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141.
Neil F. Glasser Philip D. Hughes Cassandra Fenton Christoph Schnabel Henrik Rother 《第四纪科学杂志》2012,27(1):97-104
This paper presents results of the analysis of paired cosmogenic isotopes (10Be and 26Al) from eight quartz‐rich samples collected from ice‐moulded bedrock on the Aran ridge, the highest land in the British Isles south of Snowdon. On the Aran ridge, comprising the summits of Aran Fawddwy (905 m a.s.l.) and Aran Benllyn (885 m a.s.l.), 26Al and 10Be ages indicate complete ice coverage and glacial erosion at the global Last Glacial Maximum (LGM). Six samples from the summit ridge above 750–800 m a.s.l. yielded paired 10Be and 26Al ages ranging from 17.2 to 34.4 ka, respectively. Four of these samples are very close in age (10Be ages of 17.5 ± 0.6, 17.5 ± 0.7, 19.7 ± 0.8 and 20.0 ± 0.7 ka) and are interpreted as representing the exposure age of the summit ridge. Two other summit samples are much older (10Be ages of 27.5 ± 1.0 and 33.9 ± 1.2 ka) and these results may indicate nuclide inheritance. The 26Al/10Be ratios for all samples are indistinguishable within one‐sigma uncertainty from the production rate ratio line, indicating that there is no evidence for a complex exposure history. These results indicate that the last Welsh Ice Cap was thick enough to completely cover the Aran ridge and achieve glacial erosion at the LGM. However, between c. 20 and 17 ka ridge summits were exposed as nunataks at a time when glacial erosion at lower elevations (below 750–800 m a.s.l.) was achieved by large outlet glaciers in the valleys surrounding the mountains. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
142.
Organizations involved in the governance of natural resources are challenged to adjust to the call for more holistic management approaches. This often necessitates organizational change. Here change processes in the International Council for the Exploration of the Sea (ICES) during the years 1998–2009 are investigated using semi-structured interviews combined with observations and review of documents. Several organizational reforms were implemented during the time period studied. The major drivers were the need to improve efficiency and a striving for better integration between different components within the organization. The reform processes were driven forward by individuals who navigated between opportunities and constrains embedded in the network structure of ICES. This required good leadership and communication skills. Broad consultations were important to ensure support within the ICES community. By increasing the understanding of the dynamics of change in organizations, which operate at the science–policy interface developments in desired directions can be facilitated. 相似文献
143.
O Eugster N Grögler M.D Mendia P Eberhardt J Geiss 《Geochimica et cosmochimica acta》1973,37(9):1991-2003
He, Ne, Ar, Kr and Xe concentrations and isotopic abundances were measured in three bulk grain size fractions prepared from sample L-16-19, No. 120 (C level, 20–22 cm depth) returned by the Luna 16 mission. The expected anticorrelation between the concentrations of trapped solar wind noble gases and grain size is observed. Elemental abundances of solar wind trapped noble gases are similar to those previously found in corresponding grain size fractions of the Apollo 11 and 12 fines. The trapped ratio varies in the soils from different lunar maria due to diffusion losses. A rough correlation of with the proportion of ilmenite in these samples is apparent. The elemental and isotopic ratios of the surface correlated noble gases in Luna 16 resemble those previously found in Apollo fines. Based on 21Ne, 78Kr and 126Xe a cosmic ray exposure age of 360 my was determined. This age is similar to those obtained for the soils from other lunar maria. 相似文献
144.
Peter Luger M. Gröschke M. Bussmann A. Dina W. Mette A. Uhmann H. Kallenbach 《International Journal of Earth Sciences》1994,83(4):711-727
The Jurassic and Cretaceous sedimentary history of northern Somalia and the Morondava Basin of south-western Madagascar have been studied. Both regions display an independent facial development; however, a comparison of the sequential evolution of the Mesozoic sedimentary successions in these two presently widely separated areas reveals a surprisingly high level of similarity, which probably reflects major events during the disintegration of Eastern Gondwana during the Jurassic and Cretaceous. Although in Jurassic times the onset of transgressions and regressions in both areas compares well with eustatic development, major deviations in combination with the tectonic activities of different degrees are observed in the Early and Late Cretaceous synchronously in both regions. Transgressions are observed in Toarcian, Bajocian (not dated in northern Somalia), Callovian, Valanginian (Madagascar only), Aptian and Campanian times. Tectonism is noted before the Aptian and Campanian transgressions in northern Somalia and the Morondava Basin of south-western Madagascar. 相似文献
145.
