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1.
A New Lichenometric Dating Curve For Southeast Iceland.   总被引：2，自引：0，他引：2  
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   

2.
This article presents a summary of our studies of Holocene moraines and glaciers of the Tien-Shan, Pamir, and Himalaya mountains with the purpose of providing pattern regularity of the Holocene glaciation decomposition. We developed a method for obtaining reliable radiocarbon dating of moraines with the use of autochthonous organic matter dispersed in fine-grained morainic material, as well there were shown new possibilities of isotope-oxygen and isotope-uranium analysis for the Holocene glaciations dynamics. We found that Holocene glaciations disintegrate stadially according to the decaying principle, and seven main stages may be distinguished. We achieved the absolute dating of the first three stages, identifying these periods as 8,000, 5,000, and 3,400 years ago. The application of the above-mentioned isotope methods of the Holocene glaciations and moraines study will allow researchers to improve the offered model of the Holocene glaciations disintegration; it will be great contribution to salvation of the problem of long-term climatic and glaciations forecast.  相似文献   

3.
Two lichenometric techniques were compared in a study of lichen growth–rate in northern Sweden. The first technique, based on the maximum lichen diameter on glacier moraines, was identical to the technique used in the 1970s, whereas the other utilized the lichen diameter measured on 100 randomly selected boulders. The results indicate that it does not matter which technique is chosen, as long as the technique is used consistently on both the calibration surfaces and the surfaces to be dated. The use of data from both the 1970s and the 2000s increased the number of calibration surfaces available. The new calibration curve indicates that the age of Little Ice Age moraines was underestimated by up to about 30 years in the study conducted in the 1970s.  相似文献   

4.
Two lichenometric techniques were compared in a study of lichen growth–rate in northern Sweden. The first technique, based on the maximum lichen diameter on glacier moraines, was identical to the technique used in the 1970s, whereas the other utilized the lichen diameter measured on 100 randomly selected boulders. The results indicate that it does not matter which technique is chosen, as long as the technique is used consistently on both the calibration surfaces and the surfaces to be dated. The use of data from both the 1970s and the 2000s increased the number of calibration surfaces available. The new calibration curve indicates that the age of Little Ice Age moraines was underestimated by up to about 30 years in the study conducted in the 1970s.  相似文献   

5.
Glaciers in small mountain cirques on South Georgia respond rapidly and sensitively to changes in South Atlantic climate. The timing and rate of their deglaciation can be used to examine the impact that nineteenth- and twentieth-century climate change has had on the glacial dynamics and terrestrial ecosystems of South Georgia. As part of a reconnaissance study in Prince Olav Harbour (POH), South Georgia, we measured the size of lichens ( Rhizocarpon Ram. em Th. Fr. subgenus. Rhizocarpon group) on ice-free moraine ridges around two small mountain cirques. Our aims were twofold: first, to provide age estimates for lichen colonization, and hence, deglaciation of the moraine ridges, and second, to examine the potential of applying lichenometry more widely to provide deglacial age constraints on South Georgia. In the absence of lichen age-size (dating) curves for South Georgia, we use long-term Rhizocarpon lichen growth-rates from recent studies on sub-Antarctic Islands and the western Antarctic Peninsula to calculate likely age estimates. These data suggest ice retreat from the two outermost moraines occurred between the end of the 'Little Ice Age' (post c. 1870) and the early twentieth century on South Georgia. Lichen colonization of the innermost moraines is probably related to glacier retreat during the second half of the twentieth century, which has been linked to a well-defined warming trend since c. 1950. Patterns of possible nineteenth- and twentieth-century glacial retreat identified in POH need to be tested further by establishing species- and site-specific lichen age-size (dating) curves for South Georgia, and by applying lichenometry to other mountain cirques across South Georgia.  相似文献   

6.
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   

7.
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   

8.
One of the major goals of geomorphology is to understand the rate of landscape evolution and the constraints that erosion sets on the longevity of land surfaces. The latter has also turned out to be vital in modern applications of cosmogenic exposure dating and interpretation of lichenometric data from unconsolidated landforms. Because the effects of landform degradation have not been well documented, disagreements exist among researchers regarding the importance of degradation processes in the dating techniques applied to exposures. Here, we show that all existing qualitative data and quantitative markers of landform degradation collectively suggest considerable lowering of the surface of unconsolidated landforms over the typical life span of Quaternary moraines or fault scarps. Degradation is ubiquitous and considerable even on short time scales of hundreds of years on steeply sloping landforms. These conservative analyses are based entirely on field observations of decreasing slope angles of landforms over the typical range of ages in western North America and widely accepted modeling of landscape degradation. We found that the maximum depth of erosion on fault scarps and moraines is on average 34% of the initial height of the scarp and 25% of the final height of the moraine. Although our observations are limited to fault scarps and moraines, the results apply to any sloping unconsolidated landform in the western North America. These results invalidate the prevailing assumption of no or little surface lowering on sloping unconsolidated landforms over the Quaternary Period and affirm that accurate interpretations of lichen ages and cosmogenically dated boulder ages require keen understanding of the ever-present erosion. In our view, the most important results are twofold: 1) to show with a large data set that degradation affects universally all sloping unconsolidated landforms, and 2) to unambiguously show that even conservative estimates of the total lowering of the surface operate at time and depth scales that significantly interfere with cosmogenic exposure and lichen dating.  相似文献   

