glacial conditions that contribute to the regeneration of...

HYDROLOGICAL PROCESSESHydrol. Process. 28, 2749–2760 (2014)Published online 30 April 2013 in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/hyp.9787

Glacial conditions that contribute to the regeneration ofFountain Glacier proglacial icing, Bylot Island, Canada

Pablo Wainstein,1,2* Brian Moorman1 and Ken Whitehead11 Department of Geography, University of Calgary, Alberta, Canada

2 BGC Engineering, Calgary, Alberta, Canada

*CUnE-m

Co

Abstract:

Proglacial icings are one of the most common forms of extrusive ice found in the Canadian Arctic. However, the icingadjacent to Fountain Glacier, Bylot Island, is unique due to its annual cycle of growth and decay, and perennial existencewithout involving freezing point depression of water due to chemical characteristics. Its regeneration depends on theavailability of subglacial water and on the balance between ice accretion and hydro-thermal erosion. The storage andconduction of the glacial meltwater involved in the accretion of the icing were analyzed by conducting topographic andground penetrating radar surveys in addition to the modelling of the subglacial drainage network and the thermalcharacteristics of the glacier base. The reflection power analysis of the geophysical data shows that some areas of the lowerablation zone have a high accumulation of liquid water, particularly beneath the centre part of the glacier along the mainsupraglacial stream. A dielectric permittivity model of the glacier – sediment interface suggests that a considerable portion ofthe glacier is warm based; allowing water to flow through unfrozen subglacial sediments towards the proglacial outwash plain.All these glacier-related characteristics contribute to the annual regeneration of the proglacial icing and allow for portions ofthe icing to be perennial. Copyright © 2013 John Wiley & Sons, Ltd.

KEY WORDS Bylot Island; Fountain Glacier; proglacial icing; glacial hydrology; permafrost hydrology; GPR

Received 10 September 2011; Accepted 7 February 2013

INTRODUCTION

Icings are common features often found on the proglacialplains of arctic glaciers. However, the perennial natureof the icing in front of Fountain Glacier is unusual. Itsreplenishment depends on the balance of severaldifferent factors, which include: air temperature, topog-raphy of the proglacial plain and the hydrology of thesource glacier.Studies have recognized the importance of englacial

and subglacial hydrology in the supply of water used inthe generation of such ice features (Irvine-Fynn et al.,2011). Pollard et al. (1999) and Hodgkins et al. (2004)suggest that long-term subglacial storage andinterconnected marginal ice-dammed lakes are possiblesources of water. Moreover, the balance between theglacier, acting as the source of water, and thesurrounding permafrost, acting as the environmentthrough which water flows, is crucial in determiningwater availability, flow characteristics, water pressure

orrespondence to: Pablo Wainstein, Department of Geography,iversity of Calgary, Alberta, Canada.ail: [email protected]

pyright © 2013 John Wiley & Sons, Ltd.

and routing of the icing source spring (upwelling) thatconnects the glacier’s hydrological network with theproglacial icing (Wainstein et al., 2008).In the specific case of Fountain Glacier icing, Moorman

(1998) and Moorman and Michel (2000) suggested thatthe combination of the shape of the proglacial outwashplain and the glacier’s hydrology play a significant role onthe regeneration of the icing. The dome-like shape of theproglacial outwash plain allows meltwater flow toconcentrate on the sides of the valley, decreasing thehydro-thermal erosion of the centre parts. Wainstein et al.(2008), Wainstein et al. (2010) and Wainstein (2011)have suggested that there is a temporal relationshipbetween the glacial dynamic behaviour of FountainGlacier and the changes observed in the proglacialicing. Their results suggest that glacial dynamics andespecially the recent retreat and thinning trend presentedby Fountain Glacier have had a detrimental impact onthe hydrology and replenishment of the icing. Specifi-cally, the icing has thinned, and its source spring hasabandoned its previous position observed in 1991 andmoved to a newer location closer to the current glacierterminus. Additionally, Wainstein et al. (2008) alsosuggested that the equilibrium between the glacier and

2750 P. WAINSTEIN, B. MOORMAN AND K. WHITEHEAD

permafrost is unstable, meaning that rather smallglaciological changes are likely to have considerableeffects on the conditions of the icing.Although a significant amount of work has been done

on the hydrology of polythermal glaciers and specificallyon their subglacial processes and relationship withproglacial icings (Bingham et al., 2006; Bingham et al.,2008; Elver, 1994; Moorman, 1998; Moorman andMichel, 2000; Wainstein et al., 2008; Wainstein et al.,2010; Wohlleben et al., 2009) the specific glacio-

Figure 1. a) Location ofBylot Island on the northern edge ofBaffin Island, and(b and c) differential GPS and GPR (50MHz) survey lines were collected overthe lower portion of Fountain Glacier during the summer of 2007 and 2008

Copyright © 2013 John Wiley & Sons, Ltd.

hydrological conditions that contribute to the annualregeneration of icings have not yet been fully addressed.The specific objectives of this paper are:

• describe and model the englacial and subglacial hydrophysical conditions that contribute to the annualregeneration of Fountain Glacier proglacial icing;

• show that the location of the source spring is related tothe morphology of the subglacial drainage system; and

• model the glacier’s thermal characteristics that allowwater to flow from Fountain Glacier onto the proglacialicing in spite of the surrounding permafrost and thefreezing winter air temperatures.

