Coastal permafrost landscape development since the Late Pleistocene in the western Laptev Sea,Siberia

Winterfeld, M., Schirrmeister, L., Grigoriev, M. N., Kunitsky, V. V., Andreev, A., Murray, A. & Overduin, P. P. 2011: Coastal permafrost landscape development since the Late Pleistocene in the western Laptev Sea, Siberia. Boreas, 10.1111/j.1502‐3885.2011.00203.x. ISSN 0300‐9483. The palaeoenvironmental development of the western Laptev Sea is understood primarily from investigations of exposed cliffs and surface sediment cores from the shelf. In 2005, a core transect was drilled between the Taymyr Peninsula and the Lena Delta, an area that was part of the westernmost region of the non‐glaciated Beringian landmass during the late Quaternary. The transect of five cores, one terrestrial and four marine, taken near Cape Mamontov Klyk reached 12 km offshore and 77 m below sea level. A multiproxy approach combined cryolithological, sedimentological, geochronological (¹⁴C‐AMS, OSL on quartz, IR‐OSL on feldspars) and palaeoecological (pollen, diatoms) methods. Our interpretation of the proxies focuses on landscape history and the transition of terrestrial into subsea permafrost. Marine interglacial deposits overlain by relict terrestrial permafrost within the same offshore core were encountered in the western Laptev Sea. Moreover, the marine interglacial deposits lay unexpectedly deep at 64 m below modern sea level 12 km from the current coastline, while no marine deposits were encountered onshore. This implies that the position of the Eemian coastline presumably was similar to today's. The landscape reconstruction suggests Eemian coastal lagoons and thermokarst lakes, followed by Early to Middle Weichselian fluvially dominated terrestrial deposition. During the Late Weichselian, this fluvial landscape was transformed into a poorly drained accumulation plain, characterized by widespread and broad ice‐wedge polygons. Finally, the shelf plain was flooded by the sea during the Holocene, resulting in the inundation and degradation of terrestrial permafrost and its transformation into subsea permafrost.     

Late Saalian and Eemian palaeoenvironmental history of the Bol''shoy Lyakhovsky Island (Laptev Sea region, Arctic Siberia)

Palaeoenvironmental records from permafrost sequences complemented by infrared stimulated luminescence (IRSL) and [Formula: See Text]Th/U dates from Bol'shoy Lyakhovsky Island (73°20'N, 141°30'E) document the environmental history in the region for at least the past 200 ka. Pollen spectra and insect fauna indicate that relatively wet grass-sedge tundra habitats dominated during an interstadial c. 200-170 ka BP. Summers were rather warm and wet, while stable isotopes reflect severe winter conditions. The pollen spectra reflect sparser grass-sedge vegetation during a Taz (Late Saalian) stage, c. 170-130 ka BP, with environmental conditions much more severe compared with the previous interstadial. Open Poaceae and Artemisia plant associations dominated vegetation at the beginning of the Kazantsevo (Eemian) c. 130 ka BP. Some shrubs (Alnus fruticosa, Salix, Betula nana) grew in more protected and wetter places as well. The climate was relatively warm during this time, resulting in the melting of Saalian ice wedges. Later, during the interglacial optimum, shrub tundra with Alnus fruticosa and Betula nana s.l. dominated vegetation. Climate was relatively wet and warm. Quantitative pollen-based climate reconstruction suggests that mean July temperatures were 4-5°C higher than the present during the optimum of the Eemian, while late Eemian records indicate significant climate deterioration.     

Genesis of marginal moraines in the Caucasus

Composition and fabric of different moraines in the glaciated areas of Central Caucasus were studied to elucidate sedimentary environments and geomorphological processes. Methods used included particle-size distribution, pebble and mineral counts, shape analysis of pebbles, and till fabric analysis. Samples were taken from superglacial and basal debris, marginal moraine ridges and till horizons, and quantitative parameters were established for different glacial environments and till accumulation processes. Considerable differences in composition and fabric of superglacial and basal debris were caused by their specific transport conditions. Most marginal moraines consist of material similar to basal debris and only the topmost parts of some ridges have a noticeable incorporation of superglacial debris. These results point to considerable erosion of mountain valleys by temperate glaciers. Previous geomorphological investigations have emphasized the importance of superglacial debris in the formation of marginal moraines by alpine glaciers, but this viewpoint is not supported by the studies performed.     

地震资料处理中相对保幅性讨论

地震资料的相对保幅性处理是进行岩性油气藏勘探的基础,目前关于相对保幅性处理业界没有统一的认识,缺乏对关键处理技术的系统分析.笔者根据多年的实际地震资料处理经验,重点分析了相对保幅处理的难点,提出相对保幅处理的几项判断标准,并以此标准对振幅处理、噪声压制和提高分辨率等处理技术的保幅性进行了分析,这对相对保幅处理具有一定的...     

