Quantitative interpretation of fossil pollen spectra: Dissimilarity coefficients and the method of modern analogs

Dissimilarity coefficients measure the difference between multivariate samples and provide a quantitative aid to the identification of modern analogs for fossil pollen samples. How eight coefficients responded to differences among modern pollen samples from eastern North America was tested. These coefficients represent three different classes: (1) unweighted coefficients that are most strongly influenced by large-valued pollen types, (2) equal-weight coefficients that weight all pollen types equally but can be too sensitive to variations among rare types, and (3) signal0to-noise coefficients that are intermediate in their weighting of pollen types. The studies with modern pollen allowed definition of critical values for each coefficient, which, when not exceeded, indicate that two pollen samples originate from the same vegetation region. Dissimilarity coefficients were used to compare modern and fossil pollen samples, and modern samples so similar to fossil samples were found that most of three late Quaternary pollen diagrams could be “reconstructed” by substituting modern samples for fossil samples. When the coefficients indicated that the fossil spectra had no modern analogs, then the reconstructed diagrams did not match all aspects of the originals. No modern analogs existed for samples from before 9300 yr B.P. at Kirchner Marsh, Minnesota, and from before 11,000 yr B.P. at Wintergreen Lake, Michigan, but modern analogs existed for almost all Holocene samples from these two sites and Brandreth Bog. New York.     

Stable carbon isotope analysis on fossil Cedrus pollen shows summer aridification in Morocco during the last 5000 years

Quantitative climate reconstructions from pollen typically rely on empirical relationships between pollen abundances or assemblages and climate, such as the modern analogue technique. However, these techniques may be problematic when applied to fossil sequences, as they cannot separate anthropogenic from climatic influence on pollen assemblages. Here, we reconstruct Mid‐ to Late Holocene summer aridity in the Middle Atlas, Morocco, using stable carbon isotope analysis of isolated fossil Cedrus pollen. This approach is based on well‐documented plant physiological responses to moisture stress and is therefore independent of vegetation composition. We find that there has been a general long‐term trend of increasing summer aridity in the region during the last 5000 years to the present day. The gradual decline of Cedrus atlantica forest in the Late Holocene follows this aridity trend. Additionally, we show how isolating a specific pollen type for carbon isotope analysis yields a robust climate signal, versus using pollen concentrates or bulk sediment. Our findings indicate that climate has become drier in the region and confirms the Mid‐ to Late Holocene aridification trend observed more widely in the western Mediterranean, using a novel proxy for this region with good potential for wider application in other environments.     

鄱阳湖地区过去2160年孢粉记录的植被变化及影响因素

本研究在AMS ¹⁴C测年的基础上,通过分析鄱阳湖430 cm沉积剖面44个样品的孢粉记录和有机碳（TOC）指标,同时结合江西人口密度指标及当地历史文献资料,重点探讨了该区过去2160年植被变化的影响因素及在人类活动影响下该区域植被变化的历史。研究结果显示,鄱阳湖流域过去2160年植被变迁及人类活动变化主要分为5个阶段：1）160 BC~400 A.D.,乔灌木花粉含量较低,草本植物花粉较高,指示鄱阳湖流域周围地势低平的地带已经被广泛垦殖,但周围仍分布着一定量的常绿阔叶类、落叶阔叶类以及针叶类植被。高含量的禾本科表明鄱阳湖流域的稻作农业已初具规模,但该期乔灌木花粉含量相对较高,蕨类孢子含量较低,人口密度较小,指示该时期森林覆盖度较高,人类活动相对较弱。2）400~800 A.D.,乔灌木植物花粉与草本植物含量变化不大,植被与前期基本一致。高含量的蕨类孢子指示人类活动进一步加强,但人口密度较低,指示该时期森林覆盖度仍然较高,人类活动相对较弱。3）800~1350 A.D.,乔灌木花粉含量显著下降,而与人类活动相关的草本植物花粉含量略有上升,表明鄱阳湖流域随着人类活动增强,地带性森林植被遭受到一定的破坏,森林覆盖度明显下降,该期人口密度迅速递增,农业垦殖强度进一步加强,农业生产规模继续扩大,指示该时期人类活动影响较强,森林植被覆盖度较低。4）1350~1650 A.D.,乔灌木花粉含量略有上升,草本植物花粉含量略有下降,指示该时期鄱阳湖流域常绿阔叶类、落叶阔叶类及针叶类植被较前期略有恢复,尤其是松属花粉含量增长明显,表明该区地带性植被破坏后逐渐演变为具有次生性质的松林。该时期由于气候变冷,自然灾害频发,加上战争频繁,人口密度显著下降,人类活动影响减弱,进而导致森林植被略有恢复。5）1650 A.D.至今,鄱阳湖流域植被与前期较为一致,但松属花粉含量后期呈现明显的下降趋势,表明鄱阳湖流域随着人类活动进一步加强,次生松林破坏较为严重。过去2000多年气候变化虽然对区域植被有一定的影响,但历史时期以来人口的不断增长、农业的快速发展以及国家鼓励农民垦荒政策的实施是引起当地森林植被减少以及生态环境退化的主要原因,该结果对鄱阳湖流域恢复和重建自然生态环境具有重要的理论价值。