STUDY AREA

Bylot Island (Figure 1) is located in the Canadian Arctic,north of Baffin Island, adjacent to Lancaster Sound,between latitudes 72.5�N and 74�N, and longitudes 76�Wand 81�W. It is roughly 180 km long in its NW-SE axisand 100 km wide in its NE-SW axis. The Byam MartinMountains are extensively glacierized and extend acrossthe length of the island. They are characterized by sharppeaks, deep intersecting cirque basins and narrow, steepcols. The centre of the island is covered by a 4500 km2

icefield where some of the island’s highest summits arelocated (e.g. Angilaaq at 1,844m asl and Malik at1,905m asl). The majority of the valley glaciers whichflow outwards onto the lowlands, such as FountainGlacier, have their accumulation zones within this icefield(Dowdeswell et al., 2007; Zoltai et al., 1983). A thoroughdescription of the glacial history of Bylot Island can befound in Klassen (1993).The climate of Bylot Island is dry and cold. A mean

annual air temperature of �14 �C and a mean annualprecipitation of approximate 225mm have been locallyrecorded (Irvine-Fynn, 2004; Moorman, 2003; Moormanand Michel, 2000). Local annual snow depths average lessthan 80 cm. The island is located in the zone ofcontinuous permafrost (Zoltai et al., 1983) with estimatedthicknesses being between 200 and 400m (Moorman,2003). The combined action of low precipitation andsubzero ground temperatures makes the hydrology of thearea highly dependent on glacial conditions and allowsfor the preservation of ice-related features.Fountain Glacier (Figure 1) is a polythermal glacier with a

catchment area of 72 km2 (Walter, 2003). It is 16 km long, andits elevation ranges from 255m asl to 1758m asl, with anaverage surface slope of 5.5� (Walter, 2003). Lately, theterminus of the glacier has presented severe changes in itsmorphology and position (Wainstein, 2011; Wainstein et al.,2008;Whitehead et al., 2010b, c). The combination of awarmcore covered by a layer of cold ice, frozen to the glacier’s

Hydrol. Process. 28, 2749–2760 (2014)

2751CONDITIONS THAT CONTRIBUTE TO THE REGENERATION OF A PROGLACIAL ICING

margins, allows for the storage of pressurized subglacialwaterused in the regeneration of the proglacial icing (Akerman,1982; Hodgkins et al., 2004; Moorman, 1998; Wainstein,2011; Wainstein et al., 2008; Wainstein et al., 2010).

METHODOLOGY

Glacial morphology and ice thickness

The glacio-hydrological conditions of Fountain Glacierand its proglacial icing were studied by using topograph-ical and geophysical surveys in conjunction with aerialand ground photo analysis. Topographical surveys wereconducted in the summer of 2007 and 2008 with theobjective of generating a Digital Elevation Model(DEM) of the glacier’s surface and quantify the icingextents. Surveys were conducted using a TrimbleDifferential Global Positioning System, and, in the caseof the glacier, they followed the geophysical survey linesshown in Figure 1. Surveys achieved an average horizontalaccuracy of 10 cm and 20 cm vertically, after beingcorrected against a base station installed near the terminusof the glacier. A 5m resolution DEM was generated usingthe Topo to Raster function of ArcMap Spatial Analyst.Geophysical surveys were also conducted in 2007 and

2008 using a Pulse Ekko Pro ground penetrating radar(GPR) system. Transverse and longitudinal GPR lineswere established on Fountain Glacier following thepattern shown in Figure 1. Radar data were collectedusing 50MHz antennae in parallel broadside configura-tion, which allowed for an appropriate depth ofpenetration without significantly compromising verticalresolution (Irvine-Fynn et al., 2006). Profiles wereacquired in continuous mode using a 1000V transmitterand a trace stacking of 16, resulting in an average stepsize of 13 cm. These settings were highly oversampled forresolving the base of the glacier, but it ensures a clearimaging of englacial features such as conduits. Addition-ally, a GPS unit was connected to the GPR system toallow an accurate positioning of each profile. CommonMid Point surveys were collected to determine wavepropagation velocities and to enable the generation ofdepth scales. The average calculated wave propagationvelocity was of 0.16m/ns.Radar data was prepared for interpretation by:

(i) applying a DEWOW filter and extracting repeated traces,(ii) positioning information was incorporated and the realstep size calculated, (iii) a three-point down trace averagingwas applied to reduce high-frequency noise and (iv) profileswere enhanced with an Automatic Gain Control tostrengthen weak returns. Once the GPR profiles wereprocessed, the ice thickness was obtained by delineating thebase of the glacial ice using ReflexW software. The base ofthe glacier was determined to be the first continuous linear


reflection found at an average of 1000 ns. This reflectionwas interpreted as the base of the glacier based on its highreflection amplitudes, its reflection polarity and lateralcoherence. Subsequently, the subglacial topography wasobtained by subtracting the ice thickness from the surfaceDEM, and a 40m resolution DEM was created from thesubglacial topography.

