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1.
通过对江汉平原JZ-2010湖相沉积剖面AMS14C年代的精确测定与校正,以及代用指标粒度、磁化率的分析和研究,重建了研究区12.76 cal ka BP以来的环境演变过程:1)12.76-6.70 cal ka BP,区域环境处于偏干的晚冰期向湿润的早全新世发展阶段.2)6.70-4.47 cal ka BP,区域环境湿润,为湿度配置最佳的全新世适宜期,这是大溪文化向屈家岭文化转变时期自然背景.3)4.47-3.67 cal ka BP,区域气候环境偏干,其中经历了明显的4.2 cal ka BP干旱事件.4)3.67-1.22 cal ka BP时期,环境向湿润发展;在2.5 cal ka BP左右水动力条件明显变强,此时正对应于古云梦泽扩张时期.5)1.22 cal ka BP后,研究区处于湖泊面积减小的干旱时期;人类活动带来的影响达到高峰,围湖造田等农业活动也是江汉平原湖群趋于衰退的重要原因之一.  相似文献   

2.
One of the goals for paleaoenvironmental research is to predict the tendency of future climate and environmental changes based on the understanding of the past. The key approach is to find similar pictures which happened in the past. By understanding the background and mechanism of the paleaoenvironmen- tal changes, reliable parameters and verifications can be provided for the numerical model to predict the tendency of future climate and environmental changes. The Mid-Holocene as the nearest …  相似文献   

3.
Based on the concentration and distribution pattern analysis of the rare earth elements (REEs) at the Shell Bar section from Qaidam Basin, we studied the geochemical fractionations of REE in paleolake deposits and their paleo-environmental significance. Our results show that the REE concentration in AS (acid soluble) and AR (acid residual) fractions are 20.9 μg/g and 95.4 μg/g (except element Y) individually, showing a strong REE differentiation between AS and AR fractions. However, the two types of fractions (AS & AR) have similar REE distribution patterns, which are slightly rich in light earth rare elements (LREEs), with slightly right-tilting and negative Eu anomaly. The LREE of AR is richer than that of AS. There were no significant correlations between the REE in AS, AR and other proxies. It indicated that the lacustrine deposition had different material sources and experienced varying geochemical procedures. Correlation analysis between the REE and the content of fine grain-size (<4 μm) of the sediments, Rb/Sr ratio and Mn concentration showed high correlation coefficients. Our results demonstrated that the REE in acid soluble and residual fractions bear different environmental significances, which are strongly dependent on local environment. The REE of acid soluble fraction is closely related to the paleoclimatic changes in the lake catchment and the evolutionary processes of lake itself. Our results suggest that δCe and (La/Yb)n could be used as proxies of the reduction-oxidation conditions and furthermore the temperature change and the lake level fluctuations. Using these proxies, we reconstructed the paleoclimate and water level fluctuation history during the high lake level period lasting between 43.5 and 22.4 cal. ka BP.  相似文献   

4.
中国北方季风边缘区的湖泊对环境的反应很敏感,乌拉盖高毕是位于内蒙古高原东北部的一个已干涸的内陆封闭湖泊.研究剖面总厚度为405 cm,根据取得的9个AMS14C及光释光测年数据,建立了51.9 ka BP以来的年代序列.结合岩性的变化、粒度组分分析、粒度参数(标准偏差、偏度、峰度等)及Rb/Sr比值特征进行了详细的分析,恢复了乌拉盖高毕湖泊沉积记录的5万年来的环境演化过程.将0~405 cm的剖面划分为4个较大的气候阶段:51.9-44.1 ka BP(405~343 cm),气候为干冷期,各环境指标显示为滨湖沉积,湖泊水位较低;44.1-28.5 ka BP(343~130 cm),气候为温湿期,湖泊逐渐扩张,湖泊水位升高;28.5-11.38 ka BP(130~35 cm),气候转冷,湖面萎缩,并在盛冰期出现砂楔,该阶段后期湖泊干涸;11.38 ka BP至今(35~0 cm),气温回升.  相似文献   

