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Past and present permafrost as an indicator of change

Hugh M. French

The permafrost history of the high northern latitudes over the last two million years indicates that perennially frozen ground formed and thawed repeatedly, probably in close synchronicity with the climate changes that led to the expansion and subsequent shrinkage of continental ice sheets. The early stages of the are the least known and the changes that occurred in the Late Pleistocene and early Holocene are the best known. Evidence that permafrost is degrading in response to the current global warming trend is difficult to ascertain. The clearest signals are probably provided by changes in permafrost distribution in the sub- regions. at the extreme southern fringes of the discontinuous permafrost zone.

H. M. French, Depts. of Geography and Science, Universit?, of Ottawa, P.O. Box 450, Station A, Ottawa. Ontario KIN 6N5, .

Introduction Past permafrost

At high latitudes today, permafrost forms under Permafrost has undergone growth and decay at current climatic conditions. A map recently various times during Earth's history. There is compiled and published under the auspices of convincing evidence to suggest that much of The International Permafrost Association (Brown today's permafrost probably originated during et al. 1998) indicates that permafrost occupies the fluctuating climate of the Pleistocene. Some approximately 20-25% of the Earth's land surface of the most striking evidence includes the remains in the . In the case of the of woolly mammoths and other Pleistocene North American continent, field observations in animals found preserved in permafrost in , both Canada (e.g. Brown 1967; National Atlas of and north-western Arctic Canada. Another Canada 1995) and Alaska (e.g. Ferrians 1994) line of evidence is cryostratigraphic: in some indicate that the southern limit of continuous areas, the upper boundary of permafrost lies below permafrost coincides with the general position of the depth of modem seasonal freezing and the the -6 to -8°C mean annual air temperature temperature of permafrost sometimes decreases (MAAT) isotherm. This relates to a ground with increasing depth. Both phenomena indicate temperature of about -5°C measured just beneath residual (i.e. relict) cold. Another clue lies in the the depth at which seasonal fluctuations are fact that the thickest permafrost occurs in areas minimal. The discontinuous permafrost zone lies which escaped glaciation and which were not to the south of the continuous zone. Its southern protected from cold subaerial conditions by a thick limit approximates the - 1"C MAAT isotherm and ice cover. Finally, offshore or submarine perma- ground temperatures vary from just below 0°C to frost exists on the continental shelves beneath both -3 or -4°C. The most extreme, or southern, the Kara and Beaufort seas and could only have occurrences of permafrost are found beneath peaty formed when sea level was lower (i.e. during the materials, the result of the unusual insulating cold periods of the Pleistocene). properties of such material. Cryostratigraphic evidence for Pleistocene-age

French 1999: Polar Research 18(2), 269-274 269 Fig. I. Diagram illustrating the types of ice discontinuities commonly found within perennially frozen in (a) the continuous permafrost Seasonally zone of the Arctic. or the frozen layer - Thaw region, and (b) the unconformity Palaeo- discontinuous permafrost zone active Relict of the sub-Arctic region. layer active layer Residual thaw layer - Palaeo-thaw uncontorrni ty

- Palaeo-thaw unconformity

permafrost

V V [71 T 1 O’C (seasona//y [71 T~OOC(unfrozen thawed ground) .. ground)

T< O’C (perennially T~C(seasonally frozen ground) frozen ground) permafrost is provided by the study of the ice the simplest and most obvious example of a thaw discontinuities (e.g. Murton & French 1994; unconformity . French 1998) commonly found within perennially From the viewpoint of permafrost history, it is frozen sediments. Most permafrost, especially that possible to distinguish between primary (i.e. which has formed in unconsolidated sediments, present-day) and secondary (i.e. palaeo-) thaw contains ice. This may be as much as 30-50% by unconformities. Both are shown in (a), but in (b) a volume in the upper few metres. Discontinuities in palaeo-thaw unconformity overlies permafrost the ice are usually the result of either the thaw of unrelated to the present surface conditions. As frozen material or the subsequent refreezing of such, the permafrost is “relict.” previously thawed materials. The significance of The manner in which permafrost degrades and these discontinuities, termed “thaw unconformi- subsequently reforms, and the cryostratigraphic ties.” are explained below. evidence which it leaves, is illustrated in Fig. 2, Figure 1 shows the typical permafrost condi- which considers a scenario of permafrost tions which might exist in (a) the continuous being subject to warming and subsequent cooling. permafrost zone of the Arctic, or the tundra region, It depicts an initial permafrost sequence (a), that is and (b) the discontinuous permafrost zone of the subject to degradation from the surface down- sub-Arctic, or the boreal forest () region. In wards, possibly the result of regional climate (a), the “active layer” is shown as the surficial warming (b). As thaw proceeds, a primary thaw horizon of permafrost terrain which thaws during unconformity (T-Ul) forms at depth below a the summer months. A “relict active layer” is residual thaw layer. At this time, the ground shown as ground immediately below the modem surface experiences only seasonal freezing and active layer that was once part of the active layer thawing. In the process, an is truncated but which is now perennially frozen. A “palaeo- and is no longer active. When the climate active layer” is shown as the horizon between the subsequently deteriorates, as in (c), permafrost ground surface and the base of the relict active aggrades and the base of the active layer again layer. In (b), a “residual thaw layer” is shown as becomes the primary thaw unconformity. Re- referring to an unfrozen layer, formerly perma- newed thermal contraction cracking at the ground frost, lying between the modem depth of seasonal surface permits a new ice wedge to form. During frost penetration and an underlying permafrost this process, the original thaw unconformity at body. depth becomes a secondary (i.e. palaeo-) thaw The base of the current active layer, as in (a), is unconformity (T-U2). The latter can be recognized