Subglacial water flow model

A subglacial 2D flow model was constructed to describethe theoretical paths subglacial water would take subjectedto subglacial topography and ice thickness. It was obtainedby applying ArcMap’s hydro tools to determine the flowaccumulation patterns from the calculated hydraulicpotential (Ψ) defined by Shreve (1972) as:

Ψ ffi Frighi þ rwgZ (1)

Where F is a glacier thickness factor (percentage ofglacier thickness to be accounted for); ri (kg/m

3) is thedensity of ice; g (m/s2) is the acceleration of gravity; hi(m) is the ice thickness; rw (kg/m3) is the density of waterand Z (m) is the elevation of the base of the glacier abovea base potential datum.Hydraulic potentials were calculated and mapped for

different values of F. Some differences in water routingwere observed, which suggests that ice thickness andtherefore ice overburden pressure have some control overthe distribution of the subglacial drainage network. Avalue of F= 1, which accounts for the entire thickness ofthe glacier, was chosen to create the subglacial drainagemodel, since, based on field observations (existence ofpressurised fountains), it is inferred the subglacialdrainage is pressurised. A DEM of the modelledsubglacial flow was obtained and a slope analysis wasconducted to enhance the visualization of the results.

Geophysical characterization of the glacial environment

Further characterization of the glacial environment wasdone by analyzing the variation of the englacial andsubglacial GPR reflectivity. This technique provides aneffective way of determining areas of high water contentand the distribution of warm and cold ice. Based on thework conducted by Gades (1998) and Gades et al. (2000),the return power (P) of a reflection of amplitude(A) within a time window (t2�t1) was calculated as:

P ¼ 12 t2 � t1 þ 1ð Þ

Xt2i¼t1

A2i (2)

The bed reflection power (BRP) is a relative index ofthe energy reflected from the base of the glacier and was

Hydrol. Process. 28, 2749–2760 (2014)

Figure 2. a) Hydrology of the icing and surrounding area and b) temporaextensions of icing ice in 2008


calculated using a time window (t2�t1) = 50 ns, which islong enough to include the complete basal reflectionpulse. The raw amplitude of the basal reflection wasobtained from the interface delineation. The internalreflection power (IRP), which includes all the reflectionswithin the ice column and is an index of the energyreflected englacially, used a varying time window of150 ns after the air and ground wave to 50 ns before thebeginning of the basal reflection. In this way, the surfacenoise derived from surficial cracks was excluded. In orderto filter and improve the data quality, the ambientelectromagnetic noise was determined, and traces show-ing a higher than average level of ambient noise wereexcluded from the analysis. Traces that showed a highconcentration of vertical reflection patterns resulting fromthe reverberation of areas flooded by surficial meltwaterwere also removed to avoid misinterpretations.As a result of the reflective nature of the GPR

technique, where observations are based on the reflectionof waves from subsurface features; the determination ofthe BRP is affected by: (i) the characteristics of theenglacial environment, (ii) the distance through which theGPR waves travel and (iii) the nature of the interface onwhich they reflect. Following what Gades et al. (2000)and Copland and Sharp (2001) suggested, the relationshipbetween BRP and its major influencing parameters wasdetermined. A descriptive statistics and correlationanalysis was conducted to study the multivariatecorrelation between BRP, IRP, ice thickness and thecharacter of the subglacial bed. In contrast to what Gadeset al. (2000) observed, in the case of Fountain Glacier,there was no clear correlation between variables, and assuch, the BRP and IRP were not standardized.The englacial environment was further characterized by

studying the radar signature. Radar profiles were examinedto determine the development of seasonal layers of warmand cold ice. A qualitative description of the radar profileswas performed, in addition to a probabilistic analysis of thedistribution and polarity of the hyperbolic reflectionsproduced by englacial conduits. The polarity of thereflections (+�+ or� +�) was compared with the polarityof the reflection of the basal interface, and based on theworkof Arcone et al. (1995), the hydraulic characteristics of theenglacial conduits were determined.

Inference of subglacial thermal conditions by applyingRadar Range and Looyenga dielectric mixing model

Additional characterization of the subglacial environ-ment was achieved by modelling the dielectric constant(e) of the base of the glacier. Based on this model, thesubglacial thermal conditions were inferred. The RadarRange Model toolbox (Ekko42 GPR software) was usedto calculate the theoretical reflection amplitude of an

