An extensive permanent snowfield and the possible occurrence of permafrost in till in the...

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AN EXTENSIVE PERMANENT SNOWFIELD AND THE POSSIBLEOCCURRENCE OF PERMAFROST IN TILL IN THE RIDNIT VSOHKKA

AREA, FINNISH LAPLAND

HEIKKI HIRVAS, PETRI LINTINEN and PEKKA KOSLOFF

HIRVAS, HEIKKI, LINTINEN, PETRI and KOSLOFF, PEKKA 2000. An ex-tensive permanent snowfield and the possible occurrence of permafrost in tillin the Ridnit vsohkka area, Finnish Lapland. Bulletin of the Geological Societyof Finland 72, Parts 1–2, 47–56.

An area of permanent snow and frozen ground was studied at the end of sum-mer seasons during the years 1990–1993 near the summit of Ridnit vsohkka fell,which at 1317 m a.s.l. is the second highest peak in Finland. The eastern flankof Ridnit vsohkka has the most extensive area of permanent snow (3 km2) in Fin-land, while the summit region represents the highest single area of basal till de-void of vegetation cover and displaying patterned ground features.

The ground penetrating radar results show that the form of the snowfield doesnot conform to the topography of the underlying bedrock and reveal the pres-ence of several continuous reflectors dipping down-slope. Observations basedon drilling and a single test pit indicate that the snowfield is 6.2 m thick andthat it contains discontinuous lenses or layers of ice from 2 to 30 mm in thick-ness. Plant material from the depth of 4.05 m in the drilling core yielded a radio-carbon age of 35 years, while variations in the size of lichens growing at theedge of the snowfield indicated that the snowfield was of considerably greaterextent some 100 to 150 years ago. The results suggest that the snowfield hasbeen stable for a relatively long period but the “residence time” of snow in thesnowfield is rather short, presumably less than 100 years and possibly only afew decades.

Percussion drilling on the Ridnitvsohkka summit plateau, situating 1290 m a.s.l.,revealed that the till was frozen at the depth of 1.9 m at the end of July 1993.Discovery of frozen till shows that frozen ground can locally survive from oneseason to the next, while the extent of the permanent snowfield and vegetation-free patterned ground suggest that the conditions favourable for permafrost mighthave existed over wider areas.

Key words: periglacial features, snow, firn, permafrost, till, drilling, Ridnit-vsohkka, Lapland, Finland

Heikki Hirvas and Petri Lintinen: Geological Survey of Finland, P.O. Box 96,FIN-02151 Espoo, Finland.E-mail: [email protected]; [email protected]

Pekka Kosloff: Finnish Institute of Marine Research, P.O. Box 33, FIN-00931Helsinki, Finland.E-mail. [email protected]

48 Heikki Hirvas, Petri Lintinen and Pekka Kosloff

INTRODUCTION

Perennial snowfields occur in some favourablesites in Finnish Lapland. The largest number ofperennial snowfields is situated in the east ornorth-east facing gullies and nivation hollows infell slopes at the north-western part of the Enon-tekiö district. In this area many of the fell sum-mits exceed the elevation of 1000 m above sealevel. According to Østrem (1964), the theoreti-cal glaciation limit, i.e. the minimum altitude re-quired for glaciers, in the north-western part of theEnontekiö district is 1450 m a.s.l., which is rela-tively close to the altitude of the highest fell sum-mit in Finland.

Northern Lapland is classified climatological-ly as belonging to the zone containing sporadicoccurrences of permafrost (Péwé 1979), with amean annual temperature of between 0°C and–3°C (Helminen 1987). The presence of palsabogs is the most diagnostic indication that perma-frost exists at least locally in northern Lapland andindeed the presence of palsa features is common-ly taken to delineate the southernmost occurrenceof permafrost conditions (Fig. 1). Palsa landformscan be traced across northernmost Finland fromjust north of Lake Inari through Hietatievat andfurther westwards at a latitude of 68°25’ to theSwedish border (King & Seppälä 1987).

