Geomorfologiâ i paleogeografiâ

The article presents a generalization of the results of field and analytical studies of cryogenic phenomena in the Lower Volga region. For the first time for this territory, pseudomorphs, soil wedges and cryoturbations were described and studied in detail. Their cryogenic genesis was substantiated. In the Lower Volga region, various structures have been identified in loess-soil series, alluvial and marine deposits. The development of cryogenesis in similar environmental conditions, but in different genetic types of sediments, leads to the formation of structures of different shapes, which directly depends on the humidity and composition of the sediments. The processes of ice degradation and accompanying changes in their morphology are of decisive importance in the final appearance of soil structures. Absolute dating of the deposits containing cryogenic structures made it possible to identify the time intervals of their formation. Six stages of cryogenesis in the Late Pleistocene were identified based on the structural features, their stratigraphic position, and the results of laboratory analyzes. Stage I is characterized by the spread of deep seasonal freezing in the region, recorded in coastal marine sediments in MIS 5d. For stages II-III (MIS 5b, MIS 4, respectively), the existence of a perennial permafrost zone is reconstructed, cryogenic forms are recorded in various genetic types of sediments. Stage IV (MIS 3c – MIS 3b) corresponds to the existence of a perennial permafrost zone only for the northern part of the region (Srednyaya Akhtuba and Raygorod sections) and thin sporadic permafrost or deep seasonal freezing for the southern part of the Volga River valley (Chernyy Yar section). Stages V (MIS 3a) and VI (MIS 2) are characterized by the spread of thin sporadic permafrost or deep seasonal freezing. The identified major stages of the development of permafrost in the Caspian Lowland significantly refine the available data on the cryogenic horizons of the East European Plain.

The history of studying glacial complexes in North-Eastern Siberia goes back more than 150 years. During this period, extensive geological and geomorphological features were obtained, which made it possible to determine the stages, nature and extent of glaciations. At the same time, the lack of direct dating of the glacial relief obtained by geochronological methods does not allow for full-fledged paleogeographic reconstructions. This leads to discussions in both Russian and English literature about the possibility of the existence of glaciation in the mountains of North-Eastern Siberia. In this regard, to determine the size and time of glaciation in the southern part of the Chersky Range, we carried out a complex of geomorphological and geochronological studies, which are part of the international project “Searching for the missing ice sheet in Eastern Siberia”. Because of fieldwork in the Ohandya Ridge, in the Malyk-Sien River valley, three terminal moraine ridges have been identified, reflecting different stages of glaciation. Based on the dating of exposed boulders within three terminal moraine complexes, 22 ¹⁰Be cosmogenic dates were obtained. The average exposed age for the outer moraine is 120.8±13.7 ka, for the middle one North-Eastern 37.7±4.9 ka and for the internal moraine North-Eastern 13.8±2.2 ka. The age of the terminal moraine complexes testifies to the mountain-valley character of the glaciation of the Chersky Range in the Middle and Late Pleistocene, and emphasizes the trend towards a gradual decrease in the maximum length of glaciers in Northeast Asia. The successive reduction of glaciers from MIS 6 to MIS 2 indicates an increase in the deficit of atmospheric precipitation and a significant cryoaridization of the region. The decreasing trend may be related to the sharply continental conditions observed in the interior of Eurasia and western North America. This trend contrasts with much of the glaciated areas in the Northern Hemisphere, where the maximum area of Late Pleistocene glaciers is reconstructed for LGM time (MIS 2). The obtained datings of the glacial complexes of the Chersky Ridge confirm that at the end of the Middle and Late Pleistocene glaciations here were of a limited nature and there was no single ice cover in the mountains.

