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Quaternary Science Reviews 24 (2005) 2308–2318

Hydrologic monitoring of supercooled meltwater fromIcelandic glaciers

Fiona S. Tweeda,�, Matthew J. Robertsb, Andrew J. Russellc

aDepartment of Geography, Staffordshire University, College Road, Stoke-on-Trent, Staffordshire ST4 2DE, UKbPhysics Department, Icelandic Meteorological Office, Bustajavegur 9, 150 Reykjavık, Iceland

cSchool of Geography Politics & Sociology, University of Newcastle, Daysh Building, Newcastle upon Tyne NE1 7RU, UK

Received 19 January 2004; accepted 24 November 2004

Abstract

Knowledge of how glaciers entrain sediment is central to understanding processes of glacier movement and products of glacial

sediment deposition. Previous work has shown that if the total hydraulic potential of subglacial meltwater increases more rapidly

than the resulting mechanical energy can be transformed into sensible heat, then supercooling and ice growth will result. This

process causes frazil ice to grow onto adjacent glacier ice, which acts to trap sediment in flowing meltwater eventually producing

sedimentary inclusions within glacier ice. Supercooling has been recognised as a sediment entrainment mechanism at glaciers in

Alaska, and more recently at several temperate Icelandic glaciers. Here we present short-period temperature measurements and field

evidence of glaciohydraulic supercooling from three Icelandic glaciers. Temperature measurements demonstrate that supercooling

occurs over a range of hydrological conditions and that the process does not operate continuously at all instrumented sites.

Measurements of supercooling during a small jokulhlaup are also presented. Progressive accretion of supercooled meltwater creates

sediment-laden ice exposures adjacent to active artesian vents. Understanding controls on the efficacy and pervasiveness of

hydraulic supercooling is important for decoding the sedimentary record of modern and ancient glaciers and ice sheets.

r 2005 Elsevier Ltd. All rights reserved.

1. Introduction and aims

Glaciohydraulic supercooling has been recognised asa sediment entrainment mechanism at glaciers in Alaska(Alley et al., 1998; Lawson et al., 1998; Evenson et al.,1999). Recent work also suggests that supercoolingoccurs at several temperate Icelandic glaciers (Knudsenet al., 2001; Roberts et al., 2001, 2002). However, thespatial and temporal pervasiveness of the processremains unknown (Evenson et al., 1999), and theglaciological and geomorphic significance of fluvialsediment entrainment by supercooled water has notyet been quantified. Understanding of such issues mayhelp to explain the sediment dynamics of former icesheets and glaciers. For example, it has been proposed

e front matter r 2005 Elsevier Ltd. All rights reserved.

ascirev.2004.11.020

ing author. Tel.: +441782 294113;

94114.

ess: f.s.tweed@staffs.ac.uk (F.S. Tweed).

that rapid effluxes of supercooled meltwater from theoverdeepened basins of ice sheets during the late-Pleistocene were responsible for entraining substantialamounts of sediment to ice sheets (Alley et al., 1998,1999; Roberts et al., 2002). Such debris entrainmentcould account for large volumes of sediment depositedat former terrestrial glacier margins and the depositionof IRD in marine settings (Alley et al., 1998, 1999;Andrews and MacLean, 2003).

Despite steadily increasing awareness of glaciohy-draulic supercooling, direct quantitative evidence for it(e.g. Ensminger et al., 1999) is relatively scarce, chieflydue to the expense and logistical difficulty of collectingmeltwater temperature measurements (Evenson et al.,1999). This paper presents meltwater temperature dataand evidence of associated debris-rich ice accretion fromIcelandic glaciers Skeijararjokull, Skaftafellsjokull andSvınafellsjokull.

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Fig. 1. Skeijararjokull, Skaftafellsjokull and Svınafellsjokull southern

Iceland, illustrating the location of sites mentioned in the text.

