Problems of the last interglacial in Arctic Siberia

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  • Quaternary International, Vol. 10-12, pp. 215-222, 1991. 1040-6182/91 $0.00 + .50 Printed in Great Britain. All rights reserved. 1992 INQUA/Pergamon Press Ltd

    PROBLEMS OF THE LAST INTERGLACIAL IN ARCT IC S IBERIA

    Andre i V. Sher Severtsov Institute of Evolutionary Animal Morphology and Ecology, Russian Academy of Sciences, 33 Leninskiy

    Prosp., 117071 Moscow, Russia

    The basic problem in reconstructing the last interglacial in the Siberian Arctic is to recognize correlative deposits in the Quaternary sequence. Assignment of any particular "warm" event to the last interglacial is usually based on various indirect criteria. There is no one independent criterion that could distinguish the last interglacial from earlier or later warm periods. Potentially, the most rapidly evolved mammal lineage of collared lemmings could be helpful for dating the last interglacial in the Arctic, but there are still some problems to be solved. Another important problem is climatic interpretation of warm events presumably referred to the last interglacial. Most commonly such reconstructions are based on the concept of northward shift of modern-like plant communities. But some features of pollen spectra and insect faunas suggest a special character of communities existed during the warm events in the Arctic. They do not seem to be exact analogues of modern communities, and a non- uniformitarian approach is necessary for their climatic interpretation. Survival of tundra-steppe communities and grazing mammals through the last interglacial climatic change suggests that it was not so destructive for the Arctic ecosystems as the Pleistocene/Holocene environment restructuring.

    INTRODUCTION

    Arctic Siberia is very rich with the Quaternary sequences. Most of them reflect treeless environment and harsh climate with active low-temperature perma- frost, but some include one or more horizons deposited under evidently milder conditions. Such horizons are commonly rich in organic debris which portray richer past vegetation. Pollen and plant records, as well as some features indicative of permafrost degradation suggest a warmer climate - - at least as warm as the present or even warmer. Some of these horizons are presumed to represent the last interglacial; however, such conclusions are usually based on various indirect criteria, such as: relative stratigraphic position; infinite radiocarbon dates; correlation with the sea level changes. Most of these criteria are not definitely convincing. There is no single independent criterion that enables one to distinguish last interglacial deposits from those representing earlier or later warm periods.

    In the absence of independent dating criteria (such as tephras, e.g. in Alaska) the pollen and plant macro- fossil record has become the most decisive evidence for recognition the last interglacial in the Siberian Arctic. This increases the danger of circular reasoning. For instance, it hardly would be correct to conclude that the last interglacial was the warmest in the region when such a conclusion was based on the sites where deposits are assigned to this age because they appear to represent the warmest climate in regional sequence. To avoid this, we should pay careful attention to which criteria are used for assigning warm events to the last interglacial. These criteria are different in different sectors of Siberian Arctic.

    REGIONAL CRITERIA FOR RECOGNITION OF THE LAST INTERGLACIAL SEDIMENTS IN

    ARCTIC SIBERIA

    In the western sector (Ob'-Taimyr-Lena) there is evidence of both Pleistocene glaciations and marine transgressions. The main Late Quaternary glacial complex (Zyryanian) overlies sediments of marine origin which have traditionally been assigned to the Kazantsevian transgression of the last interglacial epoch. Thus besides the pollen data, glacial-and- marine stratigraphy and marine invertebrates are addi- tional criteria. Mammalian biostratigraphy in this sector is still poorly developed.

    In the Taimyr Peninsula and North-Siberian Low- land, the marine sediments lying between two glacial tills are assigned to the Kazantsevian according to their pollen spectra and marine molluscan fauna. The latter is believed to include extinct species like Cyrtodaria jenisseae Sachs, Astarte invocata Merkl. et Petr., A. leffingwelli Dall, that are unknown from the later Karginian transgression (Andreeva, 1982). However, in general composition this fauna consists predomi- nantly of equal numbers of Arctic and Boreal-Arctic molluscan species. Arboreal pollen, including spruce and even fir, dominates the pollen spectra as is the case at the Bolshaya Rassokha River section (site 11, Fig. 1). This suggests that the northern limit of taiga was at least 2 north of its present location (Andreeva et al., 1982).

