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1.
This paper evaluates the potential usefulness of non-pollen palynomorphs or NPPs (microfossils other than pollen and spores present in palynological preparations) contained in lake sediments in the paleolimnological reconstruction of high altitude environments (>4,000 m) from the Venezuelan Andes. A synthetic, quantitative approach is employed, instead of the classical analytical and mostly qualitative approach commonly used so far for NPPs. The main sources of variation are the Pediastrum–Botryococcus alternation and the relationship between these two algae and animal remains such as Acari legs, postabdomina of Cladocera, mandibles of other invertebrates, and an unknown type called LVA-1. Other significant microfossils are remains of Rivularia-type and turbellarian oocytes, including Gyratrix. The sequence initiates around 15,000 calibrated years before present (cal BP) with the deglaciation of the lake catchment, high water levels and still cold climates. A phase of lower lake levels was recorded between about 12,000 and 6,000 cal BP. Temperatures increased by around 9,000 and 7,000 cal BP, and then decreased until 6,000 cal BP. Since that time, both lake levels and temperature increased again and stabilized at about 4,000 cal BP, when they reach modern-like values. These results show a good agreement with previous studies based on pollen, diatom and oxygen isotope analyses, and provide additional paleoecological information, as for example the possibility of a previously unrecorded Younger Dryas signal. The inclusion of quantitative NPP analysis in routine paleolimnological studies using synthetic methods is thus recommended. Possible future improvements are suggested, mainly those related with the development and use of NPP modern analogs.  相似文献   

2.
This study used organic matter in oligotrophic Lake Constance (southern Germany) to reconstruct lake environment and to disentangle the multiple factors, such as climate change and human impacts, which influence sedimentation in large lakes. A sediment core from Upper Lake Constance, which represents 16,000 years of Late Glacial and Holocene lake history, was analysed for organic biomarkers, hydrogen index and elements calcium, strontium, and magnesium. Magnetic susceptibility was measured to establish a high-resolution stratigraphic framework for the core and to obtain further information about changes with respect to relative allochthonous versus autochthonous sedimentation. Dinosterol—a biomarker for dinoflagellates—and calcium have low concentrations in Younger Dryas sediments and consistently high concentrations between 10,500 and 7,000 cal. years BP. These variations are attributed to changes in lake productivity, but are not reflected in the proportion of total organic carbon within the sediment. During the Younger Dryas and between 6,000 and 2,800 cal. years BP, concentrations and accumulation rates of land-plant-derived C29-steroids (β-sitosterol, stigmastanol and stigmasterol), in combination with a relatively low HI, indicate periods of enhanced terrigenous input to the lake. For the Younger Dryas, higher runoff can be attributed to a cold climate, leading to decreased vegetation cover and increased erosion. After 6,000 cal. years BP, high terrestrial input may be explained by enhanced precipitation. Biomarker and HI results, in combination with archaeological studies, raise the question as to whether lakeshore settlements affected sedimentation in Upper Lake Constance between 6,000 and 2,800 cal. years BP.  相似文献   

3.
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4.
选用甘青地区达连海、青海湖、苏家湾、大地湾4个典型高分辨率的钻孔资料进行对比分析,阐明了该地区末次冰消期以来气候变化规律与主要气候事件,初步探讨了该地区植被纬向时空演化规律。结果显示末次冰消期大致开始于15.2~14.6 ka BP之间,冰消期期间该地区气候表现为冷暖波动频繁,气候不稳定,植被类型由东向西为草原-荒漠化草原。全新世早期阶段10.4~8.2 ka BP气候表现为温干,植被类型由东向西为疏林草原-草原。全新世中期8.2~4.3 ka BP气候温暖湿润,植被发育良好,由东向西出现森林-森林草原植被。4.3 ka BP以后该地区气候总体向凉干方向发展,3.9~3.4 ka BP期间陇中地区气候波动较显著,植被类型草原-森林草原交替出现。晚全新世后期2.3~0 ka BP气候冷干,从东到西发育草原-荒漠化草原植被。  相似文献   

