Holocene changes in tropical wind intensity and rainfall: Evidence from southeast Ghana

Remnants of a fixed aeolian dune ridge occur along the southeast coast of Ghana, just behind the present shoreline. Aeolian sands also cover extensive areas of the Accra Plains. No dunes are present here, the sands mainly occurring as sheets which blanket an early Holocene landscape. The sediments are of mid-Holocene age and were deposited during the interval 4500 B.P.–3800 yr B.P., when the southwesterly winds were stronger than they are at present and much of tropical Africa seems to have been subject to marked aridity. The onset of drier, windier conditions around 4500 yr B.P. brought to an end the more equable climates than had characterized much of West Africa during the earlier Holocene. Aridity, intensified winds, and desert expansion between 4500 and 3800 yr B.P. parallel environmental conditions in tropical continental areas at the height of the Late Pleistocene glaciation.     

The European sand belt in eastern Europe - and comparison of Late Glacial dune orientation with GCM simulation results

A compilation is presented of the continuation of the European sand belt, east of Poland. Dune fields encompass most of the aeolian formations in eastern Europe. Supposed sand provenance, dune orientation, and the few available datings suggest initial aeolian activity during cold stages of the Upper-Pleni (= full)glacial and Late Glacial, similar to northwest and central Europe. Dune fomiation was primarily supply controlled. Comparison of dune orientation with Late Glacial surface wind directions simulated by various GCMs permits reconstruction of dune activation with the positioning of (winter) westerlies over glacial deposits during the Oldest Dryas. Final widespread aeolian activity occurred during the Younger Dryas. As compared to the smaller fields toward the centre of deglaciation, aeolian sand deposits of the intermediate and periglacial zone benefited from distinctly longer intervals of accumulation. In the zone of deglaciation, aeolian activity was restricted to relatively isolated basins.     

Ancient and modern cold-climate aeolian sand deposition: A review

Although partly active aeolian sand sheets and dunes cover large areas in the zones of (dis)continuous permafrost, little precise information is available about the influence of cold-climate conditions on modern aeolian processes. This means that palaeoenvironmental reconstructions in the stabilised, mainly Late Pleistocene dune fields and cover sand regions in the ‘sand belts’ of the European Lowlands and the Northern Great Plains of the USA and Canada, are necessarily still based on ancient evidence. Cold-climate wind deposits are typically derived from areas of abundant sediment supply like unvegetated flood plains, glacial outwash plains, till plains and lake shores. The common parabolic and transverse dune forms resemble those observed in temperate regions. Although a variety of periglacial features has been identified in Late Pleistocene dune and cover sands none of them indicate that permafrost is crucial to aeolian activity. Specific structures in aeolian strata permit tentative interpretation of the moisture content of depositional sand surfaces, the nature of annual sedimentation cycles and the processes by which strata were deposited and/or contorted. But surprisingly little is known about the role of vegetation in the process of sand accumulation. Dunes are most informative with respect to reconstructions of past wind regimes, which offer important data for verification of palaeoclimatic simulations.     

冰缘风沙地貌与沉积研究进展

冰缘风沙地貌与沉积是在冰缘寒冷干旱气候环境中,伴有冰川或冻融作用影响,主要由风沙作用形成的荒漠地貌.现代冰缘荒漠主要分布于南、北半球的高纬度地区及中低纬度的高海拔地区,部分温带沙漠也有古冰缘荒漠沉积分布.冰缘沙丘的规模一般较小,沙丘沉积富含腐殖质及冰粒,并兼具冻融和积雪层理构造等,砂的化学及物理成熟度较低.冰缘风沙沉积往往与邻近的其它沉积相伴生,如黄土、冰川、湖泊、海洋甚至火山碎屑,因而可参与区域气候、环境,尤其是古环流的恢复与重建,也可为研究温带荒漠在冰期时的演化提供参考.目前在冰缘沙漠研究中已经取得了一些可喜的成绩,但在确定冰缘荒漠的原因过程方面还没有形成系统的理论,更多工作有待进行.青藏高原腹地的普若岗日冰缘沙丘与戈壁,是我国典型的冰缘荒漠地貌与沉积,研究其地貌与环境过程,对丰富我国沙漠学研究内容和了解晚第四纪高原腹地气候、环境变化具有重要意义.     

