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191.
Kerstin?DrostEmail author Ulf?Linnemann Neal?McNaughton Old?ich?Fatka Petr?Kraft Michael?Gehmlich Christian?Tonk Jaroslav?Marek 《International Journal of Earth Sciences》2004,93(5):742-757
The Teplá-Barrandian unit (TBU) of the Bohemian Massif was a part of the Avalonian-Cadomian belt at the northern margin of Gondwana during Neoproterozoic and Early Cambrian times. New detrital zircon ages and geochemical compositions of Late Neoproterozoic siliciclastic sediments confirm a deposition of the volcano-sedimentary successions of the TBU in a back-arc basin. A change in the geotectonic regime from convergence to transtension was completed by the time of the Precambrian-Cambrian boundary. The accumulation of around 2,500 m Lower Cambrian continental siliciclastics in a Basin-and-Range-type setting was accompanied by magmatism, which shows within-plate features in a few cases, but is predominantly derived from anatectic melts displaying the inherited island arc signature of their Cadomian source rocks. The geochemistry of clastic sediments suggests a deposition in a rift or strike-slip-related basin, respectively. A marine transgression during Middle Cambrian times indicates markedly thinned crust after the Cadomian orogeny. Upper Cambrian magmatism is represented by 1,500 m of subaerial andesites and rhyolites demonstrating several geochemical characteristics of an intra-plate setting. Zircons from a rhyolite give a U-Pb-SHRIMP age of 499±4 Ma. The Cambrian sedimentary and magmatic succession of the TBU records the beginning of an important rifting event at the northern margin of Gondwana.
相似文献
Kerstin DrostEmail: |
192.
Ulf?LinnemannEmail author Neal?J.?McNaughton Rolf?L.?Romer Michael?Gehmlich Kerstin?Drost Christian?Tonk 《International Journal of Earth Sciences》2004,93(5):683-705
Neoproterozoic rocks in the Saxo-Thuringian part of Armorica formed in an active margin setting and were overprinted during Cadomian orogenic processes at the northern margin of Gondwana. The Early Palaeozoic overstep sequence in Saxo-Thuringia was deposited in a Cambro-Ordovician rift setting that reflects the separation of Avalonia and other terranes from the Gondwana mainland. Upper Ordovician and Silurian to Early Carboniferous shelf sediments of Saxo-Thuringia were deposited at the southern passive margin of the Rheic Ocean. SHRIMP U/Pb geochronology on detrital and inherited zircon grains from pre-Variscan basement rocks of the northern part of the Bohemian Massif (Saxo-Thuringia, Germany) demonstrates a distinct West African provenance for sediments and magmatic rocks in this part of peri-Gondwana. Nd-isotope data of Late Neoproterozoic to Early Carboniferous sedimentary rocks show no change in sediment provenance from the Neoproterozoic to the Lower Carboniferous, which implies that Saxo-Thuringia did not leave its West African source before the Variscan Orogeny leading to the Lower Carboniferous configuration of Pangea. Hence, large parts of the pre-Variscan basement of Western and Central Europe often referred to as Armorica or Armorican Terrane Assemblage may have remained with Africa in pre-Pangean time, which makes Armorica a remnant of a Greater Africa in Gondwanan Europe. The separation of Armorica from the Gondwana mainland and a long drift during the Palaeozoic is not supported by the presented data. 相似文献
193.
Quantification and regionalization of groundwater–surface water interaction along an alluvial stream
In the present study, groundwater seepage to an alluvial stream and two tributary streams was examined at nine field sites using hydrological, geophysical, and geomorphological observations. The data indicate that seepage enters the streams in the following ways: (i) directly through the streambed; (ii) as nearly superficial flow from diffuse discharge areas on the flood plains or; (iii) as a combination of (i) and (ii). At about 40% of the sites more than 50% of seepage flows through the streambed. Moreover, it was found that the ratio C, defined as the width of the wet zone of the flood plain divided by the effective width of the stream, can be used as an indicator of the percentage of water entering the stream directly through the streambed. When C is small streambed seepage is large, while when C is large streambed seepage is small and ground water enters the stream mainly as nearly superficial or over-bank flow from the wet zone. 相似文献
194.
