Problems of the last interglacial in Arctic Siberia
Post on 23-Nov-2016
Embed Size (px)
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.
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
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,
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;
The Last Interglacial in Arctic Siberia 217
GIN-3868 MAG-592 GIN-4017 LU-1675 GIN-4013 LU-1674 LU-1676
GIN-4018 GIN-4015 GIN-3884 MGU-470 GIN-3862 GIN-2280
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-1688 MGU-469 LU-1678 MGU-573
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 faile...