Synchrony of Southern Hemisphere Late Pleistocene arid episodes: A review of luminescence chronologies from arid aeolian landscapes south of the Equator

Luminescence-dating chronologies that have been reported in various studies of Late Pleistocene arid-land aeolian sequences from Australia, southern Africa and South America are reviewed with the objective of determining whether any pan-hemispheric synchroneity existed in the timing of landscape responses to Late Quaternary climate changes south of the equator. The results broadly show that contemporaneous regional arid-land aeolian activity occurred in Australia and southern Africa during the periods ca 65–41 and 36–9 ka. The relatively limited luminescence data from the South American subcontinent also point to recurrent arid-land aeolian deposition between 63 and 8 ka, with increasing frequency in the period ca 32–8 ka. Records from all three continents suggest a period of reduced aeolian activity between 41 and 36 ka. Overall, the data are indicative of extensive landscape instability of the low to mid-latitudes in the Southern Hemisphere during the last glacial period, particularly at the Last Glacial Maximum, which accords with mainstream opinions. Aeolian activity during the Holocene has generally been localised. The rigour with which comparisons can be made between the different studies, however, is constrained by variations in practices between laboratories and changes that have occurred to luminescence dating procedures over the years.     

State-of-the-art overview of the Loess Palaeolithic of Poland

In Poland, loess mainly occurs in the southern part of the country, in the Polish Uplands, and on the northern foreland of the Sudetes and the Carpathians. Most of these sediments come from the time of the last glaciation–the Vistulian (Weichselian) glaciation. Thin covers of loess from the penultimate glaciation have been preserved in Upper Silesia. Only a few sites from the Middle Palaeolithic remain in the loess. In the eastern part of Poland loess creates profiles several metres high. Discoveries in this zone represent the cultural sequence from the transition from the Middle to Upper Palaeolithic. We know more traces of various cultures from the Upper Palaeolithic exist. The study attempts to synchronise profiles from selected archaeological open-air sites, outlines of the history of research on loess sites, and attempts to explain terminological and conceptual issues. The issue of loess in the cave site profiles will not be discussed in the article.     

Subglacial conditions in a branching Saalian esker in north-central Poland

The results of a sedimentological study of a branching esker system near Uniszki, north-central Poland, provide a deeper insight into the depositional processes that took place within a subglacial tunnel formed during the Wartanian (= Late Saalian) glaciation. The internal structure of the esker deposits is complex, and coarse-grained debris intercalates with silt and clay layers. Ten main lithofacies types are recognized, including one formed in a tunnel-mouth environment; and three other lithofacies, which must have been deposited in a truly subglacial environment. A “subglacial-tunnel association” is defined. Fluctuations in meltwater discharge through the tunnels resulted in cyclicity in the sedimentary succession. Deposition of fine-grained facies took place during low discharge, whereas boulder and gravel lithofacies formed in the tunnel-mouth during high discharge. Changes in the hydrostatic conditions finally resulted in the collapse of the tunnel roof, so that the succession became covered with supraglacial deposits. The Uniszki deposits record deposition under highly variable sedimentary hydrostatic conditions, which is typical of environments in the Polish Lowlands. The results of the present study provide some criteria for the interpretation of subglacial meltwater debris and thus may contribute to the current discussion on subglacial-tunnel sedimentation.     

Englacial kames near Jeziorany (Warmia — western Masurian Lakeland,Poland): Morphology,internal structure and origin

Groups of hills on a regional Vistulian (Weichselian) deglaciation plateau (Warmia, NE Poland) are covered by till of up to a dozen metres thick. It is the same till of the main stadial of the Vistulian glaciation that outcrops in the upland. The internal parts of the hills consist of fine-grained sands. These glacial landforms are referred to as englacial kames; they result from infilling of englacial caverns with sand. The orientation of the kame fields and the form pattern within these fields coincide with the system of primary crevasses in the ice, as it was reconstructed on the basis of the orientation of postglacial crevasse forms. The origin of the englacial caverns at crossing points of crevasse surfaces in the glacier is discussed. Structural analysis (relationship with a circular lineament, local differentiation of the complete profile of the Pleistocene deposits, possibility of occurrence of faults — festoon glacitectonics) suggests that the origin of these landforms is related to movements in the substratum.     

