共查询到20条相似文献，搜索用时 250 毫秒
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20世纪90年代,几名农夫发现了堪称世界第九大奇迹的龙游地下石窟建筑群。于是世界各地的知名专家云集浙江龙游,对石窟进行实地考察。20余年的学术纷争也没有探寻出其中的奥秘,中央电视台“探索发现”栏目曾召集各路学者对石窟进行了历史文化全方位的破析,但却以无果而告终。目前,仍没有科学定论,至今还是一个谜。 相似文献
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风沙危害是敦煌莫高窟保护面临的主要环境问题之一,建立一个完整的防护体系是解决这一问题的有效途径。在对20世纪80年代以来建立的防沙治沙试验工程防护效应分析研究的基础上,对敦煌莫高窟风沙危害综合防护体系设计进行了讨论。根据因地制宜,因害设防;以固为主,固、阻、输、导相结合;以工程和生物措施为主,兼顾化学固沙;高新技术与常规治理技术相结合;重点治理,分阶段实施与长远目标相结合的设计原则,认为根据不同地貌特征及地表组成物质,依次建立鸣沙山前缘流动沙丘和平坦沙地阻固区、窟顶戈壁防护区、洞体崖面固结区、石窟对面流动沙丘固定区、窟区防护林带建设区及天然植被封育保护区,可使危害莫高窟的风沙灾害得到有效控制,并达到莫高窟作为世界文化遗产和全国重点文物保护单位对环境质量的要求。 相似文献
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BY NICHOLAS R. GOLLEDGE JEREMY D. EVEREST TOM BRADWELL JOANNE S. JOHNSON 《Geografiska Annaler: Series A, Physical Geography》2010,92(1):111-124
This paper presents new lichenometric population data from the Antarctic Peninsula (67°S), and describes a new approach to lichen growth-rate calibration in locations where dated surfaces are extremely rare. We use historical aerial photography and field surveys to identify sites of former perennial snowpatches where lichen populations now exist. As an independent check on lichen mortality by snowkill, and the timing of snow patch disappearance, we use a positive-degree day (PDD) approach based on monthly climate data from Rothera Research Station. We find that maximum growth rates for lichens <40 mm in diameter on Adelaide Island are around 0.8 mm/yr. Furthermore, we propose that our combined methodology may be more widely applicable to the Polar Regions where the construction of lichenometric dating (age-size) curves remains a problem. 相似文献
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The reliability of lichenometric dating is dependent on a good understanding of lichen growth rates. The growth rate of lichens can be determined from direct measurement of growing lichens or indirect methods by measuring lichens growing on surfaces of known age, although there are limitations to both approaches. Radiocarbon (14 C) analysis has previously been used in only a handful of studies to determine lichen growth rates of two species from a small area of North America. These studies have produced mixed results; a small amount of carbon turnover appears to occur in one of the species ( Caloplaca spp.) previously investigated introducing uncertainty in the growth rate, while much higher carbon cycling occurred in another ( Rhizocarpon geographicum ), making the 14 C approach unsuitable for estimating growth rates in the species most commonly used in lichenometric dating. We investigated the use of bomb-14 C analysis to determine the growth rate of a different crustose species ( Pertusaria pseudocorallina ) common to Northern Europe. 14 C-based growth rates were considerably higher than growth rates of morphologically similar species based on direct measurement made at locations nearby and elsewhere in the UK. This observation strongly suggests that a degree of carbon turnover probably occurs in Pertusaria pseudocorallina , and that bomb-14 C analysis alone cannot be used to determine lichen age or absolute growth rates in this lichen species. 相似文献
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BY GREGORY C. WILES DAVID J. BARCLAY NICOLÁS E. YOUNG 《Geografiska Annaler: Series A, Physical Geography》2010,92(1):101-109
In Alaska, lichenometry continues to be an important technique for dating late Holocene moraines. Research completed during the 1970s through the early 1990s developed lichen dating curves for five regions in the Arctic and subarctic mountain ranges beyond altitudinal and latitudinal treelines. Although these dating curves are still in use across Alaska, little progress has been made in the past decade in updating or extending them or in developing new curves. Comparison of results from recent moraine-dating studies based on these five lichen dating curves with tree-ring based glacier histories from southern Alaska shows generally good agreement, albeit with greater scatter in the lichen-based ages. Cosmogenic surface-exposure dating of Holocene moraines has the potential to test some of the assumptions of the lichenometric technique and to facilitate the development of a new set of improved lichen dating curves for Alaska. 相似文献
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Tom Bradwell 《Geografiska Annaler: Series A, Physical Geography》2004,86(1):31-41
Abstract The age of recent deposits can be determined using an intrinsic characteristic of the lichen ‘population’ growing on their surface. This paper presents a calibrated dating curve based on the gradient of the size‐frequency distribution of yellow‐green Rhizocarpon lichens. The dating potential of this new curve is tested on surfaces of known age in southeast Iceland. This particular size—frequency technique is also compared with the more traditional largest‐lichen approach. The results are very encouraging and suggest that the gradient can be used as an age indicator, at least on deposits formed within the last c. 150 years – and probably within the last c. 400 years – in the maritime subpolar climate of southeast Iceland. Using both lichenometric techniques, revised dates for moraines on two glacier forelands are presented which shed new light on the exact timing of the Little Ice Age glacier maximum in Iceland. 相似文献