Copyright © 2013 John Wiley & Sons, Ltd. Hydrol. Process. 28, 2749–2760 (2014

l

interface of varying dielectric characteristics. The interfacerepresents a rough glacial bed covered by 80m of ice. Thetransmitter voltage and wave frequency of the model wereset to 1000V and 50MHz, respectively. The output of themodel is a matrix with modelled reflection amplitudesversus dielectric constants, which were then comparedagainst field measured amplitudes and used to infer thethermal conditions of the glacial bed. Different statisticsdistributions were tested and a Gamma 3 statistical model,describing the probability of having a specific dielectricconstant at the base of the ice, was best fitted. The thermalcharacterization of the glacial base was then obtained bycomparing the dielectric constant probability distributionwith Looyenga’s mixing model (Looyenga, 1965).Looyenga’s mixing model predicts the dielectric constantof a mixed material such as till, composed by sediments andinterstitial water or ice, as a function of the porosity and thedielectric constants of its components (Equation (3)).

e2 ¼ Φ � e1=3w iþ 1�Φð Þe1=31

� �3(3)

)


Where ew _ i is the dielectric constant of water or ice,e1 is the dielectric constant for the matrix component ofthe mixture; while Φ is the mixture’s porosity.

RESULTS AND DISCUSSION

Icing hydrology and morphology

Fountain Glacier icing is located at approximately 250masl, and it extends southeast for over 1 km from the terminusof Fountain Glacier to a major valley constriction, which isthe result of the combined action of bedrock and an oldlateralmoraine of the adjacent StagnationGlacier (Figure 2).This geomorphic constriction plays a critical role in theicing’s hydrology since it naturally dams the valley, raisingthe base level of the proglacial plain and allowing water toaccumulate and the icing to accrete annually. Moreover, aswater freezes during the winter, the dam promotes theformation of thicker icing ice, which acts as an insulatinglayer that allows liquid water to flow within and under theicing despite the cold ambient temperatures.The morphology of Fountain Glacier valley promotes

the catchment of different sources of liquid waterthroughout the year, which all contribute to the hydrologyof the icing (Wainstein et al., 2010). The main sources ofwater are: (i) the glacier itself, (ii) marginal glacial lakesand (iii) mountain streams originated from snowmeltwithin the main peaks of the watershed (Figure 2a).Although it is thought that Fountain Glacier is the mainsource of water necessary for the annual accretion of theicing, ice marginal lakes also play an extremely importantrole in the supply of water. Year round, the lakes supplywater to the subglacial environment, raising the systempressure while being recharged by waters coming frommountain streams in summer. The water supplied to thesubglacial environment eventually flows down valley andreaches the proglacial icing taking part in its regenerationcycle. During winter, freezing temperatures do not allowlakes to be recharged from surficial sources, but sincethey do not freeze completely, they are still able to supplywater and pressure to the subglacial environment. It issuggested that this is done through sub-lake taliks.As a result of the high latitude location of Fountain

Glacier, summers in the area are short. Positivetemperatures are initially seen around the first week ofJune, and they last until the secondweek of Septemberapproximately. By the beginning of the summer, the icingcompletely covers the proglacial valley with an area ofapproximately 0.26 km2 and an average thickness of3.6m with local thicknesses reaching 6m. Althoughrelatively short, summers result in a significant loss of theicing’s areal coverage and volumetric ice content(Figure 2b). High-accuracy GPS surveys reveal the icingloses almost 3m of thickness in some areas, the areal


coverage decreases by 71% and it has an average volumeloss of approximately 82% over a period of 3months.The initiation of the icing ablation results from the

increase of runoff from terrestrial streams. The increasedrunoff rapidly erodes the north western margins of theicing adjacent to the glacier’s terminus. Water levelsincrease within the upper icing and so does the watercontent of the icing ice. Further downstream, the slope ofthe subicing topography does not allow much ponding ofwater. Instead, supraicing streams form along the marginsand/or the centre line. As a result, ice located on theeastern side of the icing is better drained and drier thanthe ice within the upper western icing.During July, the icing generally faces the fastest rates

of melting. Surface glacier melt and subglacial hydrolog-ical activity increase, and water is flushed onto theproglacial outwash plain via proglacial springs and lateralstreams (Wainstein et al., 2008; Wainstein et al., 2010).The increase in water flow results in the rapid melting ofthe icing that covers the upper proglacial valley. Thegeneral increase of runoff also has erosional conse-quences for the lower icing ice. High flow rates quicklyenlarge and deepen incipient channels until they reach thesediment floor.By the end of summer, erosional processes within the

lower icing are further enhanced by mechanical fracturingand collapsing of undercut icing slabs. The upper icingice is almost completely melted, and the lower icing,after facing a rapid melt, has only a few perennial patchesof ice remaining. Hydro-thermal erosion is furtherenhanced by higher precipitation rates. The melting ofthe sea ice within Eclipse Sound, the sound that separatesBylot Island from Baffin Island, provides a source forwater to evaporate, and hence, precipitation events aremore frequent.The regeneration phase of the proglacial icing is a

slow process that continues for the entire winter season. Itstarts around the beginning of October, when airtemperatures drop below the freezing point. The generalhydrology of the outwash plain decreases as the lateralstreams dry up. Although the reduction of the surficialdischarge is significant by the end of October, theproglacial icing still has liquid water flow coming fromthe proglacial springs fed by subglacial sources. Timelapse cameras in place since 2008 have captured imagesthat show liquid water flowing on the icing even in themiddle of extremely cold months such as December.