成都平原稻田甲烷排放的实验研究 总被引:9,自引:0,他引:9
任丽新 王庚辰 张仁健 段长麟 M. A. K. Khalil M. J. Shearer R. A. Rasmussen R. W. Dalluge 《大气科学》2002,26(6):731-743
根据1996~1999年四个稻季的观测资料,分析了成都平原单季稻甲烷排放的季节变化和年际变化特征.结果表明:在水稻生长季节甲烷排放通量变化很大,在分蘖期和成熟期一般会出现峰值.年际间的通量变化也很大,其年均排放通量的变化范围在2.35~33.95mg m-2 h-1之间.4年的平均排放通量为12 mg m-2 h-1,与四川乐山的7年平均值30mg m-2 h-1相比,存在着明显的地区差异.同时分析讨论了温度、施肥、水稻品种、土壤氧化还原电位(Eh)以及稻田水位等诸多因素对稻田甲烷排放的影响.结果表明:在成都平原水稻生长季节的平均气温对CH4的平均排放通量影响不大;而气温对CH4排放的日变化有相对重要的影响,但气温对甲烷排放日变化的影响与水稻植物体的生长阶段有关;发现了水稻植物体(根、茎、叶)重量对CH4排放的重要作用.讨论了合理使用肥料和施肥量,控制水位和Eh值对稻田CH4的减排作用,提出优化组合诸影响因子,以充分发挥其减排潜力. 相似文献
195.
Back-scattered electron (BSE) imaging and X-ray element mapping of monazite in low-grade metasedimentary rocks from the Paleoproterozoic
Stirling Range Formation, southwestern Australia, reveal the presence of distinct, high-Th cores surrounded by low-Th, inclusion-rich
rims. Previous geochronology has shown that the monazite cores are older than 1.9 Ga and overlap with the ages of detrital
zircon grains (∼3.5–2.0 Ga), consistent with a detrital origin. Many cores have scalloped and embayed surfaces indicating
partial dissolution of former detrital grains. Textural evidence links the growth of the monazite rims (∼1.2 Ga) to deformation
and regional metamorphism during the Mesoproterozoic Albany-Fraser orogeny. These results indicate that high-Th detrital monazite
is unstable under low-grade metamorphic conditions (<400°C) and was partially or completely dissolved. Dissolution was followed
by near-instantaneous reprecipitation and the formation of low-Th monazite and ThSiO4. This reaction is likely to operate in other low-grade metasedimentary rocks, resulting in the progressive replacement of
detrital monazite by metamorphic monazite during regional prograde metamorphism. 相似文献
196.
Petrographic and sulphur isotope studies support the long‐held contention that rounded grains of pyrite in siliciclastic sequences of the Late Archaean Witwatersrand Supergroup originated as placer grains. The grains are concentrated at sites where detrital heavy minerals are abundant within quartz‐pebble conglomerates and quartzose sandstones. Depositional sites with abundant pyrite are: (1) within the matrix of bar‐type, clast‐supported conglomerates; (2) on scoured or winnowed surfaces; and (3) on stratification planes. The grains are internally compact or porous, with truncation of internal structure at outer margins indicating fragmentation and rounding of pyritic source‐rocks during erosion and sediment transport. A large range in textures reflects source‐rock lithologies, with known varieties linked to sedimentary‐hosted diagenetic pyrite, volcanic‐hosted massive sulphide deposits and hydrothermal pyrite. Laser ablation sulphur isotope analysis of pyrite reveals a broader range in δ34S values (? 5·3 to + 6·7‰) than that of previously reported conventional bulk‐grain analyses (? 1 to + 4‰). Rounded pyrite from the Steyn Reef has significant variation in δ34S values (? 4·7 to + 6·7‰) that establishes heterogeneous sulphur compositions, with even adjacent grains having diverse isotopic signatures. The heterogeneity supports a placer origin for rounded pyrite. Euhedral pyrite and pyrite overgrowths which are undoubtedly authigenic have restricted δ34S values (? 0·5 to + 2·5‰), are chemically distinct from rounded pyrite and are probably the products of metamorphism or hydrothermal alteration. The placer origin of rounded pyrite indicates that pyrite was a stable heavy mineral during erosion and transport in the early atmosphere. Its distribution in three sequences (Witwatersrand Supergroup, Ventersdorp Contact Reef and Black Reef), and in other sequences not linked to Witwatersrand‐type Au‐U ore deposits, implies deposition of redox‐sensitive detrital heavy minerals during the Late Archaean. Consequently, rounded grains of detrital pyrite are strong indicators of an oxygen‐poor atmosphere. While not confirming a placer origin for gold in Witwatersrand Au‐U ore deposits, the palaeoenvironmental significance of rounded pyrite negates its link to hydrothermal mineralization. 相似文献
197.
