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Silicified carbonates of the latest Mesoproterozoic Sukhaya Tunguska Formation, northwestern Siberia, contain abundant and diverse permineralized microfossils. Peritidal environments are dominated by microbial mats built by filamentous cyanobacteria comparable to modern species of Lyngbya and Phormidium. In subtidal to lower intertidal settings, mat-dwelling microbenthos and possible coastal microplankton are abundant. In contrast, densely woven mat populations with few associated taxa characterize more restricted parts of tidal flats; the preservation of vertically oriented sheath bundles and primary fenestrae indicates that in these mats carbonate cementation was commonly penecontemporaneous with mat growth. Eoentophysalis mats are limited to restricted environments where microlaminated carbonate precipitates formed on or just beneath the sediment surface. Most microbenthic populations are cyanobacterial, although eukaryotic microfossils may occur among the simple spheroidal cells interpreted as coastal plankton. Protists are more securely represented by large (up to 320 micrometers in diameter) but poorly preserved acritarchs in basinal facies. The Sukhaya Tunguska assemblage contains 27 species in 18 genera. By virtue of their stratigraphic longevity and their close and predictable association with specific paleoenvironmental conditions, including substrates, Proterozoic cyanobacteria support a model of bacterial evolution in which populations adapt rapidly to novel environments and, thereafter, resist competitive replacement. The resulting evolutionary pattern is one of accumulation and stasis rather than the turnover and replacement characteristic of Phanerozoic plants and animals. 相似文献
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Filina VN 《Soviet geography》1987,28(10):728-741
"A large body of empirical data on the dynamics of local travel in the USSR is surveyed to illustrate the influence of commuter movement on the formation and spatial interaction of settlement systems. Correlations are established between commuting patterns and the population levels of cities of different size categories." 相似文献
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根据联合国沙漠化会议第20项关于培训规划组成部分的建议,受联合国环境规划署委托,中华人民共和国国务院环境保护领导小组办公室、中国科学院兰州沙漠研究所于1981年9月1日至9月26日在中国举办了第二次沙漠化防治讲习班。参加这次讲习班的,有埃及、肯尼亚、墨西哥、民主也门、秘鲁、塞内加尔、索马里、坦桑尼亚和土耳其等九个国家15名从事沙漠化研究和治理的专家与环境保护方面的官员。这次沙漠化讲习班采用讲课与野外考察相结合的方式,共分三个阶段进行。参加讲习班成员首先考察了我国北方地区干草原及荒漠草原地带沙漠化发生区的自然特征,沙漠化过程以及较成功的沙漠化治理的典型区,行程约8,000公里。 相似文献
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在芬诺斯坎迪亚地区及邻近地区建立了一个地壳及上地幔的三维地球物理模型。这个模型采用地震学结论、重力场和热流量数据进行组建。根据地震学数据得出的地层以及Ｐ波速度在这些地层中的分布,定出基本的初始地震模型。而速度、热产生和密度之间的关系的经验公式用来将地震模型变换成热学模型与密度模型。在变换过程中,压力与温度对物理性质的影响已经搞清楚并已加入计算。在解决地热和重力数据的反演问题时,使用速度、热产生和密 相似文献
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de Jonge VN 《Marine pollution bulletin》2007,55(10-12):407-414
The EU Water Framework Directive demands the protection of the functioning and the structure of our aquatic ecosystems. The defined means to realize this goal are: (1) optimization of the habitat providing conditions and (2) optimizing the water quality. The effects of the measures on the structure and functioning of the aquatic ecosystems then has to be assessed and judged. The available tool to do this is 'monitoring'. The present monitoring activities in The Netherlands cover target monitoring and trend monitoring. This is insufficient to meet the requirements of the EU. It is, given the EU demands, the ongoing budget reductions in The Netherlands and an increasing flow of unused new ecological concepts and theories (e.g. new theoretical insights related to resource competition theory, intermediate disturbance hypothesis and tools to judge the system quality like ecological network analysis) suggested to reconsider the present monitoring tasks among governmental services (final responsibility for the program and logistic support) and the academia (data analyses, data interpretation and development of concepts suitable for ecosystem modelling and tools to judge the quality of our ecosystems). This will lead to intensified co-operation between both arena's and consequently increased exchange of knowledge and ideas. Suggestions are done to extend the Dutch monitoring by surveillance monitoring and to change the focus from 'station oriented' to 'area oriented' without changing the operational aspects and its costs. The extended data sets will allow proper calibration and validation of developed dynamic ecosystem models which is not possible now. The described 'cost-effective' change in the environmental monitoring will also let biological and ecological theories play the pivotal role they should play in future integrated environmental management. 相似文献
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Systems of circular migration existing in the USSR are described. The role that such migration plays in linking settlement systems is noted, and the influence of city size on migration is discussed. 相似文献