Sediments from Lake Cheko (Siberia), a possible impact crater for the 1908 Tunguska Event

Cheko, a small lake located in Siberia close to the epicentre of the 1908 Tunguska explosion, might fill a crater left by the impact of a fragment of a Cosmic Body. Sediment cores from the lake’s bottom were studied to support or reject this hypothesis. A 175‐cm long core, collected near the center of the lake, consists of an upper ～1 m thick sequence of lacustrine deposits overlaying coarser chaotic material. ²¹⁰Pb and ¹³⁷Cs indicate that the transition from lower to upper sequence occurred close to the time of the Tunguska Event. Pollen analysis reveals that remains of aquatic plants are abundant in the top post‐1908 sequence, but are absent in the lower pre‐1908 portion of the core. These results, including organic C, N and δ¹³C data, suggest that Lake Cheko formed at the time of the Tunguska Event.     

The origin of Lake Cheko and the 1908 Tunguska Event recorded by forest trees

Several lines of evidence indicate that Cheko, a small lake close to the epicentre of the 1908 Tunguska Event (TE), fills a crater left behind by a fragment of the Tunguska Cosmic Body that impacted the ground downrange of the main explosion. It is thought that over 80 million trees were flattened or burnt as a consequence of the TE. However, a small number of trees in the devastated area survived the explosion and recorded in their growth‐ring patterns the environmental changes that followed this event. Some of those trees were found around Lake Cheko, ~10 km NW of the inferred TE epicentre. We analysed new data from the floor of Lake Cheko, including seismic‐reflection profiles, side‐scan sonar and video images, as well as dendrochronological evidence in tree samples collected along the shores, to test the hypothesis of a 1908 formation of the lake.     

New data on accounts of the 1908 Tunguska event

There has been a longstanding debate about the nature of the 1908 Tunguska event. Many eyewitness accounts were collected more than half of a century after the event. Among these are many second‐hand oral accounts after the generation of eyewitnesses faded away. However, several years ago, two valuable publications appeared containing first‐hand eyewitness accounts collected by a Russian ethnographer, Sev'yan Vainshtein, during an expedition to the settlement of Sulomay in 1948. This paper presents additional details of these accounts, based on the author's discussions with Vainshtein before he died in 2008, and demonstrates how these accounts shed new light on what happened in Tunguska in June 1908.     

Paleobiology of the Mesoproterozoic-Neoproterozoic transition: the Sukhaya Tunguska Formation, Turukhansk Uplift, Siberia

Silicified carbonates of the latest Mesoproterozoic Sukhaya Tunguska Formation, northwestern Siberia, contain abundant and diverse permineralized microfossils. Peritidal environments are dominated by microbial mats built by filamentous cyanobacteria comparable to modern species of Lyngbya and Phormidium. In subtidal to lower intertidal settings, mat-dwelling microbenthos and possible coastal microplankton are abundant. In contrast, densely woven mat populations with few associated taxa characterize more restricted parts of tidal flats; the preservation of vertically oriented sheath bundles and primary fenestrae indicates that in these mats carbonate cementation was commonly penecontemporaneous with mat growth. Eoentophysalis mats are limited to restricted environments where microlaminated carbonate precipitates formed on or just beneath the sediment surface. Most microbenthic populations are cyanobacterial, although eukaryotic microfossils may occur among the simple spheroidal cells interpreted as coastal plankton. Protists are more securely represented by large (up to 320 micrometers in diameter) but poorly preserved acritarchs in basinal facies. The Sukhaya Tunguska assemblage contains 27 species in 18 genera. By virtue of their stratigraphic longevity and their close and predictable association with specific paleoenvironmental conditions, including substrates, Proterozoic cyanobacteria support a model of bacterial evolution in which populations adapt rapidly to novel environments and, thereafter, resist competitive replacement. The resulting evolutionary pattern is one of accumulation and stasis rather than the turnover and replacement characteristic of Phanerozoic plants and animals.     

Sedimentation models for the Jurassic stratum in the Potanai oilfield (Western Siberia)

The sedimentation models for the upper part of the Tyumen Formation (the J_2?C3 productive layers) and the Abalak Formation (J ₁ ² and J ₁ ¹ ) were constructed on the basis of data from lithological studies of Jurassic stratum cores (the Potanai oilfield in the northeast Shaim megaswell). The models show that the sedimentation environment sequentially changed from the continental to the deltaic, lagoonal, and shelf types through the Middle and Late Jurassic.     