Electrical conductivity measurements carriedout on several summits above the tree line, wherewinter snow cover is relatively thin, have beenpostulated to indicate the presence of permafrosthorizons which may be as much as 10–50 m thick(King & Seppälä 1987). The thickness of the ac-tive permafrost layer in Finnish Lapland has alsobeen estimated by measuring temperature gradi-ents (Putkonen 1990) and the temperature datafrom Lapland have been compared with tempera-ture measurements from the active permafrost lay-er in the Disko area of Greenland. The interpre-tations by Putkonen (1990) were somewhat at var-iance with those of King and Seppälä (1987)pointing only to the existence of seasonal frost atsome sites. However, permafrost was explicitlyobserved on the fell summit of Ylläs in a drilledwell being frozen at 40 m depth below the bed-

rock surface (King & Seppälä 1987).During the course of till stratigraphic investi-

gations carried out by the Geological Survey ofFinland in the summer of 1974, six study pits wereexcavated along a road trending north-eastwardsfrom Kilpisjärvi (Hirvas 1990). The study pitswere excavated between 10th and 13th July, atelevations of between 490 m and 545 m and eventhough the deepest pit was excavated down to7.5 m, no signs of permafrost or frozen till werefound anywhere. Subsequent studies have likewisefailed to find direct confirmation of the presenceof frozen minerogenic ground in the proximity topalsa bogs in late summer. However, during theroad construction work at Peera, near Kilpisjärvi,peat and glacilacustrine silt underlying the roadstructure are expected to be permanently frozendue to the artificially induced heat conductionthrough the snow-free road surface (Saarelainen1990).

The purpose of our investigations has been tostudy the permanent snow and describe perigla-cial phenomena on the second highest fell in Fin-land. This area is characterized by the harshestclimatic conditions in the country and containsboth the largest permanent snowfield, 3 km2 insize (Fig. 2), and an extensive area of patternedground devoid of vegetation. A programme in-volving radar survey, drilling and excavation ofstudy pits was undertaken in order to study thehistory and internal structures of the snowfield.Drillings were also conducted to find direct evi-dence for frozen minerogenic ground at the endof the summer season, which would at least inprinciple mean that permafrost might be found onthe highest and most exposed summits in north-ernmost Lapland.

STUDY AREA

The Ridnit vsohkka fell is located in the north-west-ern extremity of the Enontekiö district, adjacentto the Norwegian border (Fig. 1). The bedrock inthe region consists primarily of allochthonoussheets of early Palaeozoic rocks that were thrustsouth-eastwards over the Fennoscandian Shield

49An extensive permanent snowfield and the possible occurrence of permafrost…

and a thin Cambrian sedimentary sequence dur-ing the Caledonian orogeny, some 400–500 mil-lion years ago. The allochthonous units dip gen-tly towards the north-west and extend for onlyabout 10–20 km into Finland from the Norwegianborder (cf. Simonen 1980, Lehtovaara 1995).

The Kilpisjärvi-Ridnit vsohkka area contains allof the Finnish peaks that have elevations greaterthan 1000 m above sea level. Relatively broad, yetsteep-sided valleys separate the higher peaks fromone another, resulting in the greatest topographi-cal relief to be found in Finland, with 200–500 m

Fig. 1. The location of the Ridnit vsohkka fell in Northern Finland, Lapland. The southern boundary of palsa bogsand the potential southern boundary of sporadic permafrost runs from north of Lake Inari through Hietatievatand further westwards at a latitude of 68°25’ to the Swedish border (King & Seppälä 1987).


differences in elevation between summits and val-ley floors being typical.

Various morphological features in the Halti-Ridnit vsohkka area are indicative for past moun-tain glaciation, untypical for the other regions ofFinnish Lapland. For example, the steep-sidedKovdajohka valley has evidently been occupied bya 5 km long valley glacier. The terminal positionsof this glacier have been recorded by a distinct setof end-moraines (Hirvas 1968). Also the presentwell-developed cirques, e.g. in the north-east faceof Kovddoskaisi and south-east slope of Rid-nit vsohkka, with associated end-moraines, are firmevidences for former mountain glaciers. Accord-ing to Kujansuu (1967, 1992), the mountain gla-ciers were active after the Scandinavian ice sheethad retreated to the south some 9000–10 000 yearsago.