D. Gebauer J. Bernard-Griffiths M. Grünenfelder 《Contributions to Mineralogy and Petrology》1981,76(3):292-300
U-Pb data on zircons from the largest mafic-ultramafic body (6×2 km) of the French Central Massif (Sauviat-sur-Vige) yield the following age results: Primary magmatic crystallization of the gabbroic and peridotitic protoliths took place in the Cambro-Ordovician (496±25/17 m.y.). Variable transformation under eclogite facies conditions was Hercynian (320±29/36 m.y.). The same age pattern, derived by U-Pb monazite analyses, was found also for the immediate country rocks, i.e. kyanite bearing, coarse-grained metagranites occurring to the W and N of the Sauviat massif. Due to the fact that there is no regional Hercynian high-grade metamorphism in this part of the French Central Massif (e.g. Duthou 1977; Bernard-Griffiths 1975), both mafic-ultramafic complex as well as immediate felsic country rocks must have been emplaced tectonically into pre-Hercynian (Acadian±Caledonian) crustal rocks. The cause for such a Hercynian tectonism is thought to be due to continent-continent collision of the Spanish with the Armorican plate. Preliminary U-Pb zircon results on one eclogite sample taken about 50 km S of the Sauviat complex indicate also an Early Palaeozoic age for the magmatic protolith and a Hercynian transformation into eclogite, combined with and/or followed by tectonic emplacement. However, opposite to the continental Sauviat massif, we are probably dealing here with oceanic material, possibly deposited in an Early Palaeozoic marginal sea basin. Thus, subduction and tectonic emplacement of oceanic crust into continental crust terminated in this area also in the Hercynian. Concerning U-Pb systematics of zircon and monazite the following conclusions can be drawn:
- U-Pb systems of primary magmatic zircons of mafic and ultramafic rocks are only weakly disturbed during static eclogite facies metamorphism (T>820° C; P> 15 kbar);
- New- and overgrowth of zircon during eclogite facies metamorphism seems to be the major cause for the degree of discordance;
- Amphibolitization of metagabbros and eclogites had no effect on the degree of discordance of zircon;
- U-Pb systems of monazites remained undisturbed during intense weathering of the mother rock.
146.
In an attempt to find the premetamorphic discordance pattern of detrital zircons extracted from Central European metasediments, unmetamorphosed or only slightly metamorphosed sediments were collected from two areas: (1) from the Montagne Noire, the southernmost part of the French Central Massif and (2) from the innerbohemian Algonkian (= Proterozoic) in the CSSR.The generally accepted hypothesis that zircons from Central European metasediments must have plotted close to or at the corresponding upper intercept between discordia trajectory and concordia curve prior to the metamorphism of the host rocks could not be supported. The zircon populations from sediments of both areas are similar in discordance to those of the numerous populations extracted from metasediments of the Central European basement complexes. However, in contrast to the latter, the data points of size fractions scatter considerably and reliable intercept ages cannot be calculated.In the case of the Cambro-Ordovician sand- and siltstones of the Montagne Noire, the ages of detrital muscovites strongly argue for a Cadomian (550–700 m.y.) provenance of the detritus. Thus, the strong discordance of the analyzed fractions most probably is caused by zircons newly formed and/or partly or completely reset during a Cadomian event in the provenance of the detritus. In addition, lattice unit parameters indicate that the detrital zircons must have been recrystallized after their primary formation more than 1.7 b.y. ago.The Algonkian sediments of Bohemia (CSSR) can be taken as the very low-grade metamorphic equivalents of the Moldanubian paragneisses from which discordia trajectories between about 2 b.y. and 460–320 m.y. are known (Gebauer and Grünenfelder, 1974; Grauert et al., 1974). Nevertheless, all analyzed zircon fractions are strongly discordant indicating that they probably recrystallized during the Assyntian (=Cadomian) very low-grade metamorphism of the host rock loosing most of their accumulated radiogenic lead. If such an interpretation is correct, the low-temperature recrystallization model of Gebauer and Grünenfelder (1976) can be applied to metamict zircons in host rocks formed at temperatures as low as 300 ° C. In our 1976-paper we gave temperatures of 350–400 ° C for the maximum temperature necessary to recrystallize metamict zircons in chlorite-grade quartzphyllites in agreement with the experimental results of Pidgeon et al. (1973).In contrast to the zircons of the Montagne Noire it can be shown that the U-Pb systems of the Algonkian zircons must have been re-opened in post-Assyntian time, probably recently or in the Tertiary. However, no plausible explanation can be given to account for this. 相似文献
147.