    Though not as thoroughly investigated, the eastern sector (Chukotka), is rather similar to the western one in retaining traces of several marine transgressions and glaciations. One of the transgressions, Valkatlen (16,

    215

  • 216 A.V. Sher

    Fig. 1) as well as some fluvial of the sequences, are believed to belong to the last interglacial. In North Chukotka, pollen from these presumed last interglacial sediments indicate mainly a tundra or shrub-tundra environment (Petrov, 1985). Birch forest-tundra and shrub-tundra existed to the south, in Anadyr region, while in the Koryak Mountains shrub pine (Pinus pumila) grew, as it does today. In more continental regions of West Chukotka, sediments assigned to the last interglacial yield pollen spectra interpreted as forest-tundra with tree birch. Sometimes larch and shrub pine are also recorded (Resolutions . . . . 1987).

    In the middle sector (Lena-Kolyma) both the glacia- tions and marine transgressions were of very limited importance and therefore these stratigraphic criteria are not useful for dating and correlation of Late Pleistocene sediments. Instead, the record of mammals and insects is more informative here. Pollen, plant macrofossils and permafrost provide the most impor- tant evidence for defining the Late Quaternary succes- sion in this region.

    PROBLEMS OF RADIOCARBON DATING IN RECOGNITION OF WARM EVENTS

    An infinite radiocarbon date, i.e. beyond the limits of the method, is often considered as an additional indirect criterion for assignment of a 'warm' organic deposit to the last interglacial. However, during the last two decades many sections formerly assigned to the last interglacial have been dated to 30-35 Ka and referred to as the Late Pleistocene 'interstadial' (Karginian). This 'radiocarbon revolution' caused a sharp decrease

    in number of the last interglacial localities. Recently, more and more scientists have expressed doubts on reliability of radiocarbon dates older than 30 Ka in the Arctic. The Karginian age of some important sites has recently been reassessed. They are once again thought to be of infinite radiocarbon age (Sher and Plakht, 1988).

    One of the most remarkable cases of this kind is the story of famous Duvannyy Yar locality in the lower Kolyma River valley (site 15, Fig. 1; Fig. 2). The more than 30 m thick silt deposit with impressive ice veins is the type section of the Yedoma Horizon. Its pollen spectra indicate treeless vegetation. The Yedoma deposit is underlain by a peat layer with 'warm' pollen spectra (e.g. dominance of pollen of trees and shrubs - - see pollen diagram in Sher et al., 1979). Abundant fossils of large mammals show that the age of the Yedoma deposit is Late Pleistocene. This fauna does not provide a more exact age; however, existence of the younger terrace of the latest Pleistocene age (Sarta- nian) in the same area suggested that the Yedoma thickness is of Zyryanian age (= Early Wisconsin) (Sher, 1974). This implied that the warm phase at the bottom of the Duvannyy Yar section might belong to the last (or an earlier ?) interglacial.

    Acquisition of two radiocarbon dates of about 37,000 years BP from the peat horizon (Fig. 2) radically changed the concept of the age of the whole section. The underlying 'warm' peat was interpreted as Kargi- nian, and the Yedoma Member as Sartanian (Kaplina et al., 1978). Since the international field trip to the locality in 1979, a new stratigraphic name for the whole Beringia Region has been introduced - - Duvannyy Yar

    FIG. 1. Quaternary mammal localities and important sections mentioned in the text. 1. Timoshkovichi; 2. Nyatesos; 3. Borisova Gora. Gralevo; 4. Cheremoshnik; 5. Ulovka; 6. Malyutino; 7. Shkurlat; 8. Kipievo; 9. Gornokazymsk; 10. Yarsino; 11. Bolshaya Rassokha; 12. Achchagyy-Allaikha; 13. Keremesit; 14. Bolshoi Khomus-Yuryakh; 15. Duvannyy Yar;