5.
Postglacial sediments from Lake Jues, located at the SW margin of the Harz Mountains, were investigated for pollen, diatoms and sediment characteristics. The paper focuses on the time period between 7600 and 1200 y cal. BP (calibrated years before present), during which human impact began to influence the environnment of the lake. Climate variability was mainly inferred from sediment characteristics and changes in algal assemblages. The observed climatic changes coincide well with those recorded from other sites. Neolithic settlement started during the warm and dry Atlantic period. Intensive land use occurred under favourable climatic conditions during the Bronze and the Iron Ages, while human activities declined during cool and wet periods around 4000 and 2700 y cal. BP and after 2300 y cal. BP. The study shows that climate strongly influenced human settlement at remote locations like Lake Jues.  相似文献   

6.
Five pollen zones are identified in Yangmu peatland of Mishan region located at 45o34’N, 132o23’E through sporo-pollen analysis. The changing process of paleovegetation and paleoclimate was obtained. Warm-inclined broad-leaved forest predominated in the environment of warm climate with a little dry 3400 yr BP. Deciduous broad-leaved and coniferous mixed forests predominated, in which Pinus, Picea and Abies were main species, together with wet meadow in the environment of cool and humid climate during 3400-1940 yr BP. Deciduous broad-leaved and coniferous mixed forests predominated in the dry and warm climate environment 1940-1090 yr BP. Broad-leaved forest was predominant, and the climate was warm and humid 1090-545 yr BP. Marsh meadow predominated when the climate changed to cool and dry 545 yr BP. The composition of the upper part of the 143-125 cm of the peat profile presented the cold period in the early Christian era through mutual identification between the records of historical material such as spores and pollens, susceptibility, organic matter and archaeological studies. The composition of the parts of 125-85 cm and 85-38 cm presented the warm climate in the Northern and Southern Dynasty and Sui and Tang dynasties. Since 3400 yr BP because of the frequent human activities in Mishan region, the amount of cultural relics in the Sui and Tang dynasties increased, which indicated that the ancients took much more woods from the forests in the warm climate environment.  相似文献   

7.
An 8000-year record of palaeoproductivity, based on the chemical and chironomid stratigraphies from Lake Päijänne, S. Finland, was assessed with respect to known morphometric, climatic and anthropogenic events. A gradual trend of dystrophication and an associated decrease in aquatic production was detected during the Holocene, with the following exceptions: (1) high diatom and chironomid production around 8000-6000 cal yr BP, (2) eutrophication around 2000 cal yr BP, and (3) an anthropogenic signal during the last few decades.The changes in chironomid assemblages, before the past few decades, have mainly been shifts in concentration, but not in species composition. Variation in chironomid production was mainly explained by the accumulations of biogenic silicon, carbon and organic matter. Nutrient availability seems to be important in controlling biogenic silicon, which we use to infer past diatom production. The high production ca. 8000-6000 cal yr BP and the fluctuation in chironomid influx after ca. 2000 cal yr BP, however, were probably caused by the proposed warm/dry and cold/wet conditions during these times, respectively. These results highlight the sensitivity of boreal shield lake ecosystems to climatic forcing. In contrast, the pronounced change in the morphometry of the basin around 7000 cal yr BP had little effect on the trophic state of the lake. The human-induced trophic change during the past few decades has affected the Lake Päijänne ecosystem to an extent never experienced before during the last 8000-years.  相似文献   