5.
Holocene peat sediment has been attached impor-tance to reconstruct the Holocene climatic variations because it can provide much palaeoclimatic informa-tion with high resolution. Analysis on the plant mac-rofossil[1], pollen[2―4], isotope ratio[5―9], element con-tent[10,11], total organic carbon (TOC)[12], humifica-tion[13,14] and grayscale[12] for peat sediment has shown its superiorities as a recorder medium for reconstruct-ing the Holocene climate. Hong et al. have done great fruitful w…  相似文献   

6.
The palaeolake evolution across the Tibetan Plateau and adjacent areas has been extensively studied, but the timing of late Pleistocene lake highstands remains controversial. Robust dating of lacustrine deposits is of importance in resolving this issue. This paper presents 14 C or optically stimulated luminescence(OSL) age estimates from two sets of late Quaternary lacustrine sequences in the Qaidam Basin and Tengger Desert(northeastern Tibetan Plateau). The updated dating results show:(1) the radiocarbon dating technique apparently underestimated the age of the strata of >30 ka BP in Qaidam Basin;(2) although OSL and 14 C dating agreed with each other for Holocene age samples in the Tengger Desert area, there was a significant offset in dating results of sediments older than ~30 ka BP, largely resulting from radiocarbon dating underestimation;(3) both cases imply that most of the published radiocarbon ages(e.g., older than ~30 ka BP) should be treated with caution and perhaps its geological implication should be revaluated; and(4) the high lake events on the Tibetan Plateau and adjacent areas, traditionally assigned to MIS 3a based on 14 C dating, are likely older than ~80 ka based on OSL chronology.  相似文献   

7.
Relict (perched) lacustrine deltas around the perennially ice-covered lakes in the Taylor Valley, Antarctica, imply that these lakes were up to 40 times larger in area than at present since the last glacial maximum (LGM). These deltas have been used to constrain ice-margin positions in Taylor Valley, and the boundaries of the proposed LGM ice-damned Glacial Lake Washburn. The timing of these high lake levels has depended on 14C chronologies of algal layers within relict lacustrine deltas. To provide additional geochronometric data for the post-LGM lake-level history, we applied photon-stimulated-luminescence (PSL) sediment dating to polymineral fine silt and sand-size quartz from 7 perched-delta and 3 active-delta sites of different elevations along 3 major meltwater streams entering Lake Fryxell. Our PSL dating of 4 quartz-sand samples from core tops in the seasonal ice-free moat of Lake Fryxell (elevation ∼18 m a.s.l.) and two core-top moat samples from the seasonal moat of Lake Vanda in nearby Wright Valley establish that adequate PSL clock zeroing (by daylight) occurs in regional, modern shoreline deposits. Minimum-age micro-hole PSL results from the moats are consistently near 100 a. Minimum-age micro-hole age estimates for the deltas range from ∼50 to 100 a near the present lake level up to 13.4 ± 1.3 ka at 240 m. These are systematically younger than the comparable, reservoir-uncorrected, 14C ages that range from 7 ka (cal yr BP) to 13 ka (cal yr BP) near lake level up to 20 ka (cal yr BP) at 220–240 m elevation. Our results indicate the occurrence of a dramatic discrepancy between PSL minimum-age and 14C age estimates that is presently unresolved.  相似文献   

