Siberian Archipelago Research Articles

Interglacial History of a Palaeo-lake and Regional Environment: A Multi-proxy Study of a Permafrost Deposit from Bol’shoy Lyakhovsky Island, Arctic Siberia

Chironomid, pollen, and rhizopod records from a permafrost sequence at Bol’shoy Lyakhovsky Island (New Siberian Archipelago) document the development of a thermokarst palaeo-lake and environmental conditions in the region during the last Interglacial (MIS 5e). Open Poaceae and Artemisia associations dominated vegetation at the beginning of the interglacial period. Rare shrub thickets (Salix, Betula nana, Alnus fruticosa) grew in more protected and wetter places as well. Saalian ice wedges started to melt during this time, resulting in the formation of an initial thermokarst water body. The high percentage of semi-aquatic chironomids suggests that a peatland-pool initially existed at the site. A distinct decrease in semi-aquatic chironomid taxa and an increase in lacustrine ones point to a gradual pooling of water in the basin, which could in turn induce thermokarst and create a permanent pond during the subsequent period. The highest relative abundance of Chironomus and Procladius reflects unfrozen water remaining under the ice throughout the ice-covered period during the later stage of palaeo-lake development. The chironomid record points to three successive stages during the history of the lake: (1) a peatland pool; (2) a pond (i.e., shallower than the maximum ice-cover thickness); and (3) a shallow lake (i.e., deeper than the maximum ice-cover thickness). The trend of palaeo-lake development indicates that intensive thermokarst processes occurred in the region during the last Interglacial. Shrub tundra communities with Alnus fruticosa and Betula nana dominated the vegetation during the interglacial optimum. The climate was moister and warmer than present. The results of this study suggest that quantitative chironomid-based temperature reconstructions from Arctic thermokarst ponds/lakes may be problematic due to other key environmental factors, such as prolonged periods of winter anoxia and local hydrological/geomorphological processes, controlling the chironomid assemblages.

Mesozoic syncollision siliciclastic sediments of the Bols’shoi Lyakhov Island (New Siberian Islands)

Graywackes and shales of the Bols’shoi Lyakhov Island originally attributed to the Mesozoic were subsequently considered based on microfossils as the Late Proterozoic in age. At present, these sediments in the greater part of the island are dated back to the Permian based on palynological assemblages. In the examined area of the island, this siliciclastic complex is intensely deformed and tectonically juxtaposed with blocks of oceanic and island-arc rocks exhumed along the South Anyui suture. The complex is largely composed of turbidites with members displaying hummocky cross-stratification. The studied mineral and geochemical characteristics of the rocks defined three provenances of clastic material: volcanic island arc, sedimentary cover and/or basement of the ancient platform, and exotic blocks of oceanic and island-arc rocks such as serpentinites and amphibolites. All the rock associations represent elements of an orogenic structure that originated by collision of the New Siberian continental block with the Anyui-Svyatoi Nos island arc. Flyschoid sediments accumulated in a foredeep in front of the latter structure in the course of collision. The Late Jurassic volcanics belonging to the Anyui-Svyatoi Nos island arc determine the lower age limit of syncollision siliciclastic rocks. Presence of Late Jurassic zircons in sandstones of the flyschoid sequence in Bols’shoi Lyakhov Island is confirmed by the fission-track dating. The upper age limit is determined by the Aptian-Albian postcollision granites and diorites intruding the siliciclastic complex. Consequently, the flyschoid sequence is within stratigraphic range from the terminal Late Jurassic to Neocomian. It appears that Permian age of sediments suggested earlier is based on redeposited organic remains. The same Late Jurassic-Neocomian age and lithology are characteristic of fossiliferous siliciclastic sequences of the Stolbovoi and Malyi Lyakhov islands, the New Siberian Archipelago, and of graywackes in the South Anyui area in Chukchi Peninsula. All these sediments accumulated in a spacious foredeep that formed in the course the late Cimmerian orogeny along the southern margin of the Arctic continental block.

Vegetation and Climate of the New Siberian Islands for the Past 15,000 Years

This article presents paleobotanical and paleoclimatic reconstructions of the New Siberian Islands for the past 15,000 years based on data from Quaternary geology and archaeology collected in the late 1980s and early 1990s as part of research conducted by the Arctic and Antarctic Research Institute, St. Petersburg, Russia. The data were obtained from 18 reference sites from Holocene and upper Pleistocene deposits on Kotelny Island, the largest island of the New Siberian Archipelago, and from the smaller and more northerly Zhokhov Island. Conclusions are also drawn from the composition of the avian fauna recovered during the excavations of the Zhokhov archaeological site on that island. The composition of bird species is an important source of information which helps interpret the results of palynological analysis because the birds are very sensitive indicators of changes in temperature and vegetation. The palynological analysis has revealed a sequence of 13 spore-pollen complexes that have been generalized as a scheme showing a broad sequence of events in the area of survey. The complexes belonging to the early Holocene time are markedly different from the others. The most favorable conditions occurred in the time span from 10,000 to 9000 BP. After 8000 yr ago the fluctuations were of lesser amplitude, and both their sequence and timing were close to those of Northern Europe, though there appear to be differences between the two areas in the range of variation.

230Th/U Dating of Frozen Peat, Bol'shoy Lyakhovsky Island (Northern Siberia)

The chronology of Quaternary paleoenvironment and climate in northeastern Siberia is poorly understood due to a lack of reliable numerical age determinations. The best climatic archives are ice-rich permafrost sequences, which are widely distributed in northeastern Siberia. For this study, 230Th/U-ages were determined by thermal ionization mass spectrometry (TIMS) from frozen peat in a permafrost deposit at the southern cliff of the Bol'shoy Lyakhovsky Island (New Siberian Archipelago), west of the Zimov'e River. These yielded a Pre-Eemian “isochron”-corrected 230Th/U-age of 200,900±3400 yr. This result is reliable because permafrost deposits behave as closed systems with respect to uranium and thorium. Our findings suggest that 230Th/U dating of frozen peat in permafrost deposits is a useful tool for the reconstruction of the Middle Quaternary environment of northern Siberia and of the whole Arctic.

Palynological Investigations of Miocene Deposits on the New Siberian Archipelago (U.S.S.R.)

New paleobotanical data (mainly palynological) are reported from Miocene beds of the New Siberian Islands. The palynoflora has a number of distinctive features: the presence of typical hypoarctic forms, the high content taxa representing dark coniferous assemblages and the considerable proportion of small-leaved forms. Floristic comparison with the paleofloras or the Beaufort Formation in arctic Canada allows interpretation of the evolution of the Arctic as a region during Miocene-Pliocene time. The paper is a preliminary analysis of the mechanisms of arctic florogenesis. The model of an adaptive landscape is considered in relation to the active eustatic drying of polar shelves. Key words: palynology, U.S.S.R., New Siberian Islands, Miocene, Arctic, florogenesis