A sedimentological and isotopic study of the origin of supraglacial debris bands: Kongsfjorden, Svalbard

A sedimentological and isotopic study of the origin ofsupraglacial debris bands Kongsfjorden Svalbard

Bryn HUBBARD Neil GLASSER Michael HAMBREY James ETIENNECentre for Glaciology Institute of Geography and Earth Sciences University ofWales Aberystwyth SY23 3DBWales

E-mail byhaberacuk

ABSTRACT Debrisbands associatedwith supraglacialmoraines andassociatedbasaldeposits have been logged and sampled for their ice and debris at three glaciers in north-west Spitsbergen Svalbard Physical properties including sediment concentrations sedi-ment particle-size distributions clast macro-fabrics and oxygen isotope compositionsindicate that all transverse and some longitudinal debris bands originate from the basalzone of these glaciersTransverse supraglacial bands are composed of extensive stratified-facies basal ice that is enriched in 18O andwhich contains polymodal debris with spatiallyconsistent clast fabrics These properties suggest initial formation as basal ice and subse-quent elevation into an englacial position by thrusting rather than formation as crevassefillsThe formation of longitudinal debris bands results from laterally compressive foldingin response to the convergence of multiple flow units into a narrow glacier tongue In com-monwith transverse debris bands longitudinal bands appear to be composed of stratifiedbasal ice The bands exposed at the surface of austre Brggerbreen comprise two sub-facies strongly suggesting that the glacier was at least partially warm-based in the pastwhen the basal ice formed

INTRODUCTION

Awide range of processes can result in the entrainment ofsediment into ice masses Supraglacial material may fallonto the ice surface and be carried with the local flow vec-tor leading to burial in the accumulation area and exhuma-tion in the ablation area Large amounts of debris can alsobe entrained at the glacier margins and bedThis typicallyoccurs through a debris-rich basal ice layer which is formedby water freezing in the presence of debris near the basalinterface (eg Hubbard and Sharp 1989 Alley and others1997 Knight 1997) Although the mechanism remains un-proven it has been argued that basal sediments may alsobe incorporated into the body of a glacier through foldingand thrusting (Goldthwait 1951 Tison and others 1989Hambrey and others 1999) and as basal crevasse-fills(Mickelson and Berkson 1974 Sharp 1985 Bennett andothers 1996 Evans and Rea 1999 Ensminger and others2001Woodward and others 2002)

Extensive linear ridges of sediment-rich ice commonlycappedwith a layer of melted-out debris have recently beenreported in the ablation area of numerous polythermal gla-ciers particularly in Svalbard These ridges are generallyaligned either parallel to the ice-flow direction or perpen-dicular to it and are respectively referred to hereafter aslongitudinal and transverse supraglacial moraine ridgesThese features are described in more detail below

Longitudinal supraglacial moraine ridges

Longitudinal supraglacial moraine ridges or medial mor-aines located at the tongues of several composite Svalbardglaciers have recently been interpreted by Hambrey andothers (1999) and Hambrey and Glasser (2003) as products

of folding along flow-parallel axes The moraines arealigned parallel to longitudinal foliation and with thisstructure form an axial-planar relationshipwith the foldingwhich develops in response to lateral compression as multi-ple flow units converge into a narrow tongue (Fig 1) Theseauthors reported that the debris-charged ridges associatedwith this folding emerge at the glacier surface near its termi-nus as fold hinges that plunge gently up-glacier

Hambrey and Glasser (2003) reported that the debrisforming supraglacial moraine ridges at vestre Lovecurren nbreenand austre Brggerbreen can be either coarse and angularindicating a supraglacial origin or (more commonly) com-posed of diamicton containing subrounded faceted andstriated clasts indicating a basal origin Orientation meas-urements by these authors indicated that these basally de-rived clasts are strongly aligned parallel to the locallongitudinal foliation and associated fold axis In such casesit was inferred by Hambrey and others (1999) that thesource material (subglacial sediment or debris-rich basalice) is folded along flow-parallel axes directly into the bodyof the glacier and reaches the surface as a consequence ofthis folding and subsequent ablation (Fig1)

Transverse supraglacial moraine ridges

Glasser and others (1998) associated transverse supraglacialmoraine ridges on the glaciers of Kongsfjorden with struc-tures interpreted as thrusts As with longitudinal moraineridges the visual appearance of this debris is consistent witha basal origin but few quantitative data support this infer-ence

Woodward and others (2002) recently presented analternative hypothesis for the origin of the debris bands

Journal of GlaciologyVol 50 No 169 2004

157

associated with transverse supraglacial moraine ridges atKongsvegenThey argued that the features originated fromdebris-filled basal crevasses that have been exposed at theglacier surface by ablation of the overlying ice However incommonwith the thrust hypothesis outlined above no new

data relating to the physical properties of the materials con-cerned were reported

An origin for debris bands by basal crevasse fillinghowever has been advanced on the basis of research at otherglaciers from where data have been reported Ensminger

Fig 1 Schematic illustration of the formation of longitudinal supraglacial moraine ridges at Svalbard glaciers (after Hambrey

and others 1999)

