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1.
The Weichselian Late Pleniglacial and Lateglacial aeolian stratigraphy (Older Coversand I, Beuningen Gravel Bed, Older Coversand II, Younger Coversand I, Usselo Soil, Younger Coversand II) in the southern Netherlands has been reinvestigated in its type locality (Grubbenvorst). Sedimentary environments have been reconstructed and related to their climatic evolution based on periglacial structures. In addition, 22 optically stimulated luminescence (OSL) ages have been determined that provide an absolute chronology for the climatic evolution and environmental changes of the coversand area. From this work it appears that, prior to 25 ka fluvial deposition by the Maas dominated. After 25 ka fluvial activity reduced and deposition occurred in a fluvio‐aeolian environment with continuous permafrost (Older Coversand I). This depositional phase was dated between 25.2 ± 2.0 and 17.2 ± 1.2 ka. The upward increase of aeolian activity and cryogenic structures in this unit is related to an increase of climatic aridity and a decrease in sedimentation rate during the Last Glacial Maximum (LGM). The Beuningen Gravel Bed, that results from deflation with polar desert conditions and that represents a stratigraphic marker in northwestern Europe, was bracketed between 17.2 ± 1.2 and 15.3 ± 1.0 ka. Based on this age result a correlation with Heinrich event H1 is suggested. Permafrost degradation occurred at the end of this period. Optical ages for the Older Coversand II unit directly overlying the Beuningen Gravel Bed range from 15.3 ± 1.0 ka at the base to 12.7 ± 0.9 ka at the top. Thus this regionally important Older Coversand II unit started at the end of the Late Pleniglacial and continued throughout the early Lateglacial. Its formation after the Late Pleniglacial (LP) maximum cold and its preservation are related to rapid climatic warming around 14.7 ka cal. BP. The Allerød age of the Usselo Soil was confirmed by the optical ages. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

2.
The nature of permafrost and related environmental conditions in the Weichselian Late Pleniglacial and Lateglacial are reconstructed based on the assessment of frost structures that are best documented in the Loess Belt and in plateau areas composed of glacial till. Investigations were conducted in the central‐eastern part of the European Sand Belt (SE Poland and NW Ukraine) on a fluvio‐aeolian sedimentary succession and took into account its chronological context given by luminescence dating. Various generations of periglacial structures found in these deposits indicate not only the development of permafrost (ice‐wedge pseudomorphs) and decreased humidity (composite wedge casts) but also the degradation of permafrost (large‐scale involutions) and, finally, the establishment of deep seasonal frost (frost cracks). The diversity of structures in the study region appears to result from local conditions rather than increasing continentality of climate towards the east.  相似文献   

3.
The Weichselian Late Pleniglacial, Lateglacial and Holocene fluvial history of the middle Tisza valley in Hungary has been compared with other river systems in West and Central Europe, enabling us to define local and regional forcing factors in fluvial system change. Four Weichselian to Holocene floodplain generations, differing in palaeochannel characteristics and elevation, were defined by geomorphological analysis. Coring transects enabled the construction of the channel geometry and fluvial architecture. Pollen analysis of the fine-grained deposits has determined the vegetation development over time and, for the first time, a bio(chrono)stratigraphic framework for the changes in the fluvial system. Radiocarbon dating has provided an absolute chronology; however, the results are problematic due to the partly reworked character of the organic material in the loamy sediments. During the Late Pleniglacial, aggradation by a braided precursor system of the Tisza and local deflation and dune formation took place in a steppe or open coniferous forest landscape. A channel pattern change from braided to large-scale meandering and gradual incision occurred during the Late Pleniglacial or start of the Lateglacial, due to climate warming and climate-related boreal forest development, leading to lower stream power and lower sediment supply, although bank-full discharges were still high. Alternatively, this fluvial change might reflect the tectonically induced avulsion of the River Tisza into the area. The climatic deterioration of the Younger Dryas Stadial, frequently registered by fluvial system changes along the North Atlantic margin, is not reflected in the middle Tisza valley and meandering persisted. The Lateglacial to Holocene climatic warming resulted in the growth of deciduous forest and channel incision and a prominent terrace scarp developed. The Holocene floodplain was formed by laterally migrating smaller meandering channels reflecting lower bank-full discharges. Intra-Holocene river changes have not been observed.  相似文献   

