Control of wind strength and frequency in the Aral Sea basin during the late Holocene   总被引：1，自引：0，他引：1  

Philippe Sorrel  Hedi Oberhnsli  Nikolaus Boroffka  Danis Nourgaliev  Peter Dulski  Ursula Rhl 《Quaternary Research》2007,67(3):371-382

Changing content of detrital input in laminated sediments traced by XRF scanning and microfacies analyses reflect prominent variations in sedimentation processes in the Aral Sea. A high-resolution record of titanium from a core retrieved in the northwestern Large Aral Sea allows a continuous reconstruction of wind strength and frequency in western Central Asia for the past 1500 yr. During AD 450–700, AD 1210–1265, AD 1350–1750 and AD 1800–1975, detrital inputs (bearing titanium) are high, documenting an enhanced early spring atmospheric circulation associated with an increase in intensity of the Siberian High pressure system over Central Asia. In contrast, lower titanium content during AD 1750–1800 and AD 1980–1985 reflects a diminished influence of the Siberian High during early spring with a reduced atmospheric circulation. A moderate circulation characterizes the time period AD 700–1150. Unprecedented weakened atmospheric circulation over western Central Asia are inferred during ca. AD 1180–1210 and AD 1265–1310 with a considerable decrease in dust storm frequency, sedimentation rates, lamination thickness and detrital inputs (screened at 40-μm resolution). Our results are concurrent with changes in the intensity of the Siberian High during the past 1400 yr as reported in the GISP2 Ice Core from Greenland.  相似文献   

Younger Dryas to mid-Holocene environmental history of the lowlands of NW Transylvania, Romania   总被引：2，自引：1，他引：2  

Angelica Feurdean  Volker Mosbrugger  Bogdan P. Onac  Victor Polyak  Daniel Veres   《Quaternary Research》2007,68(3):364-378

Pollen, micro-charcoal and total carbon analyses on sediments from the Turbuta palaeolake, in the Transylvanian Basin of NW Romania, reveal Younger Dryas to mid-Holocene environmental changes. The chronostratigraphy relies on AMS ¹⁴C measurements on organic matter and U/Th TIMS datings of snail shells. Results indicate the presence of Pinus and Betula open woodlands with small populations of Picea, Ulmus, Alnus and Salix before 12,000 cal yr BP. A fairly abrupt replacement of Pinus and Betula by Ulmus-dominated woodlands at ca. 11,900 cal. yr BP likely represents competition effects of vegetation driven by climate warming at the onset of the Holocene. By 11,000 cal yr BP, the woodlands were increasingly diverse and dense with the expansion of Quercus, Fraxinus and Tilia, the establishment of Corylus and the decline of upland herbaceous and shrubs taxa. The marked expansion of Quercus accompanied by Tilia between 10,500 and 8000 cal yr BP could be the result of low effective moisture associated with both low elevation of the site and with regional change towards a drier climate. At 10,000 cal yr BP, Corylus spread across the region, and by 8000 cal yr BP it replaced Quercus as a dominant forest constituent, with only little representation of Picea abies. Carpinus became established around 5500 cal yr BP, but it was only a minor constituent in local woodlands until ca. 5000 cal yr BP. Results from this study also indicate that the woodlands in the lowlands of Turbuta were never closed.  相似文献   

Continuous record of environmental changes in Chukotka during the last 350 thousand years     

A. V. Lozhkin  P. M. Anderson  T. V. Matrosova  P. S. Minyuk  J. Brigham-Grette  M. Melles 《Russian Journal of Pacific Geology》2007,1(6):550-555

The comprehensive study of the upper 1283 cm of sediment from Lake El’gygytgyn, which formed nearly 4 Ma ago following a meteorite impact in northern Chukotka, yielded the first continuous record of the extreme changes in the Beringian climate and vegetation from the middle Middle Pleistocene to recent time (equivalent of marine isotope stages of 1–7 and the upper part of isotope stage 8). During this period, the climate was warmer than at present between 8600 and 10 7000 ¹⁴C years and during the Late Pleistocene (isotope substage 5e, 116–128 ka ago). In 2003, the German-Russian-USA expedition continued studying sediments of Lake El’gygytgyn to obtain new evidence of the change in the vegetation cover in the Middle Pleistocene and the first information on the Middle Pleistocene interglacial (isotope stage 9; 297–347 ka ago). Pollen spectra characterizing the Middle Pleistocene interglacial are similar to spectra of the early stage of the Early Pleistocene interglacial and the climatic optimum in the Pleistocene to Holocene transitional period. The climatic history of Lake El’gygytgyn is basic for stratigraphic interpretations and correlations in the eastern sector of the Arctic. These data also expand our understanding of climatic changes that are studied within the framework of the “Pole-Equator-Pole Paleoclimate,” “Past Global Changes,” and other international projects.  相似文献   

