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Past and Present Permafrost As an Indicator of Climate Change

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Past and Present Permafrost As an Indicator of Climate Change Past and present permafrost as an indicator of climate 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 Pleistocene 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-Arctic regions. at the extreme southern fringes of the discontinuous permafrost zone. H. M. French, Depts. of Geography and Earth Science, Universit?, of Ottawa, P.O. Box 450, Station A, Ottawa. Ontario KIN 6N5, Canada. 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 Northern Hemisphere. In the case of the of woolly mammoths and other Pleistocene North American continent, field observations in animals found preserved in permafrost in Siberia, both Canada (e.g. Brown 1967; National Atlas of Alaska 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 sediments in (a) the continuous permafrost Active layer Seasonally zone of the Arctic. or the tundra 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 terrain 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 (taiga) 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 ice wedge 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 climate change 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 erosion of loess 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 Europe 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- North America 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.
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