Icing Blister Development on Bylot Island, Nunavut, Canada

Icing blister development on Bylot Island, Nunavut, Canada

F.A. Michel & S.P. Paquette Earth Sciences, Carleton University, Ottawa, Ontario, Canada

ABSTRACT: Ice blisters were discovered forming in a small valley surrounded by alpine glaciers on southern Bylot 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 displayed a pattern of progressive downward freezing. This indicates that it was formed under near equilibrium conditions in a closed 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 isotopic data demonstrate that icing blisters develop in a similar manner to frost blisters. The icing blisters have no soil cover, but instead rely on previously formed aufeis to provide a confining layer for the injected water. Carbonate precipitate 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 mounds discovered within the annual icing are not formationally related to the icing blisters.

1 INTRODUCTION differentiate between icing blisters and icing mounds, to determine the hydrological source of the icing blis- Icing blisters are mounds formed by the seasonal freez- ter water, and to determine the mechanism of forma- ing of water injected under pressure into aufeis, and tion; a single versus multiple pulse of water injection. differ from icing mounds, which contain an accumulation of thinly layered ice formed by water discharging from below river ice or from the ground. Icing blisters 2 STUDY AREA are similar in form to frost blisters except that they are not covered by a layer of seasonally frozen ground (van The study area for this research was focused on a small Everdingen 1978). Although frost blisters have been east-west oriented valley located on the southern part of widely reported in the permafrost literature (Muller Bylot Island, directly across from Pond Inlet (Figure 1). 1943, Bogomolov and Sklyarevskaya 1969, van The upper (western) portion of the valley contains Everdingen 1978, Pollard 1983, Michel 1986), icing Fountain Glacier, formally known as Glacier B26, blisters have received little attention (Muller 1943, van while Sermilik Glacier and its moraines essentially Everdingen 1978, Froehlich and Slopik 1978). block the lower (eastern) end of the valley. Two small Icings often develop in river valleys where ground- glaciers, B28 (Stagnation Glacier) and B30, occupy water discharges through taliks and river ice during tributary valleys and supply meltwater from the north periods of subzero air temperatures. For an icing blister side of the valley. to form, some of this water must become localized The floor of the 7 km long main valley is entirely within the icing and not be directly connected to a con- covered by aufeis; the uppermost one km is covered duit permitting flow to the icing surface. by a perennial icing, while the remainder of the valley As with frost blisters, the development and growth contains annual aufeis that disappears each summer to of an icing blister is expected to be a two-stage pro- expose a cobble and coarse gravel filled floor dissected cess. During the first stage, the aufeis would heave by a braided stream network (Elver 1994). The peren- rapidly as the hydrostatic pressure of the water exceeds nial icing is up to 12 metres in thickness while the the overlying lithostatic load of the ice. Slower growth annual icing thins down valley from 9 metres adjacent would occur during the second phase as the injected to the perennial icing, to less than 1 metre in thickness water gradually freezes with a 9% volume expansion. near Sermilik Glacier. Rupture (cracking) of the blister during either stage of The mean annual temperature for the area freezing could lead to water loss and collapse. is 14.7°C, with extremes of 53.9°C to 20.0°C Field investigations on southern Bylot Island iden- recorded across Eclipse Sound at Pond Inlet (AES tified the existence of a number of mounds associated 1982). Temperatures are above 0°C from June through with icings developed downstream from the snout August, although snow is possible at any time of the of alpine glaciers. The aims of this study were to year. Over 75% of the precipitation falls during the

759 period of May to October, with July to September Laboratory of the Ottawa-Carleton Geoscience Centre being the wettest months (50% of annual precipita- in Ottawa, Canada. The FI92-6 samples were also tion). The ground is normally snow free by mid June. analysed at the isotope laboratory of the Estonian Academy of Sciences in Tallinn for comparison. Reproducibility of results is 0.2‰ for d18O and 1‰ 3 METHODS for d2H analyses.

