Ionic composition of pore water in shallow shelf deposits of the Laptev Sea

The cationic and anionic compositions of pore water in shallow deposits of Buor-Khaya Bay is studied. Significant concentration heterogeneity of the vertical ionic profile in the studied drill columns is shown. It is established that the vertical ionic profile of shelf deposits of the Laptev Sea is basically formed under the influence of thawing of underwater permafrost, hydrodynamic conditions, the water mass, and heat flows and depends on the lithological and granulometric types of the deposits. The highest concentrations of ions are registered in plant detritus represented by ground grass vegetation. A relationship between the ionic composition of pore water and cryogenic state of the sequence and its variability is demonstrated using the method of major components.     

Environmental changes in the eastern shelf of the Laptev Sea in the late-glacial time

Data on the interaction of the land-sea system have been obtained based on complex palynological analysis (pollen and spores of terrestrial plants and cysts of dinoflagellates and green algae) and the SEM analysis of sediments from boreholes drilled on the Laptev Sea’s eastern shelf and dated by the radiocarbon method. The spore and pollen ensemble revealed for the first time in late-glacial sediments of the boreholes indicates that warming which started within the time interval 15.6–10.7 ka ago was accompanied by short-term cooling events. Sedge and herb phytocoenosis predominated in the vegetation cover of East Arctic Siberia during the cooling. The climate warming favored the distribution of shrub birch tundra. The results of the analysis of organic-walled phytoplankton and the low value of the AH criterion indicate cold-water conditions with a longstanding ice sheet at sea. After the interval that ended 10.7 ka ago, thermophilic species of dinocysts appeared on the outer shelf, and the AH-criterion value increased, which suggests an active inflow of relatively warm North-Atlantic water to the shelf. Based on the comparison of the obtained results, an inference has been made about frequent changes of the climate in the region and the environment in the eastern part of the Laptev Sea during the deglaciation epoch.     

Thermotomographic model and oil-and-gas forecasting for the sedimentary cover of the Laptev Sea shelf

2D and 3D geotemperature models of the sedimentary cover within the Laptev Sea shelf have been calculated. The presence of several “thermal domes” (zones of uplifted isotherms) has been found. Based on analogous sedimentary basins with domes, which correlate to the location of hydrocarbon deposits, the forecasting has been made that the Ust-Lena graben and Omoloi trough are the most promising areas for oil-and-gas prospecting.     

Detection of methane ebullition in shelf waters of the Laptev Sea in the Eastern Arctic Region

Doklady Earth Sciences -     

Characteristic of quaternary sedimentation on a shelf of the Laptev Sea according to the molecular composition of n-alkanes

The molecular composition and distribution of the concentration of n-alkanes are considered for sediments of boreholes drilled in the shallow part of the Laptev Sea, in the area of Buor Khaya Gulf. The diverse molecular composition of n-alkanes is dominated by long-chain odd homologs, which indicate terrigenous organic matter (OM). Heterogeneous distribution and burial of OM are shown under conditions of pulsating contribution of river and thermoabrasive material and multiple changes in them up to hiatuses in sedimentation.     

Locations of nonstructural hydrocarbon traps on the shelf of the northern Barents Sea

This paper considers the results of summarized integrated geophysical investigations that were carried out from 2006 to 2012. The investigations included common depth point (CDP) seismic reflection survey, over water gravity survey, and differential hydromagnetic exploration with a total work scope of 30 000 linear kilometers. The deep structural tectonic plan, the structural and lithofacies features of the sedimentary cover section on the basic reflecting boundaries, and the features of the seismogeological complexes and seismic sections on a depth scale have been studied, and geological oil-and-gas zoning of the Northern Barents shelf has been made. Seventy-nine local anticlinal highs have been revealed, and the zones with potential nonstructural hydrocarbon traps have been determined. Due to the lack of huge anticlinal highs in the northern Barents Sea region, nonstructural traps are of interest in studying and replacing the mineral raw material base of Russia, as well as for arranging marine exploration.     

