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1.
The Landscape Reconstruction Algorithm (LRA) with the two models REVEALS and LOVE is developed to transform pollen percentage data to vegetation cover. This paper presents the first study to evaluate LRA in a region with large topographic variations within a short distances. The REVEALS model estimates regional vegetation abundance based on pollen assemblages from large lakes (100–500 ha). Pollen surface samples from one large and 28 small lakes are used together with a combination of regionally derived pollen productivity estimates and available estimates from other regions of Europe. The results show a good relationship between REVEALS‐estimated forest cover and vegetation abundance based on the CORINE land‐cover data. The REVEALS results using various sets of pollen assemblages from small lakes were comparable to those using one large lake. Local vegetation abundance using the LOVE model was estimated around 26 lakes. For common taxa, such as Pinus and Poaceae, the LOVE‐based estimates of plant abundance match well with the distance‐weighted plant abundances based on vegetation maps. Our results indicate that the LRA approach is effective for reconstruction of long‐term vegetation changes in western Norway and other regions with high topographic relief when no major gradients exist in the pollen data.  相似文献   

2.
The relationships amongst modern pollen assemblages, vegetation, climate and human activity are the basis for reconstructing palaeoenvironmental changes using pollen records. It is important to determine these relationships at regional scales due to the development of vegetation under different climatic conditions and human activities. In this paper, we report on an analysis of modern pollen assemblages of 31 surface lake samples from 31 lakes (one sample per lake) on the southwestern Tibetan Plateau where the knowledge of modern pollen and their relationships with vegetation, climate and human activities is insufficient. The region includes five vegetation zones: sub‐alpine shrub steppe, alpine steppe, alpine meadow and steppe ecotone, mountain desert and alpine desert. The lakes span a wide range of mean annual precipitation (50–500 mm) and mean annual temperature (?8 to 6 °C). Modern pollen assemblages from our samples mainly consist of herb taxa (Artemisia, Cyperaceae, Poaceae, Chenopodiaceae, etc.) and some tree taxa (Pinus, Fagaceae, Alnus, etc.). The results indicate that modern pollen assemblages are able to reflect the main vegetation distribution. Redundancy analysis for the main pollen types and environmental variables shows that precipitation is the leading factor that influences the pollen distribution in the study area with the first axis capturing 13.7% of the variance in the pollen data set. The Artemisia/Chenopodiaceae ratio is valid for separating the desert component (<2) from the steppe and other vegetation zones (>2) but is unable to distinguish moisture variations. The Artemisia/Cyperaceae ratio is able to identify meadows (<1) and steppes (>1) and can be used as a moisture index on the southwestern Tibetan Plateau. Our results show that an appropriate range is needed for a modern pollen data set in order to perform pollen‐based quantitative climate reconstructions in one region. It is essential to perform modern studies before using pollen ratios to reconstruct palaeovegetation and palaeoclimate at a regional scale.  相似文献   

3.
Temperatures for the past 2700 yr are estimated using well-dated pollen data from northwestern lower Michigan. The pollen data were sampled from sediment cores of four lakes along a 75-km transect, with fine-grained morainic soils around the two western lakes and sandy outwash soils around the lakes to the east. Climatic reconstructions based on the pollen data from the sandy sites show less temperature change than the reconstructions from the other sites, because variations in the composition of the vegetation at the sandy sites are edaphically restricted. One of the cores studied was dated by counting visible annual laminations (varves). The cores from the other lakes were dated based on three radiocarbon dates per core as well as the historically determined age of the settlement horizons. All the time scales were cross-checked using pollen-stratigraphic correlation between the four sites. A calibration function was developed using a network of modern pollen and climate data covering all of lower Michigan. Based on this calibration function, the 2700-yr reconstruction for Marion Lake indicates an estimated growing-season temperature range of 1.3°C between extreme 30-yr means. Mild conditions persisted prior to ca. A.D. 400, but a cold interval occurred between ca. A.D. 500 and 800. The well-marked warm period evident from ca. A.D. 1000 to 1200 was the last time when temperatures were about equal to the 1931–1960 mean. A prolonged longed cooling occurred after A.D. 1200 and reached 1°C below the 1931–1960 mean by the 1700s. A warming of 0.5°C is indicated from ca. A.D. 1750 to 1850. The estimated temperatures for the 1830s at Marion Lake agree with the instrumental data for that period and this provides some validation of the calibration-function results.  相似文献   

