G. N. Shilova

Paleoclimatic Indicators from Permafrost Sequences in the Eastern Taymyr Lowland

Permafrost sequences in the Labaz Lake area were investigated to reconstruct their paleoenvironmental history during the Late Quaternary using geocryological field studies, sedimentological, geochemical, stable isotopic and palynological analyses in connection with radiocarbon dating. The current results show that during post-Zyryan (Early Weichselian) times denudation and accumulation in this area was dominated by processes connected with the decay of an ice cover, formed at least during the Zyryan stadial. As a result of the specific peculiarities of glacier ice thawing in a territory with continuous, low- temperature permafrost, buried glacier ice bodies were preserved until the present time. The most intensive decay of glacier ice occurred during the first warming stage of the Kargin (Middle Weichselian) Interstadial more than 40000 yr BP. The formation of extensive lake depressions within the glacial deposits is connected with these times. About 40000 yr BP, a surface stabilization occurred, and syncryogenic deposits with thick polygonal ice wedges (Ice Complexes) and peat beds were formed. Subsequent warming phases, apparently connected with increases in precipitation, led to a new activation of glacier ice decay, and to the accumulation of additional lacustrine and fluvio-lacustrine sediments. Palynological and stable isotope data from ground ice suggest a continual trend to a cooler and more continental climate from the Kargin Interstadial to the Sartan stadial times. During the maximal cooling period, polygonal ice wedges in dried-up lacustrine deposits formed. At the termination of the Pleistocene, lacustrine sedimentation seems to have reactivated in reaction to short climate improvements. Thermokarst lakes were formed in this period. Extensive peat accumulation started at the end of the Boreal and spread to flat lacustrine depressions during the Atlantic. Since the Subboreal, a drying up of lakes and gradual increasing of permafrost aggradation can be inferred.

First 230Th/U date of Middle Pleistocene peat bog in Siberia (key section Krivosheino, Western Siberia)

A 14C date older than 53900 yrs BP was obtained for the uppermost part of the buried peat bog in Krivosheino section (Middle Pleistocene of Western Siberia). These sediments also yielded 230Th/U dates of 195−9.1+10.8 ka using the leachate alone (L/L) and 204−13+17 ka using total sample dissolution (TSD) models. Peculiarities of 230Th/U dating are discussed. Palynological investigation of the buried peat bog together with underlying and overlaying sediments, and comparison with palynological data from Baikal and Elgygytgyn lakes revealed that the peat layer in Krivosheino section was formed at the end of Shirta Interglacial (Marine Isotopic-Oxygenous stages MIS-7), when climate conditions at all studied sites were more severe compared to the modern ones.

New outcrop of buried Kazantsevo peat at lower reaches of the Irtysh River

The Chembakchino outcrop has been known for a long time [3, 7, 8, and others]. Its section comprises Oligocene and almost entire Pleistocene sediments. The outcrop is confined to the steep bank slope, which extends from the Chembakchino Settlement for 7 km upstream along the Irtysh River. The Chembakchino section is most complete and best studied. Approximately 1.5‐2.0 km upstream from the settlement, the upper (half or third) part of the slope is well exposed, while its lower part is buried under landslides and observed only in rare incisions. The outcrop is approximately 60 m high. The generalized section [8] includes the Kargino, Kazantsevo, and Shirtino soils buried at depths of 6, 10‐11, and 20‐21 m, respectively. The Shirtino soil is laterally replaced by peat. Previously, the Kazantsevo soil level was assigned to peat. Two peat levels at depths of 35‐36 and 42‐44 m were defined in the Tobol (Mindel‐Riss) section. The thickness of these peats was not indicated, although the authors of [3] described a peat section 30‐50 cm thick in the Tobol (Chembakchino) Formation immediately near the eponymous settlement (now the peat section has been destroyed). During field works of 2002, we observed at all levels cited above fragments of buried sols grading into thin (from 2‐5 to 10‐15 cm) and usually intensely deformed peats. The peat (>50 cm thick) was discovered 2.5 km away from the Chembakchino Settlement. Judging from the hypsometric and stratigraphic position of this peat, it belonged to the Shirtino Horizon. Unfortunately, it had been destroyed by intense erosion and landslides by 2004. The Irtysh River bank, located 4.5‐5.0 km away from the settlement, is an accumulative structure. It is separated from the river channel by a wide beach and is shown in [7] as an entirely forested and unexposed area. We discovered several small outcrops of Pleistocene sediments in the forested slope 6 km away from the settlement. One such outcrop hosts a peat lens at 9‐10 m below the edge of the slope. The lens is more than 0.5 m thick in excavations. The upper part of the section was recovered by a ditch 7 m long and 6 m deep. The section was exposed by excavations in the depth interval of 6‐ 10 m and by a prospecting pit to a depth of more than 10 m. The section comprises the following members: (1) greenish gray massive loam (interval 0‐9.1 m) enclosing the pedological complex of two soils at a depth of 1.3‐1.8 m and peat lenses at 4.4‐5.0 m; (2) peat in the interval of 9.1‐10.0 m; (3) alternating sand, loam, and loamy sand in the interval of 10.0‐14.1 m.

The first uranium-thorium dating of the Middle Neopleistocene peat in West Siberia

For a sample from the roof of peat (0–2 cm) located in a reference section for the Middle Neopleistocene in West Siberia near the village of Krivosheino, we obtained an exorbitant value of 14C date ≥53.9 ka B.P. (LU-6024). In the peat proper, the 230Th/U dates obtained by the isochronous method are 195.2 ± 10.8/9.1 ka B.P. for the L/L model and 204.1 ± 17/13 ka B.P. for the TSD model. The palynospectra of the peat characterize, from the bottom up, birch forests with fir and spruce participation; then spruce forests with fir; next spruce-cedar forests, similar to the middle taiga subzone. In the clay, the following palynospectra have been examined: forb-gramineous grasslands and light forests with spruce and Betula fruticosa. In the upper part of clays, the palynospectra reflect the evolution of swampy, birch, light forests with spruce participation. The conclusion is made that the studied part of the section formed at the end of the Samarovo Ice Age and in the last third of the Taz Ice Age. The break in sedimentation related to the erosion contact covers a part of the Samarovo Ice Age and the first two-thirds of the Taz Ice Age.

Climatic Stratigraphy of the Kazantsevo Horizon (as an Analogue of MIS-5) in the Boreal Zone of Western Siberia

Terrestrial deposits in western Siberia, the ages of which correspond to the interval of MIS-5, have been well studied using palaeobotanical methods. However, because of the small number of isotope data, there is no possibility for identifying the palaeoclimatic events relevant to the substages within MIS-5. Therefore, the large numbers of palaeobotanical results have been underused for the purposes of climatic stratigraphy. 230Th–U dates produced during the past two decades have allowed the changes in vegetation during the first half of the Late Pleistocene to be determined. This has enabled a comparison with palaeoclimatic events reflected in the Greenland ice core, and the identification in western Siberia of the analogues of substages MIS-5a, 5b, 5c, 5d, and 5e as well as events 5e1, 5e2, 5e3, 5e4, and 5e5 within MIS-5e.