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自地球形成至寒武纪将近40亿年（距今46亿～5.4亿年,通常称为前寒武纪）的气候演变是一个具有特殊难度和挑战性的研究领域,同时也是基础和前沿的研究领域。文章选择了前寒武纪气候演化中的三个重要科学问题进行综述：大气演化、两次全球性的冰川期以及暗弱太阳问题。关于大气演化,本文首先描述了大气成分的演化历史,然后简述了影响大气成分演化的三个基本过程：大气逃逸、两次大气氧含量突然增加、碳酸盐-硅酸盐循环及其对气候系统的负反馈作用。两次全球性的冰川期分别发生在古元古代（距今24亿～21亿年）和新元古代（距今8亿～5.8亿年）,文章简述了其成因以及相关的气候模拟结果。暗弱太阳问题是地球历史气候演化的一个经典问题,论文简要地综述了一些最新的研究成果和观点。 相似文献
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《Marine Policy》2017
Spatial boundaries have become an indispensable part of regimes and tools for regulating fisheries, with examples including marine protected areas, regional fisheries management organizations and Exclusive Economic Zones. Yet, it is also widely acknowledged that boundaries are a social construct, which may be resisted by both fishers and fish ecology. The ensuing spatial and institutional mismatches have been shown to frustrate management efforts, exacerbating issues of non-compliance and ultimately leading to conflicts and overfishing. Interestingly, the often static and rigid nature of these boundaries has also led to a concomitant research interest in ‘transboundary’. This paradoxical situation of more boundary-setting entailing more transboundary thinking warrants a deeper understanding about boundaries and the role of transboundary research in fisheries. The aims of this review article are twofold: (1) a theoretical clarification on the meanings and uses of spatial boundaries drawing on geographical “boundary studies” literature; and (2) a construction of a typology that outlines how transboundary research is being articulated and envisioned. Together, the study reveals that transboundary scholarship in fisheries are mostly related to resources, fleets, trade and governance aspects and that dealing with the “boundary paradox” encompasses re-incorporating, re-scaling and re-imagining of boundaries. This article provides a conceptual basis for reflecting upon boundaries in world's fisheries and opens up discussions for a more nuanced boundary application that can better cope with multi-level interactions and dynamicity. 相似文献
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“蒸发悖论”在黄河流域的探讨 总被引：14，自引：1，他引：13
利用黄河流域72个气象站点1960-2010年的气象资料,系统分析了过去51年间气温、降水量以及潜在蒸散量的变化趋势,研究了气温、降水量与潜在蒸散量之间的长期变化趋势关系,对影响潜在蒸散量下降的主要因子进行了探讨,重点对黄河流域是否存在“蒸发悖论”进行验证.研究结果表明:(1)过去51年间,黄河流域内气温增加显著、潜在蒸散量呈下降趋势,总体上存在“蒸发悖论”;(2)“蒸发悖论”具有空间上和时间上的不一致性,随着气温增加,春、夏、冬三季潜在蒸散量呈减少趋势,减少区域主要集中于山西、河南大部分区域以及甘肃、宁夏、内蒙古、陕西等少部分区域;时间上主要表现在1960-1979年潜在蒸散量变化趋势不明显,1980-2010年气温与潜在蒸散量变化趋势在空间分布上的逆向关系更加明显;(3)过去51年间,降水量无论是年际还是夏、秋季变化趋势都不明显,降水量与潜在蒸散量时空变化分布上大体呈现逆向变化关系;(4)从气象要素变化对潜在蒸散量变化的贡献率来看,近51年来风速的明显减小是导致黄河流域潜在蒸散量减少的主导因素. 相似文献
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Deep Fractionation of Clinopyroxene in the East Pacific Rise 13°N: Evidence from High MgO MORB and Melt Inclusions 总被引：1，自引：1，他引：0
Mid-ocean ridge basalts (MORBs) from East Pacific Rise (EPR) 13°N are analysed for major and trace elements, both of which show a continuous evolving trend. Positive MgO–Al2O3 and negative MgO–Sc relationships manifest the cotectic crystallization of plagioclase and olivine, which exist with the presence of plagioclase and olivine phenocrysts and the absence of clinopyroxene phenocrysts. However, the fractionation of clinopyroxene is proven by the positive correlation of MgO and CaO. Thus, MORB samples are believed to show a “clinopyroxene paradox”. The highest magnesium-bearing MORB sample E13-3B (MgO=9.52%) is modelled for isobaric crystallization with COMAGMAT at different pressures. Observed CaO/Al2O3 ratios can be derived from E13-3B only by fractional crystallization at pressure >4 ±1 kbar, which necessitates clinopyroxene crystallization and is not consistent with cotectic crystallization of olivine plus plagioclase in the magma chamber (at pressure ~1 kbar). The initial compositions of the melt inclusions, which could represent potential parental magmas, are reconstructed by correcting for post-entrapment crystallization (PEC). The simulated crystallization of initial melt inclusions also produce observed CaO/Al2O3 ratios only at >4±1 kbar, in which clinopyroxene takes part in crystallization. It is suggested that MORB magmas have experienced clinopyroxene fractionation in the lower crust, in and below the Moho transition zone. The MORB magmas have experienced transition from clinopyroxene+plagioclase+olivine crystallization at >4±1 kbar to mainly olivine+plagioclase crystallization at <1 kbar, which contributes to the explanation of the “clinopyroxene paradox”. 相似文献