Fig 2 Kongsfjorden with sample sites numbered (1) Kongsvegen transverse supraglacial moraine ridge and nearby glacier mar-

gin (2^5) midre Lovecurren nbreen east margin (2) proglacial area (3) longitudinal supraglacial moraine ridge (4) and west margin

(5) and (6) austre Brggerbreen longitudinal supraglacial moraine ridge

Hubbard and othersThe origin of supraglacial debris bands

158

and others (2001) for example describe a finely layered(mm thick) sequence of debris-rich and debris-poor bandsinterpreted to have originated as basal crevasse fills atMatanuska Glacier Alaska USA In this case the layerswere associated with the injection of sediment-rich waterinto basal crevasses where it froze in part as a result ofsupercooling upon emergence from a basal overdeepeningThese authors also found that the debris within these basalcrevasse fills was well sorted silt-rich and fined with dis-tance above the bedconsistent with grain settlingthrough a viscous suspension during formation Not all suchdebris injections however are considered to have involvedsediment-laden water Sharp (1985) for example arguedthat crevasse-squeeze ridges can form as a result of viscoussoft-sediment injection into basal crevasses that open duringsurging In such cases the resulting features are more likelyto include massive and poorly sorted debris containingcoarse clasts than those associatedwith the less viscous flowdescribed by Ensminger and others (2001) However thesetwo cases most probably represent near end-members of arange of forms whose degree of sorting and layering corres-ponds to the extent of fluidization experienced during for-mation Sharp (1985) and Evans and Rea (1999) also pointout that the character of crevasse fills may be altered afterformation by for example ice-deformation depositionalprocesses

Processes of incorporation and transport may also be in-vestigated through analysis of the fabric of the clasts presentwithin debris bands For example a spatially variable butlocally strong fabric might be anticipated within a fluidizedcrevasse fill similar to a viscous debris flow (Lawson1979a) whereas a more extensive and consistent fabricorientation might be anticipated within a thrust or thinshear zone

Research approach

In this study we investigate the physical properties particu-larly the sedimentology and isotopic composition of debrisbands associated with longitudinal and transverse supragla-cial moraine ridges at three Kongsfjorden glaciers Kongs-vegen midre Lovecurren nbreen and austre Brggerbreen Inaddition to characterizing these ridges our aim is to usetheir physical properties to evaluate and refine theories oftheir formation

Table 1 Summary of the features observed and sampled at the

three glaciers studiedp

indicates that the feature was

observed S indicates that the feature was sampled for its sedi-

mentological characteristics I indicates that the feature was

sampled for its isotopic composition

Feature Kongsvegen Midre

Lovecurren nbreen

Austre

Brggerbreen

Transverse supraglacial morainep

Sp

STransverse debris band

pS I

pS I

Longitudinal supraglacial morainep p

Longitudinal debris bandp

S Ip

S I

Basal stratified faciesp

S Ip

S IBasal planar facies

pS

Glacier icep

Ip

Ip

I

Supraglacial meltwaterp

Ip p

Bulk meltwaterp p

Ip

Fig 3Transverse moraine ridge exposed (a) on the surface of Kongsvegen (viewing towards the north) and (b) at the nearby

glacier margin (viewing towards the south) Figures for scale are in roughly the same location in both photographs

159

Hubbard and othersThe origin of supraglacial debris bands

FIELD SITE ANDMETHODS

Fieldwork was undertaken at three glaciers austre Brg-gerbreen midre Lovecurren nbreen and Kongsvegen located onthe Brgger peninsula (Brggerhalvya) Kongsfjorden innorthwest Spitsbergen Svalbard (Fig 2) All three glaciershave been the subject of a long-term mass-balance moni-toring programme by the Norsk Polarinstitutt (eg Hagen

and others 1991b) revealing a general thinning and reces-sion (Liestl 1988) Kongsvegen is a surge-type glacierwhich last advanced in 1948 Since then it has ceased to beactive and in contrast to fast-flowing (700ma^1) Krone-breen with which it shares a common tidewater terminus ithas a maximum surface velocity of 8ma^1 (Hagen andothers1991b)

Austre Brggerbreen and midre Lovecurren nbreen are also

Fig4West lateralmargin ofmidre Lovecurren nbreen (a) general view and (b) closer view of basal solid sub-facies overlain by debris-

poor foliated glacier ice

Hubbard and othersThe origin of supraglacial debris bands

160

slow-moving glaciers with equilibrium-line surfacevelocities of only a few metres per year (Bjolaquo rnsson andothers 1996) Close to their neoglacial maxima around1890 they had vertical terminal cliffs and were consideredby Liestl (1988) to be surging an inference that is disputedby Hambrey and Glasser (2003) who cite the equivocal na-ture of Liestlrsquos (1988) evidence However proglacial geo-morphic evidence indicates that these glaciers were moredynamic at that time (Glasser and Hambrey 2001) Theircontinual recession since 1890 is a reflection of climatewarming through the 20th century All three glaciers havesubstantial parts of their bed at sub-freezing temperaturesparticularly austre Brggerbreen where no temperate en-glacial ice is evident on radar images (Macheret and Zhur-avlev 1982 Hagen and Strang 1991) and boreholetemperatures indicate that most of the base is cold (Hagenand Strang 1991) Midre Lovecurren nbreen has a warm-basedinterior and a frozen terminus region while tidewaterKongsvegen is wet-based throughout The sample sitesreferred to below are summarized inTable 1 marked in Fig-ure 2 and illustrated in Figures 3^5