The first ever application of electromagnetic sounding for mapping the submarine permafrost table on the Laptev Sea shelf

The inconsistency between the position of the submarine permafrost table in the East Arctic seas revealed by simulation and anomalies in the distribution of dissolved methane associated with ascending torchlike ejections of bubbling methane made it necessary to develop a representative geophysical express method, which allows the position of the submarine permafrost table to be determined. The method is based on sounding of the generated primary electromagnetic field in the near zone of the medium and measurement of the induced secondary EM field. The reliability of the method is confirmed by core drilling in the Laptev Sea.     

Paleogeography of the Laptev Sea eastern shelf in the first half of Holocene based on faunal and palynological evidence

The Holocene paleogeography of the Laptev Sea eastern shelf is reconstructed based on mollusks, foraminifers, ostracodes, spores and pollen found for the first time. Environmental changes in the course of the postglacial (Holocene) transgression are detectable from quantitative analysis of three ostracod assemblages. Three phases of the transgression are established based on ostracode assemblages from the AMS ¹⁴C-dated core from the ancient valley of Yana River, central part of the Laptev Sea eastern shelf. After the first phase of coastal brackish-water environment (11.3–11.1 ka), shallow-water marine taxa appeared in the transitional phase (11.1–10.3 ka), and the third phase of modern sea conditions commenced 10.3 ka ago. Spore and pollen assemblages from the core indicate that the early Holocene land vegetation was dominated by tundra plant communities adapted to climatic conditions not warmer than at present. In the interval of 9.3–8.0 ka, climate was increasingly humid and warm, and forest-tundra vegetation occupied more favorable zones of the region.     

Picophytoplankton of the Laptev Sea in Autumn

Doklady Earth Sciences - The abundance and biomass of picophytoplankton, the total concentration of chlorophyll a, and the contribution of the picofraction to chlorophyll a were studied in the...     

Probable distribution of Upper Jurassic and Lower Cretaceous deposits on the Laptev Sea shelf and their petroleum resource potential

The analysis of Upper Jurassic and Lower Cretaceous marine sections developed in surrounding structures of the Laptev Sea revealed that all of them are composed of terrigenous rocks, which enclose abundant concretions cemented by calcareous material. The Upper Jurassic portion of the section is the most variable in thickness and stratigraphic range of sediments usually including hiatuses. Its Lower Cretaceous part represented by the Boreal Berriasian (=Ryazanian) and lower Valanginian stages is most complete. The Upper Jurassic and Lower Cretaceous sections are usually composed of fine-grained rocks (clays and mudstones) in the west and coarser cemented varieties (siltstones and sandstones) with rare mudstone intercalations in the east. Practically all the investigated Upper Jurassic and Lower Cretaceous sections include readily recognizable age and facies analogs of the Bazhenovo Formation and Achimov sandstones, which are petroliferous in West Siberia. There are grounds to assume the occurrence of these formations also on the Laptev Sea shelf, which is confirmed by seismic records. Conditions favorable for the formation of potential hydrocarbon reservoirs could exist in the western part of the paleobasin along the Nordvik Peninsula coast and northeastern Tamyr Peninsula margin. Paleotectonic reconstructions presented in this work are well consistent with stratigraphic conclusions.     

Alkanes in Quaternary deposits of the Laptev Sea

The distribution and genesis of n-alkanes in sediments from the 38-m sequence obtained during core boring in the Ivashkina lagoon were studied. Sediments were formed in the Holocene as a result of thermokarst and penetration of seawater. The sequence mostly includes permafrost rocks partially molten in the upper horizons and covered by Quaternary deposits, which are mostly the products of thermoabrasion.     