4.
Despite the high potential of pollen records for climate reconstruction, pollen–climate relationships may be biased due to past and present human activities on the landscape. We use (i) transfer functions based on modern pollen–climate relationships to infer seasonal temperature and summer precipitation for the period 11 500–4500 cal. a BP and (ii) lake‐level change records based on different sedimentary proxies in multiple cores that are mainly indicative for summer hydrology at Lago dell'Accesa (central Italy). Quantitative reconstructions indicate lowest summer precipitation during two phases (8500–7700 cal. a BP and after 6000 cal. a BP) and a gradual winter temperature increase from 11 500 to ca. 8000 cal. a BP. Lowest summer precipitation was reconstructed during these phases characterised by vegetation shifts from open forests dominated by summergreen oaks (Quercus) to forests dominated by evergreen oaks (Quercus ilex), which are at present most abundant where summer drought is stronger. Similarly, the lake‐level record indicates two long‐lasting low summer precipitation phases (8800–7700 and 6400–4400 cal. a BP) that were interrupted by short‐term high summer precipitation events. Based on the broad agreement between the pollen‐inferred summer precipitation and the low‐frequency lake‐level changes, we suggest that the duration of the high summer precipitation events may have been too short to maintain drought‐sensitive trees, which may have been affected by high mortality rates when summer dry conditions returned. Although past and modern pollen–climate relationships may very likely have been affected by human activities since the Neolithic (i.e. when exploitation of the landscape started), we reject the hypothesis of a significant anthropogenic bias in the pollen‐based climate reconstruction. In addition, we suggest that pollen‐based and lake‐level reconstructions may have different inherent abilities of capturing high‐ and low‐frequency precipitation signals. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

5.
The large landmass of northern Russia has the potential to influence global climate through amplification of climate change. Reconstructing climate in this region over millennial timescales is crucial for understanding the processes that affect the global climate system. Chironomids, preserved in lake sediments, have the potential to produce high resolution, low error, quantitative summer air temperature reconstructions. Canonical correspondence analysis of modern surface sediments from high-latitude lakes, located in northern European Russia and central Siberia, suggests that mean July air temperature is the most significant variable explaining chironomid distribution and abundance. This strong relationship enabled the development of a chironomid-based mean July air temperature-inference model based on 81 lakes and 89 taxa which has a rjack2 = 0.92 and RMSEP = 0.89 °C. Comparison of taxon responses to July temperature between this Russian and existing Norwegian data-sets shows that the temperature optima of individual taxa were between 1 and 3 °C higher in the Russian data regardless of modelling technique. Reconstructions based on fossil assemblages from a Russian tundra lake core (VORK5) using a Norwegian chironomid-based inference model provide mean July air temperature estimates that are 1.0–2.7 °C colder than from the 81-lake Russian model and are also lower than the instrumental record from a nearby meteorological station. The Norwegian model also did not reconstruct decadal-scale fluctuations in temperature seen in the instrumental record. These observations suggest that chironomid-based inference models should only be applied to sediment cores which have similar climate regimes to the geographic area of the training set. In addition a 149 lake, 120 taxa chironomid-based continentality inference model was also developed from the modern Norwegian and Russian training sets. A 2-component WA-PLS model was the minimal adequate model with rjack2 = 0.73 and RMSEP = 9.9 using the Gorczynski continentality index. Comparison of reconstructed continentality indices from the tundra lake, VORK5, show close agreement with local instrumental records over the past 70 years and suggest that the model is reliable. Recent warming in the Arctic has been spatially and seasonally heterogeneous; in many areas warming is more pronounced in the spring and autumn leading to a lengthening of the summer, while summer temperatures have remained relatively stable. A continentality inference model has the potential to detect these seasonal changes in climate.  相似文献   