Sample treatment and analysis

Ice meltwater frozen debris and unfrozen debris were

Fig 5 Longitudinal moraine ridge emerging from the surface of midre Lovecurren nbreen Ice flow is directly out from the page

Fig 6 Longitudinal moraine ridge exposed at the surface of

austre Brggerbreen Ice flow is away from the viewer

161

Hubbard and othersThe origin of supraglacial debris bands

sampled Unfrozen debris was sampled by hand trowelstored and transported in sealed plastic bags Samples offrozen debris and debris-rich ice were removed from the gla-cier and broken up by ice axe and melted in clean plasticbags Samples of debris-poor ice were recovered by icescrew All samples were transported in sealed plastic bagsto a field laboratory where they were filtered dried andweighed within 24 hours of samplingWhen filtrate was re-tained for isotopic analysis at the Geophysical Isotope La-boratory Copenhagen University Denmark it was storedand transported in sealed dark-brown bottles Meltwatersampled as liquid in the field was press-filtered prior tostorage in sealed dark-brown bottles

Debris textures were determined in the laboratory at 1intervals from ^4 to 3 by dry sieving and from 3 tolsquolsquofiner than 10rsquorsquo by settling analysis (SediGraph 5100 Mi-cromeritics) Results are presented on plots of size ()against weight () and as double logarithmic plots of par-ticle diameter (d) against number of particles (Nd)The lat-ter of these allows the degree of self-similarity (expressed asthe correlation coefficient (R) of the variables) and fractaldimension (m) of the debris to be calculated Here a self-similar distribution will define a straight line (R frac14 10000)on the plot of log d against logNd according to

Nd frac14 N0d

d0

m

eth1THORN

(Hooke and Iverson1995) whereN0 is the number of parti-cles of reference diameter d0 and m the fractal dimensionis given by the negative slope of the log^log plot Althoughinsensitive to minor changes in grain-size distribution(Benn and Gemmell 2002) the value of m summarizes theratio of smaller particles to larger particles over the sizerange analyzed The analysis therefore provides a usefulsingle-value expression for the character of an entire par-ticle-size distribution and thereby provides a straight-forward means to compare samples A self-similardistribution of tessellating cubes has a fractal dimension of258 (Sammis and others 1987) and samples of basally de-rived debris generally have fractal dimensions in the range27^30 (eg Hooke and Iverson1995 Hubbard and others1996 Fischer and Hubbard1999 Khatwa and others1999)

Clast macro-fabrics were recorded in the field forsamples of 50 prolate clasts each with an a-axisc-axis ratioof 42 (Andrews 1970) The data are plotted on Schmidtequal-area lower-hemisphere projections and summarizedusing standard eigenvector analysis

Statistical testing of differences between sample data isbased on two sample t tests (large samples) orU tests (smallsamples) and results are expressed as the probability (P ) of

differences in the databeing due to chance According to thenotation usedlsquolsquosimilar at P 4 01rsquorsquomeans that there is great-er than a 10 chance of the samples being from the sameparent population and lsquolsquodifferent at Plt 001rsquorsquo means thatthere is less than a 1 chance that the samples are fromthe same parent population

RESULTS

Field sections

Transverse supraglacial moraine ridges

The transverse moraine ridge studied at Kongsvegen ex-tended from the glacier margin for several tens of metresacross the glacier surfaceThe surface debris formed an ice-cored mound of diamicton 1m highWashing off the un-consolidated surface debris revealed a 05m wide debrisband formed of thin alternating layers of debris-rich anddebris-poor ice the former containing a wide range of par-ticle sizes The debris-poor ice layers were largely bubble-freeThus it was visually similar to the stratified discontin-uous sub-facies of the basal zone as identified by Lawson(1979b) at Matanuska Glacier

At midre Lovecurren nbreen a similar transverse moraineridge extended from the glacier surface down the lateralmargin where it merged indistinctly into a layer of frozenbasal sediment composed of muddy gravel Similar to thesurface debris band sampled at Kongsvegen the band atmidre Lovecurren nbreenwas formed of multiple debris-rich layersseparated by relatively clean bubble-free ice

Table 2 Summary of sediment concentration results classified by sample source and glacier indicates number of samples xindicates the mean concentration (g L^1) and indicates the standard deviation in concentration (g L^1)

Sediment concentration (g L^1)

Sample source Kongsvegen Midre Austre All glaciers

Lovecurren nbreen Brggerbreen x x x x

Basal ice 2 3235 1146 11 7684 7108 13 7000 6708Surface transverse debris bands 6 1520 725 5 1400 315 11 1466 554Surface longitudinal debris bands 11 1259 902 11 1259 902All debris bands 22 1362 738

