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1.
The Niers valley was part of the Rhine system that came into existence during the maximum Saalian glaciation and was abandoned at the end of the Weichselian. The aim of the study was to explain the Late Pleniglacial and Late Glacial fluvial dynamics and to explore the external forcing factors: climate change, tectonics and sea level. The sedimentary units have been investigated by large‐scale coring transects and detailed cross‐sections over abandoned channels. The temporal fluvial development has been reconstructed by means of geomorphological relationships, pollen analysis and 14C dating. The Niers‐Rhine experienced a channel pattern change from braided, via a transformational phase, to meandering in the early Late Glacial. This change in fluvial style is explained by climate amelioration at the Late Pleniglacial to Late Glacial transition (at ca. 12.5 k 14C yr BP) and climate‐related hydrological, lithological and vegetation changes. A delayed fluvial response of ca. 400 14C yr (transitional phase) was established. The channel transformations are not related to tectonic effects and sea‐level changes. Successive river systems have similar gradients of ca. 35–40 cm km?1. A meandering river system dominated the Allerød and Younger Dryas periods. The threshold towards braiding was not crossed during the Younger Dryas, but increased aeolian activity has been observed on the Younger Dryas point bars. The final abandonment of the Niers‐Rhine was dated shortly after the Younger Dryas to Holocene transition. Traces of Laacher See pumice have been found in the Niers valley, indicating that the Niers‐Rhine was still in use during the Younger Dryas. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

2.
嫩江下游现代河流沉积特征   总被引：5，自引：0，他引：5       下载免费PDF全文
王俊玲  任纪舜 《地质论评》2001,47(2):193-199
本文运用河流沉积学原理，以黑龙江省富裕县塔哈乡大马岗嫩江现代沉积露头为例，利用探槽，探坑及钻井获取的野外露头资料及室内各种微观分析测试资料，对嫩江下游现代河流沉积特征进行了详细研究，研究表明，该区河流积积自上而下包括了两种河型沉积，底部是以砾石沉积为主的辫状河沉积，其砾石层厚＞5米，横向上可追踪数百米，上部为一低能量曲流河点坝沉积，以中砾质细砂及粉砂沉积为主，底部无明显冲刷面，发育低角度槽状交错层理，同沉积变形层理及波状交错层理，向上过渡为天然堤的粉砂质泥与粉矿薄互层沉积及漫滩粉砂质泥沉积，其整体沉积特征是在一个辫状河泛滥平原上发育起来的低能源曲流河沉积，这一研究为地下复杂的河流相地层识 提供了一个新的实例。  相似文献   

3.
Cosmogenic 36Cl was measured in bedrock and moraine boulders in the Za Mnichem Valley (High Tatra Mountains). The post‐LGM deglaciation of the study area occurred about 15.9 ka ago. The northernmost part of the valley slopes was ice‐free around 15 ka ago. The terminal moraine on the valley threshold was finally stabilized 12.5 ka ago during the Younger Dryas cold event (Greenland Stadial 1). At that time, the Za Mnichem glacier was 1.3 km long and had an area of 0.57 km2. The AAR equilibrium line of the glacier was located at 1990 m a.s.l., which corresponds to an ELA depression of ～500 m compared to today. The mean summer temperature was colder by 4°–4.5°C than the present‐day temperature. The mean annual temperature was colder by 6°C than today. Such conditions suggest a decrease of the annual precipitation by ～15–25% compared with the present‐day annual average. These data indicate a probable uniform temperature change across central and western Europe, with the precipitation being the most significant factor affecting the mass balance of mountain glaciers. The spatial distribution of balance data suggests increasing continentality towards the east during the Younger Dryas.  相似文献   