Glacial morphology

The supraglacial hydrology of Fountain Glacier ischaracteristic of polythermal arctic glaciers. At the begin-ning of the ablation season, meltwater percolates throughthe snow and accumulates on the cold surface of the ice

Hydrol. Process. 28, 2749–2760 (2014)

Figure 3. a and b) Early stages in the development of a collapse feature onthe north side of Fountain Glacier terminus and c) current view of the

glacier’s snout presenting two major collapsing fronts

Figure 4. Sediment-filled subglacial channels at the base of the northernmarginal dry calving escarpment

Figure 5. a) Subglacial topography contour map obtained from the50MHz GPR survey. Contour interval is 10m. b) Subglacial topographyDEM of the same location showing areas of over deepening (I), asubglacial ridge (II) and a subglacial channel (III). The contour map doesnot show the exact location of the over deepening since its depth is smaller

than the resolution of the contours


forming superimposed ice. Later in the season, airtemperatures increase, water percolation from snow meltis enhanced and melting of the superimposed ice begins. Alayer of slush develops, increasing the water content of theboundary between snow and ice. As the ablation seasonunfolds, surficial meltwater and slush flows concentrate inpre-incised supraglacial streams which are reoccupiedannually. On average, by mid July, the layers ofsuperimposed ice, on the lower ablation area, have meltedaway and supraglacial streams run sub-aerially.The lower ablation zone of Fountain Glacier is

longitudinally divided in two by a well-incised supraglacialchannel that meanders for roughly 3 km (Figure 3). At itswidest, it is approximately 70m wide and 20~ 30m deep,

Copyright © 2013 John Wiley & Sons, Ltd. Hydrol. Process. 28, 2749–2760 (2014

and it concentrates the majority of the supraglacialmeltwater in the ablation area. A second supraglacialchannel, south of the main one, has started to develop overthe last 7 years.Marginal streams running on the valley sidescollect the excess surficial runoff. The lack of well-developed moulins or crevasses on the lower ablation areaof Fountain Glacier restrict water from directly entering thesubglacial environment, and as such, it is suggested that thesubglacial environment is mainly fed by waters comingfrom up-valley sources and marginal glacial lakes.The aerial and ground photographic record shows

that the terminus of Fountain Glacier historically had a

)

Figure 6. a) Subglacial flow model. b) Ground-based photo showing collapse features (i) and (ii). c) Oblique aerial photo of the glacier’s terminus.Note the subglacial channel emerging onto the proglacial plain along the path described by the subglacial drainage model


smooth profile (Figure 3a). However, the current snoutmorphology differs considerably; glacial collapsefeatures have carved steep cliffs on the east andnorthern walls of the snout. It has been observed onthe field that, glacial conditions have changed fromslow melting to fast mass wastage through dry calving.Such collapse features began developing during the1990s; contemporarily to the maximum observedthinning of the proglacial icing and the relocation ofthe icing source spring (Wainstein et al., 2008;Wainstein et al., 2010). Collapse features, such as theone shown in Figure 3b, developed when the ice roof oflarge subglacial caves collapse inward, leaving a bowl-shaped feature. The shape of the collapse features andthe presence of supraglacial circular fractures on theupslope side indicate that the cavity continues upglacier, although with a thicker roof that has stillsustained thermal erosion and resisted the collapsingforces. Subglacial cavities of such dimensions arethought to be the result of the physical and thermalerosive power of subglacial water that annuallyreoccupies the same drainage pathways. These channelsare preserved in the ice as a result of the low creep rateof cold glacial ice. The size of the features suggests thatthe subglacial drainage responsible for their develop-ment is a fast channelized drainage system. Lately(since 2006), the incipient collapse features shown inFigure 3a and b have increased their size and haveevolved into major dry calving escarpments (Figure 3c).Field observations indicate that dry calving activity hasincreased dramatically since 2006, due to glacial retreat


and hydraulic undercutting of the ice front by marginalstreams. Currently, remnants of sediment-filled subgla-cial channels have been observed at the base of thenorthern escarpments (Figure 4). This is indicative ofabandonment of hydrological conduits, which havelikely resulted from the physical and thermal alterationsto this portion of the glacier margin.GPR surveys allowed the study of Fountain Glacier’s

subglacial hydraulic network. The derived subglacial DEMshows three distinct topographical features (Figure 5). Anover-deepened area (i) located to the northeast of aprominent subglacial ridge (II) allows subglacial meltwaterto be collected. The ridge (II) divides the lower ablation areain two and creates two main subglacial channels. Thebiggest channel (III) runs along the centre line of the glacier.The ridge (II), although not as prominent as in the lower areaof the subglacial DEM, is observed to gently continue upvalley. The resulting subglacial waterways were modelledby determining the flow accumulation of the calculatedhydraulic potential. The results of the hydraulic modelshown in Figure 6 predict two main subglacial waterways.The first conducts water from the margins of the ablationarea towards the northern side of the glacier terminus. Theresults of the model show its outlet is located where thenorthern collapse feature began developing (Figure 3a) andwhere the remnants of sediment-filled conduits have beenobserved (Figure 4). The model also shows that the secondsubglacial waterway conducts water down glacier along themid-section of the glacier, parallel to the well incisedsupraglacial stream. Both glacier collapse features shown inFigure 3 are located where the model predicts the largest