    16. Valkatlen.

  • The Last Interglacial in Arctic Siberia 217

    GIN-3868 MAG-592 GIN-4017 LU-1675 GIN-4013 LU-1674 LU-1676

    GIN-3861

    GIN-4018 GIN-4015 GIN-3884 MGU-470 GIN-3862 GIN-2280

    GIN-3866

    44 200+1100 GIN-4003

    33 500+1100 GIN-4006

    44 600+1200 GIN-4000

    35 500+700 GIN-3999

    36 800+800 GIN-3997 35 400+900 GIN-3996

    45 2001100 GIN-3852 >53 000 GIN-3857

    ~-~silt ~sand ~peat l~eolygo.nal L_~ ~ce ye,ns ~ ice-wedge pseudo- morphs

    GIN-2279

    GIN-1688 MGU-469 LU-1678 MGU-573

    MGU-468

    first 14C sampling: 1974(78) 0 second 14C sampling: 1979(82) third 14C sampling: 1984(87)

    17 fourth 14C sampling: 1985(87)

    FIG. 2. Radiocarbon dating of Duvannyy Yar key section in the lower course of the Kolyma River (site 15, Fig. 1). After Sher and Plakht, 1988. Note that dates come from several different samplings. Full references to the dates can be found in that paper.

    Interval, ranging from about 30 to 14 Ka (Hopkins, 1982).

    However, recent studies suggest earlier and earlier ages, not only for the underlying peat, but for the most part of the Yedoma Member as well. At present, Sartanian dates (i.e. younger than 30 Ka) are known only from the uppermost part of the Yedoma Member (Fig. 2), while its lower part has the dates earlier than 50 Ka. In addition to the first dates, many of the later dates appear unreasonably young, probably due to contamination (Sher and Plakht, 1988). This means that the earlier proposal of a Kazantsevian age for the underlying peat layer is once again plausible.

    The dating problems at Duvannyy Yar are repeated at many of the important sections of the Lena-Kolyma sector of Siberian Arctic, as the age of many 'warm' deposits overlaid by the yedoma silts is based on definite radiocarbon dates of about 35-45 Ka. A very high percentage of this order of dates among the total amount of dates is rather suspicious by itself (Sher and Plakht, 1988); probably, some of them should be actually referred to the age beyond the limits of t4C dating. That means that at least some 'warm' deposits

    presently dated as 'Karginian', in fact may belong to the earlier warm phases including the last interglacial.

    MAMMAL FAUNAS OF THE LAST INTERGLACIAL

    Recognition of interglacial mammal faunas in general and of those dating to the last interglacial in particular raises many problems. First of all, for the whole Soviet Arctic we do not know of any fossil mammal fauna of the Pleistocene age which could be considered interglacial based solely on its content of taxa. Mammal assemblages indisputably related with any 'warm' event are very rare and again, demonstrate no special features (e.g. presence of southern compo- nents). The problem of delimiting interglacial faunas is no less for temperate latitudes, especially in Siberia.

    In the European Russia, several recently des- cribed small mammal assemblages are assigned to the last interglacial (Mikulino) (cf. Markova, 1985, 1987, and Motuzko, 1989 for reviews). None of them is located farther North than 57N (cf. Fig. 1). They are dated according to their local geological position within

  • 218 A.V. Sher

    the well-studied stratigraphic succession of European Russia. The ecological character of Mikulino faunas varies markedly from region to region. The northern- most faunas (Cheremoshnik in Yaroslavl Region, Ulovka at the Klyazma River) are thought to be of forest character. In Byelorussia and Lithuania, at about 54N, small mammal faunas referred to the last interglacial also include forest elements. Motuzko (1989) subdivides them into early interglacial (Nyatesos, Borisova Gora) and full (optimum) inter- glacial (Timoshkovichi). The suggested environment is mixed forests with some broad-leaved trees - - not essentially different from the present vegetation of the area. However, all the faunas include lemmings. Their fossils are especially numerous in Borisova Gora, where Lemmus sibiricus dominates, and even Dicro- stonyx is present. According to Motuzko (1989), the important role of the Siberian lemming is due to its association with boggy environments. In Timo- shkovichi, lemming fossils are very few and belong to Lemmus or Myopus, the forest lemming. However, it should be admitted that this fact makes the Mikulino interglacial faunas of Byelorussia quite different from the modern mammal fauna of the region.