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This paper presents a new simulation algorithm for generating realizations of a Boolean model for sandstone reservoirs conditional on sandstone body intersections in wells. It is a statistically corrected version of the conditional simulation algorithm originally proposed by Haldorsen. In previous work it was shown that the conventional algorithm does not reproduce the correct statistics for sandstone body size at the well locations. The simulation of a Boolean model, given grain intersections on line transects, must be in accordance with the conditional distribution of the model, which implies that sandstone bodies intersected by wells and sandstone bodies in the interwell area should be simulated independently. Based on the conditional distribution a simulation algorithm is developed, which is compared to Haldorsen's algorithm by simulating an outcrop section of fluvial sandstone deposits. Simulations are conditioned on data of two fictitious wells. It turns out that the adjusted algorithm gives better results for the sand fraction that is connected to a set of wells, and also for the sand fraction that would be connected to an infill well. 相似文献
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The diverse and contested meanings of sustainable development 总被引：2，自引：0，他引：2
We provide a heuristic framework that can be used as a lens for understanding the arguments being presented in the papers which comprise this special issue on sustainable development and environmental issues in Great Britain. This framework can also be used as an introduction to the wider literature on sustainable development because it is designed to bring greater clarity to this large, diverse and rapidly expanding field of enquiry populated by heterogeneous discourses, multiple approaches and a variety of recommendations as to the ways forward. 相似文献
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20世纪70年代以来,随着经济全球化和区域经济一体化的快速发展,跨境经济合作区作为次区域经济合作的一种组织模式开始不断发展并得到学术界的重视。“一带一路”倡议提出后,边境地区越来越成为对外开放的前沿,而跨境经济合作区成为促进中国与沿线国家经贸合作的重要载体。由于涉及国家主权让渡及各种要素跨越国界流动,跨境经济合作区具有高度的复杂性和不确定性,其建设进展相对较慢。通过中老磨憨-磨丁经济合作区的案例分析,深入探讨跨境经济合作区的建设难题、形成机制及突破路径。研究认为,跨境经济合作区建设面临典型的“尺度困境”,其困境大小主要取决于区域经济一体化程度,国内多尺度治理协调程度及边境双边治理结构匹配程度等3个维度。跨境经济合作区作为多尺度汇聚的边境地区,区域经济一体化是降低跨境合作边界屏蔽效应的关键,而其治理需要从国家到地方多尺度的紧密合作以及边境双边的制度对接。打破跨境经济合作区的“尺度困境”需要边境双边协同治理,国家尺度较完善的制度安排,或给予地方较大的自主权限,同时要加强跨境基础设施建设。 相似文献
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The Aichi 2020 Targets, under the Convention on Biological Diversity (CBD), aim to halt the loss of biodiversity by 2020, in order to ensure that ecosystems continue to provide essential services. Here we apply a social–ecological systems analysis to provide insight into the diverse system interactions that pose impediments to delivery of the Aichi Targets. We applied an analytical framework of pair-wise exchanges along six axes between the social, economic, environmental and political loci of the global social–ecological system. The analysis identified that many impediments result from partial decoupling in the system through phenomena including delayed feedbacks and insufficient information flows. It suggests 15 of the Aichi Targets are unlikely to be delivered; 3 are likely to be delivered in part; and 2 in full. We considered how interventions at leverage points may overcome the impediments, and compared these to actions included within the Implementation Decision for the Aichi Targets, to find gaps. These new leverage points to fill identified gaps involve many aspects of system re-coupling: co-production of knowledge and more equitable food systems governance (environmental–social axis); support for social change movements (social–political axis); an appropriate financial target for biodiversity conservation investment, with a clear means of implementation such as a currency transaction tax (economic–political axis); and co-governance of natural resources (environmental–political axis). The recently released Global Biodiversity Outlook 4 shows that 18 of the 20 Aichi Targets are tracking in accordance with our analysis; and that current efforts are unlikely to result in an improvement in the base state of biodiversity by 2020, confirming some of our results. We argue that attention to the interactions within, and the partial decoupling of, the global social–ecological system provides new insights, and is worthy of further attention both for delivery of the Aichi Targets and for guiding longer term actions for the conservation of biodiversity. 相似文献
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张世萍 《成都信息工程学院学报》2005,20(2):223-226
通过对悖论性命题的仔细分析，发现这些命题的真值会发生周期性的变化。因此，将振荡周期函数纳入语形学，以便从形式上揭示悖论性命题真假周期性过渡这种对立统一结构，再纳入动态蒙太格赋予振荡函数里的变元以意义，这样就从语形、语义、语用的角度构建了一种动态语义模型，从而达到消解语义悖论的目的。 相似文献