Changes in the deposition of terrestrial organic matter on the Laptev Sea shelf during the Holocene: evidence from stable carbon isotopes

Stable carbon isotope ratios in the organic fraction of surface sediments from the Laptev Sea shelf were analyzed in order to study the modern distribution pattern of terrestrial organic matter. The '¹³C_org signature of the surface sediments range from -26.6‰ near the coastal margin to -22.8‰ in the north towards the outer shelf. Characterizing the possible sources of organic matter by their '¹³C_org signature reveals that the terrestrial influence reaches further north in the eastern than in the western Laptev Sea. Downcore records of the '¹³C_org, measured on three AMS ¹⁴C-dated cores from water depths between 46 and 77 m, specify the spatial and temporal changes in the deposition of terrestrial organic matter on the Laptev Sea shelf during the past 12.7 ka. The major depositional changes of terrestrial organic matter occurred between 11 and 7 ka and comprised the main phase of the southward retreat of the coastline and of the river depocenters due to the postglacial sea level rise.     

Structure and petroleum potential of the Laptev Sea region

Analysis of specific features of the structure and petroleum potential of the Aptian Laptev Sea region is presented. Age and lithological composition of the sedimentary cover of the shelf are predicted. Oil- and-gas source sequences, catagenetic zonation of organic matter, and potential petroliferous objects in large structural zones are characterized. It is supposed that oil likely played a significant role in the phase composition of naphtides. Its formation in the land and coastal shelf zone was stipulated by the presence of high-quality oil source rocks in the Riphean–Phanerozoic cover of the Siberian Craton. In the remaining part of the shelf, where the Aptian–Cenozoic rift system is widespread, oil can also be present in pools, because the rocks likely accommodate not only gas source rocks, but also deltaic and prodeltaic oil-and-gas source rocks that are located in the main oil formation zone. The above properties are specific features of the study region, and its hydrocarbon potential will be increasing steadily with the shelf development.     

Geology and hydrocarbon resources of the continental shelf in Russian Arctic seas and the prospects of their development

The overall jump in global demand for gas, and especially oil, gives rise to particular concern regarding mankind’s energy future. In the middle and late 21st century, the crucial role in securing oil and gas supply of mankind will be played by sedimentary basins in the Arctic Ocean deep-water area, including those of the continental shelf in Russia’s Arctic seas. There is a 0.90 probability that the initial in-place resources of hydrocarbons in the Arctic Ocean will be greater than 90 Btoe. The estimates predict the rise of oil and gas industries on the Arctic shelves in the near future.     

Isotope and geochemical parameters of hydrocarbon gases in bottom sediments of the shelf of the East Siberian Sea

The research results on gas geochemistry of hydrocarbon gases in bottom sediments of the East Siberian Sea are presented. The key isotope and geochemical parameters of syn- and epigenetic hydrocarbon gases are determined. The correlations between the molecular masses of hydrocarbon fractions and the carbon isotope composition on hydrocarbon gases of various origin are revealed.     

Seismic geologic structure model for the sedimentary cover of the Laptev Sea part of the Lomonosov Ridge and adjacent parts of the Amundsen Plain and Podvodnikov Basin

Seismic data on the southern (Laptev Sea) extremity of the Lomonosov Ridge were used to develop a new structural model for the sedimentary cover. It permitted a correlation between the seismic cross-sections of the ridge crest and two deep-sea basins: the Podvodnikov Basin and the Amundsen Plain. It is the first time that a seismic model has taken into account both regional seismic-reflection profiles obtained from NP drifting ice stations and recent high-resolution CDP data. Our seismic model agrees both with geological data on the Laptev Sea continental margin and the data obtained from deep-sea drilling into the Lomonosov Ridge under the IODP-302 project. The sedimentary cover of the southern Lomonosov Ridge and adjacent parts of the Amundsen Plain and Podvodnikov Basin was dated at the Aptian–Cenozoic. The sedimentary section is divided by two main unconformities, of Campanian–Paleocene and Oligocene–Early Miocene ages. The cover contains a structurally complicated graben system, which is an extension of the New Siberian system of horsts and grabens, recognized in the shelf. Sedimentation began in the grabens in the Aptian–Albian and ended with their complete compensation in the Paleocene.     