6.
When properly calibrated, Holocene pollen data provide an important source of quantitative information about Holocene climates. Multiple linear regression of modern climate and pollen data allows the development of statistical calibration functions that transform percentages of certain pollen types into quantitative estimates of climatic variables, and these functions, when applied to Holocene pollen data, yield estimates of climatic variables for past times. Confidence intervals for the climatic variables provide estimates of the statistical errors. These interval estimates are based upon the following statistical assumptions: (1) the regression model is appropriate; (2) the errors in measuring the climate variables are independent, normally distributed and have constant variance; and (3) no outliers are present. We outline the steps to be followed in calculating calibration functions, including (1) selecting the calibration region; (2) selecting a pollen sum; (3) analyzing scatter diagrams of a given climate variable against each pollen type; (4) deleting outliers and transforming pollen data; (5) performing the regression; and (6) testing the appropriateness of the statistical assumptions. We used available computer programs for most of this study. In addition, we developed new software to compute the Moran statistic to test for spatial autocorrelation among the regression residuals, using the dual of the Voronoi diagram to describe the spatial relationships among the sites. In order to illustrate the sequence of procedures, we used data from the lower peninsula of Michigan to develop a calibration function for July mean temperature and then used Holocene pollen data from central lower Michigan to estimate past temperatures.  相似文献   

7.
This paper presents the results of a high‐resolution Late‐glacial chironomid stratigraphy from Hawes Water, a small carbonate lake in northern Lancashire. The samples were from a core taken from the terrestrialised margin of the present lake, which represents an intermediate depth between the true littoral and the profundal. The chironomid assemblage showed a high degree of sensitivity to both broad‐scale and short‐term temperature changes. Comparison with an existing proxy temperature record (δ18O) for the site confirmed the presence of four temperature inversions within the Late‐glacial Interstadial. A mean July air temperature inference model, derived from acid, soft‐water lakes in Norway and Svalbard, was applied to the data. Despite the absence of carbonate lakes within the Norwegian training set, there was a close similarity between trends in estimated July air temperature and the δ18O trace, with a particularly strong correspondence in the periods of clay deposition. This suggests that this model is highly robust. The inferred maximum Interstadial temperature was 13.4°C, dropping initially to 7.5°C in the Loch Lomond Stadial. Temperatures reach a maximum of nearly 10°C in this period, cool for a short period before rising rapidly to 13.2°C at the start of the Holocene. These temperatures are similar to but slightly higher than those estimated for Whitrig Bog, southeast Scotland, and lower than those inferred from coleopteran‐based models for sites in South Wales. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

8.
Chironomids (non-biting midges) can provide accurate climate reconstructions from the Late Glacial to the present. Until now, anoxic lakes have been avoided for temperature reconstructions since chironomid assemblages are sensitive to changes in oxygen concentrations in the hypolimnion. However, anoxic lakes may have varved sediments, providing the possibility for near-annual climate reconstructions. Here, we tested the applicability of two calibration methods to reconstruct mean July air temperatures from chironomid assemblages preserved in the sediments of the anoxic Seebergsee located in the northern Swiss Alps: a calibration-in-space approach and a calibration-in-time approach. The calibration-in-space approach (i.e. chironomid assemblages from surficial lake sediments (0–1 cm) calibrated against meteorological data) provided accurate inferences (i.e. similar temperature changes as measured at the closest meteorological station, and at regional stations) in the Seebergsee stratigraphy until anoxia increased in the lake. With the increase of anoxia, the chironomid-inferred temperatures were generally colder than measured temperatures. A calibration-in-time approach (i.e. calibration of chironomid assemblages in a time series against instrumental data from the closest meteorological station) provided accurate reconstructions (i.e. similar to the regional records) for the past 100 years, including the time period of inferred anoxia. However, its applicability should be further tested on longer temporal scales.  相似文献   