Recent Seasonal Dynamics of the Flux of Charcoal Particles in the Sediments of Shira Lake (Southern Siberia) Estimated by Sedimentation Traps

Doklady Earth Sciences - Forest and steppe fires pose a threat to natural ecosystems and significantly worsen the living conditions of people living in the surrounding areas. Charcoal particles...     

Geochemistry of obsidian from Krasnoe Lake on the Chukchi Peninsula (Northeastern Siberia)

Doklady Earth Sciences - This report considers features of the geochemical composition of obsidian from beach sediments of Krasnoe Lake along the lower course of the Anadyr River, as well as from...     

Change in the circulation regime in the stratified saline Lake Shira (Siberia,Republic of Khakassia)

The in-situ data on the vertical structure and stability of the vertical stratification of saline Lake Shira over the past decade (2007–2015) are analyzed. Simplified mathematical models have shown that strong wind in the autumn of 2014 together with rather thick ice in the winter of 2015 caused a change in the circulation regime of this water reservoir from meromictic (incomplete mixing) to holomictic (compete mixing). Based on the results obtained, a circulation regime for deep saline lakes located in the continental climate zone, in particular, in the arid zones of Southern Siberia (Khakassia, Transbaikal, and Altai) can be predicted under various climate scenarios of the future.     

Magnetic properties of bottom sediments from Meromectic Shira Lake (Siberia,Russia)

Magnetic properties were studied in bottom sediments of saline meromictic Shira Lake by the methods of static magnetometry and resonance Mössbauer spectroscopy for the first time. All layers of bottom sediments contain nanosized single-domain magnetite particles produced by magnetotactic bacteria. The concentration of magnetite in bottom sediments decreased with depth, reaching a local minimum in the layer corresponding to the minimal level of the lake observed in 1910–1930. It is demonstrated that biogenic magnetite may indicate climate-related changes in the level of Shira Lake, in addition to the other biological and geochemical characteristics.     

Sedimentation Model of Rocks of the Pokur Formation: Basis for the Prediction of Filtration-Volumetric Characteristics (Upper Cretaceous,West Siberia)

The HC-rich Upper Cretaceous rocks in the West Siberian petroliferous basin comprise genetically heterogeneous sediments, resulting in the specificity of morphology, structure, and properties of natural reservoirs therein. One of the possible techniques for more reliable predictions of reservoir rock characteristics is the utilization of results of sedimentation modeling based on the complex interpretation of borehole and seismic data. The paper presents an example of the realization of such approach for refining the structure of oil-and-gas pools in the Pokur Formation of the Russko-Chasel megaswell.     

Deformation style in the damage zone of the Mondy fault: GPR evidence (Tunka basin,southern East Siberia)

The Mondy strike-slip fault connects the W-E Tunka and N-S Hovsgol basins on the southern flank of the Baikal rift system. Ground penetrating radar (GPR) surveys in its damage zone provide constraints on thicknesses, dips, and plunges of fault planes, as well as on the amount and sense of vertical slip. Strike-slip faulting in the southern segment of the Mondy fault within the territory of Russia bears a normal slip component of motion along the W-E and NW planes. These motions have produced negative flower structures in shallow crust appearing as grabens upon Pleistocene fluvioglacial terraces. The amount of normal slip estimated from the displacement of reflection events varies over the area and reaches its maximum of 3.4 m near Mondy Village. In the Kharadaban basin link, left-lateral strike slip displaces valleys of ephemeral streams to 22 m, while normal slip detected by GPR reaches 2.2 m; this normal-to-strike slip ratio corresponds to a direction of ~ 6° to the horizon. The angles of dips of faults are in the range 75°-79°; the thicknesses of fault planes marked by low- or high-frequency anomalies in GPR records vary from 2.5 to 17.0 m along strike and decrease with depth within a few meters below the surface, which is common to near-surface coseismic motions. Many ruptures fail to reach the surface but appear rather as sinkholes localized mainly in fault hanging walls. The deformation style in the damage zone of the Mondy fault bears impact of the NW Yaminshin fault lying between its two segments. According to photoelasticity, the stress field changes locally at the intersection of the two faults, under NE compression at 38°, till the inverse orientations of principal compression and extension stresses. This stress pattern leads to a combination of normal and left-lateral strike slip components.     