4.
The Weichselian deposits of the flat Dutch-Belgian coversand area are characterized by highly varying facies types. The geomorphological location and the role of water during the deposition and the transportation of the original eolian sediments may fully explain the texture and sedimentary structures of the lithostratigraphic units. Directly deposited eolian loams and sands on the dry interfluves contrast with the same, but reworked, sediments in wet valleys and depressions. The formation of periglacial phenomena is also dependent on the geomorphologic, lithologic, and hydrologic conditions. The occurrence of peaty beds is restricted to wet environments without precise climatic significance. The Pleniglacial sequence is subdivided into early and late Pleniglacial stades, both characterized by (partial) permafrost conditions, interrupted by a middle Pleniglacial interstadial complex with clearly milder conditions.  相似文献   

5.
The Late Pleistocene was characterized by rapid climate oscillations with alternation of warm and cold periods that lasted up to several thousand years. Although much work has been carried out on the palaeoclimate reconstruction, a direct correlation of ice‐core, marine and terrestrial records is still difficult. Here we present new data from late Middle Pleniglacial to Lateglacial alluvial‐fan and aeolian sand‐sheet deposits in northwestern Germany. Records of Late Pleniglacial alluvial fans in central Europe are very rare, and OSL dating is used to determine the timing of fan aggradation. In contrast to fluvial systems that commonly show a delay between climate change and incision/aggradation, the small alluvial‐fan systems of the Senne area responded rapidly to climatic changes and therefore act as important terrestrial climate archives for this time span. The onset of alluvial‐fan deposition correlates with the climate change from warm to cold at the end of MIS 3 (29.3±3.2 ka). Strong fan progradation started at 24.4±2.8 ka and may be related to a period of higher humidity. The vertical stacking pattern of sedimentary facies and channel styles indicate a subsequrent overall decrease in water and sediment supply, with less sustained discharges and more sporadic runoffs from the catchment area, corresponding to an increasing aridity in central Europe during the Late Pleniglacial. Major phases of channel incision and fan aggradation may have been controlled by millennial‐scale Dansgaard–Oeschger cycles. The incision of channel systems is attributed to unstable climate phases at cold–warm (dry–wet) or warm–cold (wet–dry) transitions. The alluvial‐fan deposits are bounded by an erosion surface and are overlain by aeolian sand‐sheets that were periodically affected by flash‐floods. This unconformity might be correlated with the Beuningen Gravel Bed, which is an important marker horizon in deposits of the Late Pleniglacial resulting from deflation under polar desert conditions. The deposition of aeolian sand‐sheet systems (19.6±2.1 to 13.1±1.5 ka) indicates a rapid increase in aridity at the end of the Late Pleniglacial. Intercalated flash‐floods deposits and palaeosols (Finow type) point to temporarily wet conditions during the Lateglacial. The formation of an ephemeral channel network probably marks the warm‐cold transition from the Allerød to the Younger Dryas.  相似文献   