4.
A south‐east Australian speleothem stable isotope record displaying an apparent cooling synchronous with the northern hemisphere Younger Dryas climate event (12.9–11.7 ka) has significantly influenced scientific thinking on the climatic response of the southern hemisphere following the Last Glacial Maximum. This is one of very few records displaying such a response, and yet the cooling was inferred from substantial extrapolation between just three uranium‐series ages. Technological advances since then have produced major improvements in both the spatial resolution and the accuracy of uranium‐series geochronologies. Re‐analysis of this sample has yielded ages of 7.96 ± 0.36 to 7.69 ± 0.33 ka for the interval previously inferred to span the Younger Dryas, and reveals a substantial hiatus in deposition from 6.93 ± 0.64 to 1.83 ± 0.16 ka. These data not only refute the original evidence for an inter‐hemispheric synchroneity of the Younger Dryas but also reject any evidence for neoglacial conditions at 3 ka. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

5.
Blomvåg, on the western coast of Norway north of Bergen, is a classical site in Norwegian Quaternary science. Foreshore marine sediments, named the Blomvåg Beds and now dated to the Bølling‐Allerød from 14.8 to 13.3 cal. ka BP, contain the richest Lateglacial bone fauna in Norway, numerous mollusc shells, driftwood, and flint that some archaeologists consider as the oldest traces of humans in Norway. The main theme of this paper is that the Blomvåg Beds are overlain by a compact diamicton, named the Ulvøy Diamicton, which was interpreted previously as a basal till deposited during a glacial re‐advance into the ocean during the Older Dryas (c. 14 cal. ka BP). Sediment sections of the Blomvåg Beds and the Ulvøy Diamicton were exposed in ditches in a cemetery that was constructed in 1941–42 and have subsequently not been accessible. A number of radiocarbon and cosmogenic 10Be exposure ages demonstrate that the diamicton is not likely to be a till because minimum deglaciation ages (14.8–14.5 cal. ka BP) from the vicinity pre‐date the Ulvøy Diamicton. We now consider that sea ice and icebergs formed the Ulvøy Diamicton during the Younger Dryas. The Scandinavian Ice Sheet margin was located on the outermost coastal islands between at least c. 18.5 and 14.8 cal. ka BP; however, no ice‐marginal deposits have been found offshore from this long period. The Older Dryas ice margin in this area was located slightly inside the Younger Dryas margin, whereas farther south it was located slightly beyond the Younger Dryas margin.  相似文献   

6.
The marine geology of Port Phillip is described in detail, based on data from seismic profiling, vibrocoring and grab sampling. Three major unconsolidated facies can be distinguished: sands and muddy sands peripheral to the present coastline, muds covering the major central region, and channel fills of muds and sands. The first two facies units result from an increase in wave sorting towards the coast, reworking of Tertiary and Quaternary sandstone outcrops around the coast, and a dominant mud supply from river sources into the central area. The distribution and thicknesses of the unconsolidated facies have been augmented by a shallow‐seismic program that reveals the thicknesses of the modern sediments overlying an older surface comprised of consolidated clays and sandy clays of Pleistocene or older age. In central Port Phillip, muds and sands up to 27 m‐thick have infilled Pleistocene channels cut into underlying consolidated units. Sediments immediately above the channel bases show characteristic seismic patterns of fluvial deposition. The presence of peat deposits together with gas phenomena in the water column suggest organic breakdown of channel‐fill deposits is releasing methane into the bay waters. Outside the channel areas, carbon‐14 dating indicates that the unconsolidated sediments largely post‐date the last glaciation sea‐level rise (<6500 a BP), with an early Holocene period of rapid deposition, similar to other Australian estuaries. Stratigraphic and depositional considerations suggest that the undated channel‐fill sequences correlate with the formation of cemented quartz‐carbonate aeolianite and barrier sands on the Nepean Peninsula at the southern end of Port Phillip. Previous thermoluminescence dating of the aeolianites suggests that channel‐fill sequences B, C and D may have been deposited as fluvial and estuarine infills over the period between 57 and 8 ka. The eroded surface on the underlying consolidated sediments is probably the same 118 ka age as a disconformity within the Nepean aeolianites. Further estuarine and aeolianite facies extend below the disconformity to 60 m below sea‐level, and may extend the Quaternary depositional record to ca 810 ka. Pliocene and older Tertiary units progressively subcrop below the Quaternary northwards up the bay.  相似文献   