Position of the base of the Quaternary in West Siberia (based on paleobotanical and paleomagnetic evidence)

This study reviews a possible new position of the base of the Quaternary in West Siberia based on paleobotanical and paleomagnetic data in view of ratification of ICS recommendations by the IUGS Executive Committee and the decision of the Interdepartmental Commission on the Quaternary Stratigraphy of Russia to lower the base of the Quaternary (and, therefore, the base of the Pleistocene) at 2.58 Ma. The paleobotanical and paleomagnetic data from two horizons on the West Siberian plain (Kulunda, attributed in part to the formerly Neogene Gelasian Stage, and Kochki, attributed to the Quaternary) were used to identify global cooling that occurred synchronously with a regional cooling phase determined at the base of the Gelasian at Monte San Nicola, Italy.     

新疆乌伦古湖的210Pb、137Cs测年与现代沉积速率

通过对乌伦古湖2个沉积柱样的²¹⁰Pb和¹³⁷Cs剖面的观测,运用²¹⁰Pb过剩法和¹³⁷Cs时标法估算了近几十年来的平均沉积速率。研究结果表明：²¹⁰Pb过剩法的CRS模式测定的沉积速率的变化范围为0.018～0.071 g/(cm²·a),137Cs时标法测定沉积速率的变化范围为0.034～0060 g/(cm²·a),两者具有较高的一致性。20世纪60年代以前,人类活动对本区影响相对较小,湖泊的沉积速率主要受自然因素的影响;20世纪60年代以后,随着乌伦古河中、下游农业的发展和调水工程的实现,人类活动逐渐成为主导湖泊演化的因素,从而影响乌伦古湖沉积速率。     

Molar tooth structures: Formation and specificity of carbonate diagenesis in the Late Precambrian,Middle Riphean Sukhaya Tunguska Formation of the Turukhansk Uplift,Siberia

A new way of formation of the problematic Molar Tooth (MT) structures, which, along with stromatolites, could be considered as a “visiting card” of the Riphean, was examined on the example of carbonate sediments of the Riphean Sukhaya Tunguska Formation from the Turukhansk Uplift, Siberia. These structures were formed due to the consequent replacement of oozy constituent of carbonate sediments with calcitic microsparite in the course of diagenesis with substantial shift of the dynamic equilibrium toward the solution in the sediment—porous solution—microsparite system. An excess of soluble magnesium significantly hampering precipitation of crystal cores of the future calcitic microsparite could be one of the possible causes of the shift. It is suggested that magnesium mole fraction of the porous solutions was related to the early dissolution of the metastable high-Mg calcite of the silty sediment brought from the areas of active stromatolite formation. The facies occurrence of the MT-structures indicates that they occurred in descending trails of the most productive zones of carbonate platforms tapering toward the basin. The synchronism of stratigraphic trends of the MT-structures and stromatolites in the Precambrian resulted from the evolution of carbonate-productive microbe-mineral systems which were the immediate suppliers of fine-grained carbonate sediment into external zones of carbonate platforms.     

New data on the lithology of coastal facies of the Turtas formation (Upper Oligocene,Southwestern Siberia)

Peculiarities of the material composition and microstructure of coastal facies of Turtas Lake–Sea were studied in its marginal southwestern part for the first time. Interpretation of the lithological data showed that the deposits considered were formed under the conditions of a slightly saline basin and nearby full-flow river network. Based on the lithological, geochemical, and micropaleontological studies of clay–siliceous rocks of the Lower Turtas Formation (the boundary zone between the Tyumen and Sverdlovsk regions), additional support for the freshwater lake regime of the Late Oligocene Turtas basin is given.     

西南太平洋新喀里多尼亚洛亚蒂盆地的现代沉积作用

洛亚蒂盆地沉积物是棕黄色软泥和生物碎屑砂。它们有5个来源:新喀里多尼亚堤礁、深海生物群落、浮游生物群体和火山碎屑。沉积物由浊流和半深海沉积物的互层组成,可以通过粒度分析,矿物成分和生物碎屑的分析将两者加以区分。通过对蒂奥和利富之间的岩心的研究,认识到在蒂奥水道对面,分布着一个展布广阔,但幅度很小、延伸50km的海底扇。     

A New Finding of a Docodontan (Mammaliaformes,Docodonta) in the Lower Cretaceous of Western Siberia

Doklady Earth Sciences - The first finding of the docodontan (a lower molar of Sibirotherium sp.) from the Bol’shoi Kemchug 3 locality (Ilek Formation, Lower Cretaceous) in Krasnoyarsk...