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61.
Else Kolstrup 《第四纪科学杂志》1992,7(2):163-172
Radiocarbon-dated pollen diagrams from four Danish sequences are presented in their sedimentological context. The pollen diagrams indicate the former presence of stands of Betula and Pinus and of Ericales, Poaceae, Cyperaceae and other herbs. From a straightforward comparison with other diagrams, one of the present series is tentatively correlated with part of the Early Weichselian Brørup Interstadial and two with parts of the Odderade Interstadial, which has not been demonstrated previously in Denmark. One diagram and part of another do not seem to match previously published records in northwest Europe. Yet, the possibility that the sediments date to hitherto unrecorded periods with environmental conditions similar to those reflected in known Weichselian deposits is not excluded. In two of the sequences high percentages of charcoal particles suggest the former occurrence of fires, which influenced the palaeovegetation and soil surface stability. It is suggested that counting of charcoal particles is included in future investigations in order to establish whether periods with fires can be identified in the sedimentary sequences. 相似文献
62.
In Ostrobothnia, western Finland, buried fossil soils have been recognised in a number of places in the sandy sediments that occur between glaciofluvial deposits and overlying till. Samples from the soil horizons as well as below and above them were taken for optically stimulated luminescence (OSL) dating. The same sites were also sampled for thermoluminescence (TL) dating. Altogether five TL dates and seventeen OSL dates were obtained. The OSL dates can be grouped into two age classes, (i) 120-163 ka and (ii) 76-106 ka, whereas all TL dates are of the order 135-155 ka. A comparison between the results obtained from the two dating methods shows that OSL dates are generally younger than the TL dates for the same sample. The discrepancy may arise partly from problems of setting a correct residual level in TL dating. If this is the case then the TL dates may indicate an upper limit for the true age. The results support the geological interpretation that the fossil soils were formed during the last interglacial, that the soil-forming processes possibly also continued during the first Early Weichselian stadial and interstadial (Brørup sensu lato), and that in general the till-covered glaciofluvial sequences, interpreted as eskers in Ostrobothnia, were deposited during the Saalian or Early Weichselian deglaciation. 相似文献
63.
Climate change during the Last Glacial is considered as a major forcing factor of fluvial system changes. A continuous succession of fluvial sediments, reflecting adaptations to climate change from the Weichselian Middle Pleniglacial (oxygen isotope stage 3) onwards, occurs in lowland river basins in the Netherlands.A comparison of the Pleniglacial and Late Glacial fluvial record in the Netherlands shows that climatic oscillations of similar magnitude did not produce changes in the fluvial sedimentary system of equal magnitude. The Late Glacial fluvial system proves to be highly sensitive to climate change. By contrast, many of the rapid climate changes that have occurred during oxygen isotope stage 3, according to the Greenland ice core record, are not detectable in the fluvial sediments. This can be explained by differences in the impact of the climate variations on drainage basin vegetation. During the Late Glacial, the tree line repeatedly shifted through the Netherlands, whereas the area remained within the tundra zone during the Middle Pleniglacial. Precipitation variations and permafrost aggradation and degradation have played a secondary role.The Weichselian fluvial succession in the Netherlands demonstrates that detection of a change in the fluvial sedimentary system and relating this change to climate change is subject to methodological limitations. The climatic significance of changes in the fluvial record should be carefully evaluated, as well as their chronology. The possibility that climate did not influence the fluvial system should always be considered as a null hypothesis in studies on fluvial successions. 相似文献
64.
V. Wennrich B. Wagner M. Melles P. Morgenstern 《International Journal of Earth Sciences》2005,94(2):275-284
An 8.40 m long sediment sequence from the Salziger See basin, Central Germany, was investigated by a multi-disciplinary approach including chronological, sedimentological, geo- and biogeochemical methods. Radiocarbon dating, conducted on ten samples of aquatic and terrestrial fossil remains, and the occurrence of the Laacher See Tephra (LST) reveal that the sediment sequence comprises the Late Glacial and Holocene lake history since ca. 13,800 cal. year BP. This onset of lacustrine sedimentation is almost 7,000 years earlier as assumed so far. The geo- and biogeochemical data indicate distinct variations throughout the Late Glacial, which can be correlated with cold and warm periods, such as the Oldest Dryas, the Bølling, the Allerød, and the Younger Dryas. During most of the Holocene, minor variations in the geo- and biogeochemical data and a high carbonate content due to the formation and precipitation of autochthonous carbonate in the lake indicate relatively stable lacustrine conditions. A slight change in the sedimentary conditions between 6,700 and 4,000 cal. year BP, with lower deposition of carbonate and fine material, can be traced back to enhanced subrosion in the basin. Increased deposition of coarser sediments and a high accumulation of heavy metals document intensive human activity in the region during the past ca. 1,000 years.This revised version was published online in March 2005 with corrections to Table 1. 相似文献
65.