8.
《国际泥沙研究》2019,34(5):432-443
The clay mineralogy of Tulare Lake sediment was examined to investigate hydroclimatic and environmental changes in the southern Sierra Nevada Mountains (SNM) since the most recent glacial maximum. Evolution of clay mineral assemblages elucidates significant changes in weathering, erosion, and hydroclimatic condition in the catchment. During the last glacial period (24.4–15.1 cal ka BP), low illite content implies less physical erosion of the granitic batholith rocks and a cold and arid environment in the southern SNM. Abrupt increases of illite content at 21.8–20.8 and 17.6 cal ka BP resulted from the glacier advances to the ablation zone and illite-rich glacier flour was transported down to the lake. The gradual increase of smectite induced by progressive depletion of illite-rich glacier flour from 17.6 cal ka BP toward the end of this period indicates climate was beginning to get warm and wet. From 11.9 to 5.3 cal ka BP, two warm and wet periods (10.7–9.4 and 8.2–5.2 cal ka BP) were characterized by high smectite/illite content ratios and low illite crystallinity values, suggesting intensive rainfall precipitation and more physical erosion in the highland and lowland catchment as well as more smectite formation in the terrace soils. Since the last glacial period, physical erosion, in comparison to the chemical weathering, was the dominant process responding to the hydroclimatic change in the Tulare Lake catchment. Moderate to weak chemical weathering was signified by the mostly low illite chemical weathering index of the core sediments. Such results suggest that vegetation cover in the southern SNM was low and limited.  相似文献   

9.
The study of climatic changes since the Late Glacial Age has become one of the hotspots of the PAGES in recent years.Deep-sea cores from the high-latitude area show that the climate was very unstable during the transitional period from the Late Glacial Age to the Holocene[1,2],which has also been testified by the geological records from ocean sediments,ice cores and terrestrial sections in different latitudes of the earth[3—8].What’s more,climatic instability also ex-isted in the Holoce…  相似文献   

10.
Changes in the vegetation and climate of the westerly-dominated areas in Central Asia during the Holocene were interpreted using pollen-assemblages and charcoal data from a 300-cm-long sediment core of the Sayram Lake,northern Xinjiang.Accele-rator Mass Spectrometry(AMS) radiocarbon dating methods were applied to bulk organic matter of the samples.Artemisia spp./Chenopodiaceae ratios and results from principal component analysis were used to infer that the lake basin was dominated by desert vegetation before ca.9.6 cal.ka BP,which suggests a warm and dry climate in the early Holocene.Desert steppe/steppe expanded during 9.6-5.5 cal.ka BP,indicating a remarkable increase both in the precipitation and temperature during the mid-Holocene.Desert vegetation dominated between 6.5 and 5.5 cal.ka BP,marking an extreme warmer and drier interval.The steppe/meadow steppe recovered,and temperatures decreased from 5.5 cal.ka BP in the late Holocene,as indicated by the increased abundance of Artemisia and the development of meadows.Holocene temperatures and moisture variations in the Sayram Lake areas were similar to those of adjacent areas.This consistency implies that solar radiation was the main driving factor for regional temperature changes,and that the effect of temperature variations was significant on regional changes in humidity.The evolution of climate and environment in the Sayram Lake areas,which were characterized as dry in the early Holocene and relatively humid in the middle-late Holocene,are clearly different from those in monsoonal areas.Dry conditions in the early Holocene in the Sayram Lake areas were closely related to decreased water vapor advection.These conditions were a result of reduced westerly wind speeds and less evaporation upstream,which in turn were caused by seasonal changes in solar radiation superimposed by strong evaporation following warming and drying local climate.  相似文献   

11.
Qinghai Lake, on the northeastern Qinghai-Tibetan Plateau, is the largest extant closed-basin lake in China, and has been the subject of numerous palaeoclimatological and palaeoenvironmental studies. In this study, 32 samples of aeolian sand, loess and palaeosol at six sites, and 1 sample of shoreline deposits underlying aeolian deposits were dated using optically stimulated luminescence (OSL). Where available, OSL ages are in agreement with previously published 14C ages. Our dating results, in combination with previous published ages on aeolian deposits showed that: (1) The oldest aeolian deposits around Qinghai Lake are in excess of 165 ka. (2) Aeolian deposition then began at ∼14 ka in the Qinghai Lake area. Periods of palaeosol formation occurred at ∼16.9 ka, ∼12.2–11 ka, ∼10–9 ka, ∼5.2–4 ka, and ∼3.9–0.7 ka. (3) The accumulation intervals of palaeosols are generally consistent with drilling-core-based environmental change proxies, indicating that palaeosols were formed during wet periods with higher vegetation cover. (4) A depositional hiatus period of ∼40–50 ka exists between the surface mantle aeolian deposits and underlying gravel deposits. (5) Lake levels during the Holocene did not exceed 3205.2 m elevation (11.8 m above recent lake level of April, 2010).  相似文献   