    Further south, at 50-52N, small mammal faunas (Malyutino, Chernyanka) are dominated by steppe species, but some forest inhabitants are present as well. A forest-steppe environment is proposed (Markova, 1985) on the basis of such evidence. Still further south and east small mammal faunas correlated with the last interglacial are entirely of steppe character (Markova, 1989). Thus, except for the Byelorussian lemming fauna, the geographic distribution of the Mikulino faunas suggests a zonal pattern very similar to the present one on the Russian Plain.

    One of very few large mammal faunas assigned to the last interglacial is Shkurlat Fauna in Voronezh Region (site 7, Fig. 1). It has been assigned to the Mikulino interglaciation because of the presence of an advanced form of the straight-tusk elephant, Palaeoloxodon antiquus, which is typical for Eemian faunas of Central Europe (Alekseyeva, 1980). Due to the presence of this species, Alekseyeva interprets the Shkurlat assemblage as a forest-steppe one, though it includes no other forest inhabitants. Among large mammals in the Shkurlat assemblage are common grazers such as mammoth, woolly rhino, bison and horse, while the small mammal component includes only steppe and even desert-steppe species (Markova, 1985). Thus, contrary to the situation in Western and Central Europe during the last interglacial, forest environment was very restricted in European Russia, at least this is the conclusion drawn from the mammal faunas. This eastward 'steppization' of the interglacial environment is probably still more pronounced in Siberia.

    According to Vangengeim (1977), there are no mammal faunas confidently dated to the last inter- glacial (Kazantsevian) in the temperate latitudes of Siberia. This fact does not seem to be in agreement with the wide distribution of forest communities

    inferred by palynologists for the last interglacial. Forests might have existed in temperate Siberia during the last interglacial, but the fossils failed to preserve due to unfavorable taphonomic conditions of forest environment. On the other hand, it is evident that essentially higher continentality of Siberia might have prevented most typical mammals of European Eemian Antiquus-fauna from dispersal far to the east (Vangengeim, 1977). This is confirmed, as indicated above, by the marked faunal gradient within European Russia.

    One example from West Siberia - - the Yarsino local fauna of small mammals at the Irtysh River (59.5N) - - comes from the Yalbynyin Suite referred to the last (Kazantsevian) interglacial (Smirnov et al., 1986). Palynologists suggest a 500-700 km northward shift of regional vegetation zones for this time. However, the faunal assemblage is dominated by tundra and steppe rodents (Dicrostonyx, Lemmus, Lagurus and various Microtus including subarctic species) and contains only single fossils of possible forest species. There are some doubts about sedimentary homogeneity and correlation of the Yarsino fauna. Nevertheless, the lack of true forest faunas in temperate Siberia seems to be real and the same is apparently also true for Arctic Siberia.

    FOSSIL MAMMALS AND THE SEARCH FOR LAST INTERGLACIAL SEDIMENTS

    Mammalian biostratigraphy gives rise for some hope in recognition of the last interglacial deposits. One lineage of small mammals, the collared lemmings (genus Dicrostonyx) demonstrates the most rapid evolution in the Quaternary. This has been used for a suggested biozonation of the Quaternary deposits of Northeastern Siberia (Sher, 1984). Though additional evidence is required, it is proposed that the transition between Middle Pleistocene D. simplicior and Late Pleistocene D. guilielmi roughly corresponds in age to the time of last interglacial. In any case a 'warm' organic deposit intercalated between two bone-bearing members - - one with D. simplicior and another with D. guilielmi - - may be assigned with rather high probabil- ity to the last interglacial.

    The region where such a succession is found is the North of European Russia within the taiga zone. In the Pechora River Basin, the so-called Moscow Glacial Till is supposed to be correlative to the Late Saalian. The till is underlain and overlain with two small mammal assemblages. The lower one, Kipievo I is dominated by Dicrostonyx simplicior. The upper one, Kipievo II, which contains more than 60% of the Late Pleistocene D. guilielrni M ~ morphotype probably marks the time of the evolutionary transition from Dicrostonyx simpli- cior to the more advanced D. guilielmi (Guslitser and Isaychev, 1976). Kipievo II is certainly not an inter- glacial. The nearest locality referred to the last inter- glacial is more than 600 km farther south (Cheremoshnik) and contains no lemmings. Early

  • The Last Interglacial in Arctic Siberia 219

    Valdai (Wisconsin) fluvioglacial sand overlying the last interglacial peat in Cheremoshnik yields only the D. guilielmi morphotype (Agadjanyan and Erbaeva, 1983). A possible speculation is that Kipievo II should be referred to as the latest Saalian. This means that in any last interglacial high Arctic fauna containing collared lemmings, the lemming fossils should exhibit a stage of evolution equal to or more advanced than those of the Kipievo II stage. In other words, such fossils should belong to the lowermost Dicrostonyx guilielmi biozone (Sher, 1984).