Weichselian and Holocene palaeoenvironmental history of the Bol'shoy Lyakhovsky Island, New Siberian Archipelago, Arctic Siberia

Cryolithological, ground ice and fossil bioindicator (pollen, diatoms, plant macrofossils, rhizopods, insects, mammal bones) records from Bol'shoy Lyakhovsky Island permafrost sequences (73°20′N, 141°30′E) document the environmental history in the region for the past c. 115 kyr. Vegetation similar to modern subarctic tundra communities prevailed during the Eemian/Early Weichselian transition with a climate warmer than the present. Sparse tundra‐like vegetation and harsher climate conditions were predominant during the Early Weichselian. The Middle Weichselian deposits contain peat and peaty soil horizons with bioindicators documenting climate amelioration. Although dwarf willows grew in more protected places, tundra and steppe vegetation prevailed. Climate conditions became colder and drier c. 30 kyr BP. No sediments dated between c. 28.5 and 12.05 ¹⁴C kyr BP were found, which may reflect active erosion during that time. Herb and shrubby vegetation were predominant 11.6–11.3 ¹⁴C kyr BP. Summer temperatures were c. 4 °C higher than today. Typical arctic environments prevailed around 10.5 ¹⁴C kyr BP. Shrub alder and dwarf birch tundra were predominant between c. 9 and 7.6 kyr BP. Reconstructed summer temperatures were at least 4 °C higher than present. However, insect remains reflect that steppe‐like habitats existed until c. 8 kyr BP. After 7.6 kyr BP, shrubs gradually disappeared and the vegetation cover became similar to that of modern tundra. Pollen and beetles indicate a severe arctic environment c. 3.7 kyr BP. However, Betula nana, absent on the island today, was still present. Together with our previous study on Bol'shoy Lyakhovsky Island covering the period between about 200 and 115 kyr, a comprehensive terrestrial palaeoenvironmental data set from this area in western Beringia is now available for the past two glacial–interglacial cycles.     

Quaternary vegetation changes derived from a loess‐like permafrost palaeosol sequence in northeast Siberia using alkane biomarker and pollen analyses

Zech, M., Andreev, A., Zech, R., Müller, S., Hambach, U., Frechen, M. & Zech, W.: Quaternary vegetation changes derived from a loess‐like permafrost palaeosol sequence in northeast Siberia using alkane biomarker and pollen analyses. Boreas, Vol. 39, pp. 540–550. 10.1111/j.1502‐3885.2009.00132.x. ISSN 0300‐9483 Alkane biomarker and pollen data were obtained from a 15‐m‐high and probably c. 240‐kyr‐old loess‐like permafrost palaeosol sequence (‘Tumara Palaeosol Sequence’, TPS) in northeast Siberia. The alkane results were corrected for degradation effects by applying an end‐member model and were evaluated by comparing them with the palynological results. The two data sets are generally in good agreement and suggest that the lower part of the TPS developed mainly under larch forests, whereas the upper part of the sequence reflects the expansion of mammoth steppes during the Weichselian glaciation and finally reforestation during the Lateglacial and the early Holocene. For the lower part of the TPS, the palaeoclimatic interpretation according to modern analogue methods would indicate warm, interglacial conditions, but this is at odds with the climate chronostratigraphy based on a multi‐proxy palaeopedological approach and numeric dating. Provided that the correlation of the discussed stratigraphic unit with the Late Saalian glaciation and the Marine Oxygen Isotope Stage 6 is correct, our results suggest that temperature was not a limiting factor for tree growth at that time. Furthermore, it seems very likely that it was not mainly temperature changes but rather increasing aridity and continentality during the course of the last glacial that favoured the expansion of the mammoth steppe.     

Holocene environmental history on the eastern slope of the Polar Ural Mountains, Russia