8.
The timing of the last deglaciation in southern Chile is re-evaluated from a calendar varve chronology (Lago Puyehue, 40° S). The climate shifts are analysed by continuous annual varve-thickness measurements through the ∼17,100 cal. year to 10,800 cal. year BP time window (∼3.5 m sediment core). The varve years are determined by the alternation of light (phytoplankton-rich) and dark (terrigenous and organic-rich) layers forming graded annual couplets (∼0.2 to 0.8 mm/year). The varve chronology is constructed by conventional varve-counting methods on thin sections after correction for instantaneous volcanic and/or seismic events detected in the thin sections. The calibrated varve-age model derived from the manual varve counting is constrained by high-resolution grey-scale (GS) semi-automatic counts of the annual light phytoplankton-rich layers (∼120 μm to 300 μm thick). Due to physical sediment properties the GS constitutes a proxy record for the phytoplankton/terrigenous varve-thickness variations through the sediment record. The varve couplets are thicker/thinner during humid/dry phases and darker/lighter (negative/positive annual grey-scale index) during cold/warm phases. Our results show that at 40° S the last deglaciation took place in two phases between ∼17,100 cal. year and ∼15,500 cal. year BP. We note a climate instability between ∼15,500 cal. year and 13,300 cal. year BP and a significant dry phase between ∼15,000 and 14,500 cal. year BP. We evidence a cold event in two phases between ∼13,300 and 12,200 cal. year BP interrupted by a dry event between ∼12,800 and 12,600 cal. year BP. The onset of a significant warmer period is observed after ∼11,500 cal. year BP. Our results provide new evidence of a Younger Dryas cool reversal in southern Chile, i.e., the Huelmo/Mascardi event Hajdas et al. (2003) associated with an abrupt dry pulse at ∼12,800–12,600 cal. year BP. The high-resolution grey-scale measurements performed on the biogenic varves from Lago Puyehue provide a reliable calibrated chronology of the regional environmental and climate shifts during the last deglaciation. This is eighth in a series of eight papers published in this special issue dedicated to the 17,900 year multi-proxy lacustrine record of Lago Puyehue, Chilean Lake District. The papers in this issue were collected by M. De Batist, N. Fagel, M.-F. Loutre and E. Chapron.  相似文献   

9.
The study of global climate change for the last 2000 years is very important for predicting climate evolution in the future. In order to explore the evidence of climate change for that period, the Chinese scientists made convincing statements using high-resolution substitution data such as tree-ring, coral and ice core. Continuous accumulated peat sediment is the better substitution data to provide climate information. Selecting the peatlands with a certain area and less human interference, th…  相似文献   

10.
A 6.48 m sediment core sequence from Erhai lake, Yunnan Province, provides a multi-proxy record of Holocene environmental evolution and human activity in southwest China. These sedimentary records provide proxy time series for catchment vegetation, flooding, soil erosion, sediment sources and metal workings. They are complemented by independent regional climate time-series from speleothems, archaeological records of human habitation, and a detailed documented environmental history. The article attempts to integrate these data sources to provide a Holocene scale record of environmental change and human–environment interactions. These interactions are analysed in order to identify the roles of climate and social drivers on environmental change, and the lessons that may be learned about the future sustainability of the landscape. The main conclusions are: lake sediment evidence for human impacts from at least 7,500 cal year BP is supported by a terrestrial record of cultural horizons that may extend back to ∼9,000 cal year BP. A major shift in the pollen assemblage, defined by detrended correspondence analysis, at ∼4,800 cal year BP marks the transition from a ‘nature-dominated’ to a ‘human-dominated’ landscape. From 4,300 cal year BP, a change in river discharge responses may signal the beginning of hydraulic modification through drainage and irrigation. Major increases in disturbed land taxa and loss of forest taxa from 2,200 cal year BP onward, also associated with the start of significant topsoil erosion, register the expansion of agriculture by Han peoples. It is also the start of silver smelting linked to trade along the SW Silk Road with Dali becoming a regional centre. Peak levels of disturbed land taxa, topsoil and gully erosion are associated with the rise and fall of the Nanzhao (CE 738–902) and Dali (CE 937–1253) Kingdoms, and the documented environmental crisis that occurred in the late Ming and Qing dynasties (CE 1644–1911). The crisis coincides with a stronger summer monsoon, but exploitation of marginal agricultural land is the main driver. These historical perspectives provide insight into the resilience and sustainability of the modern agricultural system. The largest threat comes from high magnitude-low frequency flooding of lower dry farmed terraces and irrigated valley plains. A sustainable future depends on reducing the use of high altitude and steep slopes for grazing and cultivation, maintaining engineered flood defences and terraces, and anticipating the behaviour of the summer monsoon. This article is based on a keynote address delivered by John Dearing at the 10th International Paleolimnology Symposium, June 2006, Duluth, Minnesota, USA.  相似文献   