     

A detailed pollen record of vegetation and climate changes in Central China during the past 16 000 years

Zhu, C., Ma, C., Yu, S.-Y., Tang, L., Zhang, W. & Lu, X. 2009: A detailed pollen record of vegetation and climate changes in Central China during the past 16 000 years. Boreas , 10.1111/j.1502-3885.2009.00098.x. ISSN 0300-9483.
Detailed pollen analyses, along with magnetic and loss-on-ignition (LOI) measurements, were conducted on a 3 m long peat sequence recovered from the Dajiuhu Basin, the Shennongjia Mountains in Central China. Ten AMS ¹⁴C dates provide a firm age control on this pollen record in terms of vegetation changes governed essentially by the rise and fall of the Asian summer monsoon during the past 16 000 years. Between 16 000 and 12 700 cal. yr BP, pollen assemblages were dominated by coniferous and broad-leaved trees, indicating a mixed forest landscape corresponding to the initial establishment of the monsoonal climate after the Last Glaciation. The progressive increases in percentages of evergreen tree pollen after 12 700 cal. yr BP point to a steady enhancement of the summer monsoon, which was episodically weakened during the Younger Dryas stadial. From 11 000 to 6000 cal. yr BP, values of coniferous and deciduous tree pollen decreased, while evergreen broad-leaved tree pollen increased substantially, implying a stronger than normal monsoonal climate condition corresponding to the Holocene Hypsithermal Interval. A great reduction in the values of evergreen tree pollen at about 4000 cal. yr BP indicates a sudden retreat of the summer monsoon from this area.     

Rapid warming in mid-latitude central Asia for the past 100 years

Surface air temperature variations during the last 100 years (1901–2003) in mid-latitude central Asia were analyzed using Empirical Orthogonal Functions (EOFs). The results suggest that temperature variations in four major sub-regions, i.e. the eastern monsoonal area, central Asia, the Mongolian Plateau and the Tarim Basin, respectively, are coherent and characterized by a striking warming trend during the last 100 years. The annual mean temperature increasing rates at each sub-region (representative station) are 0.19°C per decade, 0.16°C per decade, 0.23°C per decade and 0.15°C per decade, respectively. The average annual mean temperature increasing rate of the four sub-regions is 0.18°C per decade, with a greater increasing rate in winter (0.21°C per decade). In Asian mid-latitude areas, surface air temperature increased relatively slowly from the 1900s to 1970s, and it has increased rapidly since 1970s. This pattern of temperature variation differs from that in the other areas of China. Notably, there was no obvious warming between the 1920s and 1940s, with temperature fluctuating between warming and cooling trends (e.g. 1920s, 1940s, 1960s, 1980s, 1990s). However, the warming trends are of a greater magnitude and their durations are longer than that of the cooling periods, which leads to an overall warming. The amplitude of temperature variations in the study region is also larger than that in eastern China during different periods.     

