer ice growth even when air temperatures are constantly and significantly above freezing. Ice grows as frazil ice and anchor ice. Frazil ice grows unattached in high-velocity discharge water. Anchor ice grows at vent orifices, in subglacial conduits and canals, in moulins, and in fractures. Anchor ice can grow as large, debris-free platy crystals or, more commonly, as fine-grained, debris-rich laminated ice. To date, these types of summer ice growth have been reported at few glaciers and studied in detail at the Matanuska Glacier but we anticipate that glaciohydrologic supercooling and the associated ice growth are operating at numerous other glaciers with the appropriate geometries. The objective of this paper is to provide a photographic atlas, and brief explanation, of the ice growth forms and features observed at the Matanuska Glacier. We hope that it will enable other researchers to recognize the evidences of glaciohydrologic supercooling at additional glaciers and eventually establish glaciohydrologic supercooling, and the associated basal freeze-on, as a major debris entrainment and transportation mechanism.

A morphometric analysis of tunnel valleys in the eastern North Sea based on 3D seismic data

“…However, if the bed rises at an angle steeper than 1.2-1.7 of the ice surface slope, the water becomes supercooled because warming by heat dissipation is reduced by potential energy gained by altitude (Rö thlisberger, 1972;Alley et al, 1998). As the supercooling threshold is reached, debris-rich ice will start to accrete at the ice/water interface, which may lead to thickening of ice near the snout and eventually result in complete channel plugging (Hooke and Pohjola, 1994;Evenson et al, 1999). Re-activation of channels and return to erosion may be caused by an increase in ice surface steepness to above 1.2-1.7 the value of the bed slope.…”

“…Supercooled water emerging at ice margins has been observed at many modern glaciers (e.g. Lawson et al, 1998;Evenson et al, 1999Evenson et al, , 2001Roberts et al, 2002), where it typically re-freezes trapping some sediment, and it has been suggested that supercooling also operated under large Pleistocene ice sheets (Larson et al, 2006).…”

Debris entrainment and landform genesis during tidewater glacier surges

“…Glacihydraulic supercooling has been invoked as a formation mechanism for debris-rich ice that shares some physical similarities with the solid stratified subfacies, including polymodal grain size distribution, medium to high debris concentrations, interstitial ice, small areas of sorted sediment, and overall facies thicknesses of >0.5 m [Lawson et al, 1998;Cook et al, 2010]. However, these examples were also associated with additional features thought to be diagnostic of supercooling, e.g., upwelling vents, anchor ice terraces, and fractures filled with platy ice [Evenson et al, 1999], none of which are observed at Tunabreen.…”

Controls on englacial sediment deposition during the November 1996 jökulhlaup, Skeiđarárjökull, Iceland

“…Recent studies by Alley et al (1998), Lawson et al (1998) and Evenson et al (1999) at Matanuska Glacier, Alaska, USA, have demonstrated subglacial sediment accretion by freeze-on of supercooled water. Glaciohydraulic supercooling occurs when water flows up a subglacial adverse slope that has a gradient >1Ð7 times the magnitude of the glacier surface gradient (Shreve, 1985;Alley et al, 1998).…”

“…At Skeidarárjökull, the thickness of fracture fill sediment and the duration of supraglacial outbursts (<1 hour) require accretion rates of the order of decimetres to metres per hour. According to Evenson et al (1999), glaciohydraulic supercooling is most prolific in turbulent, high-velocity water flow. Estimates of palaeodischarge from supraglacial fracture outlets at Skeidarárjökull are of the order of 10 2 m 3 s 1 , which implies turbulent, high-velocity discharge, as demonstrated by the presence of poorly sorted fines and boulders within jökulhlaup fracture deposits.…”