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2. Glaciohydraulic supercooling

The theory of meltwater supercooling in glaciers iswell established (Rothlisberger, 1968; Shreve, 1985;Rothlisberger and Lang, 1987; Hooke and Pohjola,1994) and it has been summarised for water flow intemperate glaciers by Alley et al. (1998, 2003a). Viscousdissipation of latent and frictional heat usually keepsmeltwater in thermodynamic balance with ambientglaciostatic stress, but if the total hydraulic potentialof subglacial meltwater increases more rapidly than theresulting mechanical energy can be transformed intosensible heat, then supercooling and ice growth willresult (Shreve, 1985). Supercooling and associated icegrowth will occur wherever there is large enoughmeltwater flow rising at a suitably steep angle,commonly on ascent from overdeepened glacier basins(Alley et al., 1998). The minimum physical requirementfor the occurrence of supercooling is a bed slopegeometrically opposed to an ice-surface slope by afactor 41.2–1.7 times (Alley et al., 1998). Not only hassupercooling been implicated in processes of sedimententrainment (e.g., Shreve, 1985; Ensminger et al., 2001;Alley et al., 2003a), it has also been suggested thatsupercooled ice growth can constrict meltwater conduitsin glaciers, thus impacting on subglacial water pressureand the hydrodynamics of subglacial drainage (Hantzand Lliboutry, 1983; Shreve, 1985; Hooke and Pohjola,1994; Alley et al., 1998; Evenson et al., 1999; Creyts andClarke, 2003).

Field evidence of glaciohydraulic supercooling sup-ports and complements theoretical treatments of theprocess. Research by Strasser et al. (1996), Lawson et al.(1998) and Evenson et al. (1999) has been largelyresponsible for broadening awareness of field evidenceof glaciohydraulic supercooling, and for suggesting alink between supercooling and debris-rich ice formation.Research at Matanuska Glacier, Alaska, has shown thatsupercooled meltwater emerges from an overdeepenedglacier basin via artesian vents, causing the rapid growthof frazil ice at the glacier margin (e.g. Lawson et al.,1998; Evenson et al., 1999, 2001; Alley et al., 2003a),which evolves into thick sequences of debris-rich ice (e.g.Strasser et al., 1996; Lawson et al., 1998; Baker et al.,2003). Within Alaska, supercooling has also beenreported at Bering Glacier, where frazil ice rafts havebeen observed in ice-marginal lakes (Natel and Fleisher,1994; Fleisher et al., 1997).

Subglacial overdeepenings are often reported inglaciological literature (e.g. Rothlisberger, 1968; Hooke,1991; Alley et al., 1999; Spedding, 2000; Alley et al.,2003a, b); thus it is logical to expect supercooling to be awidely reported glaciohydraulic process. However, asEvenson et al. (1999) allege, it is likely that manyresearchers have been working in ignorance of featuresdiagnostic of supercooling. Attempts to remedy such

lack of knowledge have been made by Evenson et al.(1999) who describe and illustrate supercooling-relatedphenomena observed at Matanuska Glacier, thusassisting researchers to recognise supercooling signa-tures at other glaciers.

3. Field sites

Skeijararjokull (�1500km2) is a temperate, surge-typepiedmont glacier draining from the southern side of theVatnajokull ice cap, southeast Iceland (Bjornsson, 1982)(Fig. 1a). Temperate valley glacier Skaftafellsjokull(�100 km2) is fed mainly by ice from Vatnajokull, withthe addition of some ice from the northern flank ofOræfajokull (Fig. 1b). Neighbouring Svınafellsjokull(�30km2), also a temperate valley glacier, drains fromOræfajokull (Fig. 1). Skeijararjokull frequently releasesjokulhlaups from Grımsvotn crater lake and from severalice-marginal lakes including Grænalon (Bjornsson, 1974,1992). A massive jokulhlaup from Skeijararjokull inNovember 1996 caused englacial hydrofracturing andassociated sediment deposition attributed to glaciohy-draulic supercooling (Roberts et al., 2001, 2002).

