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991.
Paleolimnology of the McMurdo Dry Valleys,Antarctica 总被引:1,自引:0,他引:1
The McMurdo Dry Valleys presently contain more than 20 permanent lakes and ponds, which vary markedly in character. All, with the exception of a hypersaline pond, have a perennial ice-cover. The dry valley lakes, and lakes in other ice-free regions of continental Antarctica, are unique on this planet in that they consistently maintain a thick year-round ice cover (2.8–6.0 m) over liquid water. The persistent ice covers minimize wind-generated currents and reduce light penetration, as well as restricting sediment deposition into a lake and the exchange of atmospheric gases between the water column and the atmosphere. From a paleolimnological perspective, the dry valley lakes offer an important record of catchment and environmental changes. These lakes are also modern-day equivalents of periglacial lakes that were common during glacial periods at temperature latitudes. The present lakes are mostly remnants of larger glacial lakes that occupied the valleys in the past, perhaps up to 4.6 Ma ago. Two of the valleys contain evidence of being filled with large glacial lakes within the last 10000 years. Repeated drying and filling events since then have left a characteristic impression on the salt profiles of some lakes creating a unique paleo-indicator within the water column. These events are also marked in the sediments by the concentration and dilution of certain chemical constituents, particularly salts, and are also corroborated by carbonate speciation and oxygen isotope analysis. Stratigraphic analysis of dry valley lake sediments is made difficult by the occurrence of an old carbon reservoir creating spurious radiocarbon dates, and by the high degree of spatial variability in lake sedimentation. From a biological perspective, the lakes are relatively simple, containing various taxa of planktonic and benthic microorganisms, but no higher forms of life, which is an advantage to paleolimnologists because there is no bioturbation in the sediments. Useful biological paleo-indicators found in the sediments include cyanobacterial filament sheaths, diatom frustules and other eukaryotic algal cells, protozoan cysts, photosynthetic pigments, and minerals (e.g. carbonates) associated with microbial activity. Future work will benefit from fully characterizing the connection between the ice covers, environmental conditions, and paleo-indicators, thereby allowing refinement of inferences made concerning the paleoenvironment. New dating techniques need to be tested in this environment to overcome the problems associated with radiocarbon dating. The establishment of a detailed and focused paleolimnological campaign is proposed. 相似文献
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994.
M. K. Jafari F. Pellet M. Boulon K. Amini Hosseini 《Rock Mechanics and Rock Engineering》2004,37(1):3-23
Summary Evaluation of the effects of small repetitive earthquakes on the strength parameters of rock joints in active seismic zones is of interest of the designers of underground constructions. In order to evaluate these effects, it is necessary to study the behaviour of rock joints under dynamic and cyclic loadings. This paper presents the results of a systematic study on the behaviour of artificial rock joints subjected to cyclic shearing. More than 30 identical replicas have been tested using triaxial compression devices under different conditions of monotonic and cyclic loading. At the first stage a few samples have been tested in monotonic loading modes under various confining pressures and rate of displacement. In the second series of tests, small cyclic loads were applied on the samples for increasing number of cycles, frequency levels and stress amplitudes. These were then followed by monotonic loading again. The variations of maximum and residual shear strengths for each test have been studied. The results show increase of shear strength as a result of the increase in confining pressure and they display decrease of shear strength due to the increase of rate of loading, number of cycles, frequency levels and stress amplitudes. 相似文献
995.
GIS支持下的青藏公路沿线高含冰量冻土空间分布模型 总被引:1,自引:0,他引:1
Relationships among ground ice, Quaternary soil type, vegetation cover, digital terrain model (DTM), permafrost ground temperature and topographic units are applied. Using comprehensive evaluated method and coding interpolation, spatial distribution of ground ice in permafrost regions along Qinghai-Tibetan Highway (QTH) is modeled according to GIS spatial analysis. Comparing with result,model precision can be approached to 81.2 %. Its study precision can satisfy engineering need for a small - scale study 相似文献
996.
通过实验比较了在云室中检测冰晶的两种方法-糖盘法和玻片法,结果表明玻璃片测到的冰晶数比糖盘高数倍,讨论了两种方法检测的上下限,优缺点及应用范围。 相似文献
997.
This paper is the second in a two-part series that discusses the principal axes of M-DOF structures subjected to static and dynamic loads. The primary purpose of this series is to understand the magnitude of the dynamic response of structures to enable better design of structures and response modification devices/systems. Under idealized design conditions, the structural responses are obtained by using single direction input ground motions in the direction of the intended response modification devices/systems, and by assuming that the responses of the structure is decoupleable in three mutually perpendicular directions. This standard practice has been applied to both new and retrofitted structures using various seismic protective systems. Very limited information is available on the effects of neglecting the impact of directional couplings (cross effects of which torsion is a component) of the dynamic response of structures. In order to quantify such effects, it is necessary to examine the principal axes of structures under both static and dynamic loading. In this two-part series, the first paper is concerned with static loading, which provides definitions and fundamental formulations, with the conclusion that cross effects of a statically loaded M-DOF structure resulting from the lack of principal axes are of insignificant magnitude. However, under dynamic or earthquake loading, a relatively small amount of energy transferred across perpendicular directions is accumulated, which may result in significant enlargement of the structural response. This paper deals with a formulation to define the principal axes of M-DOF structures under dynamic loading and develops quantitative measures to identify cross effects resulting from the non-existence of principal axes. 相似文献
998.
The seismic performance of timber bridge substructures is not well established, particularly when compared to concrete highway bridges. This paper presents a dual experimental‐computational modelling program to investigate the seismic behaviour of typical braced timber bridge pile bents. For this purpose, a prototype timber bridge was used to develop a near‐full‐size physical model that was used for shaking table experiments and quasi‐static reversed cyclic loading tests on the laboratory strong‐floor. A non‐linear force‐displacement computational modelling study was also undertaken as a companion effort to the experimental investigation. On the basis of the experimental study, seismic vulnerability analysis was conducted for this kind of timber bridge principally with shaking in the transverse direction. In this analysis, a simplified fundamental mechanics‐based approach was employed from which fragility curves were derived. The study showed that braced timber pile bents have considerable strength and deformability capability. Nevertheless, they are not immune from earthquake damage. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
999.
西藏纳木错第四纪湖泊沉积与湖成地貌--兼论藏北高原古大湖问题 总被引:3,自引:2,他引:3
对西藏面积最大的湖泊——纳木错湖相沉积野外现察结果,发现了由水平层理十分发育的砂与粘土所组成的、高出湖面分别为3—12m、15—22m、25—30m与35—45m的4级湖岸阶地,覆于基岩之上、高出湖面60—150m的湖相沉积和多达50条左右、由扁圆湖滨相砾石所组成的湖岸堤,环湖广泛分布的湖成地貌。在连结纳木错与其西北的仁错约玛、仁错贡玛、久如错的分水岭宽谷底部(分别高出上述3湖20m、90m与60m)与北侧山坡,即纳木错的第二与第三级湖岸阶地,发现了组成阶地与异堤的湖相与湖滨相沉积。从而确证了纳木错与仁错-久如锴曾多次连通,即数度成为一个统一的大湖,而不是以河道相连的不同湖泊。古大湖分离时代为末次冰期间冰段。 相似文献
1000.