7.
Changing river courses and fluctuations of the water table were some of the most fundamental environmental changes that humans faced during the Late Glacial, particularly as these changes affected areas intensively used for settlement and resource exploitation. Unfortunately, only a few stratigraphies have been documented in the North European plain that show the interaction between river development, vegetation history, and occupation by Late Palaeolithic humans. Here, we present the results of detailed stratigraphical studies (pedology, archaeology, chrono‐, tephra‐, and palynostratigraphy) at the Federmesser site Grabow 15 located in the broad Elbe River valley. The research aimed to produce a model of site formation based on a multiproxy approach, relating the local evidence to the palaeoenvironmental and settlement history of the wider region. After deposition of fluvial sands during the Late Pleniglacial in a braided setting, the river course developed locally toward a meandering system at the transition from the Older Dryas to the Allerød, while periodic flooding led to the deposition of floodplain sediments during the early Allerød. The floodplain was settled by people of the earliest “Federmessergruppen,” who are believed to have chosen this open floodplain area along the river for collecting and processing amber of local origin. Their artifacts became embedded in the aggrading floodplain sediments. In the late Allerød, floodplain sedimentation ceased and a Fluvisol‐type soil developed, indicating a trend toward geomorphic stability. The Fluvisol was then covered by silty floodplain sediments due to a rising water level during the late Younger Dryas resulting in the cessation of human occupation in the area. Subsequent organic‐rich Late Glacial/Holocene sediments preserved the settlement remains to the present.  相似文献   

8.
Recent research based primarily on exposure ages of boulders on moraines has suggested that extensive ice masses persisted in fjords and across low ground in north‐west Scotland throughout the Lateglacial Interstade (≈ Greenland Interstade 1, ca. 14.7–12.9 ka), and that glacier ice was much more extensive in this area during the Older Dryas chronozone (ca. 14.0 ka) than during the Younger Dryas Stade (ca. 12.9–11.7 ka). We have recalibrated the same exposure age data using locally derived 10Be production rates. This increases the original mean ages by 6.5–12%, implying moraine deposition between ca. 14.3 and ca. 15.1 ka, and we infer a most probable age of ca. 14.7 ka based on palaeoclimatic considerations. The internal consistency of the ages implies that the dated moraines represent a single readvance of the ice margin (the Wester Ross Readvance). Pollen–stratigraphic evidence from a Lateglacial site at Loch Droma on the present drainage divide demonstrates deglaciation before ca. 14.0 ka, and therefore implies extensive deglaciation of all low ground and fjords in this area during the first half of the interstade (ca. 14.7–14.0 ka). This inference appears consistent with Lateglacial radiocarbon dates for shells recovered from glacimarine sediments and a dated tephra layer. Our revised chronology conflicts with earlier proposals that substantial dynamic ice caps persisted in Scotland between 14 and 13 ka, that large active glaciers probably survived throughout the Lateglacial Interstade and that ice extent was greater during the Older Dryas period than during the Younger Dryas Stade. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

9.
The Gulf of Tonkin coastline migrated at an average rate of ca 60 m year?1 landward during Holocene sea‐level rise (20 to 8 ka). Due to a combination of rapid coastline migration and undersupply of sand, neither coastal barriers nor tidal sand bars developed at the mouth of the Red River incised valley. Only a 30 to 80 cm thick sandy interval formed at the base of full‐marine deposits. Thus, the river mouth represented a mud‐dominated open funnel‐shaped estuary during transgression. At the base of the valley fill, a thin fluvial lag deposit marks a period of lowered sea‐level when the river did not reach geomorphic equilibrium and was thus prone to erosion. The onset of base‐level rise is documented by non‐bioturbated to sparsely bioturbated mud that occasionally contains pyrite indicating short‐term seawater incursions. Siderite in overlying deposits points to low‐salinity estuarine conditions. The open funnel‐shaped river mouth favoured upstream incursion of seawater that varied inversely to the seasonal strongly fluctuating discharge: several centimetres to a few tens of centimetres thick intervals showing marine or freshwater dominance alternate, as indicated by bioturbational and physical sedimentary structures, and by the presence of Fe sulphides or siderite, respectively. Recurrent short‐term seawater incursions stressed the burrowing fauna. The degree of bioturbation increases upward corresponding to increasing marine influence. The uppermost estuarine sediments are completely bioturbated. The estuarine deposits aggraded on average rapidly, up to several metres kyr?1. Siphonichnidal burrows produced by bivalves, however, document recurrent episodes of enhanced deposition (>0·5 m) and pronounced erosion (<1 m) that are otherwise not recorded. The slope of the incised valley affected the sedimentary facies. In steep valley segments, the marine transgressive surface (equivalent to the onset of full‐marine conditions) is accentuated by the Glossifungites ichnofacies, whereas in gently sloped valley segments the marine transgressive surface is gradational and bioturbated. Marine deposits are completely bioturbated.  相似文献   