Eastern Skeijararjokull, Skaftafellsjokull and Svına-fellsjokull are currently retreating (Sigurjsson, 2003)into overdeepened basins, inducing negative ice-surfaceslopes close to their termini (Fig. 2). Radio-echosounding data from the three glaciers reveal terminaloverdeepenings at all sites (Bjornsson et al., 1999;Bjornsson et al., 2003; Finnur Palsson, personal com-munication, 2003). In combination with ice-surfaceelevation data, it is apparent that termini of Skeijar-arjokull, Skaftafellsjokull and Svınafellsjokull fulfilthe physical conditions for glaciohydraulic supercool-ing. Additionally, at all sites discussed in this paper,

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Fig. 2. Ice descent into a terminal overdeepening at Skaftafellsjokull, as evidenced by the ice surface topography. Meltwater is fed into the ice-

contact lake under artesian conditions. This site is the location of the vent complex at western Skaftafellsjokull referred to later in the text. The arrow

marks the presence of a conspicuous, curved, water-filled trench suggesting that normal faulting may have occurred due to the edge of the glacier

being afloat. (Image taken in July 2002 at a vertical elevation of 212m above the sandur surface. Photographer, M.J. Roberts).

F.S. Tweed et al. / Quaternary Science Reviews 24 (2005) 2308–23182310

meltwater emerged under artesian conditions due to thelocal piezometric surface being slightly higher than theice margin—a thermodynamic consequence of thestrongly asymmetric variation of the pressure-deter-mined melting point with respect to an adversely slopingglacier bed (Rothlisberger, 1968).

4. Methods

Fieldwork was carried out in July 2000, July 2001,March and July 2002 and March and July 2003. TwoRBR TR-1050TM temperature probes were used torecord water temperatures in turbid artesian ventssuspected of discharging supercooled meltwater. Bothtemperature probes contained 24-bit ThermometricsTM

thermistors in sealed units calibrated at source, with anaccuracy of70.002 1C, resolution of 1� 10�5

1C, and anoperating range of �40 to 35 1C. Temperature probeswere activated using a laptop computer, then roped andweighted, and immersed into vents down to depths of atleast 2m. Field evidence of glaciohydraulic supercoolingwas recorded and vent locations were constrained usinga DGPS. The structure and sediment characteristics ofice facies near to instrumented vents were documented.

Although temperature probes were usually left inplace for as long as possible, a combination of safetyissues, financial limitations and logistical difficultiesfrequently restricted the length of meltwater tempera-ture measurements to �0.6–4 h. Access to vent locationssometimes involved crossing thin frazil ice terraces in

environments subject to frequent ice collapse. The riskof losing one or both of the probes and the safety offieldworkers had to be balanced against the need forlonger measurement periods.

5. Measurements of supercooled meltwater

Turbid artesian vents were identified at a number oflocations around the margins of Skaftafellsjokull andSkeijararjokull (Fig. 1). Although access difficultieswere sometimes encountered in attempting to reach anumber of vents, especially in summer, subglacialmeltwater temperatures were measured from upwellingsat Skaftafellsjokull and Skeijararjokull on severaloccasions between July 2000 and July 2003.

At Skeijararjokull, between 2001 and 2002, a ventcomplex located at the head of the Sula migratedeastwards to the head of the Blautakvısl from whichmost meltwater from the western margin of Skeijar-arjokull now issues (see Fig. 1). Records of meltwaterdischarge from three artesian vents at the head of theBlautakvısl in July 2002 demonstrate that discharge isslightly supercooled (Fig. 3). Meltwater temperatures,sampled at 30-s intervals over �2 h, were consistentlybelow atmospheric freezing point despite higher airtemperatures. Initial oscillations in the data are inter-preted as the time required for the submerged instru-ment to reach thermal equilibrium with the vent (e.g.Fig. 3a).

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Fig. 3. Temperature plots for three of a number of subglacial artesian

vents at the head of the Blautakvısl, July 2002 (for location see Fig. 1).

Meltwater discharging from the vents is consistently below atmo-

spheric freezing point despite air temperatures being well above 0 1C.

Data were sub-sampled at 1-min intervals for illustrative purposes.

Error bars represent 70.002 1C, signifying the specified accuracy of the

temperature probes.