P. F. Rasmussen D. Rosbjerg 《Stochastic Environmental Research and Risk Assessment (SERRA)》1991,5(1):1-16
Conventional design practice aims at obtaining optimal estimates of floods with specified exceedance probabilities. Such estimates are, however, known on the average to be exceeded more frequently than expected. Alternatively, methods focusing on the expected exceedance probability can be used. Two different methods are considered here; the first is based on the sample distribution of true exceedance probabilities. The second is a Bayesian analogue using the likelihood function and a noninformative prior to describe the variability of exceedance probabilities. Appropriate analytical solutions are presented in both cases using the partial duration series approach. 相似文献
198.
P. G. Simmonds D. M. Cunnold F. N. Alyea C. A. Cardelino A. J. Crawford R. G. Prinn P. J. Fraser R. A. Rasmussen R. D. Rosen 《Journal of Atmospheric Chemistry》1988,7(1):35-58
Seven years of daily gas chromatographic measurements of CCl4 at the five globally distributed ALE/GAGE surface sites are reported. It is determined that CCl4 has been accumulating in the atmosphere at a rate of 1.3±0.1%/yr over the period 1978–1985 and that the releases of CCl4 into the atmosphere have remained fairly constant, with the smallest releases in 1981–1982. Using an inversion scheme based on a nine box model of the atmosphere, we infer a CCl4 lifetime of approximately 40 yr, an inventory on 1 July 1978 of (2.08±0.07)×109 kg and an average rate of release over the period 1978–1985 of (0.9±0.9)×107 kg/yr. These results produce excellent agreement with a release scenario derived from global production estimates for CCl4 and the major CCl4 byproduct, the chlorofluorocarbons. However, to obtain this consistency, it is necessary that our current ALE/GAGE absolute calibration standard be reduced approximately 25% thus bringing it into agreement with measurements by Yokohata et al. (1985) and Hanst et al. (1975). 相似文献
199.
Suria Tarigan Christian Stiegler Kerstin Wiegand Alexander Knohl Kukuh Murtilaksono 《水文科学杂志》2020,65(7):1239-1248
ABSTRACTOver the last decade, monoculture plantations have rapidly developed in Jambi Province on Sumatra, Indonesia. Meanwhile, there has been intensification of discharge fluctuation in the study area. We examined the relative contribution of changes in evapotranspiration and soil compaction to the catchment discharge by using the Soil Water Assessment Tool model. Evapotranspiration values based on the catchment water balance analysis in intensively cultivated oil palm plantations, smallholder oil palm plantations, rubber plantations, and the secondary forest are 5.03 ± 0.30, 4.11 ± 0.38, 3.36 ± 0.32, and 4.50 ± 0.18 mm d?1, respectively. Infiltration rate in active interrows of oil palm, rubber plantations, agroforest, oil palm frond pile is 2.6 ± 1.7, 16.3 ± 6.8, 28.0 ± 3.9, 58.2 ± 21.8 cm h?1, respectively. We found that increased evapotranspiration and soil compaction increased the frequency of low discharge by 30%, with increased evapotranspiration contributing 10% and increased soil compaction contributing 20%. 相似文献
200.
Weekly air samples were collected at Cape Kumakahi (0 km) and at nearby Mauna Loa Observatory (3.4 km) which is above the boundary layer. EC/GC and GC/FID techniques were used to measure CH3I, CHCl3, CO and CH4, which are largely natural in origin, and C2Cl4, CCl4, CH3, CCl3, CCl3F (F-11), CCl2F2 (F-12), CHClF2 (F-22) and C2F3Cl3 (F-113), which are due to anthropogenic activities. It was found that all these gases are significantly (0.05) more abundant in the boundary layer than above it. 相似文献