Icing blisters are mounds formed by the seasonal freez-ing of water injected under pressure into aufeis, anddiffer from icing mounds, which contain an accumula-tion of thinly layered ice formed by water dischargingfrom below river ice or from the ground. Icing blistersare similar in form to frost blisters except that they arenot covered by a layer of seasonally frozen ground (vanEverdingen 1978). Although frost blisters have beenwidely reported in the permafrost literature (Muller1943, Bogomolov and Sklyarevskaya 1969, vanEverdingen 1978, Pollard 1983, Michel 1986), icingblisters have received little attention (Muller 1943, vanEverdingen 1978, Froehlich and Slopik 1978).
Icings often develop in river valleys where ground-water discharges through taliks and river ice duringperiods of subzero air temperatures. For an icing blisterto form, some of this water must become localizedwithin the icing and not be directly connected to a con-duit permitting flow to the icing surface.
As with frost blisters, the development and growthof an icing blister is expected to be a two-stage pro-cess. During the first stage, the aufeis would heaverapidly as the hydrostatic pressure of the water exceedsthe overlying lithostatic load of the ice. Slower growthwould occur during the second phase as the injectedwater gradually freezes with a 9% volume expansion.Rupture (cracking) of the blister during either stage offreezing could lead to water loss and collapse.
Field investigations on southern Bylot Island iden-tified the existence of a number of mounds associatedwith icings developed downstream from the snout of alpine glaciers. The aims of this study were to
differentiate between icing blisters and icing mounds,to determine the hydrological source of the icing blis-ter water, and to determine the mechanism of forma-tion; a single versus multiple pulse of water injection.
2 STUDY AREA
The study area for this research was focused on a smalleast-west oriented valley located on the southern part ofBylot Island, directly across from Pond Inlet (Figure 1).The upper (western) portion of the valley containsFountain Glacier, formally known as Glacier B26,while Sermilik Glacier and its moraines essentiallyblock the lower (eastern) end of the valley. Two smallglaciers, B28 (Stagnation Glacier) and B30, occupytributary valleys and supply meltwater from the northside of the valley.
The floor of the 7 km long main valley is entirelycovered by aufeis; the uppermost one km is covered by a perennial icing, while the remainder of the valleycontains annual aufeis that disappears each summer toexpose a cobble and coarse gravel filled floor dissectedby a braided stream network (Elver 1994). The peren-nial icing is up to 12 metres in thickness while theannual icing thins down valley from 9 metres adjacentto the perennial icing, to less than 1 metre in thicknessnear Sermilik Glacier.
The mean annual temperature for the areais �14.7°C, with extremes of �53.9°C to �20.0°Crecorded across Eclipse Sound at Pond Inlet (AES1982). Temperatures are above 0°C from June throughAugust, although snow is possible at any time of theyear. Over 75% of the precipitation falls during the
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Icing blister development on Bylot Island, Nunavut, Canada
ABSTRACT: Ice blisters were discovered forming in a small valley surrounded by alpine glaciers on southernBylot Island, Nunavut. The entire valley floor is covered by either perennial or annual icings that originate from gla-cial melt water and precipitation. Stable isotope profiles with depth for a completely formed icing blister displayeda pattern of progressive downward freezing. This indicates that it was formed under near equilibrium conditions in aclosed system environment. A rapid shift in the profiles, relative to the theoretical isotope curves, indicates that a rupture and partial water loss with no injection of additional water occurred during the freezing process. The iso-topic data demonstrate that icing blisters develop in a similar manner to frost blisters. The icing blisters have no soilcover, but instead rely on previously formed aufeis to provide a confining layer for the injected water. Carbonateprecipitate found within the basal zone of a blister, and the local geology, suggest that the source water for this blister was from groundwater discharging along the southern margin of the valley. Other small snow-cored moundsdiscovered within the annual icing are not formationally related to the icing blisters.
period of May to October, with July to Septemberbeing the wettest months (50% of annual precipita-tion). The ground is normally snow free by mid June.
3 METHODS
In the summer of 1992, a domed mound (FI 92-6) wasexposed along a stream channel cutting through theannual icing below the confluence of surface watersdischarging from Fountain and Stagnation Glaciers(Figure 1). Ice samples were collected at 10-cm inter-vals from a vertical section cut through the 1.6 metrethick ice section. In addition, a sample of a creamywhite precipitate layer, found at a depth of 90 to 91 cmwithin the ice, was collected.