9.
Numerical techniques were used to study chironomid distribution and abundance in lakes from a 1000 km transect in Finland, with special interest on the effect of local summer air temperatures on chironomid assemblages. The final aim of the study was to develop a chironomid‐based palaeotemperature inference model. The dataset consisted of 82 lakes (of which 77 were used in the model after deletion of outliers), with catchments spanning from boreal coniferous forests to mountain birch woodland and tundra vegetation. Numerical analysis showed that the mean July air temperature was the most significant variable explaining the distribution and abundance of chironomids in Finnish lakes. Weighted‐averaging partial least squares techniques were used to develop a palaeotemperature inference model for mean July air temperature reconstructions. The model performance statistics were favourable, with cross‐validated coefficient of determination (r2) of 0.78, root mean squared error of prediction of 0.721°C and maximum bias of 0.794°C. Based on these values, the transfer function is a valid means of performing quantitative palaeotemperature estimates in downcore studies. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

10.
Wen, R. L., Xiao, J. L., Chang, Z. G., Zhai, D. Y., Xu, Q. H., Li, Y. C. & Itoh, S. 2009: Holocene precipitation and temperature variations in the East Asian monsoonal margin from pollen data from Hulun Lake in northeastern Inner Mongolia, China. Boreas, 10.1111/j.1502‐3885.2009.00125.x. ISSN 0300‐9483. Quantitative palaeoclimatic reconstruction with the weighted averaging partial least squares method was applied to the pollen profile from Hulun Lake in northeastern Inner Mongolia. The data provide a detailed history of variations in precipitation and temperature over the northeastern margin of the East Asian summer monsoon during the Holocene. A warm and dry climate prevailed over the lake region until c. 8000 cal. BP. During the period c. 8000–4400 cal. BP, precipitation increased markedly and temperature gradually declined. The interval between c. 4400 and 3350 cal. BP was marked by extremely dry and relatively cold conditions. Precipitation recovered from c. 3350 to 1000 cal. BP, with temperatures rising c. 3350–2050 cal. BP and dropping c. 2050–1000 cal. BP. During the last 500 years, the climate of the lake region displayed a general trend of warming and wetting. While Holocene temperature variations in the mid‐high latitude monsoonal margin were controlled by changes in summer solar radiation in the Northern Hemisphere, they could also be related to the strength of the East Asian summer monsoon. The lack of precipitation during the early Holocene could be attributed to the weakened summer monsoon resulting from the existence of remnant ice sheets in the Northern Hemisphere. Changes in the monsoonal precipitation during the middle to late Holocene would have been associated with the ocean–atmosphere interacting processes occurring in the western tropical Pacific.  相似文献   

11.
Lake geomorphology and high-level lacustrine deposits since the mid-late Pleistocene are well preserved in lakes of the Qinghai-Tibet Plateau. According to geological surveys of 17 lake districts in different locations of the plateau, combined with interpretations of satellite images and topographic maps, the authors studied the timing of formation and scopes of the pan-lake areas of the plateau and their paleoclimate. The latest two high lake levels （overflow surfaces） on the Qinghai-Tibet Plateau in the Quaternary occurred at N40 to 30/35 ka and N65 to 53 ka respectively. In these time intervals, the plateau was covered by huge interconnected pan-lake systems with a total area of -36 km^2 and a total volume of lake water of 〉530 million km^2, which are about 38 times and 659 times larger than those of the modern lakes respectively. Before this pan-lake period in the late Pleistocene, there had been three high lake levels that occurred at N 132-112 ka, 110-95 ka and 91-72/-83-75 ka respectively, suggesting that the late Quaternary climate on the plateau was unstable and changed rapidly. The -40-30 ka high lake level also appeared in the Tengger desert north of the plateau, suggesting that there existed very strong summer monsoons from South Asia then; the variation in solar radiation with a 20,000 precessional period has special importance for the high-altitude Qinghai-Tibet Plateau in the low-latitude zone of the Earth. Around 30 ka, the pan-lakes at the peripheries of the Qinghai-Tibet Plateau drained out suddenly with rapid uplift of the plateau and cooling. In a short time the huge amount of cold lake water emptied into the Indian Ocean and Western Pacific. The draining event of the pan-lakes brought about the environmental changes of rivers and lakes at peripheries of the plateau.  相似文献   