    However, these inferences are disputed by some Byelorussian scientists (Motuzko, 1985). They believe that some early Valdai faunas in Byelorussia that overly the last interglacial sediments, like Gralevo-2, demon- strate fully dominating D. simplicior morphotype, and even middle Valdai assemblages, like Kobelyaki, con- tain about 50% of this ancient morphotype. Motuzko believes that Kipievo faunas are also of the Valdai age.

    In recent years, the first small mammal faunas have been discovered in the North of West Siberia (Smirnov et al., 1986). Locality Gornokazymsk near the Ob' mouth at the Arctic Circle yielded D. cf. guilielmi, more advanced than D. simplicior, but slightly more archaic than the Late Pleistocene lemmings. No paleoecological evidence on the locality itself is pre- sented, Some geological data suggest that the fossili- ferous sediments are correlated with the Yalbynyin Suite, which is assigned to the Kazantsevian intergla- cial. However, the fauna is a lemming assemblage of dry tundra type. The question is how reliable is the geological correlation? It is also worth noting that Dicrostonyx guilielmi rather than D. simplicior is the lemming species expected by Smirnov and others (1986) in the sediments of the last interglacial age.

    At present Dicrostonyx remains the only biostrati- graphic tool available in Arctic Siberia for distin- guishing Late and Middle Pleistocene deposits. Strictly speaking, there is no single section in the Lena-Kolyma region where two representative faunas with D. simpli- cior and D. guilielmi are superimposed. Rather rich faunas with D. simplicior are known from the Achchagyy-Allaikha and Keremesit Rivers at the Lower Indigirka (Kaplina et al., 1980; Resolutions . . . . 1987). At Achchagyy-Allaikha, the evolutionary stage of Dicrostonyx is even more primitive than in Kipievo II, and this is the reason that the authors concluded that the underlying warm event (Achchagyy Member) predated the last interglacial. However, the other small mammals and the large mammals in this fauna display no essential differences from the Late Pleistocene age faunas (in part, probably, due to a rather poor sample); so, the Dicrostonyx fossils constitute practically the only argument for the Middle Pleistocene age of the fauna and underlying warm event.

    It is clear that the exact time of D. simplicior/D. guilielmi transition is crucial for biostratigraphic recog- nition of the last interglacial. Further research is necessary to overcome controversies between Pechoran and Byelorussian interpretation of the stratigraphic

    significance of this very important genus. To cope with the problem, we need also new Dicrostonyx localities with good samples from Arctic Siberia.

    One such locality was discovered by the author in 1989 at the Bolshoi Khomus-Yuryakh River on the Indigirka-Kolyma Lowland (Loc. 83). It is the first site in the region to contain two stratigraphically separated D. simplicior assemblages. Moreover, they are sepa- rated by a stump and log horizon representing a pronounced warm event. The faunas are being studied now by Zazhigin and others, pollen by G. Kartashova. However, if we rely upon the Pechora interpretation, the warm event should be earlier than the last inter- glacial.

    INTERPRETATION OF POLLEN RECORD

    The next problem in defining interglacial deposits in Siberia is interpretation of palynological and plant macrofossil evidence from 'warm' deposits of supposed last interglacial age. Such spectra usually show rather high percentages of arboreal and shrub pollen. Tradi- tionally, it was thought that during the 'warm' stages treeless communities of 'tundra-steppe' type were replaced by forest-tundra or taiga communities quite similar to ones existing now in Siberia. This concept implies a rather simple northward shift of modern vegetational zones. However, the concept is at odds with the fact of successful survival of large grazing mammal populations and steppe complexes of insects through 'warm' stages.

