首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Presented are the results from pedolithological investigations and reconstructions of the naturalclimatic events in the Early Holocene (radiocarbon age 10.3?8 ka; calendar age 11.7?8.8 ka) on the coast of Middle Baikal, based on data obtained by studying the structure, composition and properties of subaerial deposits and soils in terrestrial sections. A study was made of the numerous soil profiles and sections of geoarchaeological sites. The investigation revealed common features in the layered structure of the Early-Holocene portion of the terrestrial sections and evidence of climate warming represented by buried soils, signals of a cooling in the form of cryogenic fissures, and signals of drying in the form of aeolian drifts and evidence of deflation. It is shown that the Early Holocene was the time of cardinal changes in the nature of subaerial sedimentation (calcareous deposits were replaced by noncalcareous deposits) and the pedogenesis. Considerable cold storage from inherited permafrost and humidification of soils and earth materials with moisture from the thawing of permafrost were responsible for the specific character of soil formation, and for the spread of forest vegetation under a rather low atmospheric humidity. The phases of climate warming were accompanied by an intensification of soil formation with the production of soils of two types: early boreal, and boreal. Deluvial, colluvial and sometimes (in Priol’khonie) aeolian deposits accumulated during the cooling phases (Late Preboreal and Late Boreal). Small cryogenic fissures were generated. The issues of man’s adaptation to the cardinally changed natural conditions at the turn of the Pleistocene and Holocene and over the course of the Early Holocene are associated with the problem of Baikal’s water level and human settlement on its shores. The Lake Baikal stage at the end of the Late Neopleistocene was lower than at present; during the Early Holocene it was rising to reach at the late-boreal period the present-day level (or even exceeded it). The rises of Baikal’s stage at the Mid- and Late-Holocene period were causing scouring and destruction of the Early-Holocene sites that were located at lower elevations.  相似文献   

2.
全新世内蒙古自然环境演变及其特点   总被引：9，自引：2，他引：9  
根据内蒙古东、中、西部沙地、内陆湖、冰缘现象的发育与演变，讨论了全新世以来内蒙古环境演变过程，提出了对全新世中期沙漠化的形成环境、全新世环境演变特点及对现代环境影响的认识。  相似文献   

3.
The Southern Alps lie along the convergent Pacific-Indian plate boundary. Geomorphically distinct eastern, axial and western regions reflect the east-west gradient in tectonic uplift (1 to 10 mm a−1) and precipitation (600 to 10,000 mm a−1). The eastern region is divided into front-ange and basin-and-range subregions. Soil-sequence studies on terraces established temporal contrasts in pedogenesis within and between eastern and western regions encompassing Entisols, Inceptisols and Spodosols. On Late Pleistocene and early Holocene terraces Dystrochrepts are persistent soils in the eastern region and Aquods in the western region. These soil sequences are used in the interpretation of relative soil age, stratigraphy and erosion history in hill and mountain drainage basins of the eastern and western regions. In the subhumid to humid eastern front-range subregion, simple soil forms occur as catenary sequences, and there is little evidence of erosion following the destruction of forests in the last millenium. Mollisols are dominant in the subhumid, and Dystrochrepts in humid areas, respectively. Soil-debris mantle regoliths date from the early Holocene and are still developing on slopes. The soil pattern on mountain slopes in the humid, eastern basin-and-range subregion is a complex array of simple, eroded, composite and compound soils. This pattern has resulted from erosion following forest destruction within the last millenium. The oldest surface or buried forest soils are Dystrochrepts dating from the Late Pleistocene to early Holocene. Wind erosion of these low-fertility soils contributes to the loessial sediments in which younger soils have formed. In the western region, soil patterns and soil stratigraphy indicate continous instability with a complex pattern of highly leached, shallow Orthents and bedrock outcrops on slopes. The soils are eroded from slopes within 2 ka. These contrasts in soil development and erosion periodicity in the eastern and western regions of the Southern Alps parallel the east-west contrasts in erosion rates of ca. 1–10 mm a−1.  相似文献   