There are no climatic data available for thesummit of Ridnit vsohkka itself, but inferred aver-age temperatures can be estimated from the data

recorded at Kilpisjärvi (478 m above sea level).The mean annual temperature for Kilpisjärvi is–2.6°C, while for January it is –14.6°C and forJuly +10.6°C (Finnish Meteorological Institute1991). On the basis of a decrease of 0.5°C forevery 100 m increase in altitude (Laaksonen1976), an annual mean temperature of –6.8°C isobtained for Ridnit vsohkka, with January and Julymean values of –18.8°C and +6.4°C, respectively.

The summit and upper slopes of Ridnit vsohkkaare in many places devoid of vegetation and showextensive active patterned ground, exhibiting well-developed polygonal features and evidence ofsolifluction. Evidence for contemporary patternedground formation includes the abundance of boul-ders and rocks lacking any traces of lichen growthand conversely, the presence of lichen on the un-derside of inverted boulders.

A series of distinctive solifluction terraces orlobes are developed in till on the eastern flank ofRidnit vsohkka. Further down-slope, at an elevation

Fig. 2. An oblique aerial photograph from the eastern slope of Ridnit vsohkka. The snowfield is about 5 km longand 600 m broad. The drilling location is marked with X.


of between 970–850 m, there are some 30 subpar-allel till terraces of which the largest are 2–4 mhigh and up to a kilometre in length. The upper-most parts of these till terraces are free of plantcover and very rocky, whereas lower down theyare covered by vegetation and have a typical lo-bate morphology (Fig. 3). The disposition of boul-ders on the upper slopes and the presence of patch-es of disagregated till material on top of someboulders indicate that solifluction processes areactively operating in the area.

STUDY METHODS

The thickness and internal structures of the Rid-nit vsohkka snowfield were elucidated with the aidof ground penetrating radar, using a frequency of500 MHz. The survey was carried out with a Sir-3equipment along three separate down-slope pro-files. Targets for drilling were selected on the basis

of the survey results, in areas where the snow cov-er was deepest and where the basal part of the pro-files showed prominent reflectors (Fig. 4).

Reconnaissance drilling was carried out to ac-curately ascertain the thickness of the snowfield,using a 50 mm diameter drill, while a continuoussection of core material was obtained with aCRREL ice-coring drill. The drilling site is locatedin the south-eastern part of the snowfield, aboutmidway along the slope, at an elevation of 1219 mabove sea level (Fig. 2). The snow cores weresealed in airtight plastic bags prior to storage infrozen carbon dioxide in insulated boxes.

Samples were sawn to size suitable for densitymeasurements and texture analysis at the Techni-cal Research Centre of Finland. The ice sampleswere photographed and the continuity of layeringand the distribution and shapes of air bubbles inthe sawn slices were examined.

A 2 × 2 m test pit 3.5 m deep was dug next tothe drill site in order to document the stratigraph-

Fig. 3. An example of solifluction terrace on the eastern slope of Ridnit vsohkka. One individual terrace can betraced nearly 1 km. In minor scale the terraces have morphology of numerous lobes.


ical and density variations with depth. The in situdensity measurements were performed with a Kor-honen-Melander ice/snow balance only on theupper part of snow due to too hard snow in thelower part of the test pit. Organic material foundat a depth of 4.05 m within the core was dated atthe Uppsala University using the AMS radiocar-bon method.

Possible variations in the extent of the snow-field over time were assessed by comparing a se-ries of aerial photographs taken in 1960, 1969,1975, 1979, 1985 and 1991, and also by measur-ing the diameter of lichen in the proximity to thesnowfield. The distribution of lichen growth ofdifferent size was determined visually at first, andseveral zones more or less concentric with respectto the margin of the snowfield were identified. Ateach site the diameters of 100 lichens growing onadjacent boulders were measured. Although noattempt was made to identify specific taxa, the li-chen measured evidently belonged to the speciesRhizocarpon geographicum and R. alpicola.

The till cover at the summit plateau of Rid-nit vsohkka was drilled in two places using a port-able Cobra percussion drill. The drilling sites wereselected in the centre of a patterned ground area

in the middle of nonsorted polygons free of veg-etation cover. The proportion of stones and mois-ture content, and the possible presence of frozenground were determined from penetration rate andresistance during drilling. Excavations were alsocarried out to obtain further information about thephysical properties of the till.

RESULTS

The earliest written records of a permanent snow-field near the summit of Ridnit vsohkka date fromthe latter part of the 19th century (Stjernvall 1892).A continuous snowfield is present at elevations ofbetween 1270–1180 m on the northern and east-ern flanks of the fell, with a maximum width of600 m and a length of nearly 5 km.