10.
The Magdalen Islands are a valuable terrestrial record, evidencing the complex glacial and periglacial history of the Gulf of St. Lawrence. Thirteen structures interpreted as ice‐wedge pseudomorphs or composite‐wedge casts were observed at four sites on the southern Magdalen Islands and testify to the former presence of permafrost under periglacial conditions. These features truncate Carboniferous sandstone or Last Glacial Maximum (LGM) glacial and glaciomarine diamicts, both overlain by subtidal or coastal units. Six optically stimulated luminescence (OSL) and four radiocarbon ages were obtained from both host and infilled sedimentary units. These ages provide the first absolute chronological data on these structures, shedding new light on the relationships between glacial and periglacial phases. Our chronostratigraphic data suggest that, after the deglaciation and the emersion of the archipelago, thermal contraction cracks grew during the cold period of the Younger Dryas (11–10 ka; 12.9–11.5 cal. ka BP). The Younger Dryas, which is well documented in the Maritime Provinces of Canada, occurred after a pedogenesis phase associated with the Allerød warm period evidenced by the well‐developed palaeopodzol ubiquitous on the Magdalen Islands.  相似文献   

11.
This study investigated the Grabia River valley mire in central Poland to reconstruct its palaeoenvironmental conditions from the Younger Dryas to the present. We analysed sedimentological, biological and geochemical data from the palaeo‐oxbow lake and valley mire to identify the principal hydrological trends, especially episodes of high water level. During the Lateglacial and Holocene, the Grabia River had a meandering channel, and its hydraulic parameters and the channel dimensions changed in response to climatic oscillations and vegetation development. We identified phases of high flood intensity and high groundwater level that correlate with regional and supraregional climatic events. The frequency and timing of palaeohydroclimatic oscillations show strong similarities to records from other sites in Poland and the rest of Europe. We show that various analytical methods, namely, pollen, plant macrofossils, Cladocera, Chironomidae, sedimentological, geochemical and radiocarbon data, can be effective tools for reconstructing past hydroclimatic changes in palaeo‐oxbow lakes and investigating the effects of past climate changes on river environments. The high sensitivity of the biota, especially Cladocera, to changes in water level permits the reconstruction of palaeoecological changes, especially flood episodes that occurred in the river valley. In particular, the increase in the proportion of sediment‐associated Cladocera and pelagic taxa was closely correlated with floods. Through comparisons with the palaeobiological data, geochemical data allowed the identification of humid phases within the fen associated with a rising groundwater table, direct fluvial activity (floods) and alluvial deposition. We also discuss the limitations of palaeohydrological reconstructions based on these proxies, especially on fossil aquatic invertebrates.  相似文献   

12.
Cosmogenic 10Be surface exposure ages for bedrock sites around Torridon and the Applecross Peninsula in Wester Ross, northwest Scotland, provide new insights into the Lateglacial transition. Accounting for postglacial weathering, six statistically comparable exposure ages give a late Younger Dryas (G‐1) exposure age of 11.8 ± 1.1 ka. Two further outliers are tentative pre‐Younger Dryas exposure ages of 13.4 ± 0.5 ka in Torridon, and 17.5 ± 1.2 ka in Applecross. The Younger Dryas exposure ages have compelling implications for the deglaciation of marginal Loch Lomond Stadial ice fields in Torridon and Applecross. Firstly, they conflict with predictions of restricted ice cover and rapid retreat based on modelling experiments and climate proxies, instead fitting a model of vertically extensive and prolonged ice coverage in Wester Ross. Secondly, they indicate that >2 m of erosion took place in the upper valleys of Torridon and Applecross during the Younger Dryas, implying a dominantly warm‐based glacial regime. Finally, the exposure ages have clarified that corrie (cirque) glaciers did not readvance in Wester Ross, following final deglaciation. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