Fig. 4. (a, b) Temperature plots for the Skeijara vent, March 2002 (for

location see Fig. 1). (c) Temperature measurements from the Skeijara

vent, made over a 30-h period in March 2002 which coincided with a

rainfall-induced flood in Skeijara. Meltwater discharging from the

vent on all three occasions is consistently below atmospheric freezing

point despite air temperatures being well above 0 1C. Data were sub-

sampled at 1-minute intervals (a), 2-min intervals (b) and 10-min

intervals (c) for illustrative purposes. Error bars represent 70.002 1C,

signifying the specified accuracy of the temperature probes.

F.S. Tweed et al. / Quaternary Science Reviews 24 (2005) 2308–2318 2311

The Skeijara ice-marginal outlet on the easternmargin of Skeijararjokull (see Fig. 1), comprises a largevent with summertime discharges of the order of200 m3 s�1. Meltwater discharging from this vent wasalso consistently below atmospheric freezing point whenmeasured on two occasions in March 2002 (Fig. 4a

and b). On 23 March, during a period of temperaturemeasurement at the Skeijara vent, a flood fromGrımsvotn entered Skeijara. Although subglacial melt-water temperatures in the Skeijara vent remained belowatmospheric freezing point throughout the period ofmeasurement, a slow rise in meltwater temperature was

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Fig. 5. Temperature plot for a subglacial artesian vent at the eastern

margin of Skaftafellsjokull, July 2003 (for location see Fig. 1). Meltwater

discharging from the vents is consistently below atmospheric freezing

point despite air temperatures being well above 0 1C. Data were sub-

sampled at 2-min intervals for illustrative purposes. Error bars represent

70.002 1C, signifying the specified accuracy of the temperature probes.

Fig. 6. Temperature plots from (a) the head of the Blautakvısl in March 20

during March 2002 and July 2003, and (d) the centre of the Skeijararjokull ma

occasions, temperatures are above atmospheric freezing point, despite the pr

temperature variability in Fig. 6a could have been due to ice plugging the tem

and 1-h intervals (b) for illustrative purposes. Error bars represent 70.002 1

F.S. Tweed et al. / Quaternary Science Reviews 24 (2005) 2308–23182312

recorded (Fig. 4c). We attribute this rise in meltwatertemperature (despite increasing discharge) to an influxof rainwater that was observed cascading down the sideof the glacier into Skeijara, thus diluting the strength ofthe supercooling signature. These data quantify theoccurrence of supercooling during a small flood (cf.Roberts et al., 2002).

At Skaftafellsjokull, vent complexes are locatedat the heads of the two main meltwater outletsat the eastern and western edges of the glacier terminus.The western area of the terminus is dominatedby a cluster of artesian vents. Supercooled meltwatertemperatures have previously been reported fromthis vent complex (Roberts et al., 2002). In July 2003,instrumentation submerged for over 4 h recordedtemperatures that were consistently below atmo-spheric freezing point for water emerging from a newlylocated vent at the eastern margin of Skaftafellsjokull(Fig. 5).

02, (b, c) the vent complex at the western margin of Skaftafellsjokull

rgin at the Gygjukvısl during July 2003 (for locations see Fig. 1). On all

evalence of field evidence of glaciohydraulic supercooling. The lack of

perature sensor. Data were sub-sampled at 2-min intervals (a, c & d),

C, signifying the specified accuracy of the temperature probes.

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Not all monitored vents exhibited supercooled tem-peratures each time they were measured. For example,temperature measurements made at the head of theBlautakvısl in March 2002, at the vent complex at thewestern margin of Skaftafellsjokull during March 2002and July 2003, and in the centre of the Skeijararjokullmargin, at the Gygjukvısl during July 2003 (see Fig. 1for locations) were close to atmospheric freezing point(Fig. 6), but not below, despite the existence of freshaccretions of frazil ice around the vents. Given thatfrazil ice aggregates were evident in meltwater at all ofthese locations, and such evidence is considereddiagnostic of supercooled meltwater production (Even-son et al., 1999) it is speculated that either (i) thetemperature probes were not immersed deeply enoughto pick up the supercooling signature; (ii) supercooledmeltwater was diluted subglacially by meltwater inputsfrom other sources, (iii) supercooling varies temporallydue to fluctuations in meltwater discharge (Alley et al.,1998) and was not occurring at the times during whichthe probes were immersed or (iv) a combination offactors i–iii resulted in meltwater temperatures aboveatmospheric freezing point.