Fig 7 Box plots of debris concentration data classified by

sample type from all three glaciers Markers denote the 0th

1st 5th 25th 50th 75th 95th 99th and 100th percentile

valuesThe open square denotes the mean value

Hubbard and othersThe origin of supraglacial debris bands

162

Longitudinal supraglacial moraine ridges

At midre Lovecurren nbreen the debris band associated with thelongitudinal moraine studied forms part of a fold hinge thatdips up-glacier at a shallow angle consistent with Hambreyand othersrsquo (1999) structural interpretation of these features(Fig1)This band was layered by debris concentration withdebris-rich layers containing polymodal diamicton withclasts up to boulder size

At austre Brggerbreen the debris band forming thelongitudinal moraine sampled at the surface was 50 cmthick and close inspection revealed that it was composedof alternating debris-rich and debris-poor layers This bandwas visually similar to stratified basal ice and thereforesimilar in structure to other debris-charged ridges sampledfor the present study

Fig 8 Bivariate plots of weight against size class for Kongs-

vegen debris samples (a) basal solid sub-facies (b) trans-

verse supraglacial moraine ridge (c) ice-cliff transverse

moraine ridge (d) transverse supraglacial moraine ridge

melt-out debrisThe finer than 10 size class is not plotted

Fig 9 Bivariate plots of weight against size class for midre

Lovecurren nbreen debris samples (a) basal solid sub-facies

(b) transverse supraglacial moraine ridge (c) basal planar

faciesThe finer than 10 size class is not plotted

Fig 10 Bivariate plots of weight against size class for austre

Brggerbreen debris samples (a) longitudinal supraglacial

moraine ridge (red Carboniferous debris) (b) longitudinal

supraglacial moraine ridge (grey Proterozoic debris) The

finer than 10 size class is not plotted

163

Hubbard and othersThe origin of supraglacial debris bands

The stratified-facies ice forming the longitudinal debrisband at austre Brggerbreen was also characterized by asystematic pattern of debris incorporation Here thin lami-nae defined by fine-grained red debris (of Carboniferousmudstone and sandstone) enveloped a core of massive greydiamicton (of Proterozoic metamorphic rocks) containingonly interstitial ice (Fig 6) Thus the unit incorporatingthe red mudstone was identical to Lawsonrsquos (1979b) strati-fied discontinuous sub-facies and the unit containing thegrey metamorphic material was identical to Lawsonrsquos strati-fied solid sub-facies The latter is frozen debris containinginterstitial ice and ice lenses

Basal ice

Basal ice and debris were sampled from the margins ofKongsvegen and midre Lovecurren nbreen (Table 1) In both casesthe basal zone is composed of stratified-facies ice which isprincipally solid sub-facies No basal ice was sampled at aus-tre Brggerbreen A separate planar lamination somemillimetres thick and containing only fine debris was alsosampled at the west margin of midre Lovecurren nbreenThis layerwas parallel to the debris-charged ridge at the site and wasvisually similar to planar-facies basal ice identified by Hub-bard and Sharp (1995) at Alpine glaciers

Sedimentology

Debris concentration

Debris concentration (expressed as grammes of debris perlitre of meltwater g L^1) was calculated for 35 samples ofice-borne debris (Table 2 Fig 7) The mean concentrationof 13 samples of basal ice sampled from ice-marginallocations is 7000 g L^1 whereas that of 11 transverse and 11longitudinal supraglacial moraine samples is 1466 and1259 g L^1 respectively Statistical analyses of these data in-dicate that the supraglacial debris-band concentrations aresignificantly lower than the basal ice concentrations(P lt 001) In contrast the transverse and longitudinal

debris-band concentrations are not significantly differentfrom each other (P gt 001)

Debris particle-size distribution

Forty-nine debris samples were analyzed for their particle-size distributionsThese are classified by glacier and sampletype and plotted as size against weight in Figures 8^10These data are also summarized inTable 3 in terms of weight represented in the gravel (^4 to ^1 inclusive) sand(^05 to 4 inclusive) and silt and clay (44) size classesand in terms of the correlation coefficient (R) and inverseslope (m) of bivariate plots of log number of particlesagainst log particle diameter

Data from Kongsvegen indicate broad similarity in thetextures of the basal solid sub-facies debris the transversesupraglacial debris band (whether sampled at the ice sur-face or the ice margin) and the melt-out debris forming thetransverse supraglacial moraine (Fig 8) Close inspection ofFigure 8 however indicates that the last of these has aslightly greater proportion of gravel-sized clasts (or a lowerproportion of silt- and clay-sized clasts) than the debris en-trained within the debris band Analysis of these data indi-cates significant depletion (P lt 005) of silt-sized particlesin the surface moraine (145 silt and clay) relative to thedebris bands sampled at the glacier surface (248 silt andclay) and margin (221 silt and clay) Debris entrainedwithin the latter two sample groups is statistically similarfor all three size classes (P 401)

At midre Lovecurren nbreen (Fig 9) the texture of the debriswithin the supraglacial transverse debris band is generallysimilar to that of the ice-marginal basal solid sub-faciesHowever in detail the former was significantly (P lt 001)depleted in silt- and clay-sized material (41 silt and clay)relative to both the latter (230 silt and clay) Both weresignificantly (P lt 001) depleted in silt- and clay-sized ma-terial relative to the planar facies sampled at the margin ofthe glacier which was very well sorted and fine-grained(827 silt and clay) Corresponding inverse statistical