Environmental changes during the past 2000 years in north-central Wisconsin: Analysis of pollen,charcoal, and seeds from varved lake sediments

A 2000-year accumulation of varved sediments from Hell's Kitchen Lake in north-central Wisconsin was analyzed for pollen, charcoal, and seeds. The varves provided an accurate time scale for the study. The pollen record indicates changes on two different time scales. Short-term changes lasting several decades appear to be superimposed on long-term changes lasting several centuries. The short-term changes are related to individual fires, and the long-term changes result from increases or decreases in the frequency of these perturbations. From 2000 to 1150 years ago the average interval between fires was about 100 years, and from 1150 to 120 years ago the interval increased to about 140 years. Evidence from pollen, seeds, and charcoal at Hell's Kitchen Lake suggests that at least two “moist” intervals occurred during the past 2000 years, one between 2000 and 1700 years ago and the other between 600 and 100 years ago. A third but minor “moist” period occurred about 1150 to 850 years ago. A pollen and seed diagram shows that these intervals are characterized by increased percentages of white pine pollen, hemlock pollen, and yellow birch seeds, and by decreased levels of charcoal. The “dry” interval of 1700 to 1150 years ago is characterized by increased percentages of paper birch seeds, oak pollen, and aspen pollen, along with high levels of charcoal. The times of climatic change indicated at Hell's Kitchen Lake are nearly synchronous with those based on studies of tree rings, soils, glacial activity, and other pollen studies from various regions of North America, but the direction of these inferred changes is not always the same. This result suggests that the long-wave pattern of the general circulation has been variable during the past 2000 years.     

Altitudinal and latitudinal migration of Cryptomeria japonica for the past 20,000 years in Japan

Biostatistical analysis of modern pollen assemblages in 152 Japanese surface samples shows that Cryptomeria japonica can normally grow in areas with a mean January temperature of approximately −7° to 7°C, a mean August temperature of 19° to 27°C, and an effective precipitation (total precipitation during the growing season) of over 1000 mm. The full-glacial distribution of the species on both the Sea of Japan and the Pacific coasts (35–36°N lat) indicates that in these areas the maximum possible reduction of temperature was 8.7–10.2°C in January and 6.0–7.3°C in August, and that the effective precipitation was low, being 1000–1350 mm, or 40–55% below the modern level, provided that the species has not changed its physiological characteristics. Populations of C. japonica expanded northward and upslope from their full-glacial areas of distribution immediately after late-glacial climatic amelioration. This expansion appears to have been regulated mainly by the availability of effective precipitation which became high in northeastern Honshu about 4000 yr ago. After about 2500 yr B.P., C. japonica was planted extensively by humans in moist, temperate climatic regions (excluding Hokkaido), and now has its widest distribution since at least the last full-glacial interval.     

近50年来ENSO与祁连山区气温降水和出山径流的对应关系

根据有关水文气象台、站的观测资料,分析了恩索(ENSO)与祁连山区气温、降水的对应关系,研究了祁连山区出山径流对厄尔尼诺(ElNino)现象响应.结果表明:ElNino现象对祁连山区的气温、降水和径流的影响随着发生时间和地段的不同而不同.ElNino现象发生之年,整个祁连山区均出现气温偏高、降水减少及径流偏枯的现象,尤以东段和中段最为明显.ElNino现象次年,祁连山区东段和中段气温偏高、降水减少及径流偏估的程度不如ElNino现象当年那样显著,而西段的气温、降水及径流与ElNino现象则无明显关系.     

