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排序方式： 共有4078条查询结果，搜索用时 15 毫秒
931.
932.
F. I. Zhimulev Yu. A. Kalinin V. P. Sukhorukov E. De Pelsmaeker M. Poujol K. R. Kovalev S. Glorie E. A. Naumov R. Seltmann J. De Grave 《Doklady Earth Sciences》2018,481(2):1033-1036
Accessory zircon from the polyphase Raygorodok stock (Northern Kazakhstan) has been dated by local U–Pb analysis (SIMS and LA-ICP-MS). This Au-bearing intrusion has been dated to 442–447 Ma, suggesting its emplacement at the very end of the Ordovician and Early Silurian, allowing the stock’s correlation with the Stepnyak Complex of small intrusions. Thus, control of small intrusions of the Stepnyak type over gold mineralization has been corroborated permitting their use as a regional prospecting characteristic. 相似文献
933.
P. Mañas J. De las Heras 《International Journal of Environmental Science and Technology》2018,15(2):273-280
Sewage sludge from wastewater treatment plants (WWTPs) has increased in recent years in Spain and Europe in general. Agriculture seems a viable solution when seeking alternatives for its final disposal. Sewage sludge reuse can, however, pose risks given presence of heavy metals, pathogen microorganisms or toxic compounds, which must be controlled to minimize risks for human health and the environment. In the present study, the phytotoxicity of sewage sludge from the Alcázar de San Juan WWTP using two types of seeds (Lactuca sativa L. and Lepidium sativum L.) in bioassays was analyzed. The relative germination rates (RGP), relative radicle growth (RRG) and germination index were determined for both species. Hypocotyl length was measured in lettuce. Negative (distilled water) and positive (zinc sulfate) controls were prepared. Depending on the biological material and residue phytotoxicity dose, analyses can be modified given the different sensitivities of plants. Lepidium sativum L. was more sensitive to phytotoxic compounds than Lactuca sativa L. Hypocotyl measurements provide complementary toxicity information. A general agreement between authors and researchers to standardize this type of analysis would be useful to make criteria uniform. 相似文献
934.
Alain Le Kouby Antoine Guimond-Barrett Philippe Reiffsteck Anne Pantet 《Geotechnical and Geological Engineering》2018,36(3):1463-1474
Nowadays, improving the strength and deformation properties of soft soils by deep soil mixing is a commonly used technique. There is also an increasing interest in the use of this technique for foundation/structural elements and excavation retaining walls applications. The compressive strength and elastic modulus of the soil mix material are key parameters in the design of these structures. However, there is very limited information available on the impact of exposure to air drying (in the case of retaining wall) on the strength and stiffness of cement stabilized soils. The aim of this study is to investigate the effects of different curing conditions (immersion in water, cycles of wetting and drying, continuous air drying) on the mechanical properties of soils treated with cement in the laboratory. Free–free resonance tests and unconfined compression tests were performed on specimens of silt and sand treated with blastfurnace slag cement. Strength increases more rapidly than stiffness between 7 and 30 days. The strength of stabilized soils submitted to cyclic wetting and drying before the cement hydration process is complete continues to increase. As long as the periods of drying do not induce microcracks, the stiffness of the treated soil specimens also increases with time. However, the stiffness is lower than for the specimens cured in water indicating a disruptive effect of the imposed wetting–drying cycles on stiffness. Continuous exposure to air drying inhibits strength development due to insufficient water for hydration. Significant stiffness decreases were observed on specimens of stabilized silt and are attributed to microcracking. 相似文献
935.
936.
Coralie André;Dante de Ruijsscher;Wim De Clercq;Frieda Bogemans;Bart Van de Vijver;Annelies Storme;Stephen Louwye; 《Boreas: An International Journal of Quaternary Research》2024,53(1):88-105
The region of Bruges (Flanders, Belgium) was an economic and cultural centre during the Late Middle Ages because it was connected to the North Sea via a large tidal inlet called the Zwin, along which smaller towns developed. One of these towns was Aardenburg (Zeeland, The Netherlands), which developed from a Roman castellum to become one of the important medieval towns in the hinterland of Bruges. Unfortunately, archaeological, historical and (palaeo)environmental data about the Roman to medieval evolution of the Zwin area are scarce. However, the continuous occupation of Aardenburg during the first millennium AD and its location on a Pleistocene sand ridge bordering the coastal plain provide a unique opportunity to investigate the natural dynamics of the coastal evolution and the interplay with the human impact that shaped the Zwin region before its heyday. The variable character of the depositional environments on the dynamic coastal plain makes local environmental studies essential for understanding larger patterns, which were previously reduced to a simplistic outdated model over the entire region. Palynological and diatom assemblages of two sequences in Aardenburg allow the reconstruction of the terrestrial and aquatic palaeoenvironments from the Middle Holocene to the high Middle Ages. The results reveal landscape evolution in relation to woodlands, peat bogs, coastal environments and human presence, consistent with previous regional landscape evolution. A distinct increase in marine influence during the Roman and early medieval periods, when human activities played a significant role, was followed by an increase in inland indicators from the high medieval period onwards. This evolution is in accordance with the expansion of the town and the reclamation of the coastal landscape as it evolved to become a cultural centre in the Late Middle Ages. 相似文献
937.