4.
We have investigated the river terraces on the left bank of the Irkut river in the Torskaya depression a short distance from the village of Guzhiry. On the basis of lithological-geological findings and radiocarbon datings of buried soils from two sections of the second terrace (12–14 m), we identified nine formation stages of the terrace in the latter half of the Late Neopleistocene and in the Holocene. They reflect multiple changes of the leading exogenous process, implying a variety of the genetic types of deposits (soils, and aeolian and alluvial sediments) during the Early and Mid-Holocene. The formation stages of alluvium are correlated with periods of high water. It is found that the final transition of alluvial to cover deposits is associated with incision of the river to 2–4 m and is dated to 5.2–4.5 cal. ka. We examine the alternation of the natural factors for the formation of deposits of the second terrace of the Irkut river in the Late Neopleistocene and Holocene. One (hydroclimatic) factor implies accumulation of deposits of the alluvial and cover complex depending on climate and water runoff fluctuations, landscape changes, and on variation in the base level of erosion. The other (seismic) factor is correlated with data on high activity of the Tory paleoseismogenic structure, which seems to have caused the lowering as well as the rise of the bottom of the depression at the time of strong earthquakes and, as a consequence, erosion or accumulation of deposits of the channel facies of alluvium. It is established that the chief causes for the change of the terrace’s deposit types were the natural-climatic changes, the character and directedness of tectonic movements, the variations in the base levels of erosion, and the height of floo ds.  相似文献   

5.
This article presents a summary of our studies of Holocene moraines and glaciers of the Tien-Shan, Pamir, and Himalaya mountains with the purpose of providing pattern regularity of the Holocene glaciation decomposition. We developed a method for obtaining reliable radiocarbon dating of moraines with the use of autochthonous organic matter dispersed in fine-grained morainic material, as well there were shown new possibilities of isotope-oxygen and isotope-uranium analysis for the Holocene glaciations dynamics. We found that Holocene glaciations disintegrate stadially according to the decaying principle, and seven main stages may be distinguished. We achieved the absolute dating of the first three stages, identifying these periods as 8,000, 5,000, and 3,400 years ago. The application of the above-mentioned isotope methods of the Holocene glaciations and moraines study will allow researchers to improve the offered model of the Holocene glaciations disintegration; it will be great contribution to salvation of the problem of long-term climatic and glaciations forecast.  相似文献   

6.
Mapping of late Quaternary geomorphic surfaces, and analysis of the soils and sediments buried within them, provides evidence for the history of a small study area within the Red Valley physiographic zone, Black Hills, South Dakota. Geomorphic thresholds for this grassland system are correlated with periods of major climatic change. Well-developed soils dating to the late Pleistocene and early Holocene (14,000 to 9000 yr B.P.) suggest more mesic conditions and geomorphic stability. A mid-Holocene Altithermal (ca. 8000 to 4500 yr B.P.) denudation almost completely stripped the landscape of earlier Holocene sediments and soils. A prolonged, mid-Holocene (ca. 4500 to 3600 yr B.P.) mesic period of landscape stability and soil development followed, but was abruptly terminated around 3600 yr BP. Late Holocene conditions approached stability about 1200 yr BP. After this time, alluvial terrace surfaces remained stable, while alluvial fans experienced periods of stability punctuated by midslope aggradation.  相似文献   

7.
全新世以来里下河地区古地理演变   总被引：12，自引：2，他引：12  
凌申 《地理科学》2001,21(5):474-479
里下河地区是苏北平原的重要组成部分，全新世以来，本区古地理环境经历了巨大的变化。据作者研究，大致可分为三个阶段：早全新世，本区受海侵影响，7－6kaB.P．左右海侵达到高峰，整个里下河地区成为一个大的浅水海湾；中全新世，海面相对稳定，长江、淮河等河流挟带大量泥沙入海，古海湾被岸外砂堤所包围，形成泻湖；晚全新世，海岸东迁，古泻湖演变为湖泊沼泽，在人类活动与黄河夺淮的影响下，本区逐渐演变为里下河平原。  相似文献   