6.
Advance of the Late Weichselian (Valdaian) Scandinavian Ice Sheet (SIS) in northwestern Russia took place after a period of periglacial conditions. Till of the last SIS, Bobrovo till, overlies glacial deposits from the previous Barents and Kara Sea ice sheets and marine deposits of the Last Interglacial. The till is identified by its contents of Scandinavian erratics and it has directional properties of westerly provenance. Above the deglaciation sediments, and extra marginally, it is replaced by glaciofluvial and glaciolacustrine deposits. At its maximum extent, the last SIS was more restricted in Russia than previously outlined and the time of termination at 18-16 cal. kyr BP was almost 10 kyr delayed compared to the southwestern part of the ice sheet. We argue that the lithology of the ice sheets' substrate, and especially the location of former proglacial lake basins, influenced the dynamics of the ice sheet and guided the direction of flow. We advocate that, while reaching the maximum extent, lobe-shaped glaciers protruded eastward from SIS and moved along the path of water-filled lowland basins. Ice-sheet collapse and deglaciation in the region commenced when ice lobes were detached from the main ice sheet. During the Lateglacial warming, disintegration and melting took place in a 200-600 km wide zone along the northeastern rim of SIS associated with thick Quaternary accumulations. Deglaciation occurred through aerial downwasting within large fields of dead ice developed during successively detached ice lobes. Deglaciation led to the development of hummocky moraine landscapes with scattered periglacial and ice-dammed lakes, while a sub-arctic flora invaded the region.  相似文献   

7.
Krüger, L. C., Paus, A., Svendsen, J. I. & Bjune, A. E. 2011: Lateglacial vegetation and palaeoenvironment in W Norway, with new pollen data from the Sunnmøre region. Boreas, 10.1111/j.1502‐3885.2011.00213.x. ISSN 0300‐9483. Two sediment sequences from Sunnmøre, northern W Norway, were pollen‐analytically studied to reconstruct the Lateglacial vegetation history and climate. The coastal Dimnamyra was deglaciated around 15.3 ka BP, whereas Løkjingsmyra, further inland, became ice‐free around 14 ka BP. The pioneer vegetation dominated by snow‐bed communities was gradually replaced by grassland and sparse heath vegetation. A pronounced peak in Poaceae around 12.9 ka BP may reflect warmer and/or drier conditions. The Younger Dryas (YD) cooling phase shows increasing snow‐bed vegetation and the local establishment of Artemisia norvegica. A subsequent vegetation closure from grassland to heath signals the Holocene warming. Birch forests were established 500–600 years after the YD–Holocene transition. This development follows the pattern of the Sunnmøre region, which is clearly different from the Empetrum dominance in the Lateglacial interstadial further south in W Norway. The Lateglacial oscillations GI‐1d (Older Dryas) and GI‐1b (Gerzensee) are hardly traceable in the north, in contrast to southern W Norway. The southern vegetation was probably closer to an ecotone and more susceptible to climate changes.  相似文献   

8.
Reconstructions of the Weichselian Late Pleniglacial wind direction in northwest and central Europe are reviewed and compared with palaeoclimate simulations performed with an atmospheric general circulation model. These reconstructions are based on proxy data containing information on former wind directions, such as relic dune forms, sediments and wind‐polished rock surfaces. The objective is to investigate whether: (1) the proxy information is internally consistent; and (2) in agreement with the model simulations. We find a general consensus in the proxy‐based reconstructions, indicating a dominant westerly to northwesterly wind in winter during the Late Pleniglacial. The model results indicate over the study area an atmospheric circulation in winter that is dominated by southwesterly to west‐northwesterly winds, which are stronger than the southwesterly winds in the present‐day climate. The main driving factors behind the anomalous atmospheric circulation in the Late Pleniglacial are the Laurentide Ice Sheet and a colder North Atlantic Ocean with a relatively extensive sea‐ice cover, leading to an eastward relocation of the Icelandic Low and an enhanced pressure gradient over northwest Europe. The minor difference in Late Pleniglacial wind direction between the reconstructions and model can be explained by a combination of uncertainties in the proxy data and the relatively low spatial resolution of the applied climate model. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