The results of the ground penetrating radar sur-vey revealed that the surface of the snowfield doesnot conform to that of the underlying topography.The radar profile in Fig. 4 shows a distinct seriesof reflections that dip down-slope towards theedge of the snowfield. The results of drillingshowed that the snowfield is 6.2 m thick at thepoint indicated on the profile.

Fig. 4. Ground penetrating radar profile of the Ridnit vsohkka snowfield. The sounding profile is a down-slopeprofile in which the upper slope is located at the left-hand side. Note the strong reflections dipping to the lowerflanks of the snowfield, which according to drilling results are caused by ice lenses. The surface of the snowfieldis drawn as horizontal in the profile, but it actually dips to the right at about 25 degrees. At the core site thethickness of the snowfield was 6.2 m.


The study pit revealed the presence of distinctlenses and layers of ice from 0.2 to 3.0 cm thick.These were generally discontinuous and conform-able with the surface of the snowfield. More con-tinuous layers that could be traced from one wallto another were found towards the base of the ex-cavation. In some cases a narrow vertical wall ofice was present, joining layers of ice at differentlevels and resulting in a reticulate network of icelenses surrounded by less crystalline snow. Thestacked ice lenses were typically about 5–15 cmapart, such that up to 35–45 lenses were presentin the excavation walls.

Stratigraphical information from the basal partof the snowfield was obtained by analysis of drillcore, although the dip and strike of the ice lensesremained unknown because it was not possible totake oriented samples. The core had a distinctlybanded structure, formed by numerous laminae ofice 2–20 mm thick (Fig. 5). The interface betweenthe ice lenses and snow was sharp. Ice was clear-er than snow and a few air bubbles were observedwithin the ice lenses. The diameter of the singleair bubbles was 1–2 mm. The shape of single airbubbles was rounded, but normally air inclusionswere polymorphic, because the air inclusions werecomposed of several air bubbles. The laminaeform an angle of about 25 degrees with the longaxis of the core and because the ice layers exposedin the adjacent excavation dip parallel to the sur-face, it is probable that the layering in the deepermaterial sampled by drilling is also broadly con-cordant. At the depth of 3,80 – 3,95 m some lightvertical ice lenses were observed. These layers areresults of water flow through the snowfield. Thebottom part of the snowfield consists of tightlycompressed but wet recrystallized snow/ice, un-derlain by frost-shattered angular clasts of localbedrock.

Density measurements carried out on the drillcore and in the study pits revealed a range of val-ues, from 540 to 670 kg per cubic metre (Fig. 6),which according to Paterson (1994) falls withinthe typical range of densities for firn. Densitymeasurements were not performed from the sep-arate ice lenses. Density appears to increase down-wards, which is in agreement with the concomi-

tant increase in the abundance of icy layers andlaminae with depth.

The remains of grass and leaves found at adepth of 4.05 m in the drill core were dated asbeing 35 years old. The amount of radioactivecarbon in sample Ua-2886 was so high that it mustpost-date the earliest atmospheric nuclear tests,since the value obtained, A = 109.7 ±1.1 pM, cor-responds to the mean atmospheric radioactivity inthe years 1957–1958. This result receives inde-pendent confirmation in that a small piece ofrounded styrofoam (“styrox”) was also found atthe same level as the plant remains, and styrofoamwas not widely used in Finland prior to 1952.

During the summers of 1991 and 1992 the sur-face of the snowfield was exceptionally clean,whereas in 1993 the surface was covered by fine-grained inorganic and organic debris, either wind

Fig. 5. Ice layers at the depth of 3.20–3.35 m in a corefrom the Ridnit vsohkka snowfield. Note the ice lamina-tion containing air bubbles.


blown or as accumulations after snow melting andconsolidation. However, the study pit showed noreadily discernible evidence for intervals contain-ing dust or fine-grained organic matter that mightcorrespond to former snow surfaces. The remainsof leaves and grass encountered at a depth of4.05 m in the drilling profile evidently representan isolated example of exotic wind-blown mate-rial.

The comparison of aerial photographs taken inlate summer 1960, 1969, 1975, 1979, 1985 and1990 showed no distinct variations in the extentof the snowfield.