8.
A conjugate analysis of the processes of morphogenesis and sedimentation within small fluvial basins of Dauria revealed a rhythmic character of change of the Holocene processes which are combined into cycles of a different hierarchical level. It is shown that each cycle begins with an extreme phase of intense fluvial drift and delivery of matter from the system. The fluvial horizons comprise the lower part of deposits of the Holocene macrocycle in the Vesnyanka pad’ (a valley without a permanent watercourse). A disastrous fluvial drift was recorded repeatedly for the period 7–8.5 cal. ka and at the transition time from the Atlantic to Subboreal time. A normal zonal phase of integration of matter within the basins is exhibited by buried humus horizons of soils which give clear evidence of 1800-year cycles. A regional development in Dauria is recorded for the soils with the age of about 8, 6.4 and 4.6 as well as 1.2–1.4 cal. ka. The upper part of the deposits of each cycle is characterized by aeolian horizons of extreme arid phases. The profiles of proluvial trails are indicative of 130-year cycles. Based on results from studying buried soils, data of pollen analysis and radiocarbon dating of the deposits, we determined an enhancement in climate aridization for the last 10 ka and, in general, a decreased of the sedimentation and morphogenesis rate.  相似文献   

9.
孙东怀  周杰 《中国沙漠》1995,15(4):339-344
通过黄土高原及其临区１０４个地点地表土壤磁化率值与当地的现代多年平均降水量的回归分析，建立了磁化率与降水量的转换函数。在遍布黄土高原不同地区典型的全新世黄土－古土壤剖面磁化率测量的基础上，利用所建立的转换函数，初步恢复了全新世气候适宜期，黄土高原及黄土／沙漠过渡区年降水量的分布状况。  相似文献   

10.
Environmental magnetic studies were conducted on a 9.42-m-long sediment core from Gonghai Lake, North China. Radiocarbon dating indicates that the record spans the last 15,000 cal year BP. The principal magnetic mineral in the sediments is pseudo-single domain magnetite of detrital origin with minimal post-depositional alteration. Although the variations in the concentration of detrital magnetic minerals and their grain size throughout the core reflect inputs from both soil erosion and eolian dust, it is shown that their climatic and environmental significance changes with time. In the lowermost part of the core, ~15,000–11,500 cal year BP, the magnetic minerals were supplied mainly by bedrock erosion, soil erosion and dust input when climate ameliorated after the cold and dusty last glacial maximum. The increasing magnetic susceptibility (χ) in this interval may indicate a combination of changes in the lake environment together with catchment-surface stabilization and a decreasing proportion of dust input. In the central part of the core, ~11,500–1,000 cal year BP, the detrital magnetic minerals mainly originated from dust inputs from outside the catchment when the lake catchment was covered by forest, and catchment-derived sediment supply (and thus the lake sediment accumulation rate) were minimal. The generally low concentration of magnetic minerals in this part of the core reflects the highest degree of soil stability and the strongest summer monsoon during the Holocene. In the uppermost part of the core, the last ~1,000 years, detrital magnetic minerals mainly originated from erosion of catchment soils when the vegetation cover was sparse and the sediment accumulation rates were high. Within this part of the core the high magnetic susceptibility reflects strong pedogenesis in the lake catchment, and thus a strong summer monsoon. This scenario is similar to that recorded in loess profiles. Overall, the results document three main stages of summer monsoon history with abrupt shifts from one stage to another: an increasing and variable summer monsoon during the last deglacial, a generally strong summer monsoon in the early and middle Holocene and a weak summer monsoon in the late Holocene. The results also suggest that different interpretational models may need to be applied to lake sediment magnetic mineral assemblages corresponding to different stages of environmental evolution.  相似文献   

11.
陕西扶风黄土台塬全新世成壤环境变化研究   总被引：8，自引：4，他引：4  
通过对陕西扶风全新世黄土剖面的土壤学和地层学研究、磁化率、酸碱度和粒度测定分析, 揭示了周原地区全新世成壤环境演变过程。黄土剖面中的埋藏古土壤S0形成于全新世大暖期的温湿环境中, 古土壤S0中黄土夹层(Lx)的存在, 表明6000aBP前后季风气候发生转折, 在6000~5000aBP出现一个干旱阶段, 并使得全新世大暖期分裂成为两个主要的温湿阶段。大约从3100aBP开始, 受全球降温过程的影响, 季风气候朝向干旱化方向发展, 全新世大暖期的成壤作用被黄土堆积所取代。现代表土大约从1500aBP开始发育。  相似文献   

