Late Pleistocene glacial and lake history of northwestern Russia   总被引：1，自引：0，他引：1  

EILIV LARSEN  KURT H. KJæR  IGOR N. DEMIDOV  SVEND FUNDER  KARI GRØSFJELD  MICHAEL HOUMARK-NIELSEN  MARIA JENSEN  HENRIETTE LINGE  ASTRID LYSA 《Boreas: An International Journal of Quaternary Research》2006,35(3):394-424

Five regionally significant Weichselian glacial events, each separated by terrestrial and marine interstadial conditions, are described from northwestern Russia. The first glacial event took place in the Early Weichselian. An ice sheet centred in the Kara Sea area dammed up a large lake in the Pechora lowland. Water was discharged across a threshold on the Timan Ridge and via an ice-free corridor between the Scandinavian Ice Sheet and the Kara Sea Ice Sheet to the west and north into the Barents Sea. The next glaciation occurred around 75-70 kyr BP after an interstadial episode that lasted c. 15 kyr. A local ice cap developed over the Timan Ridge at the transition to the Middle Weichselian. Shortly after deglaciation of the Timan ice cap, an ice sheet centred in the Barents Sea reached the area. The configuration of this ice sheet suggests that it was confluent with the Scandinavian Ice Sheet. Consequently, around 70-65 kyr BP a huge ice-dammed lake formed in the White Sea basin (the 'White Sea Lake'), only now the outlet across the Timan Ridge discharged water eastward into the Pechora area. The Barents Sea Ice Sheet likely suffered marine down-draw that led to its rapid collapse. The White Sea Lake drained into the Barents Sea, and marine inundation and interstadial conditions followed between 65 and 55 kyr BP. The glaciation that followed was centred in the Kara Sea area around 55-45 kyr BP. Northward directed fluvial runoff in the Arkhangelsk region indicates that the Kara Sea Ice Sheet was independent of the Scandinavian Ice Sheet and that the Barents Sea remained ice free. This glaciation was succeeded by a c. 20-kyr-long ice-free and periglacial period before the Scandinavian Ice Sheet invaded from the west, and joined with the Barents Sea Ice Sheet in the northernmost areas of northwestern Russia. The study area seems to be the only region that was invaded by all three ice sheets during the Weichselian. A general increase in ice-sheet size and the westwards migrating ice-sheet dominance with time was reversed in Middle Weichselian time to an easterly dominated ice-sheet configuration. This sequence of events resulted in a complex lake history with spillways being re-used and ice-dammed lakes appearing at different places along the ice margins at different times.  相似文献   

The influence of the organic aerosol component on CCN supersaturation spectra for different aerosol types   总被引：5，自引：0，他引：5  

MIHAELA MIRCEA  MARIA CRISTINA FACCHINI  STEFANO DECESARI  SANDRO FUZZI  ROBERT J. CHARLSON 《Tellus. Series B, Chemical and physical meteorology》2002,54(1):74-81

Abstract This paper describes the effect of the presence of water-soluble organic compounds (WSOC) in aerosol particles on the aerosol critical supersaturation as defined by the Köhler theory and on cloud condensation nuclei (CCN) number concentration. Taking into account both the soluble mass increase and the surface tension depression due to WSOC, we calculated a substantial decrease of the aerosol critical supersaturation, which results in a large increase in CCN number concentration. CCN supersaturation spectra were computed for three different aerosol types: marine, rural and urban. The increase of CCN number concentration in the presence of WSOC (with respect to the case when only the inorganic aerosol compounds are considered) varies with aerosol type, with an increase up to 13% in the marine case, up to 97% in the rural case, and up to 110% in the urban case, for the supersaturation range typical of atmospheric conditions.  相似文献   

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

MARIA WINTERFELD  LUTZ SCHIRRMEISTER  MIKHAIL N. GRIGORIEV  VIKTOR V. KUNITSKY  ANDREI ANDREEV  ANDREW MURRAY  PIER PAUL OVERDUIN 《Boreas: An International Journal of Quaternary Research》2011,40(4):697-713

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.  相似文献   

Metasomatism and Melting in Carbonated Peridotite Xenoliths from the Mantle Wedge: The Gobernador Gregores Case (Southern Patagonia)   总被引：8，自引：5，他引：8  

LAURORA  ANGELA; MAZZUCCHELLI  MAURIZIO; RIVALENTI  GIORGIO; VANNUCCI  RICCARDO; ZANETTI  ALBERTO; BARBIERI  MARIA ADELAIDE; CINGOLANI  CARLOS A. 《Journal of Petrology》2001,42(1):69-87