13.
The upwelling region off northwest Africa is one of the most productive regions in the world ocean. This study details the response of surface‐ and deep‐water environments off Mauritania, northwest Africa, to the rapid climate events of the last deglaciation, especially the Bølling–Allerød (15.5–13.5 ka BP) and Younger Dryas (13.5–11.5 ka BP). A high accumulation rate gravity core GeoB7926‐2, recovered at ～20° N, 18° W, was analysed for the grain size distribution of the terrigenous sediment fraction, the organic carbon content, diatom and benthic foraminifera communities. Humid conditions were observed during the Bølling–Allerød with a high contribution of fluvial sediment input. During the Younger Dryas intensified trade winds caused a larger sediment input of aeolian dust from the Sahara and more intense upwelling with higher primary productivity, as indicated by high diatom concentrations. The abrupt and large increase of organic matter caused low oxygen conditions at the sea floor, reflected by the poor benthic foraminiferal fauna and the dominance of the low‐oxygen‐tolerant foraminiferal species Bulimina exilis. This is surprising since low‐oxygen conditions have not been recorded during modern times at the sea floor in this region, despite present‐day intensive upwelling and high primary productivity. After the Younger Dryas, more humid conditions returned, diatom abundance decreased and B. exilis was replaced by typical deep‐sea species as found in the region today, indicating the return of more oxygenated conditions at the sea floor. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

14.
Geomorphological processes in the Thar Desert of India are largely climate driven. In the lower reaches of the River Luni (the only major drainage system in the Thar Desert) a fluvio‐aeolian sequence was located at a site called Khudala. Sediments of this sequence represented a variety of depositional environments, namely aeolian, fluvially reworked aeolian, overbank deposits, gravels, and occasional evidence of pedogenesis. This provided a good opportunity to study aeolian–fluvial interaction in the region and for deducing climatic records. From the luminescence dating standpoint these sequences offered a good opportunity for a comparative study of thermoluminescence (TL), blue‐green light stimulated luminescence (BGSL) and infrared light stimulated luminescence (IRSL) on different mineral separates of identical provenance but deposited under different environments. Broadly, within experimental errors, the TL ages agreed with BGSL and IRSL ages on aeolian sands, but differed substantially in the case of fluvially reworked and proximally deposited sands and silts. The sequence provided a record spanning more than 100 ka, with an aeolian phase at > 100 ka, a channel activation phase between 70 and 30 ka and a phase of climate instability between 13 and 8 ka. This appears consistent with the records of monsoon performance during this period, which includes the Younger Dryas. It is also inferred that during the Last Glacial epoch, geomorphological processes in the Thar (both aeolian and fluvial) were dormant largely on account of their relationship with the southwest monsoon. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

15.
The late Pleniglacial and Late-glacial Maas valley, south of Nijmegen, contains four terraces. Three river systems are described based on the morphology of channel scars on these terrace surfaces and by sediment characteristics. The River Maas reacted to climatic warming at the start of the Weichselian Late-glacial by changing its river system slowly, from a braided system to a transitional phase between braiding and meandering and finally to a highly sinuous meandering system. The Maas reacted rapidly to the Younger Dryas climate deterioration by again establishing a braiding system. At the onset of the Holocene, the river changed abruptly to a meandering river without a transitional phase. The triggering factor for change in the Maas river pattern is almost certainly the changing climate in the Late Glacial. Gradient lines on the terrace surfaces show that tectonic activity did not modify the morphology of the channels. A division of the terraces is shown, the morphological, sedimentological and petrographical characteristics are presented and the linking of changing fluvial patterns with climatic changes or tectonic movements is discussed. © 1997 John Wiley & Sons, Ltd.  相似文献   