Table 3 Summary of particle-size distribution results classified by sample source and glacierWeight in size class () relates to

standard size^weight plots and m and R are the negative slope and correlation coefficient respectively of plots of log Nd against

log d

Glacier Sample source Weight in size class () logNd^ log d

Gravel Sand Silt and clay m R

All glaciers All samples 429 367 204 276 ^09983

Kongsvegen All samples 361 426 214 281 ^09993Basal solid sub-facies 405 400 196 280 ^09990Surface transverse debris band 323 429 248 285 ^09992Ice cliff transverse debris band 367 412 221 281 ^09994Surface transverse debris band meltoutdebris

397 459 145 275 ^09993

Midre Lovecurren nbreen All samples 444 345 212 276 ^09972Basal solid sub-facies 421 349 230 279 ^09992Surface transverse debris band 590 369 41 245 ^09968Basal planar facies 08 165 827 397 ^09733

Austre Brggerbreen All samples 501 320 179 269 ^09989Surface longitudinal debris band (red) 393 368 239 278 ^09988Surface longitudinal debris band (grey) 692 235 73 253 ^09991

Hubbard and othersThe origin of supraglacial debris bands

164

Fig11 Schmidt equal-area lower-hemispheric projections of clast fabric samples presented by sample type andglacier (a)Kongs-

vegen basal solid sub-facies (unfrozen) (b) Kongsvegen moraine ridge from ice cliff (c) Kongsvegen supraglacial moraine

ridge (d) midre Lovecurren nbreen basal solid sub-facies (unfrozen) from east margin (e) midre Lovecurren nbreen basal solid sub-facies

from west margin (f) midre Lovecurren nbreen proglacial diamicton (unfrozen) from east margin (g) midre Lovecurren nbreen basal solid

sub-facies from west margin and (h) austre Brggerbreen supraglacial moraine ridge Points are contoured at 5 intervals per

1 of area and arrows indicate the local ice-flow direction

Table 4 Summary of clast macro-fabrics as plotted on equal-area lower-hemisphere projections (Figure 11) classified by sample

source and glacier

Glacier Sample source Mean

azimuth

Mean

dip

Eigenvalues Sperical

variance

Dagger Dagger 1st 2nd 3rd

Kongsvegen Basal solid sub-facies (unfrozen) 117 0 062 032 006 080Transverse debris band in ice cliff 23 20 082 015 004 027Transverse debris band on glacier surface 16 16 070 023 007 027

Midre Lovecurren nbreen Basal solid sub-facies (unfrozen) east margin 66 5 067 028 006 069Basal solid sub-facies west margin 326 5 071 025 005 043Proglacial diamicton east margin 70 1 080 014 006 077Basal solid sub-facies west margin 161 8 088 009 003 031

Austre Brggerbreen Longitudinal debris band on glacier surface 180 4 083 013 005 064

165

Hubbard and othersThe origin of supraglacial debris bands

differences exist between these sample groups in the gravel-size fraction

At austre Brggerbreen (Fig 10) our data indicate amarked difference between the texture of the red and thegrey debris within the longitudinal supraglacial debrisbandThus the red sediment is significantly (P lt 001) de-pleted in gravel-sized material (393 gravel) and enrichedin silt- and clay-sized material (239 silt and clay) relativeto the grey sediment (692 gravel73 silt and clay)

Summary data of the bivariate plots of log number ofparticles against log particle diameter (Table 3) indicateslopes or fractal dimensions (m) that are in the range 26^29 with a few notable exceptions The grey debris-chargedridge material at austre Brggerbreen has a fractal dimen-sion of 253 consistent with the general depletion in finesnoted above Similarly debris sampled from the surfacedebris-charged ridge at midre Lovecurren nbreen has a fractal di-mension of 245 Conversely the fine debris sampled fromthe planar facies at midre Lovecurren nbreen has an apparent frac-tal dimension of 397 although this is questionable since thelog^log bivariate plot is clearly not linear (R frac14 ^0973)(Table 3)

Clast macro-fabrics

Eight sets of clast macro-fabric data were recorded fromwithin the debris bands sampled at the three glaciersstudied (Fig11Table 4) At Kongsvegen the two samples re-covered from the transverse supraglacial debris band (onefrom the marginal ice cliff and the other from the glaciersurface Table 1) are similar to each other characterized bystrong unimodal fabrics (first eigenvalues = 082 and 070)with an azimuth of 20Dagger and a dip of 18Dagger These direc-tions are parallel to the plan-form orientation of the supra-glacial moraine and its associated debris band ietransverse to the direction of ice flow In contrast the localice-marginal basal diamicton is characterized by a weakerfabric (first eigenvalue frac14 062) Fabrics measured in thebasal solid sub-facies located around the margins of midreLovecurren nbreen were also spatially variable characterized byspherical variances of 067^088 (Table 4) At austre Brg-

gerbreen the longitudinal debris-charged ridge samplefrom the glacier surface was characterized by a strong uni-modal fabric with a first eigenvalue of 083

Oxygen isotope composition

Oxygen isotope data are calculated as 18O in which ex-presses the ratio of the abundance of the isotope 18O to 16Oin the sample relative to that of Standard Mean OceanWater (SMOW)