Fig. 7 summarises the variability of the meltwatertemperature data presented in this paper. Whenglaciohydraulic supercooling occurs (see also Figs.3–5), it seems that there is less variability in the data,

Fig. 7. Rank-order distribution of temperature data from Figs. 3 to 6.

The figure emphasises the extreme tails of the data set, illustrating

temperature variability of 0.08 1C. The presence of characteristic values

(i.e. �0.037 and �0.023 1C) demonstrates that a lower temperature

limit exists for supercooled meltwater. Characteristic values in the

supercooled domain can be attributed to data from Skeijara (see Fig.

4). Note that a power-law distribution would be applicable only to the

range 0.008–0.024 1C.

which could be interpreted as the system having a morestable hydraulic regime. When supercooling is absent(see also Fig. 6), the data exhibit a greater variability inmeltwater temperature.

Temperature probe housings sometimes exhibitedinterlocking platelets of attached frazil ice on removalfrom many monitored vents, and ropes left in ventsaccreted frazil ice clusters and rosettes (cf. Lawson et al.,1998). Such events are simple, but persuasive, evidencefor the occurrence of supercooling (Lawson et al., 1998;Evenson et al., 1999). At many vent locations, obtainingtemperature records over a prolonged timescale proveddifficult, especially in summer. In some cases, ice growtharound the suspended temperature probes was sovigorous that instruments could not be left in place formore than a few hours as retrieval was problematic (cf.Lawson et al., 1998). Such rapid ice growth testifies tothe efficacy of glaciohydraulic supercooling as an icegeneration mechanism.

6. Field evidence of glaciohydraulic supercooling

At Skeijararjokull, vents comprising a significantinput to the meltwater outlets of the Sula and Skeijararivers repeatedly exhibited evidence of supercooling. InJuly and August 2000, widespread terraces of anchorice, visible even under high flow conditions, were presentat the Sula outlet. Such terraces, often possessing cleartread and riser units, grow in association with subglacialartesian vents discharging supercooled meltwater (Law-son et al., 1998; Evenson et al., 1999). Terracescommonly exhibit morphologies similar to tufa ortravertine deposits (‘‘soda dams’’) formed by inorganicdegassing of carbon dioxide from carbonate saturatedwaters (e.g. Julia, 1983; Ford, 1989; Ford and Pedley,1996). Platy frazil ice was observed in turbid waterissuing from the vent complex and aggregates of frazilice were identified adhering to glacier ice. The presenceof frazil ice within vents during summer conditions isdiagnostic of supercooling (Ashton, 1979; Evensonet al., 1999). By July 2002, several vents were presentat the head of the Blautakvısl (Fig. 8). Once again, frazilice was clearly visible in turbid water emerging from thevents and excavations below the water line revealedfrazil ice accretions. Frazil ice aggregates, many similarto ‘desert rose’ gypsum in structure, retrieved fromaround the vent complex contained abundant siltparticles, trapped by interlocking ice crystals. Observa-tions made at this site from 2001 to 2003 haveestablished that, during jokulhlaups from Grænalonice-dammed lake (located on the western flank ofSkeijararjokull), multiple artesian vents at the head ofthe Blautakvısl convey floodwater into the Sula. Field-work during July 2002, on the waning stage of a floodfrom Grænalon, identified relict conduits plugged with

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Fig. 8. Subglacial artesian vent complex at the head of the Blautakvısl, summer 2002.

F.S. Tweed et al. / Quaternary Science Reviews 24 (2005) 2308–23182314

frazil ice interpreted as subglacial floodwater routewaysin which profuse ice accretion occurred due to super-cooling.