9.
Moraine ridges are commonly used to identify past glacier ice margins and so infer glacier mass balance changes in response to climatic variability. However, differences in the form of past ice margins and post-depositional modification of moraine surfaces can complicate these geomorphic records. As a result, simple relationships, such as distance from current ice margin, or linear alignments, may not necessarily indicate moraines deposited contemporaneously. These disturbances can also modify the size distribution of lichen populations, providing a distinctive signature for surfaces with similar histories and a means of identifying contemporaneous moraine surfaces. In this paper, statistical analysis of lichen size distributions is used to identify moraine surfaces with similar histories from complex suites of Little Ice Age moraine fragments in the proglacial areas of Skálafellsjökull (including Sultartungnajökull) and Heinabergsjökull, southeast Iceland. The analysis is based on a novel use of the goodness-of-fit statistic, Watson's U2 which provides a measure of 'closeness' between two sample distributions. Moraine fragments with similar histories are identified using cluster analysis of the U2 closeness values. The spatial pattern of the clustered moraines suggests three distinct phases of moraine deposition at Skálafellsjökull and Heinabergsjökull, four phases at Sultartungnajökull and a digitate planform margin at Heinabergsjökull. These spatial patterns are corroborated with tephrochronology. The success of the U2 statistical analysis in identifying surfaces with similar histories using lichen size distributions suggests that the technique may be useful in augmenting lichenometric surface dating as well as differentiating between other surfaces that support lichen populations, such as rock avalanche deposits.  相似文献   

10.
The recently observed recession of glaciers on King George Island is associated with decades of climate warming in the Antarctic Peninsula region. However, with only 60 years of glaciological observations in the study area ages of the oldest moraines are still uncertain. The goal of the study was to estimate ages of lichen colonization on the oldest moraines of the Ecology and White Eagle Glaciers on King George Island and on the Principal Cone of Penguin Island volcano. The first lichenometric studies on these islands from the late 1970s used rates that had about four to five times slower Rhizocarpon growth rates. We re‐examined the sites and measured 996 thalli diameters to establish the surface ages. To estimate the age we used (1) long‐term Rhizocarpon lichen group growth rates established by authors using data from a previous lichenometric study on King George Island, and (2) previous data of lichen growth rates from other sub‐Antarctic islands. Our results suggest growth rates between 0.5 and 0.8 mm yr–1. According to these rates the ages of the oldest moraine ridges are of the Little Ice Age and were colonized at the beginning of the twentieth century. The mid‐twentieth century age of lichen colonization on the historically active Penguin Island volcano might support the date of the last eruption reported by whalers in the end of the nineteenth and the beginning of the twentieth century.  相似文献   

11.
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   

12.
The endolithic lichen Lecidea auriculata is known to enhance rock surface weathering on the Little Ice Age moraines of the glacier Storbreen in Jotunheimen, central southern Norway. This study demonstrates the reduction in Schmidt hammer Rvalues that followed the rapid colonization by this lichen of pyroxene‐granulite boulders on terrain deglaciated over the last 88 years. In the absence of this lichen, the characteristic mean R‐value of boulder surfaces is 61.0 ± 0.3; where this lichen is present, R‐values are lower by at least 20 units on surfaces exposed for 30–40 years. A similar reduction in rock hardness on rock surfaces without a lichen cover requires about 10 ka. The rapid initial weakening of the rock surfaces is indicative of rates of biological weathering by endolithic lichens that may be two orders of magnitude (200–300 times) faster than rates of physico‐chemical weathering alone. If not avoided, the effects of this type of lichen are likely to negate the effectiveness of the Schmidt hammer and other methods for exposure‐age dating, including cosmogenic‐nuclide dating, in severe alpine and polar periglacial environments. The results also suggest a new method for dating rock surfaces exposed for <50 years.  相似文献   

13.
Radiocarbon measurements on fossilized remnants of the aeolian biome, incorporated in glacial moraines in the Sierra Nevada of California, are consistent with the relative order of moraines at Bishop Creek, on the eastern slopes of the Sierra Nevada. Holocene 14C ages correspond with periods of more effective moisture according to paleoecological and treeline data in the range, whereas Pleistocene ages are penecontemporaneous with Heinrich Events in the North Atlantic. These findings must be interpreted with caution, however, owing to a number of uncertainties, including: evidence that younger contaminants are added to till matrix; the possibility of older contaminants; the effects of pretreatment on aeolian biome remains; and processes by which organics undergo diagenesis within till matrices in different biogeochemical settings. [Key words: aeolian biome, Heinrich Events, moraines, radiocarbon dating, Sierra Nevada.]  相似文献   