18O frac14 100018O=16OethsampleTHORN 18 O=16OethSMOWTHORN

18O=16OethSMOWTHORN

eth2THORN

Analysis of 110 ice and water samples yielded a mean valueof ^1175 and a standard deviation of 076 (Table 5)There is little variation in the sample means between thethree glaciers studied the mean isotopic composition of icesamples was ^1161 (n frac14 39) from Kongsvegen ^1181(n frac14 50) from midre Lovecurren nbreen and ^1187 (n frac14 21)from austre Brggerbreen

In order to investigate these data further samples aresubdivided by glacier and by sample type summarized inTable 5 and Figure12These data reveal significant and sys-tematic patterns in sample group isotopic composition

At Kongsvegen the mean composition of glacier ice andsupraglacial meltwater is ^1214 (n frac14 23) and the meancomposition of the (debris-rich) ice within the supraglacialdebris band is ^1082 (n frac14 6) The respective values atmidre Lovecurren nbreen are ^1238 (n frac14 21) and ^1140(n frac14 20) At both glaciers ice within the supraglacial debrisbands is isotopically enriched (P lt 001) in 18O relative toglacier ice and surface meltwater samples The ice sampledfrom the supraglacial debris bands is isotopically similar(P 401) to that sampled from the debris-rich basal layer(or frozen subglacial sediment) locatedat themarginof theseglaciers ^1085 (n frac14 10) at Kongsvegen and ^1105(n frac14 4) atmidre Lovecurren nbreen

At austre Brggerbreen the isotopic composition of theice forming the longitudinal supraglacial debris band(18O frac14 ^1183 n frac14 11) is similar to (P gt 01) that ofglacier ice (18O frac14 ^1190 n frac14 10) However if the

Table 5 Summary of oxygen isotope results classified by sample source and glacier indicates number of samples x indicates themean 18Ovalue (standard deviation of 18Ovalues ()

Glacier Sample source 18O ethTHORN x

All glaciers All samples 110 ^1175 0757

Kongsvegen All samples 39 ^1161 0744Glacier ice and supraglacial meltwater 23 ^1214 0380Surface transverse debris band 6 ^1082 0294Ice-marginal basal ice 10 ^1085 0399

Midre Lovecurren nbreen All samples 50 ^1181 0671Glacier ice 21 ^1238 0542Surface transverse debris band 20 ^1140 0435Ice-marginal basal ice 4 ^1105 0196Bulk meltwater 5 ^1166 0048

Austre Brggerbreen All samples 21 ^1187 0949Glacier ice 10 ^1190 1160Surface longitudinal debris band (solid sub-facies) 4 ^1280 0090Surface longitudinal debris band (discontinuous sub-facies) 7 ^1129 0108

Hubbard and othersThe origin of supraglacial debris bands

166

samples recovered from the debris band are reclassified bysub-facies the solid sub-facies (grey debris) is depleted in18O relative to the discontinuous sub-facies (red debris)(P lt 005) Neither sub-facies has a significantly differentisotopic composition from glacier ice However if an anom-alous glacier ice sample of ^1486 in 18O is discountedfrom the analysis the solid sub-facies becomes significantlylighter than the remaining nine glacier ice samples(P lt 001)

DISCUSSION

Certain consistent relationships between the supraglacialdebris bands and other sample types emerge from the evi-dence presented above

Transverse supraglacial debris bands and moraineridges

Transverse debris bands at Kongsvegen and midre Lovecurren n-

breen contain debris that is generally of similar particle-sizedistribution to that within basal ice at these and other gla-ciers (eg Lawson1979b Hubbard and Sharp1995) At bothKongsvegen and midre Lovecurren nbreen many of the clasts en-trained within the transverse debris bands are striated andfaceted They are also characterized by a strong unimodalfabric in which the clasts are aligned parallel to the plane ofthe supraglacial moraine ridge At Kongsvegen this pre-ferred orientation is remarkably consistent at two sites oneexposed on an ice cliff and the other 30m distant on theglacier surface (Fig 11b and c) At Kongsvegen and midreLovecurren nbreen ice contained within the basal solid sub-faciesand the supraglacial debris bands (whether at the glaciermargin or glacier surface) is enriched in 18O by 1^2relative to local glacier ice and supraglacial meltwaterSince glacier ice (or basal meltwater derived from it) is themost likely source for the basal ice and debris-band ice it isprobable that these latter groups have been isotopicallyaltered during their formation andor transport Such en-richment is consistent with open-system or incompletefreezing of meltwater in the presence of debris at the glacierbed (Jouzel and Souchez 1982 Souchez and Jouzel 1984)This is supported by the absence of any significant differ-ence between the isotopic composition of the debris-bandice and that within the subglacial basal solid sub-facies atKongsvegen and midre Lovecurren nbreen

In summary these sedimentological data provide strongevidence that the debris incorporated within the transversedebris bands and supraglacial ridges at Kongsvegen andmidre Lovecurren nbreen was derived from the beds of these gla-ciers Further the isotopic data are consistent with the icematrix of these debris bands also originating by refreezingat the glacier bed