    Generalized pollen spectra of some warm stages from the Indigirka-Kolyma Lowland were compared by the author with a set of surface samples taken in the present Northeast Siberian larch taiga and studied by Kartashova (1971). The samples of recent deposits, though originating in forested areas, are actually dominated by pollen of shrub rather than arboreal plants. Some of the fossil spectra seem rather similar to recent ones in their overall composition. However, most of them have some special features. For instance, the lower peat layer at the Duvannyy Yar section displays lower percentages of tree pollen than in modern surface samples. It also differs by having spectra with higher percentages of grass pollen and lower amounts of Sphagnum, which constantly domin- ates the spore fraction of surface spectra from the present larch taiga. The other spectra representing warm intervals contain much higher percentages of tree birch and little or no larch pollen. The latter character- istic cannot be due alone to bad preservation of Larix pollen. The author's revision has shown that in some essentially treeless (grass-dominated) spectra the larch pollen content is notably higher than in 'warm' (tree- shrub) spectra.

    Analysis of pollen spectra of 'warm' stages, including those supposedly referred to as the last interglacial, has shown that forest communities of these stages in Siberian Arctic were different from the existing ones (Sher, 1988a). The main forest-forming trees of these

  • 220 A.V. Sher

    stages were different species of birch, but the 'inter- glacial' forests were probably more open, alternating with grassland communities. Pure larch forests, so typical for the present East Siberian Subarctic, with their characteristic surface plant cover and soils, were probably very restricted in distribution, if they existed at all. In any case such monodominant forests seem incompatible with mammalian grazing communities. Further work in this direction will probably result in development of a new model of 'interglacial' vegeta- tion.

    PERMAFROST CRITERIA

    Of special importance is evaluation of degree of permafrost degradation during the 'warm' stages. In Arctic Siberia, almost all 'warm' members (which are mainly peat layers - - an interesting problem in itself) are related to some features of permafrost degradation, mainly ice-wedge pseudomorphs. The sediments which are supposed to belong to the last interglacial usually include very large ice-wedge pseudomorphs. At the same time, the sections include many other horizons with ice-wedge casts, which may not all mark regional warming of climate. It is necessary to find the criteria to distinguish between local and regional permafrost degradation features. The latter should be studied further in attempt to reconstruct past climatic condi- tions.

    Despite the problems associated with climatic inter- pretation of ice-wedge pseudomorphs, there is one very important and reliable type of evidence for reconstruct- ing the history of frozen sediments. It is recognition of the boundary between sediments containing ice veins that have not since their initial formation ever com- pletely melted, and underlying sediments that contain only ice-wedge pseudomorphs, indicating that they were thawed at least once before refreezing epigeneti- cally. This boundary can be easily recognized in many sections in the Indigirka-Kolyma region. It is assumed to be correlated with an important climate amelioration event, but the age and scale of this event remains uncertain. In some sections, like Achchagyy-AIlaikha and Khomus-Yuryakh, the boundary between the two types of frozen sediment may be dated to part of the Middle Quaternary, while in the others it may be correlative with the last interglacial. Furthermore it is difficult to know whether this boundary represents a single regional climatic thaw event, or several events of minor importance.

    DISCUSSION

    The above shows that recognition of the deposits of the last interglacial in the Siberian Arctic is not an easy task. Unfortunately, fossil tephras are very rare in Arctic Siberia. Well-founded minor paleomagnetic events that could be helpful for dating are also not found in the region. Hopefully special studies will reveal additional exposures of the last interglacial

    sediments with interpretable stratigraphy. Until then it is important to sum up some general issues related to the study of the Quaternary warm events in Arctic Siberia.

    At present, almost everyone conducting research on the last interglacial is attempting to reconstruct quanti- tative interglacial climatic parameters using various types of proxy data. Most of these reconstructions use methods based on characteristics peculiar to the present ranges of animal and plant taxa. Methodologi- cally, these procedures are based on an oversimplified interpretation of the uniformitarian principle, i.e. that the ecological parameters of modern species exactly match those of Pleistocene species.

    The Pleistocene environmental history of Arctic Siberia argues against this kind of uniformitarian logic. Certainly, Arctic Siberia, like other regions, responded to the global climatic changes of the Quaternary. This is evidenced, for example, by repeated shifts from almost treeless grassland communities (tundra-steppe, or Arctic steppe) to ones dominated by trees and shrubs. Such changes are usually considered as the records of relatively 'cold' (or 'glacial') and 'warm' (or 'inter- glacial') periods, respectively.