12.
乌兰布和沙漠北部全新世地貌演化   总被引：5，自引：2，他引：3  
贾铁飞  银山 《地理科学》2004,24(2):217-221
根据对乌兰布和沙漠北部地区全新世风沙和湖泊沉积记录及其沉积时代进行分析,认为全新世以来,乌兰布和沙漠北部地区地貌演化经历了4个主要的发育时期:Q33~Q41湖泊地貌发育时期,Q41风沙地貌发育时期,Q42湖(河)(屠申泽)地貌发育时期和Q43风沙地貌发育时期。全新世中期屠申泽最为繁盛的时期,湖泊相互沟通,范围几乎占据了整个乌兰布和沙漠北部地区,之后经历了自南向北的萎缩、分化过程,而屠申泽的萎缩、分化过程正是乌兰布和沙漠晚全新世风沙地貌发育并不断扩张的过程,这是一个以自然环境变化为主因、以人为影响为辅因的变化过程。  相似文献   

13.
Deposits of debris fans and bottom gullies in Southwestern Cisbaikalia have been studied and subjected to radiocarbon dating. The study identified six development stages of a 500–1000-year-long rank for erosion-accumulation processes in gully-draw systems of Southwestern Cisbaikalia uring the Holocene.  相似文献   

14.
The Hunshandake Desert is located at the northern edge of the East Asian monsoon region,and its natural environment is sensitive to monsoonal changes.Geologic records suggest that desert evolution corresponding to climate change had experienced several cycles in the Holocene,and the evolutionary process can be distinguished by four dominant stages according to changing trends of the environment and climate.(1) Holocene Ameliorative Period(11.0-8.7 cal ka B.P.),when the desert area gradually shrank following an approaching warm-wet climate and strengthening summer monsoon.(2) Holocene Optimum(8.7-6.0 cal ka B.P.),when the majority of moving sand dunes were stabilized and vegetation coverage quickly expanded in a suitable warm-wet climate and a strong summer monsoon.(3) Holocene Multivariate Period(6.0-3.5 cal ka B.P.),during a low-amplitude desert transformed between moving and stabilized types under alternating functions of cold-dry with warm-wet climate,and winter monsoon with summer monsoon.(4) Holocene Decay Period(since 3.5 cal ka B.P.),when the desert area tended to expand along with a weakened summer monsoon and a dry climate.  相似文献   

15.
伊河龙门峡段全新世古洪水和历史洪水水文学重建   总被引：2，自引：0，他引：2  
对伊洛河流域进行水文学和沉积学野外调查,在伊河龙门峡（LMX）地点发现典型的全新世黄土—古土壤剖面,根据野外沉积特征和磁化率、粒度分析判定该剖面夹有五层洪水滞流沉积层（SWD）。通过OSL测年和地层对比,确定该地层记录了四期五次特大洪水事件,分别发生在3100~3000 a BP、1800~1700 a BP、770~610 a BP和420~340 a BP,并结合史料等初步判断其中包含了AD 223和1761年历史洪水的沉积记录。这些古洪水事件与全新世气候恶化、转折和气候事件有明确的对应关系。采用古洪水水文学方法恢复洪峰水位,并采用比降—面积法计算出古洪水和历史洪水事件的洪峰流量介于14100~15800 m3/s。同时,根据实测获得1958年7月17日伊河龙门大洪水的洪峰水位和流量数据,采用相同参数和方法恢复其洪峰流量,误差仅有-3.6%,表明在该河段古洪水水文学重建所选参数和计算结果是可靠的。这些研究成果既证明季风气候区河流水文系统对全新世气候异常事件响应敏感,又为伊洛河流域水利、交通和防洪工程建设提供基础数据,具有重要的科学意义和实践价值。  相似文献   

16.
Early Holocene, near-shore marine sediments from Visdalen, Edgeøya, eastern Svalbard contain locally abundant allochthonous remains of land plants, notably bryophytes. Wetland species indicative of mineral rich and calcareous soils are frequent, but upland plants are also well represented. The fossil assemblages are indicative of ecological and climatic conditions similar to those on Edgeøya today. The sediments contain one of the first fossil beetles reported from Svalbard. Apparently, the modern flora of Svalbard was already established in the earliest Holocene, probably following immigration from northern Europe. A few Armeria scabra remains are believed to be derived from interglacial deposits.  相似文献   