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This paper presents a critical review of previous lichenometric and lichen growth studies in southern parts of West and East Greenland. These studies include classic work from around Søndre Strømfjord, Sukkertoppen, Sermilik and Angmagssalik. Particular emphasis is placed on those studies examining the role played by climatic continentality on the growth rate of crustose lichens in Greenland. The latter part of the paper presents new data on lichen growth rates from Sermilik, between 2001 and 2006, in 22 different lichen species. Measurements show that different species grow at different rates and growth rates vary from site to site. In this study Rhizocarpon geographicum generally grows slowly (<0.2 mm a−1 ) while other species such as Pseudephebe minuscula grow more rapidly (1.0 mm a−1 ) in the same environment. Comparison with other studies shows that taxa-specific growth rates are slightly greater in West than in East Greenland – probably as a result of the slightly more favourable climate and higher precipitation levels. It is suggested that recent climate change, most marked in southern Greenland, will probably result in changed growth curves (over time) for species such as Rhizocarpon geographicum . However, only more precise growth curves and lichenometric dating curves can demonstrate such a phenomenon. 相似文献
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BY HAZEL E. TRENBIRTH JOHN A. MATTHEWS 《Geografiska Annaler: Series A, Physical Geography》2010,92(1):19-39
A unique 25-year lichen growth monitoring programme involving 2,795 individuals of the Rhizocarpon subgenus at 47 sites on 18 glacier forelands in the Jotunheimen–Jostedalsbreen regions of southern Norway is reported. The data are used to address fundamental questions relating to direct lichenometry: spatial and temporal variability in lichen growth rates, climatic effects on lichen growth rates, lichen growth models, and implications for lichenometric dating. Mean annual (diametral) growth rate ranged from 0.43 to 0.87 mm yr−1 between sites, which is attributed primarily to local habitat differences. Interannual variability in annual mean growth rate exceeded 1.0 mm yr−1 at some sites. Annual mean growth rates for all sites combined varied from 0.52 to 0.81 mm yr−1 and was positively correlated with annual mean temperature and winter mean temperature (both r = 0.64, p <0.005) but not with summer seasonal temperature: positive correlations with annual and winter precipitation were less strong and the correlation with summer precipitation was marginally significant (r = 0.41 p <0.10). Growth-rate models characterized by annual growth rates that remain approximately constant or increase with lichen size up to at least 120 mm tended to fit the data more closely than a parabolic model. This is tentatively attributed to a long 'linear/mature' phase in the growth cycle. Comparison with growth rates inferred from indirect lichenometry suggest that such high measured growth rates could not have been maintained over the last few centuries by the largest lichens used in southern Norway for lichenometric dating. Several hypotheses, such as the effects of competition and climate change, which might explain this paradox, are discussed. 相似文献
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Michael G. Loso Daniel F. Doak Robert S. Anderson 《Geografiska Annaler: Series A, Physical Geography》2014,96(1):21-41
Contemporary variants of the lichenometric dating technique depend upon statistical correlations between surface age and maximum lichen sizes, rather than an understanding of lichen biology. To date three terminal moraines of an Alaskan glacier, we used a new lichenometric technique in which surfaces are dated by comparing lichen population distributions with the predictions of ecological demography models with explicit rules for the biological processes that govern lichen populations: colonization, growth, and survival. These rules were inferred from size–frequency distributions of lichens on calibration surfaces, but could be taken directly from biological studies. Working with two lichen taxa, we used multinomial‐based likelihood functions to compare model predictions with measured lichen populations, using only the thalli in the largest 25% of the size distribution. Joint likelihoods that combine the results of both species estimated moraine ages of ad 1938, 1917, and 1816. Ages predicted by Rhizocarpon alone were older than those of P. pubescens. Predicted ages are geologically plausible, and reveal glacier terminus retreat after a Little Ice Age maximum advance around ad 1816, with accelerated retreat starting in the early to mid twentieth century. Importantly, our technique permits calculation of prediction and model uncertainty. We attribute large confidence intervals for some dates to the use of the biologically variable Rhizocarpon subgenus, small sample sizes, and high inferred lichen mortality. We also suggest the need for improvement in demographic models. A primary advantage of our technique is that a process‐based approach to lichenometry will allow direct incorporation of ongoing advances in lichen biology. 相似文献