Hydrol. Process. 28, 2749–2760 (2014)

Figure 7. Map of a) bed reflection power and b) internal reflection power

Figure 8. 50MHz GPR line collected parallel to the direction of glacierflow in June 2007. a) Shows the uncorrected profile and b) shows theinterpreted topographically corrected profile. Note that the hyperbolicreflections shown in a) have been distorted in b) due to the topographic

correction


amount of subglacial flow, supporting the idea thatchannelized subglacial flow and the development ofcollapse features are indeed related (Figure 6). Moreover,the historic photo record from 1992 onwards shows that,almost a decade ago, it was common to find subglacial flowemerging onto the proglacial plain through location ‘i’(Figure 6) where the main collapse feature is currentlylocated (William Shilts, personal communication). It shouldbe noted that the mid-section branch of the subglacialdrainage model (Figure 6), the subglacial channel shown inFigure 5b-III and the well-incised supraglacial channelare spatially aligned. Moreover, the location of the icingsource spring observed in 1991 (Wainstein et al., 2008;Wainstein et al., 2010), its remnants observed in 2007and the current active location in 2008 also lay on the mid-section of the proglacial plain, spatially correlated with theaforementioned drainage features. This spatial relationshipagrees with what Boulton et al. (2007) describe with respectto the proglacial upwellings observed in front ofBreiðamerkurjökull Glacier in southeast Iceland. Hesuggests that the observed upwellings (proglacial springs)likely occur along main subglacial channels close to portalsof major streams.

Englacial and subglacial water

The analysis of the variation of GPR reflectivityallowed a further characterization of the glacial environ-ment and its water content. On average, the calculatedIRP values (105mV2/ns) are an order of magnitude higherthan the BRP (104mV2/ns), mainly due to the reflectionsproduced englacially by internal ice structures and the


presence of water inclusions. Within these, hyperbolaeproduced by point reflectors such as englacial conduitsare most important. When encountering them, the IRPshows a distinct peak, especially if the conduit is full ofwater, which increases its dielectric contrast against thesurrounding ice, subsequently increasing the amplitude ofthe reflection. The amplitude of the reflection and hencethe calculated BRP and IRP not only depend on theenglacial water content, but also on englacial structure,ice thickness and the thermal regime of the glacier base.Areas with high englacial reflections tend to present lowbasal reflection amplitudes. In order to quantify thecorrelation between the basal reflection and its controllingparameters, a multivariate regression was conducted. Itsresults show that the BRP is inversely correlated with theIRP by 8%, inversely related with the ice thickness byalmost 19%, and directly related with the type ofsubglacial bed by 3%. An acceptable best fit curve couldnot be traced between BRP and ice thickness, and henceBRP values were not standardized against ice thickness.This is thought to be due to the small variation of icethickness relative to the total thickness within the surveyarea. However, a number of standardization algorithmswere tried, and a sensitivity analysis over the finalhydrological interpretation was conducted. It was deter-mined that the interpretations concerning distribution of

Hydrol. Process. 28, 2749–2760 (2014)

igure 10. Gamma 3 (5,1,0) relative probability distribution fitted to themodelled dielectric constants of the base of Fountain Glacier


subglacial water were not affected by the standardizationor lack thereof of the BRP.The mapping of the distribution of the BRP and IRP

(Figure 7) allowed a more spatially comprehensivecharacterization of the distribution of water content andthe thermal character of the glacial bed. In the case of theBRP, higher power values are concentrated on thenortheast side of the glacier (Figure 7a-a), and to a lesserextent along the frontal margin (Figure 7a-b). These twoareas of high BRP are located very close to the location ofglacial collapse features and close to where the subglacialdrainage model suggests higher subglacial water content(Figure 6).The IRP map shows three distinctive zones of high

power values throughout the surveyed area (a, b and g).They are the result of the high concentration of englacialreflections, especially high-amplitude hyperbolae, whichdenote the presence of englacial conduits full of water.The high values observed over the northeast side of thesurveyed area (Figure 7b-a) coincide with the location ofhigh BRP values, supporting the predictions of thesubglacial drainage model that suggest the presence of awell defined branch of the hydraulic network. Within thisarea, surficial water percolates into the ice through anetwork of small crevasses and cracks, or possibly cutand closure conduits as Baelum and Benn (2011) suggestsfor a valley glacier in Svalbard. This increases the watercontent throughout the entire ice column until it reachesthe base where it joins the main water flow near theglacier’s terminus. Zone b and g are spatially morecorrelated with the over deepened area and the subglacialchannel shown in Figure 5b.The mapping of the reflection power is mainly affected

by two sources of error: edge effects and interpolationartifacts. Edge effects result in the presence of halfhyperbolic reflections. These are produced by thereflection of GPR waves from the vertical margins ofthe glacier and/or the steep walled supraglacial streams.These were eliminated by cropping the margins from thedata set used in the reflection power calculations.Interpolation artifacts produced by the zig-zag survey