12.
We present the carbon isotopic composition of the total organic carbon (TOC) and fine roots in the sedimentary profile from the underground ancient forest in Sihui to study the climatic and environmental changes from 4.5 ka BP to 0.6 ka BP. Results show that C3 plant was the main vegetation from 4.5 ka BP to 0.6 ka BP in this region. The ancient forest began to develop in the wetland at around 4 ka BP and disappeared together with the wetland at about 3.0 ka BP, implying that the climate had changed greatly at around 3.0 ka BP. As indicated by the simulation results, the content of atmospheric CO2 increased slightly during 3.5 ka BP to 3.0 ka BP, implying climate warming during that period. The interval of radiocarbon age between 3.0 ka BP to 1.2 ka BP was possibly caused by the strong erosion when the block was lifted in the neotectonic movement. From 1.2 ka BP to 0.6 ka BP, the region remained in terrestrial sedimentary environment, and the surface plant biomass declined gradually. Drought caused by the climate change was the likely cause for the disappearance of the ancient forest. South transition of Intertropical Convergence Zone (ITCZ) was probably the main mechanism for the climate change. Supported by National Natural Science Foundation of China (Grant Nos. 40231015 and 40473002), National Basic Research Program of China (Grant No. 2005CB422004), the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. KSCX2-SW-133) and Open Funds of State Key Laboratory of Organic Geochemistry (Grant No. OGL-200607)  相似文献   

13.
Vietnam's Central Highlands in Pleiku feature numerous volcanic craters (i.e. maars) formed over 0.2 Ma ago that contain continuous high-resolution lacustrine sediment records extending from the Late Pleistocene through the Holocene. Their evolution and sedimentary proxies are potentially powerful archives for interpretation of East-Asian monsoon variability across Vietnam, as well as for local to regional changes in landcover. The focus of this study is to develop a reliable chronology for the Biển Hồ 25-m-long lake sediment sequence in Vietnam's Central Highlands. We present a combined sediment chronology extending back to 55 ka cal BP that is based on 137Cs surface sediment chronology, paleomagnetic dating, and 47 14C dates distributed evenly throughout the sequence. Based on sedimentary facies analysis and a smooth age-depth model based on 137Cs and 14C data, we infer a long-term stable rate of sedimentation in Biển Hồ lake without unconformities or erosional hiatuses, except for the uppermost section (ca. 100 years) where human activities increase the rate of sedimentation significantly. The Biển Hồ sequence thus provides a complete and uninterrupted record of the lake's depositional history. The present contribution is a geochronologic opening salvo for future contributions from Biển Hồ lake featuring multi-disciplinary evidence for longer-term paleoclimate and environmental changes.  相似文献   

14.
肖海丰  沈吉  肖霞云 《湖泊科学》2006,18(6):615-620
本文在分析鹤庆古湖泊不同演化阶段烧失量指标变化特点的基础上,采用频谱分析、滤波分析方法,探讨了该湖泊古生产力轨道尺度的演化特征．综合分析结果显示:在整个演化过程中,由于北半球夏季太阳辐射与南半球潜热输出相位关系的不同,导致斜率周期一直强于岁差周期;在987-1552 ka阶段,15 ka、10．7 ka和10 ka为较强周期,其中10 ka、10．7 ka周期是与100 ka周期强化有关的半岁差周期;在湖泊古生产力演化中存在与气候变化相一致的中更新世转型．  相似文献   