Spinel-facies mantle xenoliths occur in a diatreme cutting throughthe Neogene Southern Patagonia Plateau at Gobernador Gregores(Santa Cruz Province, Argentina). This plateau is in a back-arcposition with respect to the Chile trench. Xenoliths differin their whole-rock composition from other South America occurrences,having higher CaO/Al₂O₃ ratios and, in some samples, TiO₂ enrichment,whereas the Na₂O/Al₂O₃ variation range is similar. Three assemblagescan be distinguished. Assemblage 1, in anhydrous protogranularlherzolites and harzburgites, contains clinopyroxene with adepleted major and trace element composition, indicating pre-metasomaticdepletion processes. This assemblage fully recrystallized toAssemblage 2 (amphibole ± phlogopite ± Cl-apatite-bearing)during a metasomatic episode. This causes clinopyroxene to acquiregeochemical characteristics often attributed to carbonate-meltmetasomatism. Noticeably, amphibole is markedly enriched inNb (up to 298 ppm), especially when depleted in Ti. A furtherevent, related to decompression during xenolith uplift to thesurface, induces closed-system (perhaps with the exception ofCO₂ addition) disequilibrium melting of Assemblage 2, dominantlyof amphibole. It is found in pockets (where amphibole is a residualphase) consisting of Na–Si-rich glass and carbonate (Mg-richcalcite) drops, and in veins originating from the pockets (Assemblage3). Euhedral olivine, clinopyroxene and spinel crystallize onlyin the silicate glass. So do new, euhedral apatite crystalswhen glass is in contact with previous Assemblage 2 apatite.Textural evidence and comparison with experimental work suggestthat silicate glass and carbonates are the result of unmixingof a former homogeneous melt. Because of the different flowrates of carbonate and silicate melt, the xenoliths become enrichedin carbonate, which is found in the veins during their migration.Thus, the high CaO/Al₂O₃ ratio of whole rocks provides inconclusiveevidence of carbonatite metasomatism. This factor, and otherminor deviations from the expected results of carbonatite metasomatism,lead us to hypothesize an aqueous, Cl-rich fluid, possibly slabderived, as an alternative agent. Amphibole, resulting fromreactive porous flow of this agent in the mantle, could fullyexplain the observed geochemical features, as indicated by estimatesof its partition coefficients. KEY WORDS: carbonated xenoliths; Gobernador Gregores; LAM–ICP-MS; mantle metasomatism; silicate glass  相似文献   

Sedimentary dynamics and evolutionary history of a Late Carboniferous Gondwanic lake in north-western Argentina   总被引：1，自引：0，他引：1  

LUIS ALBERTO BUATOIS  MARIA GABRIELA MANGANO 《Sedimentology》1995,42(3):415-436

Lacustrine deposits are well represented in the lower part of the Late Carboniferous Agua Colorada Formation in the north-west Sierra de Narváez, Catamarca Province, Argentina. Lake Narváez was one of the several water bodies formed in the region immediately after the Gondwana glaciation. The lacustrine transport system has been divided into three distinct zones: delta, shallow lake and deep lake. Delta progradation proceeded from the ESE. Coarse-grained delta plain and turbidite delta front deposits suggest that the delta was formed close to the headwaters (‘short-headed stream delta’type). During periods of high discharge, river mouths acted as bypass zones and fine and very fine sands were transported further into the lake by underflow currents. The clastic material supplied by the deltaic system was partially reworked by wave action. Sands accumulated in unstable conditions at the upper delta front as a consequence of delta progradation. As a result of the addition of clastics in the steep delta front, turbidity currents were formed, spreading their load along the lower delta slope. Deep lacustrine deposits are typically stacked, forming two different kinds of progradational turbidite lobe sequences. Type I lobes were formed in a basinal setting and were probably detached from their feeder systems as a result of sediment-bypassing in a shallow lake during periods of low lake level. These turbidite lobes are replaced upwards by type II lobes, which were formed on the delta slope during periods of lake level rise that allowed the onset of delta progradation. The presence of highly deformed sandstone bodies suggests rapid depositional rates in a high slope setting, whereas the occurrence of hummocky cross-stratified sandstones indicates wave reworking of the sands initially emplaced by turbidity currents. Therefore, the inner part of type II lobes was formed above storm wave base. The depositional history of Lake Narváez can be traced through four evolutionary stages: lake transgression, formation of type I lobes, formation of type II lobes and delta progradation. Tectonic activity was probably important at the early stage of lake evolution, but the subsequent depositional history was mainly controlled by fluctuations of lake level.  相似文献   

Process length variation in cysts of the dinoflagellate Protoceratium reticulatum,from surface sediments of the Baltic–Kattegat–Skagerrak estuarine system: a regional salinity proxy     

KENNETH NEIL MERTENS  BARRIE DALE  MARIANNE ELLEGAARD  IDA‐MARIA JANSSON  ANNA GODHE  ANKE KREMP  STEPHEN LOUWYE 《Boreas: An International Journal of Quaternary Research》2011,40(2):242-255