Proglacial damlakes in eastern Transbaikalia: New insight into the Pleistocene paleogeography

Paleogeographic reconstructions for the Samarovo, Taz, Murukta, and Sartan glaciations reveal the formation conditions of proglacial lakes dammed by ice in intermontane depressions and valleys of large rivers in eastern Transbaikalia. Middle-Late Pleistocene climate change is reconstructed using spore-pollen spectra from Pleistocene sediments in northern Transbaikalia. The age and lifetime of proglacial lakes are constrained by radiocarbon, thermoluminescence, and varve chronology of their bottom sediments in the periglacial zone. The lake levels remain recorded in sediments produced by deposition and erosion along the former lake shores, as well as in morphology and lithology variations of terminal moraines. A large proglacial lake, with a maximum level of 1020 m, occupied vast areas in Transbaikalia and its surroundings during the Samarovo glaciation. After the glaciers degraded, the Amur River system expanded into the area of closed lake basins in the southeastern Baikal region, including North China and Mongolia. The obtained results have implications for the Middle-Late Pleistocene history of lake deposition.     

Aeolian activity in northern Amazonia: optical dating of Late Pleistocene and Holocene palaeodunes

Palaeodunes were examined on the eastern margin of the Rio Branco–Rupununi savanna, northeast Amazonia. Optical dating suggests that the onset of aeolian activity was between 17 000 and 15 000 yr ago, just after the Last Glacial Maximum. Both the palaeodune axes and modern dominant wind directions have northeast to east‐northeast directions, implying no significant shift in atmospheric circulation patterns over northeast Amazonia during the Late Pleistocene to Holocene. Major regional climate change events, such as the Younger Dryas, do not appear to have had any effect on the rates of aeolian deposition at the study site. Aeolian activity appears to have continued to the present day, showing a remarkably constant deposition rate of around 0.13 m kyr^?1 initially, increasing smoothly to the present. Until more palaeodunes in northern Amazonia are dated, it is impossible to determine if this record of gradual aeolian deposition is a reliable regional palaeoclimate indicator, rather than being the result of local bioclimatic and geomorphological effects. Copyright © 2004 John Wiley & Sons, Ltd.     

Late Pleistocene sediments and environmental change at Plaza Creek,Falkland Islands,South Atlantic

Late Pleistocene organic-rich sediments exposed in coastal bluffs near the head of Plaza Creek, East Falkland, have yielded conventional and AMS ¹⁴C dates of between 36 and 28 ka BP, and possess a pollen spectrum dominated by grasses, indicating a vegetation assemblage similar to that of the present day. Although some sample dates are anomalous and contamination by non-contemporaneous carbon cannot be ruled out entirely, the age estimates are consistent with evidence and dates from Antarctica, South America and the amphi-North Atlantic for climate shifts to interstadial conditions at around that time. The organic-rich units are developed in and enclosed by deposits attributed to processes of periglacial mass wasting. Grain-size characteristics suggest that these sediments may have been emplaced by solifluction, shallow translational landsliding and surface wash in at least five mass-wasting episodes. Some of the mass-wasting sediments might correlate with solifluction deposits above and below a podsolic soil dated to 26 ka BP at San Carlos, East Falkland, and with periods of cirque and valley glaciation identified in the uplands of the Falkland Islands. The similarity between late Pleistocene interstadial, Holocene and present-day pollen assemblages, and the lack of vegetation change within these periods, is characteristic of most cool temperate Southern Ocean islands, and may reflect the lack of sensitivity of the vegetation to climate change and/or a lack of climate variability for the time intervals covered. © 1998 John Wiley & Sons, Ltd.     

Pleistocene river systems in the southern peribaltic area as indication of interglacial sea level changes in the Baltic Basin

The modern drainage system of central Poland developed during the Holsteinian, but it originated from the Elsterian glacial tunnel valleys and deglacial residual overflow lakes. In spite of occupation of this area by the Wartanian ice sheet and the following formation of the landscape during deglaciation, a similar river network was renewed during the Eemian. During the Weichselian the Middle Vistula valley was subjected to widespread ice-dam deposition. This resulted in rise of the base level of erosion and in westward deflection of the runoff, connected with development of the Central European spillways. The presented reconstruction of the Middle and Late Pleistocene fluvial network shows that the Holstein and Eemian sea levels were the driving force for river system development in central Poland. The Holstein and Eemian sea levels were very close to the present water level of the Baltic Sea. They made interglacial fluvial patterns roughly similar to the contemporary one, and therefore the main watersheds have been only slightly modified since that time. However, due to the considerable southward extension of the sea during the Eemian and presumably also during the Holsteinian, buried interglacial river deposits in central Poland occur at present well beneath the Holocene alluvia.     