The Holocene environmental history of the eastern slope of the Polar Ural Mountains has been reconstructed using pollen, spores, algae and other microfossils from the Chernaya Gorka palsa section (67°05'N, 65°21'E, 170 m a.s.l.). An initial oligotrophic lake was formed at the study site c. 9800-9500 ¹⁴C yr BP. Although tundra communities dominated the vegetation in the area, birch and larch trees might have grown at lower elevations. Dry and disturbed soil habitats also occurred around the lake. Algae (mostly Pediastrum and Botryococcus) started to expand in the lake as climate gradually improved after c. 9500 ¹⁴C yr BP. However, the role of mosses (mostly Calliergon and Drepanocladus) was most important for the infilling of the lake basin. Increased temperatures and subsequent improvement of hydrological conditions resulted in vegetation changes: stands of willows developed rapidly and the role of tree birch in the local vegetation increased. The lake was completely filled at c. 8600 ¹⁴C yr BP. Peat accumulation started with Bryales mosses and, later, Sphagnum became dominant. Stands of Larix, Picea and Betula became well developed during the Boreal climate optimum. Tree birch began to spread into the tundra. Different Bryales mosses formed peat c. 8000-6500 ¹⁴C yr BP. Cyperaceae later became the main peat-forming element. Dense spruce canopies with Larix sibirica and Betula pubescens surrounded the study site during the Atlantic period, pointing to the warmest climate during the Holocene. Summer temperatures might have been up to 3-4°C higher than today. However, a decline of spruce and an increase of birch around 6700-6300 ¹⁴C yr BP may reflect some climate deterioration. There are no dated deposits younger than 6000 ¹⁴C yr BP. It is assumed that Subboreal climate deterioration resulted in the development of permafrost and formation of the palsa at the site. The deposits, now protruding above the surrounding terrain, were eroded by wind, water and cryogenic processes.     

Intra-shoreface erosion in response to rapid sea-level fall: depositional record of a tectonically uplifted strand plain, Pacific coast of Japan

This study documents lowering of the surf zone (i.e. the upper shoreface) leading to intra-shoreface erosion, following two rapid relative sea-level falls along a tectonically uplifted coast during the Holocene, and the characteristics of the resultant prograding shoreface deposits. These findings are based on high-resolution analysis and radiocarbon dating of three new drill cores obtained from the Kujukuri strand plain, Pacific coast of eastern Japan, combined with previously published borehole data and information on modern shoreline profile adjustments. A shallowing-upward sandy succession composed of lower and upper shoreface facies, foreshore and backshore facies was recognized in the drill cores. Two rapid falls in relative sea-level at 2·3 to 2·6 and 1·8 to 2·0 ka are recorded by downstepping of the base of the foreshore facies, and farther seawards by the lowering of an erosional boundary between the upper and lower shoreface facies. Superimposed bed profiles of an adjacent modern beach define an envelope, the base of which reflects shore-normal migration of longshore bars and troughs. The base of the envelope represents an erosional surface that divides the surface mobile layer above from preserved deposits beneath. The surface is concave upwards and steeper than the mean beach profile, and exhibits a flat platform approximately at the lower limit of the upper shoreface equating to the storm surf zone. The seaward transition of this surface, rather than the mean equilibrium profile, controls the metre-scale to decimetre-scale internal structure of the Kujukuri shoreface deposits. Depositional models for sea-level fall based on an exponential equilibrium profile do not adequately account for the presence and migration of longshore bars and troughs.     

Late Pleistocene and Holocene vegetation and climate on the northern Taymyr Peninsula, Arctic Russia

Pollen data from a Levinson-Lessing Lake sediment core (74°28'N, 98°38'E) and Cape Sabler, Taymyr Lake permafrost sequences (74°33'N, 100°32'E) reveal substantial environmental changes on the northern Taymyr Peninsula during the last c. 32 000 [Formula: See Text]C years. The continuous records confirm that a scarce steppe-like vegetation with Poaceae, Artemisia and Cyperaceae dominated c. 32 000-10 300 [Formula: See Text]C yr BP, while tundra-like vegetation with Oxyria, Ranunculaceae and Caryophyllaceae grew in wetter areas. The coldest interval occurred c. 18 000 yr BP. Lateglacial pollen data show several warming events followed by a climate deterioration c. 10 500 [Formula: See Text]C yr BP, which may correspond with the Younger Dryas. The Late Pleistocene/Holocene transition, c. 10 300-10 000 [Formula: See Text]C yr BP, is characterized by a change from the herb-dominated vegetation to shrubby tundra with Betula sect. Nanae and Salix. Alnus fruticosa arrived locally c. 9000-8500 [Formula: See Text]C yr BP and disappeared c. 4000-3500 [Formula: See Text]C yr BP. Communities of Betula sect. Nanae, broadly distributed at c. 10 000-3500 [Formula: See Text]C yr BP, almost disappeared when vegetation became similar to the modern herb tundra after 3500-3000 [Formula: See Text]C yr BP. Quantitative climate reconstructions show Last Glacial Maximum summer temperature about 4°C below the present and Preboreal (c. 10 000 [Formula: See Text]C yr BP) temperature 2-4°C above the present. Maximum summer temperature occurred between 10 000 and 5500 [Formula: See Text]C yr BP; later summers were similar to present or slightly warmer.