11.
Variations in the oxygen-isotope composition of paleo-water bodies in the Lake Superior Basin provide information about the timing and pathways of glacial meltwater inflow into and within the Lake Superior Basin. Here, the oxygen-isotope compositions of Lake Superior have been determined using ostracodes from four sediment cores from across the Basin (Duluth, Caribou and Ile Parisienne sub-basins, Thunder Bay trough). The δ18O values indicate that lake water (Lake Minong) at ~10,600–10,400 cal [~9,400–9,250] BP was dominated by glacial meltwater derived from Lake Agassiz and the Laurentide Ice Sheet (LIS). From that time to ~9,000 cal [~8,100] BP, a period associated with formation of thick varves across the Lake Superior Basin, the δ18O values of Lake Minong decreased even further (−24 to −28‰), symptomatic of an increasing influx of glacial meltwater. Its supply was reduced between ~9,000 and ~8,900 cal [~8,100–8,000] BP, and lake water δ18O values grew higher by several per mil during this period. Between ~8,900 and ~8,800 cal [~8,000–7,950] BP, there was a return to δ18O values as low as −29‰ in some parts of the Lake Superior Basin, indicating a renewed influx of glacial meltwater before its final termination at ~8,800–8,700 cal [~7,950–7,900] BP. The sub-basins in the Lake Superior Basin generally displayed very similar patterns of lake water δ18O values, typical of a well-mixed system. The final stage of glacial meltwater input, however, was largely expressed near its input (Thunder Bay trough) and recognizable in dampened form mainly in the Duluth sub-basin to the west. Water in the easternmost Ile Parisienne sub-basin was enriched in 18O relative to the rest of the lake, particularly after ~10,000 cal [~8,900] BP, probably because of a strong influence of local precipitation/runoff, and perhaps also enhanced evaporation. By ~9,200 cal [~8,250] BP, lake water δ18O values in the Ile Parisienne sub-basin were similar to the adjacent Lake Huron Basin, suggesting a strong hydraulic connection between the two water bodies, and common responses to southern Ontario’s shift to warmer and dry climatic conditions after ~9,000 cal [~8,100] BP.  相似文献   

12.
Environmental changes of the last 9,300 years were reconstructed by geochemical and pollen analyses of a 14-m-long, laminated sediment core from Lago Aleixo, south-eastern Brazil. Fossil pollen assemblages indicate open savannah vegetation (campo cerrado) and gallery forests until approximately 6,900 cal. BP. During that time, siderite laminae were deposited under anoxic conditions at the lake bottom. Then, increased rainfall and a shorter annual dry period allowed gallery forests and semi-deciduous forests to expand, leading to more closed cerrado vegetation. High-intensity rainfall events during this period are recorded as peaks in K and Ti concentrations. The sediment facies during this period consists of alternating layers of diatoms and minerogenic matter. C/N ratios imply that algae and perhaps soils, too, were the main contributors to sediment organic matter. Biogenic silica and δ13Corg variations indicate increasing primary productivity, which was related to higher nutrient flux from intensified leaching of soils, as shown by rising K/Al ratios. Around 800 cal. BP, a closed, semi-deciduous forest developed under present-day climate conditions. Slope stabilization diminished erosion processes in the catchment and caused reduced input of minerogenic matter into the basin. Human impact is evident in the topmost homogeneous sediments, as removal of the stabilizing forest cover amplified soil erosion. The continuous trend to more humid conditions during the Holocene probably reflects increased influence of the Amazon Basin as a moisture source. We conclude that the Lago Aleixo sediment archive was a sensitive recorder of environmental dynamics in tropical South America, which were mainly controlled by changes in precipitation patterns.  相似文献   