In the summer of 1992, a domed mound (FI 92-6) was exposed along a stream channel cutting through the 4 ISOTOPE FRACTIONATION DURING annual icing below the confluence of surface waters FREEZING discharging from Fountain and Stagnation Glaciers (Figure 1). Ice samples were collected at 10-cm inter- The relative abundances of the various stable isotopes vals from a vertical section cut through the 1.6 metre of oxygen and hydrogen in precipitation fluctuate with thick ice section. In addition, a sample of a creamy the seasons due to a variety of factors (Dansgaard white precipitate layer, found at a depth of 90 to 91 cm 1964). On a global basis, Craig (1961) found that pre- within the ice, was collected. cipitation values define a meteoric water line (GMWL) In 1993, a large number of domed mounds were with the relationship: found throughout the central section of the annual icing. Two mounds (FI93-4 and FI93-5), located about 218HO80. (1) 150 metres north of the FI92-6 site, were sectioned, described and sampled from the top of the ice to the Precipitation collected at any given site over the underlying gravel. The ice in this area was 1.25 to 2.1 length of a year will form a local meteoric water line metres thick. Four domed mounds were also located (LMWL) that is close to the GMWL but usually with in the perennial icing adjacent to the terminus of a slightly lower slope. Moorman et al. (1996) defined Fountain Glacier; two of these mounds were cored. In the LMWL for Pond Inlet as being the same as the addition, samples of local streams, lakes and precipi- GMWL. tation were collected for comparison. Groundwater will usually possess an isotopic com- All ice samples were allowed to melt in closed plastic position that closely reflects the average annual pre- bags before being transferred into 25 or 50-ml polyeth- cipitation input, which in permafrost regions ylene bottles for shipment to Carleton University. correlates with fall precipitation. As air temperatures Samples were analysed for their oxygen-18 and deu- drop below 0°C, reduced melting of glaciers and terium isotope concentrations at the G.G. Hatch Isotope snowpack results in decreased stream flow. However,

Figure 1. Location map of study area on southern Bylot Island.

760 this effect is delayed when the streams are fed by water from the internal plumbing of glaciers, lake water, or groundwater discharge. Depending on the size of the system involved, flow may continue throughout the winter (e.g., Pollard 1991). Decreased air temperatures also cause the surface water to freeze gradually as it flows, resulting in the formation of icings (Slaughter 1990). As water freezes, the heavy isotopes, 18O and 2H, are preferentially incorporated into the solid ice phase, while the residual liquid becomes depleted. The fractionation factors for 18O and 2H are 1.0028 and 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 isotopic composition, the isotopic composition of the ice remains relatively constant throughout the thickness of the ice mass (e.g. lake ice), but is shifted relative to the composition of the source water. A similar uni- form profile would form during rapid freezing; however, negligible fractionation would occur and thus the ice and original water would have a similar isotopic composition. On the other hand, ice formed in equilibrium with water in a closed system (slow freezing of a diminish- ing reservoir) would result in a progressive depletion of heavy isotopes in the residual water. Michel (1986) described this process for the growth of frost blisters. He also demonstrated that the bulk isotopic composition of ice formed in a closed system would reflect the original composition of the source water. Icing blisters 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 signatures Figure 2. Isotopic and stratigraphic profiles for FI92-6 for the two types of blisters. However, to date no iso- (a) and IB93-5 (b) shown in Figure 1. topic analysis of icing blisters has been reported.

paste layer separated the milky ice from 9 cm of 5 RESULTS AND DISCUSSION massive clear ice that contained carbonate inclusions in the upper 2 cm. Below 1 metre, ice types alternated 5.1 Mound morphology between candled layers and milky or clear massive layers. All of the mounds examined had a similar domed Sectioning of FI93-5 (Figure 2b), located appro- shape, although the size varied; diameters ranged ximately 150 m north of the FI92-6 site, exposed a from 1 to 10 metres and height above adjacent icing 1.15-m thick mound capped by 2 to 3 cm of coarse from 0.25 to 1.5 metres. However, the internal struc- crystalline snow. The upper 60 cm contained candled ture of the sectioned mounds in the central annual ice, which was underlain by 20 cm of massive ice. icing area differed substantially. Below the massive ice was a soft dirty brown snow The interior of mound FI92-6 was exposed by a with harder poorly candled ice from 110 to 115 cm. stream dissecting the icing to the underlying gravel. Icing mounds located within the perennial icing The crest of the mound rose about 0.5 metres above were larger than those down valley. The two investi- the surrounding icing. The internal structure, shown gated mounds contained an ice cap (approximately in Figure 2a, contained a series of layers of candled 1.0 to 1.3 m thick) overlying a 2.4 to 2.6 metre deep ice and clear massive ice. The upper arched 75 cm water-filled cavity. Thus, they were still in the process contained only candled ice, which changed to massive of stage 2 growth where the injected water was begin- milky ice from 75 to 90 cm. A 1-cm thick carbonate ning to freeze.