9.
New sections in the coversand of the Landes region, southwestern France, show at least two main depositional phases corresponding to the Upper Pleniglacial and the Lateglacial, which are separated by palaeosols. The lower palaeosol, a gleyic to histic cryosol overlying a net of sand wedges and dated to ca. 23 14C ka BP, testifies to a short occurrence of permafrost. Impeded drainage due to the frozen subsoil is assumed to be the main factor involved in lowered aeolian transport and soil formation. Pollen analysis indicates a shrub tundra‐type environment. The overlying coversand unit is associated with small transverse ridges or sheet‐like deposits, and corresponds to the maximal extension of the sands, Upper Pleniglacial in age. An incipient podzol developed on the dunes under a boreal pine forest, and has been dated to 11.5–12 14C ka BP, i.e. to the Allerød period. This has been buried by the second coversand unit during the Younger Dryas, typified by abundant denivation features and root imprints. Although preliminary, the chronology of sand deposition in the Landes region appears thus to be roughly similar to that found for the other European coversands, showing that all were the result of similar western European climatic changes, i.e. repeated episodes of increasing aridity related to the Upper Pleniglacial and the Younger Dryas episode. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

10.
A Weichselian Late Pleniglacial fluvio-aeolian deposit has been investigated in the southern Netherlands. Three main structural lineaments have been distinguished: (1) very small, vertical platy structures (microjoints), in a parallel and a columnar configuration; (2) large joints and normal faults with minor displacement (‘Grubbenvorst type’), arranged in a conjugate fault-system; (3) large joints and normal faults (‘wedge-type’), located adjacent to ice-wedge casts. Since clay is absent, the occurrence of the vertical platy structures cannot be attributed to desiccation cracking. The vertical platy structures are interpreted as the result of thermal contraction cracking of a relatively thin layer, due to a sudden temperature drop. The large joints and normal faults of the Grubbenvorst type are the result of failure of the sediment due to the melting of the permafrost in the Late Pleniglacial, just before the formation of the Beuningen Gravel Bed. In other areas large periglacial convolutions have been formed during the same period. The normal faults and joints of the wedge type are more generally known. They are the result of failure of the sediment adjacent to a melting ice wedge.  相似文献   

11.
《Sedimentary Geology》2007,193(1-4):221-231
The palaeogeographical development of South Lithuania during the last ice age (Nemunas = Weichselian) was reconstructed by various methods. The recurring permafrost and cryogenic structures in the ground were an important phenomenon of the southeastern periglacial zone. The 3–4 lithocomplexes of the extraglacial cover correlate with the Lithuanian and Mid-European Late Pleistocene Weichselian (Nemunas) biostratigraphic divisions. In the northwest, the palaeogeography is influenced by the deglaciation during the Žiogeliai (Frankfurt) Phase and the Baltija (Pomeranian) Stage of the last ice age. The deglatiation process is shown in a series of palaeogeography maps. During the Weichselian, the SW–NE oriented middle part of the area, commonly regarded as part of the Vilnius-Warsaw-Berlin Urstromtal (ice-marginal streamway), underwent intensive interstadial fluvial erosion and accumulation, glacial erosion and sedimentation, followed by subsequent glaciofluvial accumulation on sandurs and glaciolacustrine sedimentation in a series of small basins.  相似文献   

12.
Vandenberghe, Jef, Bohncke, Sjoerd, Lammers, Wim & Zilverberg, Liesbeth 1987 03 01: Geomorphology and palaeoecology of the Mark valley (southern Netherlands): geomorphological valley development during the Weichselian and Holocene. Boreas , Vol. 16, pp. 55–67. Oslo. ISSN 0300–9483.
The actual area of the Mark valley is limited by the borders of an Early Weichselian erosion phase. The subsequent accumulation has resulted in the formation of a Weichselian Pleniglacial terrace which has been deeply dissected by Late Glacial erosion. The present alluvial plain is formed by Late Glacial and Holocene infilling. The maximum incision of the Late Glacial fluvial phase was reached slightly before 11,780 B.P. and involved locally dry conditions which have given rise to aeolian activity during this period (Older Dryas). On the deepest parts of the Pleniglacial terrace, a backswamp environment was established until the end of the Alleröd. At the beginning of the Younger Dryas the river invaded the terrace but shortly afterwards aeolian activity progressively increased. At the climax of the Younger Dryas, deep seasonal frost or local permafrost characterized the Mark valley.  相似文献   