Sea level history in Beringia during the past 250,000 years

This paper attempts to relate current knowledge of sea-level history in Beringia to the Broecker-van Donk “Termination” concept of climatic and sea-level history. The Einahnuhtan transgression is thought to represent Termination III, which according to Broecker and van Donk, took place about 225,000 y.a. The Kotzebuan transgression is thought to represent a positive fluctuation that modulated the generally falling sea level during the ensuing 100,000 yr. Sea level probably fell to about ?135 m in the Bering Sea area during the maximum phase of the penultimate glaciation. The two Pelukian shorelines probably represent Termination II (about 125,000 yr BP in the Broecker-van Donk chronology) and one of the two positive fluctuations that modulated the generally falling sea level of early Wisconsinan time, about 105,000 and 80,000 y.a. according to Broecker and van Donk. Another positive modulation brought sea level to at least ?20 m, about 30,000 y.a. Sea level evidently fell to between ?90 and ?100 m during the late Wisconsinan regression, but a substantial part of the outer Bering shelf remained submerged. Submerged shoreline features at ?38m, ?30 m, ?24 to ?20 m, and ?12 to ?10 m represent stillstands or slight regressions that modulated Termination I, the late Wisconsinan, and early Holocene recovery of sea level.     

勘查地球化学80年来重大事件回顾

勘查地球化学诞生于20世纪30年代初期,经历了80年的发展,已经成为一门重要的地球化学分支科学,为矿产勘查做出了巨大贡献。本文对勘查地球化学过去80年发展历程的标志性事件进行了回顾;对勘查地球化学基础理论研究进展和对矿产发现做出的贡献进行了概括总结;对近年勘查地球化学最为活跃的研究领域进行了简要介绍。在20世纪70年代以前勘查地球化学作为一门独立学科已经形成,它的标志性事件是经典教科书的出版、勘查地球化学专业设立、勘查地球化学杂志的创刊、国际勘查地球化学家协会成立和勘查地球化学学术会议定期召开。在基础理论研究方面体现在局部地球化学分散模型和区域分布模式的建立。一些国家大规模区域或国家地球化学调查或填图计划的实施,为全球3次批量矿床发现作出了巨大贡献。近些年勘查地球化学最为活跃的研究领域集中在:(1)在微观尺度上,从纳米水平和分子水平研究化学元素的分散和迁移机理,发展覆盖区找矿的地球化学技术;(2)在区域尺度上,继续开展区域地球化学调查或填图理论与技术研究,为成矿带地球化学评价和靶区圈定提供高质量数据和图件;(3)在全球尺度上,致力于全球地球化学基准建立,为了解全球化学元素分布背景、过去地球化学演化和预测未来全球化学变化提供定量评价标尺。     

The contemporary distribution of pollen in eastern North America: A comparison with the vegetation

By mapping and summarizing 478 pollen counts from surface samples at 406 locations in eastern North America, this study documents the relationships between the distributions of pollen and vegetation on a continental scale. The most common pollen types in this region are pine, birch, oak, and spruce. Maps showing isopercentage contours or isopolls for 13 important pollen types reflect the general N-S zonation of the vegetation. The maps and tabulations of average pollen spectra for the six major vegetational regions indicate high values for the following pollen types in each region: (1) tundra-nonarboreal birch, sedge, and alder; (2) forest/tundra-spruce, nonarboreal birch and alder; (3) boreal forest-spruce, jack pine (type), and arboreal birch with fir in the southeastern part; (4) conifer/hardwood forest-white pine, arboreal birch, and hemlock with beech, maple, and oak in the southern part; (5) deciduous forest-oak, pine, hickory, and elm, with beech and maple in the northern part, and highest values of oak and hickory west of the Appalachian crest; and (6) southeastern forest-pine, oak, hickory, tupelo, and Myricaceae. In some cases, less abundant pollen types are diagnostic for the region, e.g., bald cypress in the southeast. In the conifer-hardwood region and southward, pollen of weeds associated with deforestation and agriculture is abundant. The maps also show that much of southeastern U.S. and the area just to the east of Hudson Bay are in need of additional sampling. At 51 of the sites, absolute pollen frequencies (APF; grains/ml lake sediment) were obtained. These confirm the major conclusions from the percentage data, but differences are evident, e.g., the percentages of alder pollen peak in the tundra whereas alder APFs peak in the boreal forest, and spruce percentages peak in the forest-tundra whereas spruce APFs peak in the boreal forest. Because the APF data reflect the patterns of absolute abundance of individual taxa in the vegetation as well as the overall forest densities, future counts of modern pollen should include APF determinations. The effects of sedimentation processes on APF quantities indicate that APF samples should be obtained from moderate size lakes of similar morphology and hydrology and that, in each lake, several samples from the profundal zone should be pooled to create a sample representative of that lake.     