210 Past and present permafrost as an indicator of Fig. 2. Diagram illustrating the cryoatmigraphic evidence associated with the degradation A and subsequent aggradation of Permafrost. (a) An initial pennafrost sequence is (b) layer subjected to degradation from the surface downwards. possibly the result of regional climate warming. As thaw T- 7 proceeds, a primary thaw U unconformity (T-U 1) forms at depth below a residual thaw layer. ic) When the climate -'0 suhsequently deteriorates, ". - permafrost apgrades and the base of the active layer again becomes the primary thaw unconformity. i"' ice wedge Permafrost - Permafrost- reticulate cryostructure lenticular cryostructure by both the truncated ice wedge and by different that the last interglaciation of the North American ice structures (cryostructures) in the sediments continent, about 12.5 000 years ago, was a major above and below. warm period when there was of and Using this kind of evidence, Russian geogra- the deep and rapid thawing of previously formed phers (e.g. Gerasimov & Velichko 1987,; Rozen- permafrost. During the l00Ky that followed, a baum & Shpolyanskaya 1998) have undertaken treeless steppe-like tundra environment returned to large-scale palaeo-environmental reconstructions northern Alaska and the deposits were refrozen as for the last two million years. For and permafrost once again developed. northern Eurasia they suggest four stages of In this time period, permafrost attained its permafrost evolution, the earliest being the least greatest thickness and its lowest temperature in known. those high-latitude areas of the Northern Hemi- During the earliest stage, between approxi- sphere that escaped glaciation. Thicknesses in mately 2.0-0.7 Mya. permafrost probably existed excess of 500 m formed with temperatures at the without interruption throughout much of Yakutia depth of zero annual amplitude of greater than and north-eastern Siberia. By contrast. in northern - IS -C. In addition, and in response to the lower Europe. western Siberia and mainland North sea level in the Arctic Basin, the continental America, permafrost probably formed and thawed shelves of the Beaufort and Kara seas, together on several occasions, but little is known of these with the Bering Strait, were exposed to perma- conditions. frost forming, cold-climate subaerial conditions. Between approximately 190 Kya and 10 Kya. At this time, the extreme maximum southern extensive regions dominated by permafrost con- limits of permafrost probably extended as far ditions existed once again in both Eurasia and south as 50" in European Russia and Kazakh- to the south of the Late Pleistocene stan. ice sheets. Isotopic data (Klimenko et al. 1996) During the Holocene, or the last 10 Ky, there is indicate that global air temperatures probably evidence to indicate that the climate ameliorated, dropped by as much as 4'C on several occasions causing permafrost to partially thaw but to then during the last 223 Ky. This was sufficient to cause subsequently refreeze towards the end of the significant expansion of areas underlain by Holocene. These conditions can be demonstrated permafrost at about 150 Kya, 70 Kya and 25 Kya. with reference to the lowlands of the western One example of all these changes is found near Canadian Arctic. There, observations indicate the Fairbanks in central Alaska. There, the Eva Forest existence of a thick palaeo-thaw layer (see e.g. Bed (PCwC et al. 1997) lies buried within frozen Burn 1988, 1997; Harry et al. 1988; Murton & loess-like materials. The organic remains indicate French 1994). It occurs at a depth of 125-1SOcm

French 1999: Pokir Reseurch 18(2). 269-274 27 1 Fig. 3. Perennially frozen sediments exposed at Crumbling Point. Pleistocene Mackenzie Delta. western Canadian Arctic. The early Holocene thaw unconformity truncates icy sediments penetrated by wedges. Photo by J. B. Murton.