The boulder field immediately below the Rid-nit vsohkka snowfield is characterized by a 50–70 mwide zone of greyish boulders with only sparseand small growths of lichen, whereas below thiszone, lichen tends to be larger and merge togeth-er, giving an overall greenish appearance to therocks. Lichen diameters were measured within thezone of greyish boulders at distances of 5 m, 10 mand 50 m from the margin of the snowfield butthe results failed to show any significant differ-ences in the maximum lichen size at each of thethree sites; the largest were about 40 mm in di-ameter, while the median size range was 20–25 mm (Fig. 7).

A study pit was excavated by shovel to a depthof 0.9 m in the till cover of the Ridnit vsohkka sum-mit plateau. The sequence comprised massivesandy till with scattered large clasts and showedno evidence of frozen ground.

Drilling was carried out at two sites on the sum-mit plateau on 29–30 July 1993. The two siteswere situated in patterned ground and spacedabout twenty metres apart. The individual drillingsites were situated in the centres of nonsortedpolygons that were devoid of vegetation cover. Inboth cases the drill penetrated to a depth of 0.5 mwith minimal effort, the till at this level being sat-urated with water and leaving the drill rods ex-tremely wet and dirt-covered. Below this depthpenetration rate was slower and the material re-maining in the flow-through bit was dry. Penetra-tion rate slowed even further at 1.9 and 2.0 m, re-spectively, in both holes and ceased altogether at1.97 and 2.0 m. In both cases the drill bits con-tained 7–10 cm pieces of frozen till with numer-ous subparallel, gently dipping laminae of ice 1–2 mm thick.

CONCLUDING REMARKS

The ground penetrating radar results showed thatthe Ridnit vsohkka snowfield is situated in a bed-rock depression. The age determinations from or-ganic material obtained from a depth of 4.05 min the snowfield indicate that the snow is ratheryoung, being at most only a few tens of years old.

Fig. 6. Density of the drilling and study pit samples fromthe Ridnit vsohkka snowfield. All the densities measuredfall in the range of firn.


Reliable historical records show that the snowfieldwas in existence in the latter part of 19th century(Stjernvall 1892), suggesting that it is stable un-der the current climatic regime.

Investigations of lichen growth and distributionat Kebnekaisa in nearby Swedish Lapland suggestthat diameters of 40 mm correspond to ages of130–140 years (Karlén 1975). There are numer-ous uncertainties in comparing the Kebnekaiseresults with data from Ridnit vsohkka, such as var-iable growth rates of lichen on different substratesand the effects of microclimatic variations in tem-perature and precipitation. In general however,climatic conditions at Ridnitvsohkka resemble thoseat Kebnekaisa and the results of lichen studies canbe taken to infer that the Ridnit vsohkka snowfieldwas more extensive in the middle part the 19th

century than at present. This period correspondsto a documented event of the Little Ice Age, dur-ing which time a number of glaciers in Norwayand Sweden advanced significantly (Karlén 1982,1984).

The presence of a perennial snowfield, solifluc-tion terraces and actively forming patternedground devoid of vegetation cover all indicate theseverity of the current climatic regime at the sum-mit of Ridnit vsohkka. The discovery of frozen tillnear the summit in late summer also shows that

frozen ground can locally survive from one sea-son to the next, while the extent of the permanentsnowfield and vegetation-free patterned groundsuggest that permafrost might have existed overwider areas. The summit itself is commonly al-most free of snow during winter (P. Leinonen, oralcommunication), due to its exposed aspect, whichallows freezing to proceed to a greater depth thanin areas beneath a thick snow cover.

The frozen till found at the end of the summerseason at an elevation of 1290 m on Ridnit vsohkkais evidently the first record of at least semi-per-manent frozen ground at such altitudes in the fellcountry of northern Finland, the summits of fellsbeing in general either bare rock or frost-shatteredboulder fields. In the present case, however, theproximity of frozen ground and the largest perma-nent snowfield in Finland, together with associ-ated solifluction lobes and actively forming pat-terned ground, suggest that our results may alsobe a candidate for being the first recorded occur-rence of permafrost in till in Finnish Lapland.