761 5.2 Isotope composition the range for aufeis in the valley as reported by Elver (1994). Variation in 18O composition with depth is displayed Although there is a trend to more negative d18O val- in Figure 2a for FI92-6. The graph shows that the d18O ues with depth, the stratigraphy of the mound sug- values become progressively more negative to a depth gests that freezing of injected water did not form it. of 90 cm. Upward freezing is indicated from 100 to The core of the mound is an accumulation of drifted 90 cm. Below 100 cm, the 18O composition is rela- snow that has been encased by aufeis. The thin layer tively constant, with an average of 22.7‰ 1‰, of massive ice overlying the snow core probably and is similar to average aufeis values (Elver 1994). formed by water saturation of the uppermost snow Hydrogen isotope analyses yielded a similar picture. and rapid freezing. The candled ice above is normal The profile indicates that as freezing progressed aufeis that has accumulated by the accumulation of downward, the ice was enriched in 18O (and 2H) relative relatively thin layers of slush and water. Thus, this to the cavity water, similar to that found by Michel second group of mounds was not formed by water (1986) for frost blisters. Isotope fractionation follows injection and cannot be classified as icing blisters. the Rayleigh distillation process and allows the data curve to be compared with theoretical fractionation curves. This type of analysis for FI92-6 by Paquette 6 CONCLUSIONS (1999) found that the 10 to 40 cm interval formed under equilibrium (slow freezing) conditions (Figure 3). At Isotopic profiling and stratigraphic analysis demon- this point, the blister ruptured and lost some of its resi- strate that icing blisters do in fact form like frost blis- dual water. Freezing of the smaller water reservoir ters, with heave due to injection and freezing of a pool resulted in a negative shift in isotope composition to of water confined within the icing. a new equilibrium curve for freezing of the remaining Icing mounds found on southern Bylot Island can water. be subdivided into two groups. The isotope profile of The layer of carbonate precipitate indicates that the one icing blister studied recorded an episode of rup- residual water became saturated with respect to cal- ture and partial water loss during freezing. The cite during the final stages of freezing. Based on geol- remaining blisters examined were still in the process ogy, the only source of carbonate is groundwater of forming and contained a water filled cavity. The discharging from Cretaceous/Tertiary sediments on water source for the icing blisters is subsurface water the south side of the valley. discharging through the icing. A carbonate precipitate By comparison, the d18O-depth profile for IB93-5 found in FI92-6 suggests that groundwater from the (Figure 2b) shows a different history of formation. south side of the valley is the source. The uppermost snow sample and the deeper dirty A second group of icing mounds contains aufeis that brown snow interval (75 to 105 cm) yielded the most encases a core of snow. These mounds do not involve negative d18O values. The remainder of the ice in the water injection. Their isotope profile differentiates mound is candled and fluctuates in 18O composition between the snow and aufeis, and shows that sequen- between 20.5 and 26.5‰. This is again similar to tial freezing of an isolated water pocket did not occur.

ACKNOWLEDGEMENTS

Financial support for this research was provided to the senior author by NSERC and logistical support was provided by PCSP. The authors would also like to thank Dr. Rein Vaikmae for his assistance in the field and for the additional isotope analyses. Our thanks are also extended to all of the students who participated in the Carleton field program, especially Mark Elver and Dr. Brian Moorman. We would also like to thank the two reviewers for their constructive comments.

REFERENCES

Figure 3. Comparison of isotope depletion in FI92-6 AES 1982. Canadian Climate Normals, 1951–1980. profile with theoretical Rayleigh distillation curve for an Temperature and precipitation. The North – Y.T. and equilibrium fractionation factor of 1.0028. N.W.T., Environment Canada. 55p.

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