Climatic change and human activities of northeastern sahara in Holocene   总被引：1，自引：0，他引：1  

张虎才 《中国地理科学(英文版)》1998,8(4):377-382

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“TO CIVILITY AND TO MAN'S USE”: HISTORY,CULTURE, AND NATURE*     

I. G. SIMMONS 《Geographical review》1998,88(1):114-126

ABSTRACT. An attempt is made to give a 10,000-year perspective on the relations of history, culture, and the nonhuman world called nature. A Holocene narrative of processes separates human societies and their cultures: things that individuate and pull apart to the point of fragmentation versus those that are binding. These tendencies toward convergence may coalesce, as with the natural sciences or global electronic technology. Equally, they may involve centrifugal processes, as in the creative arts or in forms of representation accessible only to certain groups or cultures. The environment resonates in a series of segmented channels, considerably complicated by a binary Western culture, often with 1 as acceptable and o as the Other. Is a purposeful path laid down by someone else and followed to its predetermined end in Utopia, or do we cherish something more open and contingent?  相似文献   

香港全新世沉积物中的风暴潮记录     

黄光庆 《地理学报》1998,53(3):216-227

香港是受台风暴潮影响最严重的地区之一。本项研究是根据近岸及浅海地区沉积物的特征性来分析和获取全新世的时期的台风暴潮信息。根据沉积结构和构造特征,发现有５种类型的沉积层：粉砂质粉土、砂质粒序层、水平或波状层理层、贝屑富集层和含贝屑粉砂层,其中后４种类型与台风暴潮有关。根据沉积物的磁化率分析,上部深度的高磁化率层可能与台风暴潮对现代受污染沉积物的改造致使新老沉积物的混合所致。有孔虫分析显示,风暴沉积层  相似文献   

Holocene sedimentation in Lake Winnipeg, Manitoba, Canada: implications of compositional and textural variations     

P. J. Henderson  William M. Last 《Journal of Paleolimnology》1998,19(3):265-284

Two seismic facies were recognized in the sedimentary sequence overlying acoustic basement in Lake Winnipeg. The upper facies, which overlies a regional unconformity, is termed the Lake Winnipeg Sequence. Based on the seismostratigraphy, lithostratigraphy, and radiocarbon dates of approximately 4000 and 7000 yr BP from material collected directly over the unconformity in the southern and northern parts of the lake, respectively, this facies has been interpreted as representing Holocene sedimentation. Results of compositional and textural analyses of the Holocene sediment (Winnipeg sediment) from thirteen long (>2 m) cores indicate a transgressional sequence throughout the basin. In the South Basin, the generally fining upward sequence is characterized at the base by silt-sized detrital carbonate minerals, quartz and feldspar which decrease in concentration upward. In this basin, the high carbonate content and V/Al and Zn/Al ratios are indicative of a Paleozoic and Cretaceous provenance for sediment derived from glacial deposits through shoreline erosion and fluvial transport, via the Red River. Sedimentation in the central part of the lake and the North Basin is attributed to shoreline erosion of sand and gravel beaches. Consequently, the texture of these sediments is generally coarser than in the South Basin, and the composition primarily reflects a Paleozoic and Precambrian provenance. The basin-wide decrease in Ca, total carbonate minerals, dolomite and calcite concentrations upward in the cores is reflected by a decrease in the detrital carbonate component in all but the most northern cores. Other basin-wide trends show an upward increase in organic content in all cores. An increase in grain size near the top of most cores suggests a major, basin-wide change in sedimentation within the last, approximately 900 years in the South Basin.  相似文献   

Thecamoebians in Lake Winnipeg: a tool for Holocene paleolimnology     

Susan M. Burbidge  Claudia J. Schröder-Adams 《Journal of Paleolimnology》1998,19(3):309-328