13.
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   

14.
Permafrost degradation influences the morphology, biogeochemical cycling and hydrology of Arctic landscapes over a range of time scales. To reconstruct temporal patterns of early to late Holocene permafrost and thermokarst dynamics, site‐specific palaeo‐records are needed. Here we present a multi‐proxy study of a 350‐cm‐long permafrost core from a drained lake basin on the northern Seward Peninsula, Alaska, revealing Lateglacial to Holocene thermokarst lake dynamics in a central location of Beringia. Use of radiocarbon dating, micropalaeontology (ostracods and testaceans), sedimentology (grain‐size analyses, magnetic susceptibility, tephra analyses), geochemistry (total nitrogen and carbon, total organic carbon, δ13Corg) and stable water isotopes (δ18O, δD, d excess) of ground ice allowed the reconstruction of several distinct thermokarst lake phases. These include a pre‐lacustrine environment at the base of the core characterized by the Devil Mountain Maar tephra (22 800±280 cal. a BP, Unit A), which has vertically subsided in places due to subsequent development of a deep thermokarst lake that initiated around 11 800 cal. a BP (Unit B). At about 9000 cal. a BP this lake transitioned from a stable depositional environment to a very dynamic lake system (Unit C) characterized by fluctuating lake levels, potentially intermediate wetland development, and expansion and erosion of shore deposits. Complete drainage of this lake occurred at 1060 cal. a BP, including post‐drainage sediment freezing from the top down to 154 cm and gradual accumulation of terrestrial peat (Unit D), as well as uniform upward talik refreezing. This core‐based reconstruction of multiple thermokarst lake generations since 11 800 cal. a BP improves our understanding of the temporal scales of thermokarst lake development from initiation to drainage, demonstrates complex landscape evolution in the ice‐rich permafrost regions of Central Beringia during the Lateglacial and Holocene, and enhances our understanding of biogeochemical cycles in thermokarst‐affected regions of the Arctic.  相似文献   

15.
A total of 45 subfossil reindeer (Rangifer tarandus) antlers and bones - artefacts excluded - have been found over the years in the Baltic countries of Estonia, Latvia and Lithuania. The relatively high number of specimens suggests a stable residence of the species in the eastern Baltic region. For the first time, 12 of these finds were radiocarbon-dated. The ages of the samples range between 12 085 and 9970 14C yr BP (14 180-11 280 cal. yr BP), and cover the Lateglacial and early Holocene, a time period during which climatic conditions shifted from periglacial to temperate. The dates suggest a rapid colonization of the area during the deglaciation period and a local extinction around the Pleistocene-Holocene boundary. The results of the study do not support the theory that the recent wild reindeer populations of northern Europe had their origin in the Late Weichselian reindeer populations of the eastern Baltic region.  相似文献   

16.
Huvial systems from the Dutch and Polish lowlands are compared for their evolution during the Weichselian deglaciation period. In both regions the Pleniglacial braided-river pattern changed to a meandering pattern in the beginning of the Lateglacial. Several transitional stages between braided and high-sinuosity meandering systems have been recognized. A difference in fluvial activity is found during the Younger Dryas when the Maas transformed to a braided pattern while the Warta maintained its meandering pattern. Phases of erosion and aggradation are more or less synchronous in the different river basins, but their intensity may differ greatly. The major characteristics of the fluvial systems are obviously a response to the general climatic evolution and related to changes in vegetation. The regional differences are of minor importance and can be explained by site-specific factors. Threshold values within the latter factors define the changes in river dynamics.  相似文献   