V. Kárník D. Procházková Z. Schenková L. Ruprechtová A. Dudek J. Drimmel E. Schmedes G. Leydecker J. P. Rothé B. Guterch H. Lewandowska D. Mayer-Rosa D. Cvijanović V. Kuk F. Giorgetti G. Grünthal E. Hurtig 《Pure and Applied Geophysics》1978,116(6):1307-1313
Summary The isoseismal map of the North-Italian earthquake of 6 May 1976 was compiled using the contributions of the European countries lying within the shaken area. The characteristic feature of the macroseismic field is discussed. 相似文献
148.
Henrik Friis 《Sedimentary Geology》1978,21(3):169-188
Heavy-mineral analyses have been performed on marine sandy and silty deposits of Miocene age from the western part of Denmark. The heavy-mineral associations are dominated by epidote and amphibole, but the mutual distribution of these minerals is variable. Studies of surface textures show a large, but variable, proportion of the grains to be corroded. In a few cases the surface textures are indicative of post-depositional processes; however, most of these may have existed prior to deposition. It is suggested that the variations in heavy-mineral distribution and surface textures result from mixing of freshly supplied, unweathered detritus from Fennoscandia, with weathered detritus from the same provenance. This weathering took place during a temporary deposition in fluvial systems between the source area and the North Sea Basin. The relation to transgressive and regressive phases is discussed. 相似文献
149.
A. Stettler P. Eberhardt J. Geiss N. Grögler 《Earth and Planetary Science Letters》1974,23(3):453-461
39Ar-40Ar ages and37Ar-38Ar exposure ages of samples representing four different lithologies of the Apollo 17 station 7 boulder were measured. The age of the dark veinlet material77015of3.98 ± 0.04AE is interpreted as representing the time of intrusion of this veinlet into the 77215 clast. The data obtained so far indicate that the vesicular basalt 77135 formed 100–200 m.y. later. However, this has to be confirmed by39Ar-40Ar investigations on separated mineral and/or grain-size fractions. A small clast enclosed in the 77135 basalt gives a well-defined high temperature age of3.99 ± 0.02AE. A sample of the noritic clast 77215 gave4.04 ± 0.03AE, the highest age found so far in this boulder. The39Ar-40Ar ages obtained are in agreement with the age relationships deduced from the stratigraphic evidence.Taking into account the shielding by the boulder itself, an average37Ar-38Ar exposure age of(27.5 ± 2.5)m.y. is obtained for the samples collected from the boulder. 相似文献
150.
Henrik Friis 《Sedimentary Geology》1974,12(3):199-213
Exemplified by young Tertiary sandy sediments from Denmark, general aspects of the study of heavy-minerals are discussed. It is demonstrated that three different heavy-mineral associations of the sediments have originated from one single association by successive chemical weathering of amphibole and epidote. Physical processes active during deposition may influence the composition within associations, but differences of this origin are shown to be insufficient to explain the differences between the associations. It has not been possible to demonstrate major influence on the heavy-mineral associations from differences in the composition within the source area, which is assumed to be part of the Scandinavian Shield. 相似文献