In 1993, a large number of domed mounds werefound throughout the central section of the annualicing. Two mounds (FI93-4 and FI93-5), located about150 metres north of the FI92-6 site, were sectioned,described and sampled from the top of the ice to theunderlying gravel. The ice in this area was 1.25 to 2.1metres thick. Four domed mounds were also locatedin the perennial icing adjacent to the terminus ofFountain Glacier; two of these mounds were cored. Inaddition, samples of local streams, lakes and precipi-tation were collected for comparison.
All ice samples were allowed to melt in closed plasticbags before being transferred into 25 or 50-ml polyeth-ylene bottles for shipment to Carleton University.Samples were analysed for their oxygen-18 and deu-terium isotope concentrations at the G.G. Hatch Isotope
Laboratory of the Ottawa-Carleton Geoscience Centrein Ottawa, Canada. The FI92-6 samples were alsoanalysed at the isotope laboratory of the EstonianAcademy of Sciences in Tallinn for comparison.Reproducibility of results is �0.2‰ for d18O and �1‰for d2H analyses.
4 ISOTOPE FRACTIONATION DURINGFREEZING
The relative abundances of the various stable isotopesof oxygen and hydrogen in precipitation fluctuate withthe seasons due to a variety of factors (Dansgaard1964). On a global basis, Craig (1961) found that pre-cipitation values define a meteoric water line (GMWL)with the relationship:
(1)
Precipitation collected at any given site over thelength of a year will form a local meteoric water line(LMWL) that is close to the GMWL but usually witha slightly lower slope. Moorman et al. (1996) definedthe LMWL for Pond Inlet as being the same as theGMWL.
Groundwater will usually possess an isotopic com-position that closely reflects the average annual pre-cipitation input, which in permafrost regionscorrelates with fall precipitation. As air temperaturesdrop below 0°C, reduced melting of glaciers andsnowpack results in decreased stream flow. However,
� � � �2 188 0H O.
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Figure 1. Location map of study area on southern Bylot Island.
this effect is delayed when the streams are fed by waterfrom the internal plumbing of glaciers, lake water, orgroundwater discharge. Depending on the size of thesystem involved, flow may continue throughout thewinter (e.g., Pollard 1991). Decreased air temperaturesalso cause the surface water to freeze gradually as itflows, resulting in the formation of icings (Slaughter1990).
As water freezes, the heavy isotopes, 18O and 2H,are preferentially incorporated into the solid icephase, while the residual liquid becomes depleted.The fractionation factors for 18O and 2H are 1.0028and 1.0206, respectively (Suzuoki and Kimura 1973).In an open system where there is a large and continu-ous replenishment of water with a constant isotopiccomposition, the isotopic composition of the iceremains relatively constant throughout the thicknessof the ice mass (e.g. lake ice), but is shifted relative tothe composition of the source water. A similar uni-form profile would form during rapid freezing; how-ever, negligible fractionation would occur and thus theice and original water would have a similar isotopiccomposition.
On the other hand, ice formed in equilibrium withwater in a closed system (slow freezing of a diminish-ing reservoir) would result in a progressive depletionof heavy isotopes in the residual water. Michel (1986)described this process for the growth of frost blisters.He also demonstrated that the bulk isotopic composi-tion of ice formed in a closed system would reflect the original composition of the source water. Icingblisters are believed to form in the same way as frost blisters, but without a frozen ground cover. Therefore,one would expect to find similar isotopic signaturesfor the two types of blisters. However, to date no iso-topic analysis of icing blisters has been reported.
5 RESULTS AND DISCUSSION
5.1 Mound morphology
All of the mounds examined had a similar domedshape, although the size varied; diameters rangedfrom 1 to 10 metres and height above adjacent icingfrom 0.25 to 1.5 metres. However, the internal struc-ture of the sectioned mounds in the central annualicing area differed substantially.
The interior of mound FI92-6 was exposed by astream dissecting the icing to the underlying gravel.The crest of the mound rose about 0.5 metres abovethe surrounding icing. The internal structure, shownin Figure 2a, contained a series of layers of candledice and clear massive ice. The upper arched 75 cmcontained only candled ice, which changed to massivemilky ice from 75 to 90 cm. A 1-cm thick carbonate
paste layer separated the milky ice from 9 cm of massive clear ice that contained carbonate inclusionsin the upper 2 cm. Below 1 metre, ice types alternatedbetween candled layers and milky or clear massivelayers.