17.
为深入理解热喀斯特湖与多年冻土间的相互作用, 本文以青藏高原北麓河盆地典型热喀斯特湖区域为例, 构建考虑热传导和热对流过程的水-冰-热耦合模型, 对热喀斯特湖作用下的多年冻土退化特征及热喀斯特湖的水均衡进行模拟,计算地质环境和气候变暖对热喀斯特湖水均衡和冻土的影响。研究结果表明: 热喀斯特湖周围冻土逐步退化并形成贯穿融区, 导致地下水循环模式发生改变;在地表温度作用下, 形成的活动层厚度为3.35 m;热喀斯特湖在整个模拟时段内表现为负均衡, 其排泄量在285~388 a间显著增加;地层渗透性能决定了热喀斯特湖和生态环境的发展方向;气候变暖加速多年冻土向季节冻土转变。研究结果可为进一步认识寒旱区生态水文过程提供科学依据。  相似文献   

18.
Climatic instability during the late Pleistocene has been reflected in the pattern of groundwater recharge. This report summarizes palaeoclimate knowledge during the late Weichselian in Europe. During this period the majority of northern Europe was covered by thick ice sheets and permafrost, preventing aquifers from recharging. In contrast, southern Europe was generally free of these palaeoclimatic features. Palaeoclimatic information has been combined with isotope data to better understand the palaeorecharge conditions and recharge timing across the European continent. The 18O and 2H relationship shows latitudinal plus climatic influences. Radiocarbon data show that while southern European aquifers have generally been recharged continuously during the last 40,000 years, northern European aquifers typically show a recharge gap during the Last Glacial Maximum. Areas that underwent continuous recharge during the entire late Pleistocene period can also be distinguished from areas where recharge to aquifers was prevented during the Last Glacial Maximum. Finally, several examples are presented of melt-water recharge or subglacial recharge. The identification of such diversity in the groundwater palaeorecharge in Europe is of great importance for modellers developing management schemes for groundwater resources.  相似文献   

19.
Two malacological sequences sampled in loess sections P1 and P3 of Nussloch (Rhine Valley, Germany) provide the most complete and precise molluscan record of western Europe for the Weichselian Lower and Middle Pleniglacial from about 70 to 34 cal. kyr BP. Qualitative and statistical analyses were performed on 134 mollusc samples. In the most complete Lower Pleniglacial record (P1), malacofauna changes reflect three short phases of vegetation development and climatic improvement related to soils and probably interstadials. A steppe to herb/shrub tundra shift characterizes the Lower-Middle Pleniglacial transition and is followed in both malacological records by the same general environmental trend (decline in vegetation and humidity increase) ending with a new increase in temperature and vegetation cover at the top of P3. In the Middle Pleniglacial, the impact of each shorter climatic change on the malacofauna is less recognizable due to a higher sediment compaction and also to being differently recorded in both sequences as the local topography affects soil water resources, soil and vegetation development and malacofauna adaptation. A comparison shows that the western European biostratigraphical framework can thus be improved by coupling molluscan records from loess sections to pollen sequences.  相似文献   

20.
The last glacial shows large variations in climate, which are reflected in the fluvial record in the Niederlausitz, eastern Germany. The entire sequence resembles the fluvial development in other river basins in northwestern Europe, which show contemporaneous changes in depositional style at the onset of a climatic change. During the Middle and the Late Pleniglacial, permafrost conditions resulted in an episodic river discharge. The presence or absence of vegetation, in combination with such ephemeral stream conditions, determined the type of river during each period. A relatively well-developed vegetation cover on the flood plains during the Middle Pleniglacial resulted in a low sediment yield. In combination with the intermittent discharge, this caused the development of an ephemeral anastomosing river system, a river with stable channels and extensive sandy overbank areas. The decline in vegetation cover at ca. 28 ka BP caused an increase in sediment yield and peak discharges, which resulted in the development of a sandy braided river in adjustment to these new conditions. Following the coldest period at around 20 ka, precipitation was so low that fluvial activity was limited and aeolian deposition took place in the valley. © 1997 by John Wiley & Sons, Ltd.  相似文献   

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