15.
利用埃及北部Faiyum盆地获得的高取芯率沉积物岩芯,进行沉积物多种磁性参数的测量,结合有机碳、介形虫、粒度等分析,在AMS14C加速器测年的基础上,建立全新世以来湖泊沉积物磁性特征变化的时间序列.结果表明,粒度效应以及沉积后的各种次生作用对沉积物的磁性特征没有明显的影响,磁性变化主要反映了沉积物不同来源组成的相对变化.全新世前沉积物磁性较弱,主要含不完全反铁磁性矿物,与周边沙漠的物质相似,结合其粒度特征,沉积物来源应以近源物质为主.全新世早中期(约10 5.4 ka BP)沉积物磁性变化相对稳定,有机质含量也较高,指示了来自尼罗河较为稳定的物质供应;而大约5.4 ka BP尤其最近约4.2 ka BP以来,磁性的明显变化反映了流域降水减少情况下,来自青尼河物质贡献的相对增加;最近约2.0 ka BP以来沉积物的磁性变化,则更多地与盆地人类活动的强化有关.总体而言,Faiyum盆地全新世以来的环境演变主要受控于全新世以来尼罗河与盆地的水力学联系.即:全新世前盆地未与尼罗河连通时,沉积物主要来源于周边沙漠的风成物质;而受全新世早-中期来流域季风降水增加的影响,泛滥的尼罗河为盆地提供了相对稳定的物质供应,湖泊也处于高湖面;全新世晚期以来,随着流域干旱化的加剧,尼罗河与盆地的连通性开始减弱,来自高磁性的青尼罗河物质贡献开始相对增加.最近约2.0 ka BP以来,虽然仍有人工运河连接尼罗河与盆地,但沉积物磁性的显著变化更多地反映了盆地人类活动的不断强化.  相似文献   

16.
The problem of insufficient age-control limits the utilisation of the 8.2 ka BP event for modelling freshwater forcing in climate change studies. High-resolution radiocarbon dates, magnetic susceptibility and lithostratigraphic evidence from a lake sediment core from Nedre Hervavatnet located at Sygnefjell in western Norway provide a record of the early Holocene. We use the method of radiocarbon wiggle-match dating of the lake sediments using the non-linear relationship between the 14C calibration curve and the consecutive accumulation order of the sample series in order to build a high-resolution age-model. The timing and duration of Holocene environmental changes is estimated using 38 AMS radiocarbon dates on terrestrial macrofossils, insects and chironomids covering the time period from 9750 to 1180 cal BP. Chironomids, Salix and Betula leaves produce the most consistent results. Sedimentological and physical properties of the core suggest that three meltwater events with high sedimentation rates are superimposed on a long-term trend with glacier retreat between 9750 and 8000 cal BP. The lake sediment sequence of Nedre Hervavatnet demonstrates the following: only a reliable high-resolution geochronology based on carefully selected terrestrial macrofossils allows the reconstruction of a more refined and complex environmental change history before and during the 8.2 ka event.  相似文献   

17.
Changes of the deep-water circulation and chemicalcompositions in related water masses in the oceanshave played a significant role in the evolution of theglobal climate system[1].Many studies have shownthat the characteristics of the deep-water during theglaciations were distinctly different from that of to-day[2,3].In the northern Pacific,Duplessy et al.[4]foundthat the ventilation of the deep-water(at water depthbetween700―2600m)during the last glaciation wasstronger than that of today,and …  相似文献   