13.
华北平原晚冰期以来气候环境演变研究对该地区社会发展、灾害风险评估和科学应对未来全球增温背景下极端降水和洪涝事件具有重要意义。本文以华北平原中部白洋淀地区高阳剖面（BG-2019）为研究对象,通过高精度AMS14C、OSL定年技术和高分辨率孢粉组合、粒度组分分析,恢复和重建了白洋淀地区晚冰期以来（距今13710 a—今）区域植被演替和气候环境变化历史。结果显示：BG-2019剖面在距今10270~13710 a和距今4630~5400 a发育湖相沉积,距今3470~3700 a发育沼泽相沉积;距今7130~8000 a发育河流—入湖三角洲相沉积,距今3700~4630 a和距今3230~3470 a发育河流相沉积;距今8000~10270 a和距今5400~7130 a存在明显的沉积间断/地层缺失;表明采样剖面所在位置缺乏连续的湖相地层。晚冰期白洋淀为局地小湖沼;中全新世湖沼较发育、范围广,但也不是连续广袤的湖相沉积;晚全新世湖泊范围收缩。晚冰期和全新世白洋淀流域植被景观存在显著差异;晚冰期气候寒冷干燥,平原发育以蒿属、藜亚科、禾本科和菊科等为主的草地,周围山地森林覆盖度低;中全新世气候温暖湿润,平原大部仍发育以蒿属、藜亚科和禾本科为主的草地,湖区水蕨和水生植物繁盛,周围山地生长松属、栎属为主的针阔混交林,森林覆盖度增高;晚全新世气候温和偏干,平原仍是以蒿属、藜亚科和禾本科等为主的草地,西部山地生长以松属为主的针阔混交林,森林覆盖度较高。  相似文献   

14.
Lake Ohrid is considered to be of Pliocene origin and is the oldest extant lake in Europe. A 1,075-cm-long sediment core was recovered from the southeastern part of the lake, from a water depth of 105 m. The core was investigated using geophysical, granulometric, biogeochemical, diatom, ostracod, and pollen analyses. Tephrochronology and AMS radiocarbon dating of plant macrofossils reveals that the sediment sequence spans the past ca. 39,500 years and features a hiatus between ca. 14,600 and 9,400 cal. year BP. The Pleistocene sequence indicates relatively stable and cold conditions, with steppe vegetation in the catchment, at least partial winter ice-cover of the lake, and oxygenated bottom waters at the coring site. The Holocene sequence indicates that the catchment vegetation had changed to forest dominated by pine and summer-green oak. Several of the proxies suggest the impact of abrupt climate oscillations such as the 8.2 or 4.0 ka event. The observed changes, however, cannot be related clearly to a change in temperature or humidity. Human impact started about 5,000 cal. year BP and increased significantly during the past 2,400 years. Water column mixing conditions, inflow from subaquatic springs, and human impact are the most important parameters influencing internal lake processes, notably affecting the composition and characteristics of the sediments.  相似文献   

15.
豫西渑池盆地湖泊沉积特征与古环境   总被引：1，自引：0，他引：1  
翟秋敏  郭志永  沈娟 《地理科学》2011,31(8):922-928
对渑池盆地池底村古湖泊沉积剖面进行连续采样,在粒度组成和有机碳含量分析的基础上,结合14C年代测定(校正为日历年龄),重建了盆地区末次冰盛期以来的气候环境演化进程:19543~9240cal.aB.P.,气候比较干燥,古湖泊尚未形成;9240~8039cal.aB.P.,气候由干冷向暖湿化方向转变,古湖泊开始形成;8039~3473cal.aB.P.期间,气候温暖湿润,湖泊水位最高;3439~2931cal.aB.P.期间,气候波动剧烈,总体上气候较前一段干旱,湖泊水位下降;2931~2423cal.aB.P.期间,气候干旱,古湖泊干涸。盆地区的人类文化演进对气候环境变化的响应十分明显,二者之间存在一定的耦合关系。  相似文献   