16.
Thick bay‐fill sequences that often culminate in strandplain development serve as important sedimentary archives of land–ocean interaction, although distinguishing between internal and external forcings is an ongoing challenge. This study employs sediment cores, ground‐penetrating radar surveys, radiocarbon dates, palaeogeographic reconstructions and hydrodynamic modelling to explore the role of autogenic processes – notably a reduction in wave energy in response to coastal embayment infilling – in coastal evolution and shoreline morphodynamics. Following a regional 2 to 4 m highstand at ca 5·8 ka, the 75 km2 Tijucas Strandplain in southern Brazil built from fluvial sediments deposited into a semi‐enclosed bay. Holocene regressive deposits are underlain by fluvial sands and a Pleistocene transgressive–regressive sequence, and backed by a highstand barrier‐island. The strandplain is immediately underlain by 5 to 16 m of seaward‐thickening, fluvially derived, Holocene‐age, basin‐fill mud. Several trends are observed from the landward (oldest) to the seaward (youngest) sections of the strandplain: (i) the upper shoreface and foreshore become finer and thinner and shift from sand‐dominated to mud‐dominated; (ii) beachface slopes decrease from >11° to ca 7°; and (iii) progradation rates increase from 0·4 to 1·8 m yr?1. Hydrodynamic modelling demonstrates a correlation between progressive shoaling of Tijucas Bay driven by sea‐level fall and sediment infilling and a decrease in onshore wave‐energy transport from 18 to 4 kW m?1. The combination of allogenic (sediment supply, falling relative sea‐level and geology) and autogenic (decrease in wave energy due to bay shoaling) processes drove the development of a regressive system with characteristics that are rare, if not unique, in the Holocene and rock records. These findings demonstrate the complexities in architecture styles of highstand and regressive systems tracts. Furthermore, this article highlights the diverse internal and external processes and feedbacks responsible for the development of these intricate marginal marine sedimentary systems.  相似文献   

17.
A detailed shoreline displacement curve documents the Younger Dryas transgression in western Norway. The relative sea‐level rise was more than 9 m in an area which subsequently experienced an emergence of almost 60 m. The sea‐level curve is based on the stratigraphy of six isolation basins with bedrock thresholds. Effort has been made to establish an accurate chronology using a calendar year time‐scale by 14C wiggle matching and the use of time synchronic markers (the Vedde Ash Bed and the post‐glacial rise in Betula (birch) pollen). The sea‐level curve demonstrates that the Younger Dryas transgression started close to the Allerød–Younger Dryas transition and that the high stand was reached only 200 yr before the Younger Dryas–Holocene boundary. The sea level remained at the high stand for about 300 yr and 100 yr into Holocene it started to fall rapidly. The peak of the Younger Dryas transgression occurred simultaneously with the maximum extent of the ice‐sheet readvance in the area. Our results support earlier geophysical modelling concluding a causal relationship between the Younger Dryas glacier advance and Younger Dryas transgression in western Norway. We argue that the sea‐level curve indicates that the Younger Dryas glacial advance started in the late Allerød or close to the Allerød–Younger Dryas transition. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

18.
Point bars formed by meandering river systems are an important class of sedimentary deposit and are of significant economic interest as hydrocarbon reservoirs. Standard point‐bar models of how the internal sedimentology varies are based on the structure of small‐scale systems with little information about the largest complexes and how these might differ. Here a very large point bar (>25·0 m thick and 7·5 × 13·0 km across) on the Mississippi River (USA) was examined. The lithology and grain‐size characteristics at different parts of the point bar were determined by using a combination of coring and electrical conductivity logging. The data confirm that there is a general fining up‐section along most parts of the point bar, with a well‐defined transition from massive medium‐grained sands below about 9 to 11 m depth up into interbedded silts and fine–medium sand sediment (inclined heterolithic strata). There is also a poorly defined increase in sorting quality at the transition level. Massive medium sands are especially common in the region of the channel bend apex and regions upstream of that point. Downstream of the meander apex, there is much less evidence for fining up‐section. Finer sediment accumulated more readily after the establishment of a compound bar in the later stages of construction, at the terminal apex and in the bar tail. This work implies that the best reservoir sands are likely to be located in the centre of the point bar, deposited in a simple bar system. Reservoir quality decreases towards the bar edge. The early‐stage channel plug is largely composed of coarsening‐upward cycles of silt to clay and is dominated by clay and clayey silt material with poor reservoir characteristics.  相似文献   