12.
中国盐湖资源与生态环境   总被引：12，自引：0，他引：12  
郑绵平 《地质学报》2010,84(11):1613-1622
中国盐湖区受干旱-半干旱气候控制,现代盐湖主要分布于年降水量500mm范围内。依据中国盐湖地貌、地质构造条件及物质成分特点,将中国盐湖划分为4个盐湖区,并简述第四纪以来各区由于古气候变化和地质构造活动性不同,而具有各自的盐湖演化特点。由于各区盐湖规模、经济价值和交通技术条件不同,开发利用程度有差异,以吉兰泰盐湖和运城盐湖综合利用和环境保护较好;以察尔汗为代表的钾盐湖开发规模最大。目前中国盐湖区环境还存在很多需要改善的问题,尤其是地处边远的中小型盐湖,采富弃贫、回收率低和单一矿种开采使大量伴生有用组分贫化等。同时,由于气候环境因素也引起盐湖环境的重大变化,在全球变暖的背景下,西北盐湖区,自1987年以来,出现向暖湿转型,湖泊有扩大上升趋势;青藏高原盐湖区则大致从1992年以来,由东往西,相继出现高湖面,显现向暖湿转型;而西北盐湖区仍处于暖干阶段,湖水位下降,盐湖沙化。随着中国全面建设小康和和谐社会的实施,生态环境保护日益受到重视,建议进一步吸取国内外在盐业综合利用、整体开发和环境保护的经验和先进技术,加强盐湖环境变化观察和监测,建设环境友好型的绿色大盐湖产业。  相似文献   

13.
Fossil diatom assemblages from a 12-m core from Kirchner Marsh were compared with modern surface assemblages from 159 Minnesota and labrador lakes using cluster analysis. The deepest levels of the core (spruce pollen zone 13,000 to 10,200 yr B.P.) resemble modern diatom assemblages from deep oligotrophic lakes of northeastern Minnesota. Diatom assemblages of the pine pollen zone (about 10,200 to 9500 yr B.P.) have few modern analogs. In the oak zone (9500 yr B.P. to present) after a brief pulse of diatom species indicative of eutrophication, the assemblages are dominated by species characteristic of shallow lakes, suggesting a drop in the lake water level during the prairie period (5500 to 7500 yr B.P.). Macrofossil data of W. A. Watts and T. C. Winter (1966, Geological Society of America Bulletin77, 1339–1360) show that this shift to shallow-water diatoms occurred when aquatic macrophytes appeared at the site in abundance.  相似文献   

14.
Thermal dynamics of lakes has a key role in chemical and biological processes in lakes including nutrient distributions and phytoplankton growth. Applications of hydrodynamic models to lakes can provide insights into possible future alterations in thermal dynamics induced by climate change. In this study, we present the calibration and validation of the newest version of the open-source hydrodynamic model GLM (General Lake Model) to the dimictic Lake Ammersee, located in south-east Germany. The simulation of lake water temperatures for the calibration period revealed an overall root mean square error of 0.65 °C and a mean error of 0.08 °C. The seasonal stratification pattern and the annual thermal structure of this dimictic lake were reproduced by the model. The model simulated the presence of winter ice cover for the only year out of 8 years simulated, when ice cover was observed. Elevated lake water temperatures were also reproduced in model simulations during a period in 2003 of unusually high air temperatures. Statistical analysis of the model calibration results for Lake Ammersee indicates a fit comparable to or better than most other well-established hydrodynamic models and provides an opportunity for continuous simulations through periods of ice cover. Our results indicate a major improvement in GLM compared with earlier model versions and demonstrate the applicability of GLM for limnophysical studies, particularly for altered forcing conditions such as climate change.  相似文献   