Sectioning of FI93-5 (Figure 2b), located appro-ximately 150 m north of the FI92-6 site, exposed a 1.15-m thick mound capped by 2 to 3 cm of coarsecrystalline snow. The upper 60 cm contained candledice, which was underlain by 20 cm of massive ice.Below the massive ice was a soft dirty brown snowwith harder poorly candled ice from 110 to 115 cm.
Icing mounds located within the perennial icingwere larger than those down valley. The two investi-gated mounds contained an ice cap (approximately1.0 to 1.3 m thick) overlying a 2.4 to 2.6 metre deepwater-filled cavity. Thus, they were still in the processof stage 2 growth where the injected water was begin-ning to freeze.
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Figure 2. Isotopic and stratigraphic profiles for FI92-6(a) and IB93-5 (b) shown in Figure 1.
5.2 Isotope composition
Variation in 18O composition with depth is displayedin Figure 2a for FI92-6. The graph shows that the d18Ovalues become progressively more negative to a depthof 90 cm. Upward freezing is indicated from 100 to90 cm. Below 100 cm, the 18O composition is rela-tively constant, with an average of �22.7‰ �1‰,and is similar to average aufeis values (Elver 1994).Hydrogen isotope analyses yielded a similar picture.
The profile indicates that as freezing progresseddownward, the ice was enriched in 18O (and 2H) relativeto the cavity water, similar to that found by Michel(1986) for frost blisters. Isotope fractionation followsthe Rayleigh distillation process and allows the datacurve to be compared with theoretical fractionationcurves. This type of analysis for FI92-6 by Paquette(1999) found that the 10 to 40 cm interval formed underequilibrium (slow freezing) conditions (Figure 3). Atthis point, the blister ruptured and lost some of its resi-dual water. Freezing of the smaller water reservoirresulted in a negative shift in isotope composition to a new equilibrium curve for freezing of the remainingwater.
The layer of carbonate precipitate indicates that theresidual water became saturated with respect to cal-cite during the final stages of freezing. Based on geol-ogy, the only source of carbonate is groundwaterdischarging from Cretaceous/Tertiary sediments onthe south side of the valley.
By comparison, the d18O-depth profile for IB93-5(Figure 2b) shows a different history of formation.The uppermost snow sample and the deeper dirtybrown snow interval (75 to 105 cm) yielded the mostnegative d18O values. The remainder of the ice in themound is candled and fluctuates in 18O composition between �20.5 and �26.5‰. This is again similar to
the range for aufeis in the valley as reported by Elver(1994).
Although there is a trend to more negative d18O val-ues with depth, the stratigraphy of the mound sug-gests that freezing of injected water did not form it.The core of the mound is an accumulation of driftedsnow that has been encased by aufeis. The thin layerof massive ice overlying the snow core probablyformed by water saturation of the uppermost snowand rapid freezing. The candled ice above is normalaufeis that has accumulated by the accumulation ofrelatively thin layers of slush and water. Thus, this second group of mounds was not formed by waterinjection and cannot be classified as icing blisters.
6 CONCLUSIONS
Isotopic profiling and stratigraphic analysis demon-strate that icing blisters do in fact form like frost blis-ters, with heave due to injection and freezing of a poolof water confined within the icing.
Icing mounds found on southern Bylot Island canbe subdivided into two groups. The isotope profile ofone icing blister studied recorded an episode of rup-ture and partial water loss during freezing. Theremaining blisters examined were still in the processof forming and contained a water filled cavity. Thewater source for the icing blisters is subsurface waterdischarging through the icing. A carbonate precipitatefound in FI92-6 suggests that groundwater from thesouth side of the valley is the source.
A second group of icing mounds contains aufeis thatencases a core of snow. These mounds do not involvewater injection. Their isotope profile differentiatesbetween the snow and aufeis, and shows that sequen-tial freezing of an isolated water pocket did not occur.
ACKNOWLEDGEMENTS
Financial support for this research was provided to thesenior author by NSERC and logistical support wasprovided by PCSP. The authors would also like tothank Dr. Rein Vaikmae for his assistance in the fieldand for the additional isotope analyses. Our thanks arealso extended to all of the students who participated inthe Carleton field program, especially Mark Elver andDr. Brian Moorman. We would also like to thank thetwo reviewers for their constructive comments.
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