19.
Werner, K., Tarasov, P. E., Andreev, A. A., Müller, S., Kienast, F., Zech, M., Zech, W. & Diekmann, B. 2009: A 12.5‐kyr history of vegetation dynamics and mire development with evidence of Younger Dryas larch presence in the Verkhoyansk Mountains, East Siberia, Russia. Boreas, 10.1111/j.1502‐3885.2009.00116.x. ISSN 0300‐9483. A 415 cm thick permafrost peat section from the Verkhoyansk Mountains was radiocarbon‐dated and studied using palaeobotanical and sedimentological approaches. Accumulation of organic‐rich sediment commenced in a former oxbow lake, detached from a Dyanushka River meander during the Younger Dryas stadial, at ～12.5 kyr BP. Pollen data indicate that larch trees, shrub alder and dwarf birch were abundant in the vegetation at that time. Local presence of larch during the Younger Dryas is documented by well‐preserved and radiocarbon‐dated needles and cones. The early Holocene pollen assemblages reveal high percentages of Artemisia pollen, suggesting the presence of steppe‐like communities around the site, possibly in response to a relatively warm and dry climate ～11.4–11.2 kyr BP. Both pollen and plant macrofossil data demonstrate that larch woods were common in the river valley. Remains of charcoal and pollen of Epilobium indicate fire events and mark a hiatus ～11.0–8.7 kyr BP. Changes in peat properties, C31/C27 alkane ratios and radiocarbon dates suggest that two other hiatuses occurred ～8.2–6.9 and ～6.7–0.6 kyr BP. Prior to 0.6 kyr BP, a major fire destroyed the mire surface. The upper 60 cm of the studied section is composed of aeolian sands modified in the uppermost part by the modern soil formation. For the first time, local growth of larch during the Younger Dryas has been verified in the western foreland of the Verkhoyansk Mountains (～170 km south of the Arctic Circle), thus increasing our understanding of the quick reforestation of northern Eurasia by the early Holocene.  相似文献   

20.
Knowledge of the glaciation of central East Iceland between 15 and 9 cal. ka BP is important for the understanding of the extent, retreat and dynamics of the Icelandic Ice Sheet. Crucially, it is not known if the key area of Fljótsdalur‐Úthérað carried a fast‐flowing ice stream during the Last Glacial Maximum; the timing and mode of deglaciation is unclear; and the history and ages of successive lake‐phases in the Lögurinn basin are uncertain. We use the distribution of glacial and fluvioglacial deposits and gradients of former lake shorelines to reconstruct the glaciation and deglaciation history, and to constrain glacio‐isostatic age modelling. We conclude that during the Last Glacial Maximum, Fljótsdalur‐Úthérað was covered by a fast‐flowing ice stream, and that the Lögurinn basin was deglaciated between 14.7 and 13.2 cal. ka BP at the earliest. The Fljótsdalur outlet glacier re‐advanced and reached a temporary maximum extent on two separate occasions, during the Younger Dryas and the Preboreal. In the Younger Dryas, about 12.1 cal. ka BP, the outlet glacier reached the Tjarnarland terminal zone, and filled the Lögurinn basin. During deglaciation, a proglacial lake formed in the Lögurinn basin. Through time, gradients of ice‐lake shorelines increased as a result of continuous but non‐uniform glacio‐isostatic uplift as the Fljótsdalur outlet glacier retreated across the Valþjófsstaður terminal zone. Changes in shoreline gradients are defined as a function of time, expressed with an exponential equation that is used to model ages of individual shorelines. A glaciolacustrine phase of Lake Lögurinn existed between 12.1 and 9.1 cal. ka BP; as the ice retreated from the basin catchment, a wholly lacustrine phase of Lake Lögurinn commenced and lasted until about 4.2 cal. ka BP when neoglacial ice expansion started the current glaciolacustrine phase of the lake.  相似文献   

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