以人口流动为表征的空间相互作用是热点议题,然而关于其影响因素的非线性机制尚未被充分揭示。论文利用2018年腾讯迁徙大数据,以梯度提升决策树模型改进重力模型,分析距离衰减系数的非线性特征。研究发现：(1)城际联系的距离衰减系数呈非线性,偏依赖图曲线存在2个“平台区”(距离衰减系数接近于0)和2个“衰减区”(距离衰减系数约为2.8),将直线距离替换为路网距离,2018年数据替换为其他年份数据后,结果仍然稳健;(2)与基准模型相比,航空、铁路及公路交通下的距离衰减系数非线性特征存在显著差异,春运期间的距离衰减系数基本保持一致。建议根据城际联系距离衰减规律布局都市圈和城市群,优化国土空间发展格局。 相似文献
938.
Elke Vandekerkhove Maarten Van Daele Nore Praet Veerle Cnudde Peter J. Haeussler Marc De Batist 《Sedimentology》2020,67(1):364-389
Lake sediments in Eklutna Lake, Alaska, reveal the presence of turbidites within varved sequences. These turbidites, which result from flood events and earthquakes, show a similar macroscopic appearance. In order to use turbidites to reconstruct flood variability and/or seismic history in the lake basin, it is crucial to determine the trigger of the turbidity currents. This study examined the turbidite caused by the ad 1964 Great Alaska earthquake as well as turbidites linked to historical flood events in order to differentiate between these earthquake-triggered and flood-triggered turbidites. In a suite of samples from throughout the lake, distinctive proxies are identified that can be associated with event-specific flow characteristics. The study presents straightforward discrimination methods related to the sedimentology and geochemical components of the turbidites. These methods are also applicable to other lakes, particularly proglacial lakes where the sediment composition of onshore and offshore sources is similar. Finally, the discrimination of the turbidite trigger can be used to reconstruct the palaeoflood and seismic history. 相似文献
939.
Alberto Ceccato Philippe Goncalves Giorgio Pennacchioni 《Journal of Metamorphic Geology》2020,38(8):881-903
Temperature and fluid content are critical parameters that control rock rheology and strain localization in the continental crust. Here, we determine by thermodynamic modelling the of localized ductile shearing during cooling of three different granitoid plutons: the Rieserferner and the Adamello plutons in the Italian Alps, and the Lake Edison pluton in the Sierra Nevada—USA. Shear zones exploited precursor joints, associated veins and alteration zones. and P−T phase diagram sections were computed with Perple_X in the system MnO−Na2O−CaO −K2O−FeO−MgO−Al2O3−SiO2−H2O−Fe2O3. The phase diagram sections show that the nucleation of the brittle precursors (joints, veins) occurred at T» 450°C at fluid-saturated conditions. Localized ductile shearing likely occurred at temperature ranging between 420 and 460°C evolving from initially fluid-saturated to fluid-undersaturated conditions in a closed system. In this temperature range, granitoid rocks are potentially subject to a series of retrograde metamorphic reactions replacing the load-bearing feldspars with weaker phyllosilicates. Metamorphic reactions occurred in spatial association with the precursory structures, leading to localized shearing. Decreasing temperature and fluid-undersaturated conditions likely hampered progressive strain accommodation in shear zones by slowing down metamorphic reactions, thermally activated dislocation creep processes, fluid-mediated deformation mechanisms and weakening mechanisms. Polyphase granitoid ultramylonite and mylonitic quartz veins have been affected differently by the fluid-undersaturated conditions of the system, as consequence of different dominant deformation mechanisms and syn-kinematic paragenesis during localized shearing. Localized ductile shearing in cooling plutons effectively occurs in a limited temperature range (420–460°C) in which the strain accommodation capacity of the shear zone is controlled by the negative feedback between the cooling rate, the kinetics of metamorphic reactions and deformation mechanisms, and the consumption of the limited amount of available fluids. 相似文献
940.
We review the current knowledge about Southeast Asian volcanoes and their eruption histories, and focus on identifying tephrochronologic markers representing major explosive eruptions in order to further future palaeoclimate and volcanological studies. Forty-one volcanic edifices in Southeast Asia have been classified as large calderas by Whelley et al. (2015) and thus have, or are likely to have, produced large explosive eruptions with a Volcanic Explosivity Index (VEI) of 6–8. Unfortunately, only 20 such eruptions have known ages, spanning from 1.2 Ma to 1991 ad , and fewer have geochemical data that can be used for tephrostratigraphic correlations. Volcanic products from different geodynamic regions and different sources can generally be distinguished on major element plots (e.g. K2O versus CaO) of matrix glass composition. However, the distinction of multiple eruptions from the same source often requires additional data such as trace element compositions of matrix glass and/or mineral compositions. Biotite, but also magnetite compositions (MgO and TiO2 content in particular) appear to be very discriminating. Up to nine tuffs in addition to the three to four Toba tuffs can be utilised as widespread tephrochronologic markers and span a range from 1.2 to 1.6 Ma to recent. As only a few Holocene major eruptions have been well characterised and dated, many large calderas are still unstudied, and many distal tephra layers are still lacking a source, more tephrochronologic markers can certainly be defined in the future. 相似文献