Aeolian construction and alluvial dismantling of a fault‐bounded intracontinental aeolian dune field (Teruel Basin,Spain); a continental perspective on Late Pliocene climate change and variability

An aeolian dune field migrating to the east encroached on the toes of alluvial fans in the Teruel Basin (eastern Spain) during a short interval in the Late Pliocene (ca 2·9 to 2·6 Ma), when Northern Hemisphere glaciation and strong glacial–interglacial cycles began. Preservation of the dune field was controlled by syn‐sedimentary activity of a normal fault. Ephemeral water discharge eroded aeolian sands and formed V‐shaped channels in which aeolian sandstone blocks accumulated. The incorporation of loose aeolian sand in wadi waters modified the sediment/water ratio, changing the physical properties of the flows as they penetrated the aeolian dune field. The erosion and cover of aeolian dune foresets by sheetflood deposits suggest that dune‐damming caused the intermittent ponding of water behind the dunes and its flashy release. The arid climate in the Late Pliocene western Mediterranean realm favoured the transport of windblown sediments from northern Africa and western Mediterranean land masses into the Mediterranean. The formation of the studied aeolian dune field (2·9 to 2·6 Ma) and possibly others (for example, the Atacama, Namib and Sahara deserts) correlates with a strong increase of the influence of obliquity, which can be attributed to the combination of a regional expression related to the reduced effect of precession due to a minimum in the long‐period (2·3 Ma) eccentricity cycle and a remote expression of the onset of the Northern Hemisphere glaciation.     

The timing of last-glacial periglacial and aeolian events,Twente, eastern Netherlands

Sequences of last-glacial age contain valuable palaeoclimatic information but are often difficult to date because the environment has been unfavourable for deposition of datable organic material. This paper presents age-estimates, determined by optically stimulated luminescence, for sediments from the type site of the so-called ‘coversands’ (periglacial aeolian deposits) in The Netherlands. These improve the chronology of this type site considerably, allowing age limits (22–17 kyr BP) to be set, for the first time, for the important phase of widespread permafrost degradation and aeolian deflation recorded in these deposits. Aeolian deposition occurred intermittently for most of the Last Glacial Maximum and the Late-glacial phases. Based on luminescence dating, sand-sheet deposition was concentrated between ca. 17–14 kyr ago, and dune formation was dominant during the Younger Dryas. The Younger Dryas was sufficiently cold to allow the first stages of ice-wedge-cast development in The Netherlands. Copyright © 1999 John Wiley & Sons, Ltd.     

腾格里沙漠西北缘青土湖中更新世晚期以来沉积环境变迁

以腾格里沙漠西北缘青土湖钻孔ZK1为研究对象,通过对钻孔沉积物的光释光年代、粒度、磁化率等研究,揭示了该湖泊自中更新世晚期以来的环境变迁。研究结果表明：该地区中更新世晚期经历了干冷→暖湿两个阶段,晚更新世经历了湿暖→干冷两个阶段,与深海氧同位素的末次间冰期、末次冰期气候旋回特征吻合,全新世主要经历了干冷→暖湿的气候波动。反映了腾格里沙漠西北缘晚第四纪的气候波动变化特征,特别是为本区中更新世晚期以来的气候环境变化的研究提供了依据。     

A re-evaluation of Late Devensian glacial Lake Humber levels in the Vale of York,UK

Proglacial Lake Humber formed in the Vale of York and Ancholme Valley during the Late Devensian (Weichselian) glaciation, but its lake levels and their precise ages are uncertain. Three-dimensional geological modelling, based on 193 borehole sediment logs from the eastern part of the Vale of York, indicates that glaciolacustrine sediments extend no higher than 10?m O.D. By contrast, recent palaeoenvironmental reconstructions for the region that suggest Lake Humber had eight recessional shorelines, extending from 42?m to 5?m O.D. Above 10?m O.D., the sediments become more discontinuous, and comprise clay with occasional chalk and flint gravel, and matrix-rich diamicton interdigitated with sands and gravels. Sedimentary and geochemical analyses of sands and gravels from one of the putative shorelines, at 25?m O.D., indicate an easterly provenance for these sediments. They are interpreted here as colluvial deposits, sourced largely by periglacial weathering of Jurassic and chalk bedrock. Collectively, the geological evidence suggests that the highest level of Lake Humber during the Late Devensian did not exceed 10?m O.D., and therefore reconstructions invoking higher lake levels are thought to be unlikely.     

