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Records of past vegetation and fire history can be complicated by changes in the depositional environment of a sampling location. However, these changes can alternatively be used as a measure of climate variability. Our study site, ca. 18.0 cal. ka BP record from Little Brooklyn Lake, Wyoming, located near the crest of the Snowy Range, records three moisture states. Initially, the lake was likely a glacier‐fed pond indicated by the presence of Pediastrum algae colonies. Around 13.0 cal. ka BP this pond transitioned to a meadow environment, suggested by the loss of Pediastrum algae colonies and slow sedimentation rates. Meadow conditions were maintained until ca. 5.0 cal. ka BP when Pediastrum algae colony abundance increased,indicating the formation of a shallow lake. From 18.0 to ca. 5.0 cal. ka BP, the pollen record is suggestive of alpine vegetation conditions with relatively high spruce and herbaceous taxa. Low charcoal influx also characterizes the period between 18.0 and 5.0 cal. ka BP. After 5.0 cal. ka BP, the coincidence of the formation of shallow lake and pollen data, indicating a shift to a spruce and fir forest, suggests an increase in effective moisture. Fire remained rare in this basin over the entire record, however, once the lake established sedimentation rates and charcoal influx increased. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
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AbstractThe transition from the Alpine tectonic assembly to the exhumation of the units in the Rhodope metamorphic province in northernmost Greece has been refined by 40Ar/39Ar laserprobe mica analyses. Preservation of pre-Alpine (~ 280 Ma and 145 Ma) muscovite cooling ages at the western margin of the Rhodope indicate that subsequent events failed to reset the argon system thermally in white mica in the outcropping basement of this region. The central and eastern Rhodope are characterized by white mica cooling ages of 40–35 Ma with ages gradually decreasing to ca. 15 Ma near the eastern margin of the Strymon Valley. The Eo-Oligocene ages reflect the regional exhumation of the metamorphosed units to shallow crustal levels, with corresponding temperatures below ca. 350 °C, by 40–35 Ma. The younger cooling ages are attributed to the initiation and subsequent operation of the Strymon-Thasos detachment system since ca. 30 Ma. This study provides a crucial contribution to future regional tectonic models for the Rhodope region as it recognizes an early stage of development of the Strymon-Thasos detachment system, and has constrained the regional exhumation of the Rhodope metamorphic province since 40 Ma indicating that the regionally observed amphibolite facies metamorphism had terminated by this time. © 2000 Editions scientifiques et médicales Elsevier SAS 相似文献
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John Davison Simon Y.W. Ho Sarah C. Bray Marju Korsten Egle Tammeleht Maris Hindrikson Kjartan Østbye Eivind Østbye Stein-Erik Lauritzen Jeremy Austin Alan Cooper Urmas Saarma 《Quaternary Science Reviews》2011,30(3-4):418-430
This review provides an up-to-date synthesis of the matrilineal phylogeography of a uniquely well-studied Holarctic mammal, the brown bear. We extend current knowledge by presenting a DNA sequence derived from one of the earliest known fossils of a polar bear (dated to 115 000 years before present), a species that shares a paraphyletic mitochondrial association with brown bears. A molecular clock analysis of 140 mitochondrial DNA sequences, including our new polar bear sequence, provides novel insights into the times of origin for different brown bear clades. We propose a number of regional biogeographic scenarios based on genetic data, divergence time estimates and paleontological records. The case of the brown bear provides an example for researchers working with less well-studied taxa: it shows clearly that phylogeographic models based on patterns of modern genetic variation alone can be substantially improved by including data on historical patterns of genetic diversity in the form of ancient DNA sequences derived from accurately dated samples and by using an approach to divergence-time estimation that suits the data under analysis. Using such approaches it has been possible to (i) establish that the processes shaping modern genetic diversity in brown bears acted recently, within the last three glacial cycles; (ii) distinguish among hypotheses concerning species’ responses to climatic oscillations in accordance with the lack of phylogeographic structure that existed in brown bears prior to the last glacial maximum (LGM); (iii) reassess theories linking monophyletic brown bear populations to particular LGM refuge areas; and (iv) identify vicariance events and track analogous patterns of migration by brown bears out of Eurasia to North America and Japan. 相似文献
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R. Roy D. Cassard P.R. Cobbold E.A. Rossello M. Billa L. Bailly A.L.W. Lips 《Ore Geology Reviews》2006,29(3-4):260-286