Figure 9. Exponential relationship between the modelled reflectionamplitude of a rough interface and its dielectric constant in a modelled

50MHz GPR survey


F

pattern, which had to be chosen due to topographicconstraints, were decreased by making the interpolationresolution coarser. Coarsening of the resolution generatedbetter results because the change in point density as thesurvey transects approached each other resulted in ‘noisy’interpolation after the algorithm was applied.The qualitative analysis of the radar data shows that

Fountain Glacier can be described by a three-layer hydro-thermal model. The GPR line shown in Figure 8 wasacquired parallel to the direction of ice flow near theglacier’s centre line. The uppermost layer presents a radarsignature typical of wet ice (Irvine-Fynn et al., 2006),with a high concentration of small hyperbolic reflections,which is common for ice with large amounts of smallwater-filled fractures. Although cold ice is generallyassumed to be impermeable, the high concentration ofsmall fractures likely allows water to percolate down,increasing the water content and the noise level of theGPR signature of the uppermost layer of ice (Irvine-Fynnet al., 2006; Plewes and Hubbard, 2001). This indicates thatalthough the uppermost layer of the glacier is consideredcold, it seasonally develops a thin layer of wet ice duringthe ablation season. Deeper into the glacier, the GPRprofile shows a second layer of ice which is characterizedby the lack of radar noise, suggestive of colder and drierice. The main observed reflections here are large loweramplitude hyperbolae, which are suggestive of englacialconduits, and linear reflections suggestive of ice foliation

Figure 11. Looyenga’s dielectric mixing model, showing area of greatesprobability for porosities between 20 and 40%

Hydrol. Process. 28, 2749–2760 (2014

t

)


marks, which indicate a high rate of ice deformation.Finally, the deepest layer is characterized by a strongpresence of higher amplitude hyperbolic reflections whichsuggest an extensive presence of deep englacial conduitsand a high water content. This is not thought to be basalsediment layers because there are no visual indications ofthem along the margins or terminus of the glacier.Within the ice, other types of reflections were observed.

The most notable are long vertical reflection patterns witha surficial origin that extend to almost the glacier base.Field observations show that these reflections werelocated over places flooded by supraglacial meltwater.The reflections have been interpreted as high-frequencyreverberations of surficial reflectors that extend in theradargram’s time axis. Irvine-Fynn et al. (2006) observedthe same phenomena during GPR surveys over the nearbyStagnation Glacier in addition to similar changes in watercontent of the upper most layer of the glacier.The general probabilistic analysis of the distribution and

polarity of themain hyperbolic reflections supports the three-layer model suggested by the GPR line shown in Figure 8. Itwas observed that the shallowest and the deepest layers havethe largest amount of reflections, whereas the mid englacialarea is largely devoid of them. Moreover, the polarityanalysis show that within the entire ice column, 55% of thehyperbolic reflections have a (+�+) polarity, whichindicates that more than half of the englacial conduits arefilled with water or a combination of water and sediments.

Subglacial thermal regime

Although aforementioned results have shown thatFountain Glacier has a number of geomorphic conditionsthat enable the storage and conduction of water, anappropriate basal thermal regime is fundamental to assurethat the stored water is, in fact, delivered to the proglacialplain. In order to ensure both the storage and delivery ofwater, Fountain Glacier not only needs cold glacier fringesthat act as thermal dams and restrict water flow, but alsotemperate areas that allow water to flow from the subglacialenvironment to the proglacial plain year round. Theapplication of the radar range modelling toolbox generateda theoretical relationship between the reflection amplitude ofa rough interface and its dielectric constant, under similarfield conditions as the ones seen on Fountain Glacier(Figure 9). The relationship between the two is exponentialand presents a rapid increase of the dielectric constant as afunction of the reflection amplitude.The statistical analysis of the distribution of the

dielectric constants calculated for the base of the iceshows that a Gamma 3 probability distribution with fivedegrees of freedom can be best fitted (Figure 10). Theirrelative probability curve shows that dielectric constantsbetween 3 and 12 are most likely to be found at the


glacier’s base. Moreover, the highest probability(p = 23%) is held by a dielectric constant of ~ 4. It is alsoobserved that dielectric constants greater than 12 are veryunlikely to be found, which would denote a considerablyhigh spatial distribution of water.Figure 11 shows Looyenga’s mixing model (Looyenga,

1965) against varying porosities. The graph presents threemain scenarios where the voids of the subglacial till arefilled purely with water, ice or a mix between the two. Itwas calculated that frozen subglacial tills with porositiesbetween 20% and 40% would have maximum dielectricconstants between 6.5 and 8.5. In contrast, a glacial basewith temperate saturated ice would result in minimumdielectric constants of at least 29. Within these twoextremes, voids filled with a mix of water and ice willpresent dielectric constants between 8.5 and 18. Whencomparing Looyenga’s mixing model with the calculatedprobability distribution of dielectric constants (Figure 10),it is observed that the glacier’s base is most probablycomposed of areas presenting a frozen till and warmerareas having a partially frozen till where its voids arefilled with a mix of water and ice (Figure 11). Thedielectric constant that shows the highest probability inFigure 10 (e = ~4) corresponds to a frozen glacial till forporosities varying between 20% and 40%.