14.
Alpine lakes near Mount Rainier are dammed by late-glacial moraines. Multiple cores show that sedimentation started less than 350 years after deglaciation. Numerous tephra layers provide excellent marker horizons that allow comparison of the stratigraphy without the errors associated with radiocarbon dating. Cores from different locations within a lake display the same stratigraphy and basal sediments. Core location therefore is not crucial to obtaining the oldest sediments. Continuous sedimentation in all cores is interpreted as evidence for stable lake levels throughout the Holocene. The cores are valuable tools for reconstruction and dating of glacier advances and volcanic activity.  相似文献   

15.
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 (14C) 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 14C approach unsuitable for estimating growth rates in the species most commonly used in lichenometric dating. We investigated the use of bomb-14C analysis to determine the growth rate of a different crustose species ( Pertusaria pseudocorallina ) common to Northern Europe. 14C-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-14C analysis alone cannot be used to determine lichen age or absolute growth rates in this lichen species.  相似文献   

16.
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   

17.
Because of its potential for establishing chronologies far beyond the range of C14 thermoluminescence (TL) dating has made a significant contribution to the study of the Quaternary history of many Australian landscapes. But, as the reliability of the technique requires the removal by sunlight of any residual TL from quartz grains during transport, inadequate bleaching may yield ages for depositional events that are too old. Inadequate bleaching often can be detected by the shape of curves showing the ratio of natural TL vs laboratory-induced TL with increasing temperature. We use this technique here, together with C 14 dating and pedogenic evidence, to assess the reliability of TL determinations for alluvium in the valleys of the Clyde River and Termeil Creek. The Pleistocene TL ages from these valleys seem reliable, but Holocene dates do not. However, we demonstrate that, even where inadequate bleaching is demonstrable, TL analysis can still yield important insight to depositional processes.  相似文献   

18.
新疆阿尔泰山东段冰碛物光释光测年研究   总被引：3，自引：0，他引：3  
贾彬彬  周亚利  赵军 《地理学报》2018,73(5):957-972
冰川是塑造地表形态最积极的外营力之一,对冰川地貌的年代学研究是重建古冰川发展史的关键,也是研究气候变化的重要途径。冰碛物是冰川作用的直接产物,代表过去发生的冰川事件,对冰碛物进行准确测年能够为重建古冰川的进退、理解区域古气候变化提供年代学支撑。本文在新疆阿尔泰山东段采集了8个冰碛物样品以进行光释光测年,利用单片再生剂量法对90~125 μm的石英颗粒进行等效剂量的测定。通过等效剂量值频率分布特征及De（t）坪区图分析得出大部分冰碛物的光释光信号晒退不彻底,所以利用一阶动力学公式对持续激发的光释光信号晒退曲线（CW-OSL）进行多组分拟合拆分,得到快速、中速、慢速3种组分,依据分离出的快速组分确定等效剂量值。研究结果显示,距今32 ka以来阿尔泰山东段区域在MIS3阶段、MIS2阶段、8.2 ka左右、全新世大暖期及新冰期等5个时段有冰川发育,冰川发育与气候变化密切相关。  相似文献   

19.
Botanists make yearly measurements of lichen sizes that describe highly variable radial expansion of young, and old, Rhizocarpon subgenus Rhizocarpon that is a function of thallus size and age. Such non‐uniform growth would negate use of lichens to date geomorphic events, such as landslides and moraines, of the past 1000 years. Fortunately, many crustose lichens tend toward circular shapes, which can be achieved only when overall uniform radial growth prevails. Largest lichen measurements on rockfall blocks that accumulate incrementally as hillslope talus in earthquake‐prone California plot as distinct peaks in frequency distributions. Rockfall surface‐exposure times are known to the day for historical earthquakes and to the year where mass movements damage trees. Lichenometry consistently dates regionally synchronous rockfall events with an accuracy and precision of ±5 years. Only historical records and tree‐ring dating of earthquakes are better. The four crustose lichens used here have constant long‐term growth rates, ranging from 9.5 to 23.1 mm per century. Growth rates do not vary with altitude or climate in a 900 km long mountainous study region in California, USA. Linear growth regressions, when projected to the present, constrain estimates of colonization time and possible styles of initial lichen growth.  相似文献   

20.
《Geomorphology》1995,14(2):149-156
We have used cosmogenic 36Cl surface exposure dating to determine apparent construction ages of late Pleistocene moraines in the Sierra Nevada, the White Mountains, and the Wind River Range, all in the western United States. The inferred glacial chronologies from the various localities all exhibit certain characteristics: (1) Local records are fragmentary and deposits of some glacial advances are always missing; no location has deposits of all glaciations and no glacial advance is recorded at all locations. This is due either to unfavorable conditions for glacier development at some times or to obliteration of earlier deposits by later, more extensive glaciers. (2) Most moraines have young apparent exposure ages, with only a few older than the last glacial cycle. This may be due to young true ages of these deposits, erosion of moraine surfaces, or obliterative overlap and covering of older deposits by younger ones. (3) Many deposits that were previously correlated (e.g., based on their stratigraphic positions) are not correlative; they may represent different stades and, sometimes, even different glaciations. Similarly, some previously uncorrelated deposits have the same exposure ages and may be correlative. (4) Clusters of several distinct moraines of nearly the same age are present at most locations. These clusters suggest that alpine moraines are formed during short deposition episodes that last between several hundred and several thousand years.  相似文献   

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