16.
Combined analyses of pollen, seeds, woods, micro-charcoal and non-pollen palynomorphs from Stagno di Maccarese, an artificially dried out coastal basin north of the Tiber delta now occupied by the Fiumicino Airport (Rome, Italy), document marked vegetation and environmental changes during the last 8300 years. Between 8300 and 5400 cal. a BP dense mixed deciduous and evergreen forests surrounded a eutrophic freshwater basin. An abrupt change around 5400 cal. a BP marks the transition to a marshy environment, due to a lowering of the water table. An increase of cereals and micro-charcoals matches the presence of a nearby Eneolithic settlement. Between 5100 and 2900 cal. a BP there is a remarkable expansion of riparian trees, indicating an increase of the water level. Between 2900 and 2000 cal. a BP, a new development of marshlands points to a progressive lowering of the lake. After 2000 cal. a BP, during the Roman exploitation of the area, an expansion of arboreal vegetation is recorded, characterized by evergreen and deciduous oak-dominated forests, while an extensive chenopods marshland matches the presence of saltworks. On the whole, the Stagno di Maccarese area appears very unstable, due to changes in lake level, introgression of marine water, eutrophic phases, flood events, desiccations and openings of the forest vegetation.  相似文献   

17.
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18.
A sedimentary sequence from the Mediterranean coastal basin of Lago di Massaciuccoli (Tuscany, Italy) was analyzed for diatoms, covering two periods over the past ca. 7,000 years. The site was selected because it is situated in a sensitive position at the limit between Mediterranean and Central European climates and biomes. Our focus is on the impact of accelerated human activity during the recent past (water uptake in the catchment, sand extraction, wastewater discharge) and on a phase of evident change between 6,600 and 5,400 cal. BP. The diatom record suggests fresh-water conditions and rather high lake levels until ca. 6,000 years ago. The subsequent shift towards brackish conditions peaked at around 5,500 cal. BP. We relate this shift to a pervasive change towards a drier climate that has been observed elsewhere in the Mediterranean and Northern African regions, and stands in contrast to the shift towards a cooler and more humid climate in the nearby Alps (200–350 km distant) and in central Europe. Pollen and charcoal records from a previous study on the same sedimentary sequence were used to gain additional insights about the causes of the changes in the diatom assemblages and apply numerical methods to search for common trends and correlations.  相似文献   

19.
青海湖湖东风成剖面化学元素特征及其环境指示意义   总被引：2，自引：0，他引：2  
通过对青海湖湖东沙地风成沉积剖面化学元素特征的分析,结合光释光测年结果,并和已有研究进行对比,探讨了青海湖区12.5 ka BP以来的气候环境变化过程,将其划分为5个阶段：12.5 ka BP前气候寒冷干燥,青海湖应处于冰川消退的寒冷期,风沙活动强烈;12.5~11.9 ka BP气候向暖湿转变,其中12.2~11.9 ka BP发生一次寒冷事件,对应于新仙女木事件;11.9~8.0 ka BP气候冷暖波动频繁,期间出现了3次寒冷事件;8.0~2.6 ka BP是一个持续时间较长的温暖湿润期;2.6 ka BP至今,气候以干冷为主,与现代气候相近。  相似文献   

20.
Two lake-sediment cores from the western and central Canadian Arctic were used to investigate late Holocene climate variability in the region. Both cores were analyzed for pollen, organic matter, biogenic silica, and magnetic susceptibility, and were dated using a combination of 210Pb and 14C techniques. Core MB01, from southwestern Victoria Island, provides a 2600-year-long record. Fossil pollen percentages, along with other parameters, suggest the occurrence of a cold period around 2400 cal year BP (450 BC), followed by slightly warmer conditions by 1800 cal year BP (150 AD), and a return to cooler conditions throughout much of the last millennium. Core SL06, from southern Boothia Peninsula, shows more subtle changes in pollen percentages over its 2500-year duration, but an increase in Cyperaceae and decrease in Oxyria pollen around 1400 cal year BP (550 AD) are indicative of warmer conditions at that time. Quantitative climate reconstructions from these pollen sequences were compared to two other pollen-based climate records from the region and indicate the presence of a widespread wet period ~1500 cal year BP (450 AD), and a cool and dry Little Ice Age. In the reconstructions based on pollen percentage data, the twentieth century summer temperature and annual precipitation in the central and western Canadian Arctic were comparable to that which occurred over the last 2500 years. However, pollen-influx values increase in the most recent sediments, suggesting high plant productivity during the late twentieth century.
Matthew C. PerosEmail:
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