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A New Lichenometric Dating Curve For Southeast Iceland. 总被引：2，自引：0，他引：2
Tom Bradwell 《Geografiska Annaler: Series A, Physical Geography》2001,83(3):91-101
This paper presents a new lichenometric dating curve for southeast Iceland. The temporal framework for the curve is based on reliably dated surfaces covering the last 270 years, making it the best constrained study of this nature conducted in Iceland. The growth of lichen species within Rhizocarpon Section Rhizocarpon is non-linear over time, with larger (older) thalli apparently growing more slowly. The linear 'growth' curves derived previously by former authors working in Iceland represent only part of a curve which has an overall exponential form. Reasons for the non-linearity of the new dating curve are probably physiological, although climatic change over the last three centuries cannot be ruled out. Use of linear 'growth' curves in Iceland is problematic over time-spans of more than c . 80 years. Pre-20th century moraines dated using a constant, linear relationship between lichen size and age are probably older than previously believed. Those moraines lichenometrically 'dated' to the second half of the 19th century in Iceland may actually pre-date this time by several decades (30–100 years), thus throwing doubt on the exact timing of maximum glaciation during the 'Little Ice Age'. 相似文献
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BY TOM BRADWELL 《Geografiska Annaler: Series A, Physical Geography》2009,91(2):61-69
This commentary article discusses the relative merits of new mathematical approaches to lichenometry. It highlights their strong reliance on complex statistics; their user unfriendliness; and their occasional mistreatment of existing lichenometric techniques. The article proposes that the success of lichenometric dating over the past 50 years has stemmed from its relative simplicity, transparency, and general field applicability. It concludes that any new techniques which ignore these principles are likely to be unjustified, unsuitable to the user community and inappropriate for the subject matter. Furthermore, the article raises a more general philosophical question: can statistical complexity and high precision in a ‘geobotanical’ dating technique, fraught with high degrees of environmental variability and inbuilt uncertainty, ever be scientifically valid? 相似文献
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This paper highlights the importance for dating accuracy of initial studies of delay before colonization for both trees and lichens and tree age below core height, particularly in recently deglaciated terrain where colonization and growth rates may vary widely due to differences in micro-environment. It demonstrates, for the first time, how dendrochronology and lichenometry can be used together in an assessment of each other's colonization and growth rates, and then cross-correlated to provide a supportive dating framework. The method described for estimating tree age below core height is also new. The results show that on the east side of the North Patagonian Icefield in the Arco and Colonia valleys, Nothofagus age below a core height of 112 cm can vary from 5 to 41 years and delay before colonization may range from a maximum of 22 years near water to a minimum of 93 years on the exposed flanks of the Arenales and Colonia Glaciers. Tree age plus colonization delay supplied a maximum growth rate of 4.7 mm/year for the lichen Placopsis perrugosa and lichen colonization is estimated to take from 2.5 to approximately 13 years. A minimum lichenometric date of 1883 was estimated for an ice-formed trimline at the junction of the Arenales and Colonia glaciers and a maximum dendrochronological date of 1881 for a water-formed trimline in the Arco valley. Tree and lichen ages around the valley suggest that a glacial outburst drained the 1881 high level lake releasing approximately 265 million cubic metres of water. Repeated flooding, with a minimum of 38 high lake levels, is suggested by horizontal sediment lines on the Arco valley walls and moraine flanks. Dating confirmed diminishing flood levels with a last minor flood in 1963. The wider significance of the work is that it should produce more accurate dating of recent glacier fluctuations around the North Patagonia Icefield, an area where dated reference surfaces are extremely scarce. 相似文献