Soft-sediment deformation structures in NW Germany caused by Late Pleistocene seismicity

New data on seismically triggered soft-sediment deformation structures in Pleniglacial to Late Glacial alluvial fan and aeolian sand-sheet deposits of the upper Senne area link this soft-sediment deformation directly to earthquakes generated along the Osning Thrust, which is one of the major fault systems in Central Europe. Soft-sediment deformation structures include a complex fault and fold pattern, clastic dikes, sand volcanoes, sills, irregular intrusive sedimentary bodies, flame structures, and ball-and-pillow structures. The style of soft-sediment deformation will be discussed with respect to brittle failure, liquefaction and fluidization processes, and was controlled by (1) the magnitude of the earthquake and (2) the permeability, tensile strength and flexural resistance of the alluvial and aeolian sediments. It is the first time in northern Germany that fluidization and liquefaction features can be directly related to a fault. The occurrence of seismicity in the Late Pleistocene and in the seventeenth century indicates ongoing crustal movements along the Osning Thrust and sheds new light on the seismic activity of northern Germany. The Late Pleistocene earthquake probably occurred between 15.9 ± 1.6 and 13.1 ± 1.5 ka; the association of soft-sediment deformation structures implies that it had a magnitude of at least 5.5.     

Dartmoor's overlooked glacial legacy

Traditionally regarded as a relict permafrost and periglacial landscape that lay beyond the limits of Pleistocene glaciation, the granite uplands of northern Dartmoor in south‐west England in fact contain geomorphological evidence for the former existence of a plateau ice cap, making the area the location of the southernmost independent glacier mass in the British Isles. In addition to weakly U‐shaped valleys, the most prominent evidence comprises arcuate and linear bouldery ridges and hummocky valley floor drift, which are interpreted as latero‐frontal moraines deposited by the outlet glacier lobes of a plateau ice cap. Inset sequences of these depositional landforms, in association with meltwater channels, demarcate the receding margins of the glacier lobes. A numerical model of ice cap development shows that a predominantly thin plateau icefield type glaciation is required in order to produce significant ice flow into surrounding valleys. The highest and most extensive plateau areas were occupied by ice for the longest cumulative period of time throughout the Pleistocene, thereby explaining: (1) the lack of tors in such areas as the product of ‘average’ glacial conditions preferentially removing tors or dampening their production rates, (2) the survival of high relief tors during glaciation if they occupied summits too narrow to develop thick and erosive glacier ice, and (3) the survival of subdued tors in areas glaciated less regularly during the Pleistocene.     

Drainage evolution in south and east England during the Pleistocene

Two major river systems operated in southern and eastern England throughout the Pleistocene: the river Thames and the Solent river. Both rivers are axial streams of comparable size draining major basinal structures comprising similar Tertiary and Mesozoic rocks. Although the modem Thames flows broadly W-E in the London Basin, upstream of Reading it flows from the north to drain the south Midlands. It was diverted to its present course through London by glaciation in the Anglian (Elsterian) before which it flowed across East Anglia into the southem North Sea. The Solent river no longer exists since most of its course was drowned by eustatic sea-level rise during the Flandrian Stage (Holocene). Previously, it flowed eastwards across SE Dorset and S Hampshire as an extension of the modem river Frome in the Hampshire Basin. During periods of low sea-level (cold stages) it was a tributary of the 'Channel River'. Fluvial aggradations provide evidence of the former courses of these substantial rivers and their tributaries. The facies and sedimentary structures indicate that the bulk of the deposits in both systems accumulated in braided river environments under periglacial climates. Fossiliferous sediments provide biostratigraphical frameworks. During temperate periods the rivers adopted singlethread courses. Evolution of both rivers reflect their responses to climatic change, local geological structure and long-term tectonic activity. Both rivers are undoubtedly of considerable antiquity, their records potentially extend from the Early Pleistocene or Late Pliocene, but they may have originated in the early Tertiary.