18.
High-resolution peat humification records were obtained from Dajiuhu of the Shennongjia Mountains and Qianmutian of the Tianmu Mountains to study climate changes in East China. The analyses of pollen, organic matters, TOC, and Rb/Sr indicate a high degree of peat humification and thus strong decomposition of organic matter when climate was dry. Conversely, when climate was humid, the degree of humification is low because peat was preserved in a waterlogged condition. Peat humification from Dajiuhu occurred not only during the Younger Dryas (about 11.4–12.6 cal ka BP), the Bølling-Allerød Warm Period (12.6–15.2 cal ka BP), and the Oldest Dryas (about 15.2–16.0 cal ka BP), but also during the early Holocene (about 11.4–9.4 cal ka BP), the 8.2 cal ka BP cold event, and the Holocene Optimum (about 7.0–4.2 cal ka BP). Both peat humification records since nearly 5 ka BP are consistent, showing that mountain peatland has synchronous responses to the East Asia monsoon-induced precipitation. The LOI data confirm the above observation. The monsoon precipitation since nearly 5 ka BP recorded in these two peat profiles can be divided into three phases. During 4.9–3.5 ka BP, precipitation amount was high but fluctuated greatly. During 3.5–0.9 ka BP, precipitation amount was low. During 0.9–0 ka BP, degree of humification reduced gradually, indicating the increase of monsoon precipitation. Contrast to other high-resolution records from East China monsoon region shows that the monsoon precipitation records of the two peat profiles since nearly 16 ka BP are controlled by a common forcing mechanism of summer solar radicalization in the Northern Hemisphere.  相似文献   

19.
李育  李卓仑  王乃昂 《湖泊科学》2012,24(3):474-479
湖泊不同位置沉积物重建的古环境变化过程往往具有差异,而这种差异可能揭示了不同的古环境信息.本文以河西走廊石羊河流域终端湖泊猪野泽为例,探讨湖泊不同位置沉积物所代表的古环境意义.位于湖盆边缘的三角城剖面可能反映了石羊河上游地区的降水及其古径流量的变化,这种变化在千年尺度上对应了北大西洋Bond事件,显示该区域通过季风环流系统与全球气候系统的联系.位于湖泊中部的QTH01、QTH02、QTL-03剖面反映了湖泊演化的信息,显示中全新世期间(约7.0-约3.5 cal ka BP)湖泊水位较高的特征,但是这种特征并不能和该流域的古径流相联系,说明该区域湖泊演化除了受千年尺度大气环流特征的影响外,湖泊表面蒸发可能也起了重要作用.该研究为中亚地区普遍存在的中全新世高湖面问题提供了研究思路.  相似文献   

20.
Large numbers of peatlands were developed in volcanic landform regions, which would provide valuable long-term records of lake–mire ecosystem shifts and act as significant carbon pool in regional carbon cycle. To investigate lake–mire ecosystem transformations and driving mechanisms in volcanic landform regions, the developmental history of Gushantun peatland in northeast China was reconstructed in this study. Results indicate that Gushantun peatland initiated in the deepest portions of the basin, and subsequently experienced expansions outward. Peat initiated from approximately 12 ka cal. bp to present. The developmental patterns of Gushantun peatland can be divided into four stages: the stable stage 1 (12–10 ka cal. bp ), maximum stage (10–7 ka cal. bp ), stable stage 2 (7–4 ka cal. bp ) and stable stage 3 (4–0 ka cal. bp ). The possible forcing mechanisms for the development of Gushantun peatland were different during different periods. From 12 ka cal. bp to 10 ka cal. bp , autogenic process was probably the major controlling factor for the expansion of this peatland. From 10 to 7 ka cal. bp , flat basin morphology was the major influence factor for fast expansion. However, the development of Gushantun peatland was probably controlled by the dual effects of high moisture and autogenic process during the period of 7 to 4 ka cal. bp . From 4 ka cal. bp to present, steep basin morphology was the major influence factor, while moisture might be the secondary factor for development of Gushantun peatland. These features indicate that lake–mire ecosystem transforms in volcanic landform regions of Changbai Mountains were probably triggered by the complex effects of autogenic process, hydroclimate and underlying basin morphology. © 2020 John Wiley & Sons, Ltd.  相似文献   

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