Geographic Information Systems (GISs) are very useful tools for managing, checking, and organizing spatial information–from many sources and of many types–in thematic layers. Processing of these data enables exploration-oriented GISs to produce potential and predictive maps for a given commodity, which constitute documents of real use in decision-making. Integration of all information in a single reference system enables a better understanding of the parameters controlling a region's metallogeny, in terms of both time and space. But what scale should be used for developing a mineral exploration GIS? Should preference be given to systems with high spatial resolutions (scale < 1:500,000), or to more general systems with scales of around 1:1,500,000 or 1:2,000,000? Will the gain be worthwhile relative to the additional work generated by compilation at a higher scale? In order to make greater use of previous predictive studies performed on gold-rich epithermal and porphyry systems at the scale of the entire Andes, an expert-guided data-driven approach is now applied to a regional-scale GIS of NW Argentina, between the Puna and the Sierrras Pampeanas, where known deposits like Bajo de la Alumbrera, Agua Rica, and others, account for a metal potential of over 10 Mt Cu and 750 t Au. In developing this new predictive map, three criteria that were likely to be connected to the mineralizing event were selected and quantified: (i) lithostratigraphy, because of its role as a favourable environment for the development of mineralization, based on its physico-chemical properties; (ii) lithostratigraphic contacts, based on the rheological properties of the formations in contact; and (iii) the orientation of structural discontinuities, which channel source magmas and encourage the circulation of hydrothermal fluids. Assigning a score enables classification of the favourabilities calculated for each of the criteria considered. This approach is employed here to check and standardize the statistical results obtained by methods such as Weight of Evidence Modelling or an algebraic approach. For each criterion, four classes were distinguished: very favourable: score = 3; favourable: score = 2; slightly favourable: score = 1; and unfavourable: score = 0. The predictive map is obtained by adding the scores for the three favourable criteria defined above.The regional-scale work identified 20 anomalous envelopes with cumulative scores greater than 5. They correspond to mining areas that are active (e.g., Bajo de la Alumbrera), under development (e.g., Agua Rica), or abandoned (e.g., La Mejicana), or to new areas (e.g., the Vicuña Pampa Volcanic Complex). Structural analysis of the region, integrating the orientation of the favourable envelopes, suggests that the mineralizing fluids were emplaced under extensional conditions, sub-parallel to the principal directions of shortening: (i) WNW–ESE, found along the southern edge of the Puna; and (ii) E–W, seen in the Sierra de Famatina. It appears that a regional-scale information system is a tool that is well suited to the definition of areas for mineral prospecting, and to the study and confirmation of metallotects usable for mineral exploration.Comparison with work conducted on the basis of a 1:2,000,000 geological compilation shows that the principal mining districts can indeed be found at continental scale. On the other hand, the lack of detail inherent at a scale of 1:2,000,000 may lead to inaccuracies, in particular fictitious favourabilities assigned to formations that are genetically unrelated to the mineralization, but that contain, for example, small Tertiary intrusive bodies that cannot be recorded at this scale. This comparison therefore shows that the use of a continental-scale GIS is effective, and well suited to the definition of prospective areas at a strategic level. 相似文献
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Benjamin J.C. Laabs David W. Marchetti Jeffrey S. Munroe Kurt A. Refsnider John C. Gosse Elliott W. Lips Richard A. Becker David M. Mickelson Brad S. Singer 《Quaternary Research》2011,76(2):272-284
Understanding the timing of mountain glacier and paleolake expansion and retraction in the Great Basin region of the western United States has important implications for regional-scale climate change during the last Pleistocene glaciation. The relative timing of mountain glacier maxima and the well-studied Lake Bonneville highstand has been unclear, however, owing to poor chronological limits on glacial deposits. Here, this problem is addressed by applying terrestrial cosmogenic 10Be exposure dating to a classic set of terminal moraines in Little Cottonwood and American Fork Canyons in the western Wasatch Mountains. The exposure ages indicate that the main phase of deglaciation began at 15.7 ± 1.3 ka in both canyons. This update to the glacial chronology of the western Wasatch Mountains can be reconciled with previous stratigraphic observations of glacial and paleolake deposits in this area, and indicates that the start of deglaciation occurred during or at the end of the Lake Bonneville hydrologic maximum. The glacial chronology reported here is consistent with the growing body of data suggesting that mountain glaciers in the western U.S. began retreating as many as 4 ka after the start of northern hemisphere deglaciation (at ca. 19 ka). 相似文献