15.
This study investigates the distribution of modern pollen assemblages in the Qinghai-Tibetan Plateau, China, based on surface soil samples collected at 1202 sites along an altitudinal gradient of 10–5500 m asl, where mean annual precipitation (MAP) ranges from 12 to 1840 mm and mean annual temperature (MAT) from ?7 to 21.5 °C. A total of 153 pollen taxa were found with relative abundances greater than 1% in at least two samples. Canonical correspondence analysis (CCA) was used to determine the main environmental variables influencing pollen distributions. The results reveal that MAP is the most significant dominant variable. However, MAT, altitude (ALT), July temperature (MT7), and relative humidity (HHH) are also significant variables that clearly follow the gradients in the CCA ordination, suggesting that pollen assemblages probably not only reflect a single climatic parameter, but also a variety of other climatic inter-related parameters.Transfer functions, based on locally weighted weighted averaging (LWWA), were developed for MAP (R2-boot = 0.89, RMSEP = 109 mm), MAT (R2-boot = 0.78, RMSEP = 2.3 °C), ALT (R2-boot = 0.73, RMSEP = 597 m), HHH (R2-boot = 0.82, RMSEP = 4.5%), and July mean precipitation (MP7) (R2-boot = 0.87, RMSEP = 23 mm). Overall, our results confirm that pollen can provide reliable estimates of the primary climatic parameters. The application of the LWWA model to the fossil records of Chen Co Lake allowed quantitative inferences to be made about Holocene climatic changes in the southern Tibetan Plateau, suggesting that LWWA is a robust calibration method for quantitative palaeo-environmental reconstruction based on pollen data in the regions.  相似文献   

16.
We address the issue of spatial autocorrelation, an occurrence that may introduce biases in the evaluation of the performance of transfer functions, by using two fundamentally different approaches, one based on calibration (weighted averaging partial least squares regressions; WA-PLS) and the other based on similarities (modern analogue technique; MAT). The tests were made after spatial standardization of 700 North Atlantic surface data points, which include 29 dinocyst taxa and 4 climate parameters. The evaluation of transfer function performance was made by defining a verification dataset that was gradually isolated from the calibration or comparison datasets. Although strong spatial autocorrelation characterizes the original climate parameter distribution, the results show that the spatial structure of data has relatively low effect on the calculation of the error of prediction. They also show that the performances of MAT are generally better than those of WA-PLS, with lower error of prediction. The better performance of MAT in the present study can be explained by the non-modal distribution of salinity and temperature in the studied marine environments, which is not appropriate for the application of WA-PLS. The two methods yield equivalent results about the spatial structure of the residuals based on empirical semi-variograms. The analyses we performed include the comparison of reconstructions based on original raw data and gridded data. Results suggest that the gridding of the reference database may reduce the noise and thus improve the performance of the techniques.  相似文献   

17.
We analysed pollen from a sediment core from Fiddaun, a small Lateglacial lake basin in western Ireland. Results reflect the general Lateglacial vegetation development in Ireland, as reconstructed from other pollen records. The Fiddaun diagram shows a number of short‐lived regressive vegetation phases during the Interstadial. The close similarity between two pollen records from the same region (Fiddaun and Lurga) indicates that these fluctuations probably reflect regional rather than local changes. Comparison with a previously published climate reconstruction, based on a chironomid‐inferred mean July air temperature reconstruction, lithology, and oxygen and carbon isotopes of lake marl from the Fiddaun record, allowed us to establish the relationship between summer temperature and vegetation changes. Results reveal that two temporary regressive shifts in the pollen record correspond to cold oscillations, which have been correlated to Greenland Interstadial 1b and 1d. It seems that the first cold oscillation (GI‐1d) had the most distinct effect on vegetation in Ireland. In contrast, it appears that the transition from Juniperus shrubland and Empetrum heath to grassland, which is estimated at ～13.7 ka BP, was not caused by decreasing summer temperatures, as no substantial change is observed in the climate proxies. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