ACKNOWLEDGEMENTS. We would like to ex-press our appreciation to all those people and or-ganisations who assisted us, in particular the To-pographic Service and the Finnish Air Force forhelicopter air-lifts, and Tele and the Finnish Bor-

Fig. 7. Typical lichens atthe bare rock zone aroundthe Ridnit vsohkka snowfield.


der Guard for making cabin accommodationavailable. We also thank the staff of the Kilpis-järvi Biological Research Station for allowing fro-zen storage of the sample material and the Tech-nical Research Centre of Finland for providinglaboratory facilities for sample investigation. TheFinnish Meteorological Institute also kindly al-lowed us to use a snow density meter.

REFERENCES

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Helminen, V.A. 1987. Temperature conditions. In: Alalam-mi, P. (ed.) Atlas of Finland, Folio 131, Climate. Na-tional Board of Survey and Geographical Society of Fin-land, Maanmittaushallitus, Helsinki.

Hirvas, H. 1968. Haltin alueen kvartäärisestä kehityksestä.Unpublished M.Sc. thesis, University of Helsinki, De-partment of Geology. 67 p. (in Finnish)

Hirvas, H. 1990. Pleistocene stratigraphy of Lapland. Ge-ological Survey of Finland, Bulletin 354. 123 p.

Karlén, W. 1975. Licenometrisk datering i norra Skandi-navien – metodens tillförlitlighet och regionala tillämp-ning. Stockholms Universitet, Naturgeografiska Institu-tionen, Raport 22. 67 p.

Karlén, W. 1982. Holocene glacier fluctuations in Scan-dinavia. Striae 18, 26–34.

Karlén, W. 1984. Dendrochronology, mass balance and gla-cial front fluctuations in northern Sweden. In: Mörner,N.-A. & Karlén, W. (eds.) Climatic Changes on a Yearlyto Millennial Basis. Dordrecht: D. Reidel PublishingCompany, 263–271.

King, L. & Seppälä, M. 1987. Permafrost thickness anddistribution in Finnish Lapland – results of geoeletricalsoundings. Polarforschung 57, 127–147.

Kujansuu, R. 1967. On the deglaciation of western Finn-ish Lapland. Bulletin de la Commission Géologique deFinlande 232. 98 p.

Kujansuu, R. 1992. The deglaciation of Finnish Lapland.In: Kauranne, K. (ed.) Glacial stratigraphy, engineeringgeology and earth construction. Geological Survey ofFinland, Special Paper 15, 21–31.

Laaksonen, K. 1976. The dependence of mean air temper-atures upon latitude and altitude in Fennoscandia (1921–1950). Annales Academiae Scientiarum Fennicae A. III119. 19 p.

Lehtovaara, J. 1995. Kilpisjärven ja Haltin kartta-alueidenkallioperä. Summary: Pre-Quaternary rocks of theKilpisjärvi and Halti map-sheet areas. Geological mapof Finland 1 : 100 000, Explanation to the maps of pre-Quaternary rocks, Sheets 1823 and 1842, GeologicalSurvey of Finland. 64 p.

Østrem, G. 1964. Ice-cored moraines in Scandinavia. Geo-grafiska Annaler XLVI, 282–337.

Paterson, W.S.B. 1994. The Physics of Glaciers. Third edi-tion. Oxford: Elsevier. 480 p.

Péwé, T.I. 1979. Permafrost and its affects on human ac-tivities in Arctic and Subarctic regions. Geo Journal 3,333–344.

Putkonen, J. 1990. Ikiroudan aktiivikerros. Summary: Per-mafrost active layer. Geologi 42, 161–166.

Saarelainen, S. 1990. Routavaurioiden korjausrakenteetvaltatiellä 21 Peerassa. Mitoitus, koerakentaminen jaseuranta v. 1986–1988. Summary: Repair structures offrost damages on main road 21 at Peera, Northern Fin-land. Design, construction and monitoring during 1986–1988. Technical Research Centre of Finland, ResearchNotes 1102. 62 p.

Simonen, A. 1980. The Precambrian in Finland. Geologi-cal Survey of Finland, Bulletin 304. 58 p.

Stjernvall, H.J. 1892. Bidrag till Finska Lappmarkens geo-gnosi II. Försök till framställning af de geologiska för-hållandena mellan Könkämäeno och norska gränsen.Meddelanden från industristyrelsen 17, Helsingfors, 95–146.

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