in the region and can be recognized days above 0°C. Using such assumptions, the by distinct cryostructural contrasts and, in places, Stefan solution can be used to predict the depth of by truncated ice bodies. A good example is from thaw (see e.g. Mackay 1995; Burn 1997). Accord- the cliffs at Crumbling Point, in the Pleistocene ing to the Stefan solution, a doubling of active- Mackenzie Delta (Fig. 3). Recent years have layer thickness corresponds to a fourfold increase exposed a massive icy body penetrated by in thawing index, and an active layer 2.5 times as remarkably large inactive sand wedges and large thick as today implies an increase in the thawing active ice wedges, and overlain by sand and factor by 6.25. diamicton. Radiocarbon dates from here and This analysis can be applied to the climatic elsewhere in north-western Arctic Canada suggest records obtained from the five settlements of that the active layer was deepest about Whitehorse, , Tuktoyaktuk, 8.0-9.0 Kya and that it was approximately 2.5 and Rae Point, the latter being an oil company times thicker than the present active layer. A logistics base on eastern Melville Island (Table 1). similar early Holocene thaw unconformity can be These localities typically record thawing indices recognized on eastern Melville Island, at 77"N, at a today of 1900,1200,800,400 and 300 degree-days depth of approximately 113 cm (French et al. per year, respectively. The current active layer at 1986). each locality is approximately 150 cm, 100 cm, 50 cm and 30-25 cm, respectively. These data reflect the progressively shorter and cooler summers with increasing latitude. The current Permafrost and past climate treeline is located just north of Inuvik and south of the Arctic coast. However, during the early The early Holocene thaw unconformity in the Holocene climatic optimum, trees probably ex- Western Canadian Arctic can be used to illustrate isted at the location of the present Arctic coast. the relationship between permafrost and inferred Therefore, the most appropriate thawing-index past climate. Bum (1997) has shown that, with comparison for the present Arctic coast (i.e. care, it is possible to make inferences about the Tuktoyaktuk) is with Inuvik. The Stefan solution palaeo-climate prevailing at this time of maximum for the maximum depth of the thaw unconformity thaw. He stresses that certain assumptions are observed at Tuktoyaktuk indicates a thawing index critical to the analysis. One must assume that (a) of about 1.5 times current conditions at Inuvik; in any increase in the active layer is the result of other words, approximately 1800 thawing degree- summer thaw, and (b) that the thawing of frozen days. Thus, permafrost considerations enable one ground is linked to the thawing index of degree- to conclude that, in the early Holocene, the

272 Past and present permafrost as an indicator of climate change Tuhk I. Data showing average active layer depths, the depth of the Early Holocene thaw unconformity (where recognized), typical annual thawing degree-days, and bio-climatic zonations for S localities in the western and high Arctic of Canada (sources: French et al. 1986; Bum 1998).

Thawing Active Early Holocene thaw Locality Latitude degree-day.; ( c‘) layer (cm) unconformity Ecozone

Whitehorse 6 I ~‘N I900 125-1SO Boreal forest Inuvik 68‘N I200 100 Tree1i ne Tuktoyaktuk 69’” 800 so 125-150* Tundra Sachq Harbour I2N 400 30-50 Eastern Melville Island 77”N 300 25-SO 113** Polar semi-

Stefan solution: z = bI 1/2 where L = depth of thaw (cm) b = thermal properties 1 =thawing degree-daya (-0 Thaw unconformity at 125-150 cm depth corresponds to npprox. 1800 thawing degree-days. **Thaw unconformity at 113 cm depth corresponds to approx. 600 thawing degree-days. summer climate at the location of the present by restricting the depth of snow on the ground in Arctic coast was typical of that which exists today winter, it enhances winter frost penetration. Subtle near Whitehorse, in central . Likewise. the variations in surface characteristics can thaw unconformity that can be observed at a depth also produce significant permafrost changes. as of 113 cm on eastern Melville Island corresponds illustrated by the permafrost degradation that to approximately 600 thawing degree-days; that is, follows upon forest fires (e.g. Mackay 1995; Bum a value typical of the climate that forms the low 1998). Therefore, if climate warming is occurring, Arctic tundra in the southern region the marginal permafrost present at the extreme (i.e. between Tuktoyaktuk and Sachs Harbour). southern fringes of the discontinuous zone appears to be especially sensitive.