18.
A consensus on Holocene climate variability at the modern northern fringe of the East Asian summer monsoon (EASM) region remains elusive. Here, we present a pollen-based reconstruction of vegetation history and associated climate variations of a sediment core from Huangqihai Lake, central Inner Mongolia. During 10.7 to 8.8 cal kaBP, typical steppe with small patches of forest dominated the lake area, suggesting a moderately wet climate, followed by ameliorating climatic conditions until 8.0 cal kaBP as deduced by the expansion of forest. Typical steppe recovered the lake area between 8.0 and 7.2 cal kaBP, reflecting a deterioration of climatic conditions; in combination with other proxy records in the study region, we noticed that severe aridity was prevailed in the lake area between 8.0 and 7.6 cal kaBP. During 7.2 to 3.2 cal kaBP, abundant tree pollen indicated dominance of forest-steppe around the lake, marking regionally wet conditions. A notable absence of broadleaved trees after 5.2 cal kaBP reveals a slight drying trend, and climate deterioration from 4.5 to 4.1 cal kaBP might be linked to the 4.2 ka event. After 3.2 cal kaBP, a transition to steppe was associated with dry conditions in the region. Based on our pollen record and prior paleoclimatic reconstructions in the Huangqihai Lake region, there was a generally-accepted, stepwise shift to a wet climate during the early Holocene, an overall humid climate from 7.2 to 3.2 cal kaBP, and then severe drought for the rest of the Holocene. Moreover, regional comparisons among pollen records derived from lakes situated in the temperate steppe region suggested a roughly synchronous pattern of vegetation and climate changes during the Holocene and demonstrated an intensified EASM during the middle Holocene.  相似文献   

19.
Hidden within the vast Bolivian Altiplano are archives of past climate change in the form of remarkable carbonate rocks surrounding lakes long since disappeared. Beyond the Salar de Uyuni, the largest salt flat in the world, lies a relatively untouched realm of volcanoes and salt lakes. Ancient shorelines from intervals in the Altiplano history, when large lakes were more abundant, may hold important information about a time when the climate in this region was punctuated by much wetter phases before present day aridity took a hold. Previous studies in this region have reconstructed robust chronological timelines for such events and highlight two large lake phases over the last 18 thousand years (the Tauca and Coipasa lake phases); however higher resolution climate data are scarce. Current studies on climate proxies from smaller lakes in southern Bolivia may shed light on some of these higher resolution climate events including El Niño–Southern Oscillation (ENSO) events. Laminated tufa found around the palaeoshorelines of the West Lípez Lakes is one such proxy, and can be analysed to investigate the potential roles of annual versus shorter‐term climatic variation in the evolving Altiplano climate at the time.  相似文献   

20.
High-resolution proxy records from the circum-Caribbean region indicate significant variation in Late Holocene climate, especially precipitation, attributed primarily to shifts in the mean annual position of the Intertropical Convergence Zone (ITCZ). The paleoenvironmental and cultural impacts of this Late-Holocene climate variability have been analyzed intensively in the western Caribbean, and to a lesser extent in the southern Caribbean. However, the occurrence and impacts of Late Holocene climate shifts in the eastern Caribbean, especially in island interiors, has not been well documented. Here we present sediment records of Late-Holocene paleoenvironmental change from two lakes located on the Caribbean slope of the Cordillera Central in the Dominican Republic that span the last ～3000 years. Sediment characteristics, pollen, charcoal, biogenic carbonate assemblages and isotopic composition, and bulk sedimentary carbon isotope values in Laguna Castilla and Laguna de Salvador indicate extreme shifts in hydrology, vegetation, and disturbance regimes in response to climate change and human activity in the lake watersheds. Close correspondence between the hydrological histories of the lakes and trace metal concentrations in sediments of the Cariaco Basin indicate that precipitation variability here responds to the same controls, and may similarly reflect shifts in the mean annual position of the ITCZ. Human occupation of the watersheds appears to be closely linked to severe dry periods and may indicate larger scale cultural responses to precipitation variability on the island of Hispaniola. Prehistoric human populations strongly affected vegetation and disturbance regimes in the lake watersheds. Impacts may have lasted several centuries and may have been more severe than impacts of modern populations.  相似文献   

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