17.
A comprehensive investigation of the current state of soil-vegetation cover has been conducted. A pattern of reconstruction of the temporal dynamics for 4 small islands in the Gulf of Peter the Great has been obtained. Sequential changes in vegetation from polydominant broad-leaved to coniferous-broad-leaved forests on Engel’m, Lavrov and Shkot Islands observed are probably bound up with climate factors in the beginning of the Late Holocene. The transition from coniferous-broad-leaved to sparse broad-leaved forest has been probably caused by recent anthropogenic influences. The coniferous-broad-leaved forests growing on podsol soils (Naumov Island) are presently dominant, what is one of characteristics of an anthropogenically transformed ecosystem.  相似文献   

18.
吉兰泰盐湖的形成,发育和演化   总被引：12，自引：1，他引：12  
耿侃  陈育峰 《地理学报》1990,45(3):341-349
吉兰泰盐湖的演化过程有着明显的阶段性。根据地貌结构、沉积特征、盐化学特征以及人为活动对盐湖的影响,可以认为:全新世以来,吉兰泰盐湖演化大致经历了五个发展阶段:早全新世的咸水湖、中全新世早期的硫酸盐型盐湖、中全新世晚期的硫酸盐-氯化物型盐湖、晚全新世的氯化物型盐湖和现代干盐湖。现今由于人为活动的影响,盐湖正向沙下盐湖转化。  相似文献   

19.
中国干旱、半干旱区末次冰期以来气候变化规律   总被引：18，自引：11，他引：7  
以临夏塬堡末次冰期中晚期黄土剖面为基础,选用我国干旱、半干旱区末次冰期以来同期异相或同期同相的气候记录与研究剖面进行对比分析后表明：位于西风气流控制的新疆地区全新世气候总体表现为偏干偏冷,晚期较为温暖的特点．其气候波动的幅度和频率较低。而位于季风环流控制的其它地区．全新世气候变化反映了同样的变化规律,即早全新世的升温波动期、中全新世的温暖稳定期和晚全新世的变冷期,其气候波动的幅度和频率较快。而无论是西风气流控制的新疆地区,还是季风环流影响的其它我国干旱、半干旱区,末次冰期气候变化均表现为早晚期的干冷期和中期的温湿期,尽管其干冷和温湿的程度不尽相同。  相似文献   

20.
We utilized paleoecological techniques to reconstruct long-term changes in lake-water chemistry, lake trophic state, and watershed vegetation and soils for three lakes located on an elevational gradient (661–1150 m) in the High Peaks region of the Adirondack Mountains of New York State (U.S.A.). Diatoms were used to reconstruct pH and trophic state. Sedimentary chrysophytes, chlorophylls and carotenoids supplied corroborating evidence. Pollen, plant macrofossils, and metals provided information on watershed vegetation, soils, and biogeochemical processes. All three lakes were slightly alkaline pH 7–8 and more productive in the late-glacial. They acidified and became less productive at the end of the late-glacial and in the early Holocene. pH stabilized 8000–9000 yr B.P. at the two higher sites and by 6000 yr B.P. at the lowest. An elevational gradient in pH existed throughout the Holocene. The highest site had a mean Holocene pH close to or below 5; the lowest site fluctuated around a mean of 6. The higher pH and trophic state of the late-glacial was controlled by leaching of base cations from fresh unweathered till, a process accelerated by the development of histosols in the watersheds as spruce-dominated woodlands replaced tundra. An apparent pulse of lake productivity at the late-glacial-Holocene boundary is correlated with a transient, but significant, expansion of alder (Alnus crispa) populations. The alder phase had a significant impact on watershed (and hence lake) biogeochemistry. The limnological changes of the Holocene and the differences between lakes were a function of an elevational gradient in temperature, hydrology (higher precipitation and lower evapotranspiration at higher elevation), soil thickness (thinner tills at higher elevation), soil type (histosols at higher elevation), vegetation (northern hardwoods at lower elevation, spruce-fir at higher), and different Holocene vegetational sequences in the three watersheds.This is the thirteenth of a series of papers to be published by this journal that was presented in the paleolimnology sessions organized by R. B. Davis and H. Löffler for the XIIth Congress of the International Union for Quaternary Research (INQUA), which took place in Ottawa, Canada in August 1987. Dr. Davis is serving as guest editor of this series.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号