The western margin of Skaftafellsjokull is characterisedby a major vent complex (Fig. 2). During the summer,high meltwater discharges largely conceal the form of thevents, but under winter conditions, during lower flow,anchor ice terraces can be seen surrounding locations ofupwelling. Vent-related upwellings sometimes occurseveral tens of metres up-glacier from the terminus.Debris-rich ice accumulations of 2–5m were observed inflow parallel crevasses around the dominant vent com-plex in 2000, 2001 and 2002 (Fig. 9a and b). We believethat such accretions record frazil ice propagation andassociated sediment entrainment in crevasses in whichsupercooled meltwater previously upwelled (cf. Robertset al., 2001, 2002, Fig. 6). Discs and blades of frazil ice arefrequently present around vent locations, and anchor iceterraces have been observed prograding from the glaciermargin as supercooled water flows away from the centresof vents. Anchor ice terraces have also been identified atsites of meltwater upwelling further up-glacier.

At the eastern margin of Skaftafellsjokull in 2003,highly turbid meltwater discharged from the centre of alarge, well-defined vent, over structures that are strik-ingly similar to the vertical walls (‘risers’) of terracedtravertine systems (Ford and Pedley, 1996). Clearplatelets of frazil ice were observed adhering to iceterraces, trapping sediment from turbid discharge andcreating walls of debris-rich anchor ice (Fig. 9c). Frazil

ice accretions were evident below the water line andfrazil ice aggregates containing copious sand and siltparticles were repeatedly observed in the meltwateroutlet into which the vent discharged. In little over aweek, the form of this large vent had degraded, the ventorifice had decreased to approximately one-third of itsoriginal size, and intercalated units of debris-rich andclear ice were partly visible plugging the vent orifice.However, in a downstream zone within 20 m of theoriginal vent site, seven smaller upwellings were noted,all of which had developed in the intervening week. Wespeculate that the plugging of the original vent by frazilice accretion led to the development of the subsequentarray of vents (cf. Hooke and Pohjola, 1994; Creyts andClarke, 2003).

Field evidence for supercooling was also identified atnearby Svınafellsjokull (see Fig. 1b for location) duringflooding. In response to an intense period of rainfall inMarch 2002, subglacial meltwater burst from the centreof Svınafellsjokull into the Skrama stream channelon 23 March (Fig. 10). Observations made before theflood confirmed that Skrama was dry immediatelybefore the rainfall event in March. Floodwaterdischarge into the Skrama was fed by a temporarysubglacial artesian vent, (Fig. 10c) observations of icearound the mouth of which attest to the occurrence ofsupercooled meltwater discharge during this small flood.Although a temperature probe was installed in theSkrama outlet, a technical problem prevented data frombeing recorded.

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Fig. 9. Skaftafellsjokull, July 2002: (a) Debris-rich ice accreting to the walls of a crevasse with frazil ice floating in an upwelling of turbid water. (b)

Debris-rich englacial accretions sealing a crevasse close to a subglacial artesian vent. (c) Subglacial artesian vent at the eastern margin of

Skaftafellsjokull, with figures for scale. Two temperature probes are weighted, roped and immersed in the vent.

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7. A meltwater origin for debris-laden and ‘basal’ ice?

A link between supercooling and the development ofthick sequences of debris-rich basal ice has beensuggested by other workers (e.g. Strasser et al., 1996;Lawson et al., 1998). We suggest that the developmentof debris-rich ice in open crevasses at Skaftafellsjokull(Fig. 9a and b) is a direct result of supercooledmeltwater accretion. Debris-rich basal ice exposuresalso exist close to subglacial artesian vents at Skeijar-arjokull and Skaftafellsjokull, and are metres thick.Units are devoid of any signs of structural deformationsuch as faulting, folding or tectonic thickening.Instead, the sections contain bedforms, channel

fills and coarse particle clusters, representative ofgrain-by-grain deposition from turbulent, high-energyflows. Observations made at Skaftafellsjokull alsosuggest that in situ melting of basal ice creates thicksedimentary sequences, as sediment structures present inthe basal ice can be traced into ice-marginal ridges.However, the link between supercooling, freeze-on ofdebris-laden meltwater and the development of thicksequences of basal ice continues to be suppositious andremains a hypothesis to be rigorously tested. It seemsprobable that glaciohydraulic supercooling constitutesone of many processes responsible for basal icedevelopment in temperate environments (see Alleyet al., 1997).