The Lena River provides important records for understanding the Quaternary history of North-East Siberia. At present, the structure, origin and age of the elements of the Lena River valley remain unresolved. This article presents the results of lithofacies analysis and absolute dating of the Ust-Buotama section exposing the Fourth (Bestyakh) fill Terrace in the middle Lena River valley. Three stratigraphic units have been recognized in the section: lacustrine–alluvial deposits at depth of 120–85 m depth from the surface correlated with the Middle Pleistocene Mavra formation of Central Yakutia; eolian deposits of the Late Pleistocene D’olkuma formation (depth 85–23 m), and eolian deposits of the Holocene wind-blown dunes (from the depth of 23 m to the surface). First quartz and K-feldspar ages have been obtained for the section using luminescence dating. The age relations and standard tests have shown the reliability of the chronology obtained. This chronology suggests that sediments of the Mavra formation were deposited no later than 300 ka, and their stratigraphic position implies a preliminary correlation with the Tobolsk time of the Middle Pleistocene (MIS 11-9). Deposition of the D’olkuma formation took place from late MIS 3 (29–30 ka) to the late MIS 2 (14–15 ka), reflecting the period of eolian activity when sand dunes and sheets were formed. The periods of eolian accumulation alternated with deflation periods at the end of the Late Pleistocene. Short periods of stabilization of the eolian landscape are indicated in the section by poorly developed paleosoils. The uppermost part of the section consists of the Late Holocene dune sediments which accumulation started ~400 years ago during the Little Ice Age. These findings infer that the Bestyakh Terrace is not a fluvial terrace in the classic sense, but rather the remaining part of a complex deflational and depositional plain. Much of the terrace sediments appear to have been formed under subaerial conditions in cold, dry environments of the late Pleistocene.

The morphology, structure and composition of sediments at low terraces which occur in the form of non-extended fragments in the Geysernaya River valley have been studied. Coarse, poorly sorted and weakly rounded debris flow material of different age generations are dominated in the sections. Layered sand and gravel deposits that accumulated under dammed reservoir conditions were exposed in some areas. Alluvial deposits are represented by thin layers of pebbles with boulders of better roundness and sorting with sand and gravel filler, underlying and/or overlying debris flow deposits. Some fragments of terrace-like surfaces are characterized by a smaller slope compared to the longitudinal profile of the river: apparently, they represent areas of former debris flow – landslide dams. Sediments of modern debris flows can be traced from 0 up to 50 m and of ancient once from 0.5 to 12 m above the river, which indicates the absence of a direct dependence of the age of sediments from the level of their occurrence. The change in loose material is due to the proximity and activity of thermal manifestations of the Geysernoe thermal field. Gas-hydrothermal processes lead to a significant transformation of the composition and properties of the analyzed sediments – mainly to their cementation, which makes it difficult to determine the time of sediment formation. The structure of the studied sections indicates the repeated occurrence of debris flows along the valley and the formation of temporary dammed reservoirs there as a result of the landslides and debris flow dams. The active supply of material from the slopes and its redeposition by debris flows causes poor rounding and sorting of sediment, and its weak disintegration. Among the rock-forming minerals of the fine sand fraction, magnetite and pyroxenes dominate with the participation of ilmenite. The light fraction is represented mainly by opal-smectite-zeolite aggregates, and to a lesser extent by geyserite. In the mineralogical spectra of sediments accumulated in dammed lake conditions, the set of secondary minerals and aggregates is expanding. In the alluvium units underlying the mudflow material there are signs of redeposition of ancient well-rounded sediments.

The article focuses on the paleoclimatic reconstruction of Holocene environmental changes. To address this issue, a study of the bottom sediments of Lake Geographensee, located on the Fildes Peninsula, King George Island, West Antarctica, was conducted. The lake, located above the maximum Holocene marine transgression limit, preserves an undisturbed sediment record spanning the last 8500 cal. yr BP. The results of lithological, loss-on-ignition, grain size, diatom, and geochemical analyses, along with statistical data processing and radiocarbon chronology of the bottom sediments, are presented. The study allows to identify significant and minor stages of climate change. A prominent warming occurred between ca. 4800–3400 cal. yr BP. Minor warming intervals were identified at ca. 8500–8000 cal. yr BP, ca. 5600–5300 cal. yr BP, ca. 5130–4800 cal. yr BP, ca. 3400–2400 cal. yr BP, and ca. 1200–800 cal. yr BP. A notable cooling stage transpired at ca. 7500–5600 cal. yr BP, with a peak cold period around 7300–7000 cal. yr BP, and possibly at ca. 1800–1200 cal. yr BP. Minor relative cooling phases took place during next periods: ca. 8000–7500 cal. yr BP, ca. 5300–5130 cal. yr BP, and ca. 2400–1800 cal. yr BP. Additionally, short-term relative cooling and warming are suggested to have occurred during the period ca. 800–600 cal. yr BP. Taking into account the absence of suitable glaciers for obtaining the ice core for paleoclimatic records in the considered maritime Antarctic region, this paleolimnological study provides a foundation for broader understanding of the Holocene climate change in the West Antarctica.