Holocene sediments in Lake Winnipeg are expressed in the lower Lake Agassiz sequence which is unconformably overlain by the Lake Winnipeg sequence. Nine sites, covering the North and South basins and the connecting Narrows, were selected for analysis of Holocene changes in thecamoebian faunae. Only the Lake Winnipeg sequence contains thecamoebians. This study indicates that biologic productivity and consequently the type of organic material in the sediments is the main control on thecamoebian taxa in Lake Winnipeg. Other factors controlling the distribution of thecamoebians are water chemistry and turbidity. Inorganic sediment geochemistry and water temperature do not appear to significantly influence the thecamoebian fauna of Lake Winnipeg. Variations in the abundance of key thecamoebian species along a north-south transect divide Lake Winnipeg into three distinct areas. The North Basin has remained relatively unchanged since the retreat of Lake Agassiz as indicated by the domination of Difflugia manicata throughout its history. This species appears to prefer Cyanophyta and diatoms as its food source. In the Narrows harsh conditions created by turbid waters and lack of algal food taxa result in Centropyxis aculeata replacing Difflugia manicata as the dominant species. In the South Basin three thecamoebian assemblages are recognized. Cucurbitella tricuspis, indicative of eutrophic conditions, dominates the most recent sediments of the South Basin. The underlying sediments are characterized by Difflugia globulus. In Lake Winnipeg this species is not a cold climate (arctic) indicator as suggested elsewhere but instead seems to prefer sediments containing green and yellow-green algal material. A Centropyxis-Arcella Assemblage occurs only at the base of the southernmost core where it is indicative of an early phase of hyposaline conditions as developed in shallow pools during the southward transgression of Lake Winnipeg. This study illustrates the usefulness of thecamoebians as paleolimnological indicators. Environmental changes are more significant in the restricted South Basin resulting in distinct thecamoebian assemblages. In contrast, the North Basin provided a stable environment throughout the late Holocene reflected in only subtle faunal changes.  相似文献   

新疆博格达山北麓山前断裂西段的断裂组成及其在全新世时期的活动   总被引：6，自引：2，他引：4  

栾超群  胡军  罗福忠  李锰  胡伟华 《内陆地震》1998,12(4):355-361

博格达山北麓山前断裂西段全长１４０ ｋｍ ,由５ 条雁列状排列的逆断裂组成。全新世时期,古牧地断裂的最新活动发生在６．８２±０．５４～４．８７±０．３９ Ｋａ Ｂ．Ｐ．之间;四工河口断裂切割Ⅱ级河流阶地,最新活动时间与古牧地断裂大体相同;泉泉子断裂切割Ⅰ级河流阶地,最新活动发生在３．７５±０．４１ Ｋａ Ｂ．Ｐ．;热水河断裂及东风牧场断裂由于缺乏断代年龄实测数据,无法对其最新活动时间作出判断,但是推断它们也是全新世活断裂  相似文献   

Sea level,climate change and coastal evolution in Morar,northwest Scotland     

Shennan  Ian  Tooley  Michael  Green  Frances  Innes  Jim  Kennington  Kevin  Lloyd  Jeremy  Rutherford  Mairead 《Geologie en Mijnbouw》1998,77(3-4):247-262

Analyses of geomorphologically contrasting sites in Morar, NW Scotland, describe the forcing mechanisms of coastal change. Isolation basins (i.e. basins behind rock sills and now isolated from the sea following isostatic uplift) accumulated continuous marine and freshwater sediments from c.12 to 2 ka BP. Raised dune, marsh and wetland sites register breaching, migration and stability of dunes from c. 9 to 2 ka BP. High-resolution methods designed to address issues of macroscale and microscale sea-level changes and patterns of storminess include 1-mm sampling for pollen, dinocyst and diatom analyses, infra-red photography, X-ray photography and thin-section analysis. The data enhance the record of relative sea-level change for the area. Major phases of landward migration of the coast occurred during the period of low sea-level rise in the mid-Holocene as the rate of rise decreased from c. 3 to < 1 mm/year. Relative sea-level change controls the broad pattern of coastal evolution at each site; local site-specific factors contribute to short-term process change. There is no record of extreme events such as tsunami. Within a system of dynamic metastable equilibrium, the Holocene records show that site-specific factors determine the exact timing of system breakdown, e.g. dune breaching, superimposed on regional sea-level rise. The global average sea-level rise of 3 to 6 mm/yr by AD 2050 predicted by IPCC would only partly be offset in the Morar area by isostatic uplift of about 1 mm/yr. A change from relative sea-level fall to sea-level rise, in areas where the regional rate of uplift no longer offsets global processes, is a critical factor in the management of coastal resources.  相似文献