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Fig. 10. Svınafellsjokull flood, March 2002: (a) Plan view of Svınafellsjokull illustrating principal geomorphic features and floodwater routing during

March 2002. (b) Mean daily rainfall recorded at Fagurholsmyri, March 2002. Data is recorded at a rate of five measurements every 24 h; the weather

station is 20 km from Svınafellsjokull (rainfall data courtesy of the Icelandic Meteorological Office). (c) Source of the flood in the Skrama—discharge

from a temporary subglacial artesian vent discharging supercooled floodwater.

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8. Conclusions

Records of supercooled meltwater temperatures,combined with field observations, constitute evidencefor the occurrence of supercooling at Skeijararjokull,Skaftafellsjokull and Svınafellsjokull over a range ofglaciohydrological conditions. Supercooling occurs dur-ing ice-melt-dominated conditions and during bothcomparatively small and high-magnitude jokulhlaups(the latter previously documented by Roberts et al.,2002). Further work is required to establish controls ontemporal variations in supercooled meltwater signa-tures, especially in circumstances where measured melt-water temperatures are above atmospheric freezingpoint, despite the prevalence of abundant frazil andanchor ice.

At Skeijararjokull and Skaftafellsjokull the efficacyof glaciohydraulic supercooling as a debris entrainment

mechanism is clearly demonstrated; supercooled sedi-ment accretion produces debris-rich ice accumulationsin both sub- and englacial locations. However, thepotential evolution of such debris-rich ice into maturebasal ice requires further work. This research adds toknowledge of glacier hydrology and debris entrainmentin temperate glaciers, and to theories of ice-marginallandform and landscape development.

There is a need to continue to monitor sedimententrainment at overdeepened piedmont glaciers such asSkeijararjokull as they are the closest modern analogueto outlet glaciers of the Laurentide Ice Sheet (cf.Gustavson and Boothroyd, 1987). It has been speculatedthat rapid discharge of supercooled meltwater from thebase of over-deepened ice sheet basins during theQuaternary constitutes a plausible mechanism forentraining extensive amounts of sediment in theLaurentide Ice Sheet (Alley et al., 1998, 1999; Roberts

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et al., 2002). Such debris entrainment could account forlarge volumes of sediment deposited at former terrestrialglacier margins and the high concentrations of carbo-nate-rich IRD deposited in marine settings (Alley et al.,1998, 1999; Andrews and MacLean, 2003). Findingsolutions to both of these palaeoglaciological puzzles islargely reliant on our ability to constrain processesresponsible for current glacial debris entrainment inglaciers and ice sheets; understanding such processestherefore may, in turn, shed light on the hydrologicaldynamics of former ice masses as well as establishing themechanisms by which they entrained debris.

Acknowledgements

Thanks to all Earthwatch ‘Icelandic Glaciers’ volun-teers who assisted us with fieldwork from 2000 to 2003and to the Earthwatch Institute for funding fieldwork byFST, MJR, & AJR. FST is particularly grateful to PaulRitzma, Matthew Key and Simon Wallace for theirenthusiastic field assistance in monitoring the vent ateastern Skaftafellsjokull during the summer of 2003.Thanks to Dan Lawson, Grahame Larson, Ed Evenson,Jay Fleisher, Tim Harris, Oskar Knudsen and PhilMarren for their contributions to discussions in the field,and to Malfrıjur Omarsdottir for her help with fieldlogistics and translation of Icelandic glaciologicalliterature. We are grateful to Ragnar Frank Kristjans-son, warden of Skaftafell National Park, for repeatedlyallowing us to use the premises and information servicesof the park and to Kjartan Bollason for his assistancewith our field research in 2003. The comments ofGrahame Larson and an anonymous reviewer, alongwith editorial recommendations from Tim Fisher,significantly improved this paper. Rosie Duncan kindlyprepared the images.

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