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91.
92.
Bands of large (up to 4 cm long) three-dimensional crystallographic dendrites form the terrace fronts in an old travertine mound exposed near Clinton, British Columbia. The dendrites, with their long axes perpendicular to the terrace front, are characterized by numerous levels of branching. Each branch is formed of multitudes of skeletal rhombs, four- and six(?)-sided bipyramidal crystals, or prismatic hexagonal crystals that are precisely aligned along crystallographic precepts. Although individual branches are formed of one type of subcrystal, neighbouring branches may be formed of different subcrystal types.Highly supersaturated waters that were generated by rapid CO2 degassing of the spring water during its turbulent flow over the steep terrace fronts probably drove dendrite precipitation. The presence of growth lines indicates that growth was episodic. Type I growth lines probably formed annually in response to seasonal climate changes whereas Type II growth lines, which formed less frequently, may reflect changes in the flow velocity and/or flow patterns of the spring waters.Early diagenetic modification of the dendrites involved crystal face enlargement, cements formed of trigonal prisms or needle-fiber crystals, microbial infestation that mediated substrate dissolution, and/or deposition of detrital calcite crystals that formed in the water column. Much of the diagenetic modification may have taken place during the periods when the dendrites had temporarily stopped growing.The dendrites in the Clinton travertine are an excellent example of complex, episodic calcite crystal growth that was extensively modified by early diagenetic processes in a surface environment. The same spring waters from which the dendrites were precipitated mediated much of the early diagenesis. 相似文献
93.
喀喇昆仑断裂带走滑过程中伴随的快速隆升作用:热年代学证据 总被引:4,自引:0,他引:4
喀喇昆仑断裂带是青藏高原西部的一条大型右旋走滑断裂带,它是喜马拉雅山脉西段北侧重要的地质边界.本文在岩石学、变形构造的研究基础上,对喀喇昆仑断裂带东南段阿伊拉日居山-噶尔盆地地区的喀喇昆仑韧性剪切带中变质岩石的同构造矿物进行了40Ar/39Ar热年代学研究.显微构造研究表明,剪切带中的矿物记录了从高温(>600℃)到低温(<250℃)条件下的连续变形,表现为近水平的右旋剪切运动转变成斜向的右旋正滑,使绿片岩相的变形作用叠加在中-高温变形之上.暗示出走滑过程中存在隆升作用,热年代学结果显示其连续剪切变形作用从早中新世以来至少持续到4Ma,并且出现三个快速冷却阶段:第一个快速冷却阶段为从25~22Ma到21~18Ma期间,可能代表的是浅部高温剪切过程中变形局部停止或减慢的过程;第二个快速冷却时期为从15Ma到12~10Ma,是喀喇昆仑断裂带走滑过程中,阿伊拉日居山的快速隆升、噶尔盆地开始形成以及主要河流深切过程阶段;9Ma以来是第三个快速冷却过程,使阿伊拉日居山脉进一步快速隆升、噶尔盆地定形过程.根据不同年代地表地貌特征的右旋错位距离以及不同层次变形特征,估算出喀喇昆仑断裂带长期滑移速率为8~10mm/a,伴随的隆升速率为1mm/a.从显微构造和热年代学证据表明,晚第四纪以来该断裂经历了强烈的右旋走滑运动的同时伴随强烈的隆升作用. 相似文献
94.
Philip?E.?BettEmail author Hazel?E.?Thornton Robin?T.?Clark 《Theoretical and Applied Climatology》2017,127(1-2):61-80
We characterise the long-term variability of European near-surface wind speeds using 142 years of data from the Twentieth Century Reanalysis (20CR), and consider the potential of such long-baseline climate data sets for wind energy applications. The low resolution of the 20CR would severely restrict its use on its own for wind farm site-screening. We therefore perform a simple statistical calibration to link it to the higher-resolution ERA-Interim data set (ERAI), such that the adjusted 20CR data has the same wind speed distribution at each location as ERAI during their common period. Using this corrected 20CR data set, wind speeds and variability are characterised in terms of the long-term mean, standard deviation and corresponding trends. Many regions of interest show extremely weak trends on century timescales, but contain large multidecadal variability. Since reanalyses such as ERAI are often used to provide the background climatology for wind farm site assessments, but contain only a few decades of data, our results can be used as a way of incorporating decadal-scale wind climate variability into such studies, allowing investment risks for wind farms to be reduced. 相似文献
95.
High‐resolution evolution of terrigenous sediment yields in the Provence Basin during the last 6 Ma: relation with climate and tectonics
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Estelle Leroux Marina Rabineau Daniel Aslanian Christian Gorini Stéphane Molliex Francois Bache Cécile Robin Laurence Droz Maryline Moulin Jeffrey Poort Jean‐Loup Rubino Jean‐Pierre Suc 《Basin Research》2017,29(3):305-339
Basin‐wide correlation of Messinian units and Plio‐Quaternary chronostratigraphic markers (5.3 Ma, 2.6 Ma, 0.9 Ma and 0.45 Ma), the mapping of total sediment thickness and the determination of overall sedimentary volumes enabled us to provide a high‐resolution quantitative history of sediment volumes for the last 6 Ma along the Gulf of Lions margin. The results point to (i) a dramatic increase in terrigenous sediment input during the Messinian Salinity Crisis. This increased sedimentation reflects enhanced regional fluvial erosion related to the dramatic fall of Mediterranean base‐level. Stronger weathering due to a regional wetter climate probably also increased erosional fluxes. (ii) A sediment input three times higher during the Plio‐Quaternary compared to the Miocene seems in agreement with published measurements from World's ocean. However, the timing of this increase being uncertain, it implies that the trigger(s) also remain(s) uncertain. (iii) A decrease in detrital volume around 2.6 Ma is attributed to a regional change in the drainage pattern of rivers in the northwestern Alps. (iv) This study also highlights the Mid‐Pleistocene Revolution around 0.9 Ma, which resulted in an almost doubling of sediment input in the Provencal Basin. 相似文献
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Modifications, resulting in increased simplicity and ruggedness, have been made to the design of a micro-erosion meter to permit it to be used under water by SCUBA divers. In its modified form the instrument has been used successfully in a study measuring rates of nearshore erosion in overconsolidated glacial till. 相似文献
100.
J. David Smith Lorraine Plues Mireille Heyraud Robin D. Cherry 《Marine environmental research》1984,13(1):55-68
Ag, Al, Ca, Cd, Cu, Fe, Mg, Mn, Pb and Zn concentrations and 210Po and 210Pb activities were measured in 26 specimens of the squid Nototodarus gouldi taken from the waters of Bass Strait in one jigging operation. All the elements show wide ranges in concentrations in specimens apparently subject to the same environmental conditions. Copper concentration was 27-1 200 μg/g, and 210Po activity 4·8–24·2 Bq/g. The animal wet weights, the elements Ag, Al, Cd, Fe and Zn, and the radionuclide 210Po have coefficients of variation in the range 40–60%; Ca, Mg and Mn show the smallest variability (CV = < 30%), and Cu the greatest (CV = 12%). Significant correlations (p < 0·001) were found between the following pairs of elements: Cd-Zn, Cd-Cu, Zn-Cu, Mg-Mn, Fe-Mn, Ca-Mg and Fe-210Po. 相似文献