Submarine push moraine formation during the early Fennoscandian Ice Sheet deglaciation 总被引:2,自引:0,他引:2
The glacial history in the topographically confined paleo-ice stream drainage route of Vestfjorden, North Norway, was analysed based on bathymetric data, high-resolution seismology and 14C AMS-dated gravity cores. The inner part of the fjord is characterised by axial-parallel mega-scale lineations whereas the outer part is dominated by two marginal morainal bank systems. The Værøy (inner) and Røst (outer) marginal moraine systems comprise several transverse, zigzag-shaped ridges. Seismic records show thrusted and folded sediment blocks within the ridges. The landforms are inferred to reflect basal processes and the transition from warm-based (inner fjord) to cold-based (outer fjord) conditions, i.e. fast flow followed by basal freeze-on, sediment deformation and morainal bank formation. The moraines formed during the final part of two paleo-ice sheet re-advances. 14C AMS dating indicates a maximum age of 13.7 14C ka BP (16.2 cal ka BP) for the Røst system whereas the Værøy system is inferred to be slightly older than 12.5 14C ka BP (14.5 cal ka BP). This demonstrates that the northern part of the Fennoscandian Ice Sheet behaved in a much more dynamic way during the early deglaciation than previously assumed. 相似文献
66.
Little is known concerning climate changes in the Eastern Baltic region during the last interglacial–glacial cycle and in particular, climate changes during the Weichselian. In this study, a quantitative reconstruction of the mean January and July temperature for the Medininkai-117 site in Lithuania is presented. The reconstruction is based on pollen and plant macrofossils from this site, which reveal that the vegetation was characteristic of many northern Europe sites during the Eemian and Early Weichselian. Gradual evolution of the vegetation suggests that relatively uniform climate conditions existed during the Eemian. Our reconstructions support the view of a relatively stable Eemian, with short cooling phases of low amplitude. A strong increase in temperature was apparent during the beginning of the interglacial and decrease during the transition to the Weichselian. Reconstructed July temperatures of the Eemian interglacial were approximately 2 °C higher than today (18.5 °C; today: 16.2 °C) and were similar to today for January (− 5.2 °C; today: − 5.1 °C). July temperatures during the Early Weichselian were only ~ 2°C lower than during the Eemian, whereas the January temperatures gradually decreased. Winter temperatures were relatively high (above − 10 °C) during the Early Weichselian. 相似文献
67.
This paper reports the application of optically stimulated luminescence (OSL) dating to relatively coarse grains (100–200 μm) of quartz extracted from loess. Two approaches were tested – a multiple aliquot technique and a new single aliquot approach, which is especially suited to the dating of small quantities of such grains found in some loess. The results are compared with infra‐red stimulated luminescence and thermoluminescence (TL) measurements on the finer‐grained (4–11 μm) polymineral fraction and TL measurements on the relatively coarse grains of quartz. The advantages of the new single aliquot (OSL) approach are described and the iterating, computer‐controlled measurement procedure is given. The automated iterative single aliquot regeneration (AISAR) method corrects for changes in luminescence sensitivity during the measurement sequence by monitoring the response to a test dose. Some initial measurements are presented for the loess section at Zemechy, Czech Republic, and these show that the AISAR method gives more precise results than those using a multiple aliquot method. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
68.
69.
Fluvial and eolian successions of oxygen isotope stage 3 are compared with global (GCM) and regional climate (RCM) modeling experiments of the stage 3 and last glacial maximum climate in Europe. Differences in precipitation between stage-3 stades and interstades were minor, which is confirmed by the fluvial successions. The fluvial response to climate variation is non-uniform, and in southern Europe more pronounced than in northern Europe. The model simulations indicate a strong western winter circulation over Europe during stage 3, which is supported by the eolian deposits data. Wind speeds in the last glacial maximum simulation appear modest compared with those of stage 3, which contrasts with the abundance of eolian deposits. This suggests that during glacial climates the stabilizing effect of vegetation determines eolian sedimentation rates, rather than wind speed. Stage 3 can be divided into an older part (>45,000 cal yr B.P.) with a relatively stable landscape and moist climate and a younger part with more frequent climate change and decreasing landscape stability. 相似文献
70.
《Norsk geografisk tidsskrift. Norwegian journal of geography》2012,66(1):30-51
Data from different disciplines are integrated and used to propose an alternative hypothesis regarding the Upper Weichselian environmental developments in the central Scandinavian mountains. This hypothesis, which tries to integrate new and controversial finds and explain them in a new light, is far from proven and needs much further research. It is proposed that parts of the mountains have been glacio-isostatically more depressed than previously thought and that the glacial dynamics could have been much more pronounced than traditionally assumed. As a consequence, it is proposed that Baltic-Bothnian water crossed temporarily ice-free areas with low-altitude thresholds across the Scandinavian mountain range, sporadically allowing drainage towards the Atlantic Sea, and even that Atlantic water could temporarily have entered into Sweden from the west. Such land-water scenarios could have resulted in a water transport system that steered floating tree seeds and sprouts, transported by rivers from central Europe and/or by seawater from the Atlantic, into the mountain range. There, high isostatic rebound rates during early deglaciation phases could quickly have made coastal areas become land, allowing plants to start their life cycle almost immediately. 相似文献