Late Quaternary climatic history of the Chilean Channels based on fossil pollen and beetle analyses,with an analysis of the modern vegetation and pollen rain

Fossil beetles and pollen were examined from an intermorainal bog at Puerto Edén, Isla Wellington, Chile (latitude 49°08'S, longitude 74°25'W). Wood from near the base of the section has an age of 12 960 ± 150 yr BP. Occurrence of flightless beetle species in the basal peat sample is evidence that some members of the biota survived the last glacial maximum in refugia. The assumption that the Chilean Channels were entirely ice-covered is incorrect. Plants and insects that invaded the deglaciated terrain were those of an Empetrum heathland in which patches of Nothofagus forest were restricted to sheltered locations. The climate supporting the heathland is inferred to have been windier and probably drier than that of the present day. From 13 000 yr BP to 9500 yr BP Nothofagus forest expanded, possibly in response to less windiness and more available moisture. Neither the fossil beetle nor pollen data support a return to significantly colder conditions between 11 000 and 10 000 yr BP at the time of the Younger Dryas Stade. From 9500 to 5500 yr BP the climate was as wet as that of the present day, based on an increased representation of the pollen of moorland plants and of aquatic beetle species. From 5500 to 3000 yr BP the climate was drier, as indicated by the expansion of Empetrum heath and the reduction in mesic habitats. From 3000 yr BP to the present-day mesic habitats dominated as the climate returned to a wetter mode. The alternatively wetter and drier episodes are attributed to latitudinal shifts in the position of storm tracks in the belt of Southern Westerlies.     

中亚地区过去2000年气候变化研究的数据挖掘

中亚地区是古丝绸之路的主要廊道,气候变率时空差异大,随着"一带一路"倡议的逐步推进,该地区过去2000年气候变化研究备受关注。文章参照世界数据中心古气候资料共享网收录文献的关键词,从Web of Science数据库检索该地区过去2000年气候变化研究文献;并结合中亚自然资源图集的政区、湖泊、山脉等地名信息,构建了面向该类文献的数据挖掘技术;据此提取了研究区（点）、时间、代用资料与研究结果等信息,分析了其间该地区冷暖与干湿变化研究的主要进展。主要结论有：1）中亚地区过去2000年气候变化的已有研究重点为局地温度、降水/干湿变化重建与冷暖、干湿、湖泊水位等的阶段性差异分析,其中尤以干湿相关研究居多;所用代用资料主要有树轮（占44.2%）、湖泊沉积物（占41.9%）、冰芯（占9.3%）等,其中湖泊沉积物、冰芯、石笋可覆盖整个2000年,而最长的树轮仅覆盖过去1100年,且多数集中在1700年之后;研究区集中分布于西天山及帕米尔高原、巴尔喀什湖、咸海及周边地区;研究时段虽覆盖整个2000年,但高分辨率重建工作主要集中于16世纪之后。2）中亚地区公元1~3世纪末、15世纪中期~19世纪中期气候相对寒冷,5世纪中期~9世纪末、12世纪中期~14世纪末和19世纪中期以后气候总体温暖,但在1230年前后和1680年前后也分别出现过显著转冷和回暖;20世纪后期的温暖程度可能与其前千年的最暖时段相当。西天山、帕米尔和咸海三地干湿的百年尺度阶段变化并不同步;其中在百年以上尺度,咸海及周边地区公元1~5世纪、10~12世纪中期相对偏干,5世纪~9世纪、12世纪后期~15世纪中期相对偏湿,此后缓慢转干持续至今;而在年代际尺度上,西天山和帕米尔山地降水自1980年起曾显著增加,但至2000年以后又显著转干。     

Modern pollen spectra from the tundra-boreal forest transition in northern Newfoundland, Canada

Percentage pollen diagrams are presented and analysed by step-wise discriminant analysis. Arboreal taxa show little variation between communities. Non-arboreal, local taxa display substantial variability, but are good indicators of community composition and structure, and provide reliable elements for the palynological characterization of communities. The discriminant analysis indicates a distinctiveness for most communities that is surprisingly high considering the small size of many of the vegetation units within the mosaic.     

