Modern geochronometric techniques and palaeonthological methods applied for sequences of the northern Pleistocene beyond the Urals by the PECHORA project members and other international teams has led to paleoclimatic and chronostratigraphic results differing from the conventional wisdom. First of all it concerns the position of the principal stratigraphic marker - the sedimentary series of the last interglacial deposited in climates warmer than present day and therefore correlative with MIS 5e. The main sections, that used to be exemplary for the so-called Karginsky interglacial (or interstadial), formerly correlated with MIS 3, have lately yielded non-finite AMS radiocarbon dates, optically stimulated luminescence (OSL) dates in the range of 100-140 kyr, uranium/thorium dates ca 130-140 kyr, etc. These sediments include peats and fluvial sands of the Lower Ob (Astakhov et al., 2004, 2005), marine silt and sand with boreal molluscs on the Taz and Gydan peninsulas (Nazarov, 2005), peat on river Angara (Arslanov et al., 2004), all not covered by till. There are also subtill sediments of the Karginsky stratotypes on the Yenissei: the marine formation with ESR date of 122 (Arkhipov, 1990) and alluvial sequence with OSL dates ca 100-122 kyr (Astakhov, Mangerud, 2005). I.e., deposits of the Eemian interglacial occur much closer to the surface than heretofore was assumed in regional stratigraphic schemes. Another implication is disappearance of the material foundations for the "Karginsky Interglacial" which has for a long time been treated as a Siberian counterpart of MIS 3.

A good idea of what actually happened in northern Siberia during MIS 3 is given by fine-grained Yedoma-like sediments with long ice wedges not disturbed by glaciers. Thick sequences of such silts with many finite conventional and AMS radiocarbon dates have long been known in Taimyr, Yamal and Gydan (Anthropogen..., 1982; Vasilchuk et al, 1984, 1992; Moller et al, 1999). To the east the best studied sequence of this type is in the Lena delta, where scores of successive radiocarbon dates elucidate the sedimentation process from 60 to 10 kyr (Schirrmeister et al, 2002). Initially such icy silts were misinterpreted as waterlain sediments. However, mammoth bones and long-grown syngenetic ice wedges imply basically subaerial environments. The decisive evidence of dry and cold climates of MIS 3 and 2 with very low sea level is provided by xeric floras with no hydrophilous species, abundance of mammalian grazers and predominance of steppe insects in the Taimyr and Lena yedomas (Kienast, 2001; Sher et al., 2005). In western Yamal, where a MIS 3 climate was relatively more humid, the Varyakha horizon of peaty limnic sediments with radiocarbon ages 33 to 28 kyr and luminescence ages 45 to 36 kyr occurs within the icy sequence (Forman et al., 2002).

The well-dated iceless intervals of mild interglacial climate and of the subsequent interstadial with continental climate make firm chronostratigraphic markers. Basal glacial diamicts with thick bodies of fossil glacial ice, which occur close to the surface from Yamal to Taimyr, are sandwiched between the two non-glacial horizons and therefore should correlate with cold events of MIS 5d to MIS 4. Sediments beneath the strata with boreal shells, diatoms, forams and taiga plants should relate to the Middle Pleistocene.

The obtained results call for updating the chronostratigraphic scheme of the Siberian Upper Pleistocene. To preserve the existing quadruple subdivision the regional horizons should have new stratotypes. The first, warmest interval of the Eemian level can be inferred from the Karginsky marine strata, peats or the Malaya Heta alluvial formations with dates within the range of 100 to 140 kyr. The interval of the following inland glaciation may retain its old Zyryanka name. The Varyakha chron is a cold interstadial which is a counterpart of the Middle Pleniglacial and MIS 3. The final Syoyakha cryochron is a non-glacial interval of the most continental climate corresponding to the Late Weichselian and MIS 2.

Reference:

Astakhov V. Correlation of Late Pleistocene events in northern Siberia: modern results. Correlation of Pleistocene Events in the Russian North. International Workshop Abstracts. 4-6 December 2006. Saint-Petersburg, 2006, p. 14.