Early postglacial environmental conditions in the northeastern Laptev Sea region

A comparison of the first results of a comprehensive micropaleontological analysis (pollen, spores, organic-walled microfossils, diatoms, ostracods) and radiocarbon ages (AMS¹⁴C) from sediment core recovered in the northeastern outer shelf of the Laptev Sea (51 m water depth) revealed a temporal coincidence between terrestrial and marine environmental changes that occurred between 11.2–10.3 cal ka. This interval provided evidence for a landscape transition from grass tundra to shrub tundra and the development of a freshwater estuarine basin with the strong influence of riverine discharge and a minor advection of North Atlantic waters. The establishment of a warmer and wetter climate promoted the expansion of shrub tundra habitats. The interval of 9.5–7.5 cal ka recorded a transition from a shrub tundra environment to forest-tundra vegetation. This interval also revealed a series of short-term temperature fluctuations, when summer temperatures were 3–4°C higher than today. The active advection of North Atlantic waters and the increase in salinity were also recorded by this interval.     

Environmental conditions of the Laptev Sea region in the late postglacial time

The comparison between the first results of comprehensive micropaleontological analysis (pollen, spores, foraminifera, and ostracods) and those of radiocarbon dating (AMS¹⁴C) for the sediments of the eastern inner shelf of the Laptev Sea (the core collected from depth of 37 m) indicates that considerable changes in natural conditions in the sea and on land coincide in time and refer to the time period of 1500–1700 years B.P. This period is characterized by changes in microfossils: appearance of thermophilic pollen and planktonic foraminifera and increase in total number of benthic foraminifera and ostracods. Intense warming and humidification of the climate reconstructed for this 200-year period promoted the expansion of large-shrub tundra. Summer air temperatures were lower than that in the peak mid-Holocene climatic optimum by 2°–3°C, but 1°C higher than the present-day temperature. An estuary freshwater basin developed: it was strongly affected by river discharge, but North Atlantic waters also intensely penetrated here in short-term intervals. In general, the studied microfossil complex reflects the relatively stable environmental conditions and decrease in seawater salinity in the eastern part of the Laptev Sea shelf during the last 2300 years.     

Triassic stratigraphy of the Eastern Laptev Sea coast and New Siberian Islands

Studies of the paleontology and stratigraphy of the Triassic strata from the Lena–Olenek interfluve area and Kotelny Island have important implications for improving the accuracy of interregional and global correlations, refining the Boreal standard and international stratigraphic scale for the Triassic System. The importance of this study is also underlined by the necessity of refining the stratigraphic basis for regional geological exploration in Arctic zone that now became the focus for building the country’s strategic resource base. Analysis of recent paleontological and stratigraphic data from key Triassic sections in the Laptev Sea coastal region provides new age constraints for the Triassic strata based on different faunal groups. The Triassic stratigraphic scheme for the region has been refined using new data on the paleontology, thickness variations, and boundaries of local stratigraphic subdivisions.     

The Upper Jurassic of the Laptev Sea: interregional correlations and paleoenvironments

The Late Jurassic evolution of Boreal and Arctic basins is reflected in the widespread deposition of organic-rich black shales (source rocks). In this connection, the priority should be placed on the development and refinement of zonal schemes for the Upper Jurassic of the Laptev Sea coast based on ammonites, foraminifers, ostracods, dinocysts, and spores and pollen from reference sections as the basis for stratigraphic, paleogeographic, and facies studies. The Upper Jurassic and Lower Cretaceous reference section of interest is located on the left side of the Anabar Bay of the Laptev Sea (Nordvik Peninsula, Urdyuk-Khaya Cape). An uninterrupted and continuous section from Upper Oxfordian to Lower Valanginian is exposed in coastal cliffs and consists mainly of silty clay deposits with abundant macro- and microfossils. A reliable biostratigraphic subdivision of the Upper Jurassic interval of this section was taken as the basis for the assessment of the correlation potential of different fossil groups and subsequent interregional correlations, facies analysis, and detailed paleogeographic reconstructions of the study area. The analysis of variations in the composition of macrobenthic communities and microphytoplankton and terrestrial palynomorph assemblages and the biofacies analysis allowed the reconstruction of the evolution of marine paleoenvironmental settings in the western part of the Anabar–Lena sea and in the terrestrial settings in the adjacent land area of Siberia.