Permafrost and climate change Conclusions Today. clear evidence that permafrost is degrading in response to current global climate change is The history of permafrost over the last two million limited. More time is required before definite years suggests that permafrost has grown and trends can be detected. Possible indicators of decayed in response to global climate change. The climate warming in the permafrost regions of techniques of cryostratigraphy can be used to infer northern latitudes are summarized by Maxwell past climate changes from the permafrost record. If (1997). They include: (a) increases in the thickness global climate warming is occumng today, and of the active layer; (b) increases in the frequency once the buffering thermal effect of the active of occurrence of active layer failures and slope layer has been overcome, marginal permafrost instability; and (c) increased activity, bodies located at the extreme southern fringes of especially related to an increased frequency of the discontinuous permafrost zone will be the first forest fires in the summer in the boreal forest and to disappear. taiga. In addition to these parameters, however, probably one of the signals will be Acknow&-dgernenrs. -Research in over the last 30 years has been supported by the following branches of the provided by changes in permafrost distribution in Canadian Federal Government: the Natural Sciences and the sub-Arctic regions, at the extreme southern Engineering Research Council (NSERC); the Geological fringes of the discontinuous permafrost zone. In Survey of Canada and the Polar Continental Shelf Project (both of Natural Resources Canada); and the Department of Indian such areas the permafrost is typically less than - 1 and Northem Affairs. The research has also been supported by to -20c’ There’ in addition to the the Aurora Institute of the N.W.T. (formerly the lnuvik influence of bodies, the boreal forest to Research Centre) at Inuvik. the Arctic Petroleum Operators maintain permafrost in marginal situations since, Association (APOA), and the University of Ottawa.

French 1999: Polar Research 18(2), 269-274 213 References Gerasimov, 1. P. & Velichko, A. A. (eds.) 1982: Palaeogeo- graphy of Europe during the last one hundred thousand years. 155 pp. Moscow: Institute of Geography. Academy of Brown. J., Ferrians, 0. J., Heginbottom. J. A. & Melnikov, E. S. Sciences of the U.S.S.R. 1998: Circum-Arctic map qf permafrost and ground-ic.e Harry. D. G.. French, H. M. & Pollard, W. H. 1988: Massive conditions. Scale: 1 : 10,000.000. Circum-Pacific map srrirs. ground ice and ice-cored terrain near Sabine point. Yukon CP-45. Washington. D.C.: United States Geological Survey. Coastal Plain. Can. J. Earth Sci. 25, 184&-1856. Brown. R. I. E. 1967: Permafrost in Canada. Map 126JA. Klimenko, V. V., Klimanov, V. A,. Kozharinov, A. V. & Ottawa: Geological Survey of Canadmational Research Fyodorov, M. V. 1996: Global climate and the thousand year Council of Canada. temperature trend during the postglncial epoch and Holocene. Burn, C. R. 1988: The development of near-surface ground ice Mrteorol. Hydro/. 7, 2C-35. during the Holocene at Mayo. J. Quat. Sci. 3. 31-38. Mackay, J. R. 1995: Active layer changes (1968-1993) following the forest-tundra fire near Inuvik. N.W.T.. Canada. Bum, C. R. 1997: Cryostratigraphy, paleogeography. and Arct. Alp. Res. 27, 323-336. climate change during the early Holocene warm interval, Maxwell. B. 1997: Resportdir~g 10 global climarr change in weatern Arctic coast, Canada. Can. J. Earth Sci. 34, 9 12-925. Cunuda's Arctic. The Canadu Countn Study: clirnute Bum. C. R. 1998: The response (1958-1997) of permafrost and iinpacts and adaptation. Vol. 11. 82 pp. Downsview, Ontario: near-surface ground temperatures to forest fires. Takhini Atmospheric Environment Service. Environment Canada. River valley, southern Yukon Territory. Can. J. Earth Sci. 35, Murton. J. B. & French, H. M. 1994: Cryostntctures in 184-199. permafrost. Tuktoyaktuk coaqtlands, western Arctic Canada. Ferrians, 0. J., Jr. 1994: Permafrost in Alaska. In G. Plaafker & Can. J. Earth Sci. 31, 737-747. H. C. Berg (eds.): The geolo,qy if Alaska. The if National Atlas of Canada 1995: Permafrost. Map MCR 3177F. North America. Vol. G-I. Pp. 845-851. Boulder, CO: Scale: 1 :7.500.000. Ottawa: Natural Resources Canada. Piwe, T. L.. Berger. G. W.. Westgate. J. A., Brown, P. M. Geological Society of America. R: Leavitt. S. W. 1997: Eva Interglaciatiott Forest Bed, French. H. M. 1998: An appraisal of cryoatratigraphy in north- ringluciuted east-central Alaska: global ~~'artnitig125.000 west Arctic Canada. Perniafrost and Perigluciul Process. (1. yarr ago. Geologiccil So&& of America, Spccicil Publica- 291-311. tion 319. 54 pp. French. H. M.. Bennett. L. & Hayley, D. W. 1986. Ground ice Rozenbaum, G. & Shpolyanskaya, N. 199X: Late Cenozoic investigation\ near Rae Point and on Sahine Peninsula, permafrost history of the Arctic. Perintifrost and Prri,q/ucictl eastern Melville Ialand. Can. J. Eurth Sci. 23. 13x9-1400. Process. 9. 247-274.

Past and present permafrost as an indicator of climate change