塔里木盆地南缘喀拉米兰河克里雅河流内流区近30a来的冰川变化研究

对1999年的喀拉米兰河-克里雅河流内流区的Landsat ETM+影像运用非监督分类、监督分类,雪盖指数域值法和比值域值法提取其中176条冰川的边界信息,并与目视解译结果进行定量比较,结果表明:比值域值法精度最高,达98.32%;将比值域值法提取1999年的冰川边界与1970年航测地形图勾绘的冰川边界进行叠加分析,得到整个流域29 a来的冰川变化信息.统计表明,流域冰川条数减少了0.6%,总面积萎缩了2.86%,总冰储量减少了3.10%,大多数冰川处于稳定状态.     

A methodological framework for assessing and reducing temporal uncertainty in paleovegetation mapping from late-Quaternary pollen records

Mapping past vegetation dynamics from heterogeneous databases of fossil-pollen records must face the challenge of temporal uncertainty. The growing collection of densely sampled fossil-pollen records with accurate and precise chronologies allows us to develop new methods to assess and reduce this uncertainty. Here, we test our methods in the context of vegetation changes in eastern North America during the abrupt climate changes of the last deglaciation. We use the network of fossil-pollen records in the Neotoma Paleoecology Database (www.neotomadb.org) and data contributed by individual investigators. Because many of these records were collected decades before the current generation of ¹⁴C and age-model technologies, we first developed a framework to assess the overall reliability of ¹⁴C chronologies by systematically evaluating individual ¹⁴C ages and associated chronologies. We developed a qualitative ranking scheme for individual ¹⁴C ages that combines information about their accuracy and precision. ‘Benchmark’ pollen records were defined to have at least one ¹⁴C age with an accuracy within 250 years and a precision less than 500 years that is within 1000 years of the time interval of interest, and at least five pollen samples per 1000 years across this time period. Only 22 of >350 late-Pleistocene pollen cores in eastern North America met the benchmark criteria.We then used Bayesian change-point analysis to identify widespread ecological events (Picea decline, Quercus rise, and Alnus decline), and interpolated the ages of these events from the benchmark sites to non-benchmark sites. Leave-one-out cross-validation analyses with the benchmark sites indicated that the spatial error associated with interpolation was less for inverse distance-weighting (IDW) than thin-plate splines (TPS) and was about 500 years for the three biotic events. By comparison, the difference between the original ages of events at poorly constrained sites and the biostratigraphic ages interpolated from the benchmark sites was close to 1000 years, suggesting that the use of biostratigraphic ages can significantly improve the age models for poorly constrained sites. Overall, these analyses suggest that the temporal resolution of multi-site syntheses of late-Pleistocene fossil-pollen data in eastern North America is about 500 years, a resolution that allows analysis of ecological responses to millennial-scale climate change during the last deglaciation.     

回顾与展望——土力学室30年研究工作总结

本文是土力学研究室过去30年研究工作的总结,内容包括:粘土的流变特性;土的本构关系和其它基本性质;某些特殊土类的特性及其在工程中的应用;展望我室今后的进一步研究问题。     

Review and Prospect-sunmary Report of Research work for soil Mechanicd Division in the past 30 years

This paper is a summary report research work for soil mechanics division of our institute in the past 30 years. The contents of this report are rheological behavior of clay, constitutive relationship and other basic properties of soil, characteristics of some special kinds of soil and their influence on engineering, and prospect of further study for soil mechanics in this division.