Implications for icing regeneration

The annual regeneration of Fountain Glacier icingstrongly depends on the behaviour and hydrology ofFountain Glacier. Specifically, the preservation of theicing’s perennial character (Wainstein et al., 2008,Wainstein et al., 2010) depends on the ability of the glacierto conduct water from its deep englacial and subglacialenvironment, where it is kept unfrozen year round, to theproglacial outwash plain where it freezes and forms theicing. It has been observed that Fountain Glacier has severalfavourable hydrological and morphological characteristicsthat contribute to the replenishment of the icing. The factthat the subglacial environment has a well-definedchannelized hydraulic network through which water isefficiently conducted, areas of over-deepening where wateris collected during the winter, and an adequate polythermalregime that allows pressurized deep englacial and subglacialwater to flow, ensures the replenishment of such cryo-hydrological feature.Changes in glaciological behaviour, observed during

the last decades, have had severe detrimental impacts onthe conditions of the icing. It has been observed that theicing has experienced a drastic thinning trend followingthe latest increase in the retreat rate of Fountain Glacier(Whitehead et al., 2010a). It has also been observedthat the icing’s source spring has been relocated topositions closer to the glacier’s terminus due to changes

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in glacial behaviour (Wainstein et al., 2008). It is limitedpermeability of the cold margins of the glacier ice andthe permafrost which determine where the water willflow and the rates that it will flow at. As such, it ispossible that further retreat of Fountain Glacier couldtrigger a hydraulic disconnection between glacier andicing which would lead to a loss of water supply, a loss ofthe icing’s perennial character or eventually to a itscomplete disappearance.

CONCLUSIONS

The growth and regeneration of the Fountain Glaciericing and the location of its source springs depend on thebalance between the glacial and the permafrost system.As suggested in Wainstein et al. (2008) and Wainstein(2011), the glacier acts as the main source of water andtherefore is one of the main components that controls thegrowth and replenishment of the icing. Moreover, theglacier acts as a dominant factor in the balance of the twosystems; it can drive the icing out of equilibrium byaltering the hydrological conditions required to preservean open proglacial talik and hence a constant supply ofwater. The morphology and hydrology of FountainGlacier present specific characteristics that promote thewinter regeneration of the proglacial icing by providingan un-interrupted supply of water throughout the year.The main conclusions of the study are:

1. This paper presents a new technical approachtowards the understanding of the glacial hydro-thermalrelationships that contribute to the regeneration of anarctic proglacial icing.

2. The subglacial morphology, in combination with thelow creep rates of cold ice, has lead to the preservationof a well-defined channelized subglacial drainagesystem that allows the collection and conduction ofpressurized water used in the regeneration of theproglacial icing.

3. The subglacial drainage model predicts two mainwaterways through which subglacial water predomi-nantly flows. The termini of both drainage pathwaysmatch the location of ice collapse features and theremnants of sediment-filled subglacial conduits at thebase of the current avalanche escarpments.

4. The location of the fountain remnants observed in 1991and the location of the current spring, closer to theglacier’s terminus, are aligned with the central branchof the drainage model. This suggests that proglacialupwellings (springs) are mainly fed by subglacialwaters coming through the mid-section of the glacier.

5. The reflection power analysis suggests that thesurveyed area has a significant accumulation of liquid


water along the centre portion of the glacier, beside thewell-incised supraglacial stream and at the terminus ofthe northern subglacial drainage pathway.

6. The qualitative analysis of the GPR signature indicatesthat Fountain Glacier contains a considerable numberof well-preserved englacial and deep englacialconduits, with more than half of them filled with water.

7. The dielectric permittivity model indicates that the baseof the glacier ice is most probably composed of areasof frozen till and areas of mixed till, water and ice. Thispolythermal character not only allows the storage ofpressurized subglacial water, as a result of the thermaldamming effect of cold glacial margins, but also theconduction of water onto the proglacial plain.

Finally, Fountain Glacier’s proglacial icing has under-gone severe changes during the last 20 years (Whiteheadet al., 2010a, b, c). The glacier’s retreat rate has increasedsubstantially and so has its mass wastage by dry calving.The regeneration of the icing as a cryo-hydrologicalsystem depends on its adaptation capacity towards thecurrent glaciological conditions; failing to do so mighttrigger an end to its perennial character.

ACKNOWLEDGEMENTS

This work was supported by the Natural Sciences andEngineering Research Council of Canada (NSERC)through a research grant held by Dr. Brian Moorman,by the Polar Continental Shelf Project (PCSP), ParksCanada, Alberta Innovates Technology Future through adoctoral scholarship held by Dr. Pablo Wainstein and theNorthern Scientific Training Program (NSTP).

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