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Thomas Van Der Hammen Ligia E. Urrego Nohora Espejo Joost F. Duivenvoorden Johanna M. Lips 《第四纪科学杂志》1992,7(1):57-67
The sequence of Late-glacial and Holocene alluvial sedimentation in the middle Caquetá River Basin of Colombian Amazonia is described, based on the study of the sediments and palynology of several river bank sections and on 30 radiocarbon dates. An early Late-glacial sedimentation cycle is recognised, followed by a minor late Late-glacial erosion phase. The Holocene valley fill consists of grey clays (often present in the lower part of the sections) deposited in open water and silty clays often with faint yellow mottling, deposited under a regime of seasonal flooding. The base of the Holocene sections is formed by sands, where exposed. In two places the transition of sand to open-water grey clay was dated around 10 000 yr BP and there is a suggestion that open water may have been more common at the beginning of the Holocene than later, when sedimentation by seasonal flooding became important. In many places much of the earlier Holocene sediments may have been removed by erosion and replaced by younger sediments, by a process of lateral aggradation. A considerable part of the present valley fill is younger than ca. 3500 yr. However, in several places older Holocene sediments are found, apparently only little affected by later erosion, lying below younger varzea silty clays. During the Holocene more organic sediments were formed in periods with reduced river discharge, related to drier climates in the Andes and possibly in Amazonia. These dry periods, deduced from data in the Caquetá River area, correspond well with dry phases in other parts of northwestern South America (e.g. between approximately 2700-1900 yr BP and approximately 3200-3800 yr BP). Rates of average net sedimentation, calculated from dated sections that apparently lack major hiatuses caused by erosion, were high in the lower Holocene, low during the middle Holocene and increase again in the upper Holocene. Levee deposits became coarser and the high river level of the Caquetá increased during the late Holocene. These phenomena may be explained by the increasing influence of man on the vegetation cover in the Andean headwater areas and possibly also in the Amazonian catchment area of the Caquetá River. 相似文献
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Thomas Van Der Hammen Joost F. Duivenvoorden Johanna M. Lips Ligia E. Urrego Nohora Espejo 《第四纪科学杂志》1992,7(1):45-55
The late Quaternary history of the middle Caquetá River area in Colombia, northwestern Amazonia is described, based on observations of river bank sections, radiocarbon dates and palynological analyses of organic layers in floodplain and low terrace sediments of the Caquetá River. It is shown that the Late Pleistocene and Holocene climatic changes that took place in the Andean Cordilleras, were related to the depositional and erosional history of the Caquetá River in the Colombian Amazonian lowlands. The low terrace sediments consist of sandy and gravelly deposits covered by clays that sometimes contain lenses of peaty material. From these organic low terrace sediments, seven finite radiocarbon dates were obtained of Middle Pleniglacial age, between 56 000 and 30 000 yr BP. The coarse textured basal deposits of the low terrace apparently stem from the early part of the Middle Pleniglaciai period, during which the effective rainfall in the Andes was relatively high and the Andean glaciers had a considerable extension. Palynological data from silty sediments with organic remains at one site, show an interval when drier and more open types of vegetation on poor soils must have covered a larger area than today, but Amazonian forest was still the dominating type of vegetation. This interval might correspond to one of the Middle Pleniglacial savanna intervals from eastern Amazonia (Carajas). No organic sediments from the Upper Pleniglacial period were found and hence radiocarbon dates were not obtained. In the Andes this period had a very cold climate with low effective rainfall and in the east Amazonian Carajas area it is characterised by the relative extension of open savanna vegetation. The river run-off and sediment transport must have been much lower than in the Middle Pleniglacial and the Caquetá River cut itself down in its own sediments. Two Late-glacial radiocarbon datings obtained at one site (ca. 12 500 yr BP) indicate the existence of a Late-glacial sedimentation phase, separated from the Holocene sequence by a minor erosional phase. Organic layers in the Holocene floodplain sediments yielded 28 radiocarbon dates between 10 000 and 355 yr BP. Holocene sedimentation started with the rapid deposition of (sandy) clay possibly in a partly permanently inundated Caquetá valley. During the major part of the Holocene (silty) clays were deposited, with a dominant seasonal inundation cycle. 相似文献