    Many previous studies have shown that the com- munities that existed during the 'cold' periods had only very remote modern analogues. Now, more and more evidence is accumulating which shows that "inter- glacial" communities in the Arctic also largely lack modern analogues. Despite a wider spread of tree and shrub plants during the 'interglacials', various kinds of the tundra-steppe-grassland communities probably persisted, and were far more important in the Arctic than are their patchy and impoverished modern relicts. It can be also supposed that in Arctic Siberia the surface cover in the 'interglacial' forests included more of a grass/herb component than in modern taiga, which is dominated by mosses and lichens.

    In view of these assumptions, the whole set of plant communities dominating Arctic Siberia during the Pleistocene can be considered as a flexible but stable biotic system tolerant to all climatic fluctuations that took place during that time. If so, it explains the fact that grazing mammals survived during all the inter- glacials in Arctic Siberia. Only in the Early Holocene did environmental change have an adverse and irrever- sible effect on the grazing communities. We still know too few details about this restructuring, but we do know its final result - - replacement of the Pleistocene biotic system by the present one, which is at least as stable as the former one. This modern system, which is peculiar for the Hypoarctic tundra and taiga (Yurtsev, 1966), of most Northeast Siberia, is characterized by vegetation comprised of low shrub and shrublet plants, mosses and lichens at the expense of herbaceous plants.

    The most important feature of the Pleistocene/ Holocene revolution was that while several kinds of communities disappeared completely, except for a few grazing mammals almost no plant or animal species became extinct. Some species shrank their ranges

  • The Last Interglacial in Arctic Siberia 221

    dramatically, and can be found now in zonal or intrazonal communities distinct from the Pleistocene ones. For example Saiga antelope survives today in zonal desert steppe, while some xerophilous insects and probably plants are now restricted in Arctic Siberia to the communities of southern-facing slopes. Other species, like moose and caribou, became dominants within a wide range of communities.

    This kind of radical environment restructuring can- not be explained as a unified response of communities to climatic change. Rather an explanation involving individual species response seems much more appro- priate. In a recent paper (Sher, 1990) I discussed this problem in more detail. Earlier some North American paleoecologists had come to the conclusion that only the individualistic theory explains the peculiarities of the Pleistocene communities in eastern and central parts of the U.S.A. (Guilday et al., 1977; Graham, 1979). Hence, a non-uniformitarian approach to the environmental reconstructions in the Quaternary is necessary.

    An inevitable inference to be drawn from application of this approach to the Pleistocene biota is that the ecological parameters of modern species and climatic characteristics of their present ranges in particular, cannot be used automatically to reconstruct past conditions. Reviews of both Siberian and North American evidence show that direct comparison of the ecological requirements of modern species and com- munities with their Pleistocene counterparts is very dangerous (Sher, 1988b, 1990). The assumption of such exact analogies has given birth to most of the delusions in Quaternary paleoecology.

    An urgent task for paleoecologists is to develop principal qualitative models of various Pleistocene communities based on a non-uniformitarian analysis of the proxy data. Of course, these models will not immediately give us the exact values for temperature and precipitation, which are to be seen in some publications. Later, however, it should be possible to compare these proxy models with climatic models, and this will probably allow approximate quantitative esti- mates of past climate. In Arctic Siberia we are now only in the very first phase of this attempt to understand and quantify the environmental pattern of the last inter- glacial.

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    Rodents and lagomorphs of the U.S.S.R. Moscow. Nauka, 1-190. (In Russian).

    Alekseyeva, L.I. (1980). Peculiarities of the last interglacial therio- complex on the Russian Plain. In: Mammals of Eastern Europe in the Anthropogene, pp. 68-74. Proc. of Zoological Institute, U.S.S.R. Academy of Sciences, 93, Leningrad. (In Russian).

    Andreeva, S.M. (1982). Kazantsevo deposits of the North-Siberian Lowland. In: The Anthropogene of the Taimyr Peninsula, Moscow. Nauka, pp. 33-51. (In Russian).

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