Mertens, K. N., Dale, B., Ellegaard, M., Jansson, I.‐M., Godhe, A., Kremp, A. & Louwye, S. 2010: Process length variation in cysts of the dinoflagellate Protoceratium reticulatum, from surface sediments of the Baltic–Kattegat–Skagerrak estuarine system: a regional salinity proxy. Boreas, 10.1111/j.1502‐3885.2010.00193.x. ISSN 0300‐9483. Results are presented from a regional comparison of average process length variation in cysts of Protoceratium reticulatum and Lingulodinium polyedrum, extracted from surface sediments in the Skagerrak–Kattegat–Baltic estuarine system, with the environmental variables of seawater temperature and salinity. Although too few cysts of Lingulodinium polyedrum were recovered from the sediments to make reliable correlations, cysts of Protoceratium reticulatum were well represented, and average process length was correlated significantly with both salinity and temperature. Owing to dominant summer surface production, and regional covariation between salinity and density, we propose the use of the significant correlation with summer sea surface salinity (SSS_summer) by the equation SSS_summer=3.16 × average process length ?0.84 (R²=0.8). Application of this equation down‐core in Limfjord (northern Denmark) shows its usefulness as a regional palaeosalinity proxy.  相似文献   

Mediterranean Neogene stratigraphy: development and evolution through the centuries     

MARIA BIANCA CITA 《Sedimentology》2009,56(1):43-62

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CT 双期增强扫描在甲状腺腺瘤及结节性甲状腺肿鉴别诊断中的价值?          下载免费PDF全文

薛峰  孙丹  厉玛萱萱  乔忠云  单华 《CT理论与应用研究》2016,(4):471-476

目的：探讨 CT 双期增强扫描对甲状腺腺瘤和结节性甲状腺肿鉴别诊断中的价值。资料与方法：回顾性比较25例经手术后病理证实的甲状腺腺瘤和结节性甲状腺肿患者 CT 平扫及增强扫描结果,并进行分析。结果：25例患者共取出甲状腺结节35枚,其中甲状腺腺瘤18枚,结节性甲状腺肿17枚。腺瘤组平扫 CT 值30～60 HU,结节性甲状腺肿组平扫 CT 值25～55 HU,两者差别不明显。腺瘤组增强扫描 CT 值120～160 HU,结节性甲状腺肿组增强扫描 CT 值60～100 HU,两者有显著差异。结论：甲状腺 CT 双期增强扫描对甲状腺腺瘤和结节状甲状腺肿在鉴别诊断上可以提供重要的参考。  相似文献   

Distinguishing the Regional Atmospheric Controls on Precipitation Isotopic Variability in the Central-Southeast Portion of Brazil     

Vinícius dos SANTOS  Peter MARSHALL FLEMING  Luís HENRIQUE MANCINI  Stela DALVA SANTOS COTA  Graziele Beatriz de LIMA  Rafaela RODRIGUES GOMES  Roberto Eduardo KIRCHHEIM  Ricardo SANCHéZ-MURILLO  Didier GASTMANS 《大气科学进展》2022,39(10):1693-1708

Precipitation isotope ratios (O and H) record the history of water phase transitions and fractionation processes during moisture transport and rainfall formation. Here, we evaluated the isotopic composition of precipitation over the central-southeastern region of Brazil at different timescales. Monthly isotopic compositions were associated with classical effects (rainfall amount, seasonality, and continentality), demonstrating the importance of vapor recirculation processes and different regional atmospheric systems (South American Convergence Zone-SACZ and Cold Fronts-CF). While moisture recycling and regional atmospheric processes may also be observed on a daily timescale, classical effects such as the amount effect were not strongly correlated (δ¹⁸O-precipitation rate r ≤ –0.37). Daily variability revealed specific climatic features, such as δ¹⁸O depleted values (~ –6‰ to –8‰) during the wet season were associated with strong convective activity and large moisture availability. Daily isotopic analysis revealed the role of different moisture sources and transport effects. Isotope ratios combined with d-excess explain how atmospheric recirculation processes interact with convective activity during rainfall formation processes. Our findings provide a new understanding of rainfall sampling timescales and highlight the importance of water isotopes to decipher key hydrometeorological processes in a complex spatial and temporal context in central-southeastern Brazil.  相似文献   

Changes in aerosol size- and phase distributions due to physical and chemical processes in fog   总被引：4，自引：0，他引：4  

KEVIN J. NOONE  JOHN A. OGREN  ANNELI HALLBERG  JOST HEINTZENBERG  JOHAN STRÖM  HANS-CHRISTEN HANSSON  BIRGITTA SVENNINGSSON  ALFRED WIEDENSOHLER  SANDRO FUZZI  MARIA CHRISTINA FACCHINI  BEATE G. ARENDS  AXEL BERNER 《Tellus. Series B, Chemical and physical meteorology》1992,44(5):489-504

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