These data may also be used to shed some light on theprocesses responsible for forming the transverse debrisbands concerned in particular on the competing hypoth-eses of formation as thrusts or as basal crevasses The mainobstacle to such an interpretation is that both processescould produce features with physical and compositionalsimilarities Both for example involve the same subglacialdebris and water source and both can result in the develop-ment of strong clast fabrics within the bands they formHowever we believe the data from this study are more con-sistent with an origin as thrusts than with an origin as basalcrevasses for the following reasons

Fluidized flow however viscous of soft sediments intobasal crevasses would be characterized by some degreeof local debris sorting In this study we neither observednor measured any such sorting At Kongsvegen forexample almost identical polymodal diamicton wasrecovered from samples of the transverse supraglacialdebris band located at the glacier surface and in anice-cliff section tens of metres distant Although thesebands were layered by variations in debris concentra-tion the debris was not sorted in terms of its grain-sizedistribution

Fluidized flow of soft sediments into basal crevasses (atdebris^water concentrations of 41000 g L^1 Table 2)would be unlikely to result in spatially extensive planarlayering such as was observed in the present study Thetransverse debris bands investigated at the surface ofKongsvegen and midre Lovecurren nbreen were formed ofextensive debris-rich layers separated by clean and

Fig 12 Box plots of 18O composition of ice facies by sample

type and glacier (a)Kongsvegen (b) midre Lovecurren nbreen and

(c) austre Brggerbreen Markers denote the 0th 1st 5th

25th 50th 75th 95th 99th and 100th percentile valuesThe

open square denotes the mean value SDB in axis labels stands

for supraglacial debris band

167

Hubbard and othersThe origin of supraglacial debris bands

bubble-free ice identical to stratified-facies basal iceThese properties therefore indicate that the transversesupraglacial debris bands sampled at these glaciers areformed of pre-existing stratified-facies basal ice that hasbeen elevated from the glacier bed to the surface withoutnoticeable alterationWhile such a mechanism is incom-patible with the formation of these debris bands by basalcrevasse filling it is compatible with their initial forma-tion as basal ice and their subsequent englacial transportby thrusting

It is likely that fluidized flow of soft sediments into basalcrevasses would be characterized by some degree of fin-ing with distance from source as identified by Ens-minger and others (2001) This effect was not observedin the present study

Basal crevasses would be expected to cut sharply acrossother basal ice layers at a high angle (consistent withcrevasse orientationbeingbroadly orthogonal to the gla-cier bed and basal ice layers being broadly parallel to it)This effect was not observed in the present study Con-versely we did observe continuity in the structure of in-dividual transverse debris bands between the surfaceand margins of midre Lovecurren nbreen In this case thebands merged indistinctly into the debris-rich basal icelayer present at the base of the lateral margin of the gla-cier (Fig 4) This pattern is consistent with local ductiledeformation contributing to and occurring between in-itially low-angle thrusts initiating near or at the ice^bedinterface

The heavy-isotope enrichment of the debris bands bylt3 in 18O relative to glacier ice and supraglacialmeltwaters is consistent with basal ice formation byopen-system refreezing at the glacier bed Indeed suchenrichment has commonly been reported in basal icestudies (Lawson and Kulla 1978 Hubbard and Sharp1989) In contrast once injected into a basal crevasse ameltwater suspension is more likely to freeze without re-newed water turnover essentially closing the systemSampling ice frozen in a closed system should result ina wide range of isotopic values from slightly heavier(43 in 18O) to substantially lighter (46 in 18O asfreezing nears completion) than the composition of thewater in the slurry from which they formed (Jouzel andSouchez1982)This effect was not measured in the pres-ent study

Although none of the individual lines of evidence presentedabove can be interpreted as unequivocal proof of transversesupraglacial debris-band formation as thrusting of basal icefrom the glacier bed the weight of evidence favours such amechanism over that involving formation as sediment-filledbasal crevasses Indeed Hubbard and Sharp (1995) inter-preted planar facies basal ice sampled in the Alps as healedcrevasses probably containing aeolian debris sourced fromthe glacier surface The planar facies sampled from midreLovecurren nbreen is similar to these features and we interpret itsimilarly However it is possible in both cases that the faciesforms as a basal fracture into which fine subglacial debrismay be introduced by flushing in suspension (Knight andKnight1994)

One further observation at Kongsvegen was that theunconsolidated material sampled from the surface of thesupraglacial moraine ridge lacked fines relative to that

sampled from the underlying and ice-marginal debrisband We interpret this effect in terms of the preferentialeluviation of fine particles from surface moraine ridges byrainfall and meltwater Similar effects were reported byBoulton and Dent (1974) and Fischer and Hubbard (1999)

Longitudinal supraglacial debris bands and moraine ridges

The longitudinal supraglacial debris band sampled at austreBrggerbreen contains debris that is polymodal has a typ-ically basal particle-size distribution and contains clasts thatare faceted and striated As with transverse debris bands atKongsvegen and midre Lovecurren nbreen therefore we interpretthis material as being basally derived

The longitudinal supraglacial debris band at austreBrggerbreen is formed of two sub-facies a central solidsub-facies enveloped by a discontinuous sub-facies (Fig 13)Associating this pattern with Hambrey and othersrsquo (1999)structural interpretation of longitudinal debris bands(Fig 1) indicates the presence of a basal ice layer composedof two sub-facies at the bed of this glacier Moreover the po-sition of the sub-facies at the surface of austre Brggerbreenindicates that at the glacier bed the discontinuous sub-faciesoverlies the solid sub-facies (Fig 13) This implies that theformer was incorporated up-glacier of the latter andorbefore the latter This interpretation is consistent with thestrong lithological contrast between the debris incorporatedwithin the different sub-facies

It is generally accepted that solid sub-facies basal iceforms by the net adfreezing of unconsolidated subglacialsediments (Hubbard and Sharp 1989) At polythermal gla-ciers this is associated with temporal variations in the posi-tion of the freezing isotherm at the boundary between sub-freezing basal conditions at the ice margins and temperatebasal conditions beneath thicker ice up-glacier (Weertman1961) In contrast thinly layered discontinuous sub-facies

Fig 13 Schematic illustration of the distribution of basal ice

sub-facies associated with the longitudinal supraglacial

moraine ridge sampled at austre Brggerbreen (depicted in

Fig 6)

Hubbard and othersThe origin of supraglacial debris bands

168

basal ice forms from repeated freezing events more likely tobe associated with generally temperate basal conditionsSuch freezing may involve a number of processes including(i) the initial formation of finely laminated ice by closed-system regelation (Kamb and LaChappelle1963 Hubbardand Sharp 1993 1995) (ii) more extensive freeze-on asso-ciated with ephemeral patches of cold basal ice (Robin1976) or (iii) the freezing of supercooled waters emergingfrom basal overdeepenings (Alley and others 1998 1999Lawson and others 1998) We therefore infer from the pat-terns we record at austre Brggerbreen that temperate basalconditions existed upflow of marginal freezing conditions atthe time of the formation of the ice now exposed in the lon-gitudinal debris band at the glacierrsquos surface Since austreBrggerbreen is currently largely cold-based (Hagen andStrang1991 Hagen and others1991a) it is likely that thesebasal ice sub-facies formed 4100 years ago when the gla-ciers of the area were generally thicker and more dynamicthan at present (Glasser and Hambrey 2001)

The discontinuous sub-facies debris band is isotopicallysimilar to glacier ice at austre Brggerbreen and both areisotopically heavier than the solid sub-facies debris bandsampled at the glacierThe isotopic similarity of the discon-tinuous sub-facies to the glacier ice must be explained in thelight of the size of the sample collected relative to the scale ofindividual freezing events (the latter being a unit of iceformed from a closed and isotopically uniformwater body)Since the discontinuous sub-facies at austre Brggerbreencontains millimeter-scale laminae and the ice screw usedto sample it was 10mm in diameter no isotopic enrich-ment would be expected if the sub-facies formed by closed-system refreezing of water that was isotopically similar tocurrent glacier ice (Jouzel and Souchez1982 Hubbard andSharp 1993) This and the physical structure of the discon-tinuous sub-facies are consistent with initial formation byWeertman regelation (Weertman 1964) implying that theice formed in an area of the glacier bed that was temperateand probably bedrock-based (Kamb and LaChapelle1963Hubbard and Sharp1993)

Two interpretationsmaybe advanced for the relative iso-topic lightness (by1 in 18O) of the solid sub-facies rela-tive to glacier ice at austre Brggerbreen First the sub-faciesmay have formed by the open-system freezing of sourcewater that was at the time of formation gt1 lighter in18O than current glacier ice Second the sub-facies mayhave formed by the closed-system freezing of source waterthat was at the time of formation1 lighter in 18O thancurrent glacier ice In the latter case for isotope samples tobe of the restricted range in 18Omeasured the scale of eachfreezing event would have tobe smaller than our sample size(10mm vertically) This is unlikely given the massive andundifferentiated nature of the solid sub-faciesWe thereforefavour formation of the solid sub-facies ice at austre Brg-gerbreen by the open-system freezing of water that was atleast 1 lighter in 18O than current glacier ice Howeverthese competing hypotheses can really only be evaluatedwith confidence in the light of more ice and water samplesfrom the glacier particularly from its base

CONCLUSIONS

Physical properties of debris bands fromwhich supraglacialmoraine ridges are formed suggest all transverse bands and

some longitudinal bands are sourced from the glacier bedThe sedimentology and isotopic composition of transversebands indicate formation from pre-existing basal ice thathas been elevated with little bulk modification into an en-glacial position Our evidence suggests the process respon-sible for this elevation is more likely to be related tothrusting than to the filling of basal crevasses

Longitudinal debris bands can also be sourced from theglacier bed and one such bandwas observed at austre Brg-gerbreen to be formed of two distinct sub-facies Isotopicanalysis of these sub-facies indicates that the glacier waspolythermal with a temperate interior and a frozen mar-gin at the time of basal ice formation

ACKNOWLEDGEMENTS

We thank T Knudsen (University of Aarhus Denmark)and C Hammer (Geophysical Isotope Laboratory Copen-hagen University) for arranging the isotope sample analy-ses We also thank D Evans and D Lawson forcommenting on the manuscript as a result of which it hasbeen greatly improved This work was partly funded by aUK Natural Environment Research Council (NERC)grant (GST022192) JE acknowledges funding by NERCstudentship NERSA200003690

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