首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   243篇
  免费   8篇
测绘学   10篇
大气科学   18篇
地球物理   54篇
地质学   93篇
海洋学   22篇
天文学   32篇
自然地理   22篇
  2022年   3篇
  2021年   7篇
  2020年   8篇
  2019年   6篇
  2018年   10篇
  2017年   5篇
  2016年   11篇
  2015年   11篇
  2014年   9篇
  2013年   19篇
  2012年   12篇
  2011年   16篇
  2010年   20篇
  2009年   21篇
  2008年   14篇
  2007年   6篇
  2006年   8篇
  2005年   11篇
  2004年   5篇
  2003年   10篇
  2002年   11篇
  2001年   5篇
  2000年   2篇
  1999年   2篇
  1997年   2篇
  1996年   2篇
  1994年   2篇
  1993年   1篇
  1992年   1篇
  1991年   1篇
  1988年   1篇
  1980年   1篇
  1979年   1篇
  1978年   2篇
  1976年   1篇
  1974年   2篇
  1973年   2篇
排序方式: 共有251条查询结果,搜索用时 31 毫秒
11.
12.
In this study, we employ wavelength‐dispersive X‐ray fluorescence (WDXRF) to characterize construction materials from Formative civic architecture (1000 B.C.E.–C.E. 400), ethnographic mudbricks, and off‐site controls from the Taraco Peninsula, Bolivia. The preparation of earthen construction materials for civic buildings can shed light on aspects of community development such as labor organization, resource management, and architectural technologies. We apply geochemical results to reconstructing how public buildings were made as communities moved toward socio‐political complexity in this region. However, there are few geochemical studies in the Andes, and little prior scientific analysis of earthen architecture. We therefore tested the efficacy of WDXRF for this region, and developed control materials. Our archaeological samples were selected from two Formative villages, Chiripa and Kala Uyuni. In this study, we performed WDXRF analyses on 63 archaeological and control samples including archaeological floors, walling, plasters, and mortars, as well as contemporary ethnographic walling and topsoils. Elemental signatures for 28 elements clearly distinguished the archaeological flooring, walling, plaster, and mortars from ethnographic and off‐site controls. More subtle variations were detected that distinguish study sites and the different material types. Laboratory‐calibrated multi‐element XRF effectively supports efforts to reconstruct the pathways to social complexity in the Titicaca Basin.  相似文献   
13.
14.
15.
The presence of domoic acid (DA) toxin from multiple species of Pseudo-nitzschia is a concern in the highly productive food webs of the northern Gulf of Mexico. We documented the Pseudo-nitzschia presence, abundance, blooms, and toxicity over three years along a transect ~100 km west of the Mississippi River Delta on the continental shelf. Pseudo-nitzschia were present throughout the year and occurred in high abundances (>104 cells l?1) in the early spring months during high Mississippi River (MSR) flow (~20,000 m3 s?1) but were most abundant (>106 cells l?1) when MSR discharge was relatively lower among the spring months. A high particulate toxin production (maximum reaching 13 μg DA l?1) was associated with the high cell abundances and exceeded, by an order of magnitude, prior reports of particulate DA concentrations in Louisiana coastal waters. Differences in Pseudo-nitzschia peak times and its toxicity were correlated mainly with the timing and magnitude of MSR discharge and changes in associated parameters such as nutrient stoichiometry and salinity. A negative relationship between high MSR discharge and Pseudo-nitzschia and particulate DA concentrations was documented. These riverine dynamics have the potential to influence DA contamination in pelagic and benthic food webs in the coastal waters of the northern Gulf of Mexico.  相似文献   
16.
Danian (Paleocene) reefs formed by ahermatypic scleractinian corals in relatively deep water are known in a few localities in southern Scandinavia. Reflection and shallow seismic profiles, and samples from drilling and scuba diving in bridge pier excavations in Øresund, the strait between Denmark and Sweden, for the first time allow interpretation of the factors that controlled the localisation of the reefs.  相似文献   
17.
The declining health of marine ecosystems around the world is evidence that current piecemeal governance is inadequate to successfully support healthy coastal and ocean ecosystems and sustain human uses of the ocean. One proposed solution to this problem is ecosystem-based marine spatial planning (MSP), which is a process that informs the spatial distribution of activities in the ocean so that existing and emerging uses can be maintained, use conflicts reduced, and ecosystem health and services protected and sustained for future generations. Because a key goal of ecosystem-based MSP is to maintain the delivery of ecosystem services that humans want and need, it must be based on ecological principles that articulate the scientifically recognized attributes of healthy, functioning ecosystems. These principles should be incorporated into a decision-making framework with clearly defined targets for these ecological attributes. This paper identifies ecological principles for MSP based on a synthesis of previously suggested and/or operationalized principles, along with recommendations generated by a group of twenty ecologists and marine scientists with diverse backgrounds and perspectives on MSP. The proposed four main ecological principles to guide MSP—maintaining or restoring: native species diversity, habitat diversity and heterogeneity, key species, and connectivity—and two additional guidelines, the need to account for context and uncertainty, must be explicitly taken into account in the planning process. When applied in concert with social, economic, and governance principles, these ecological principles can inform the designation and siting of ocean uses and the management of activities in the ocean to maintain or restore healthy ecosystems, allow delivery of marine ecosystem services, and ensure sustainable economic and social benefits.  相似文献   
18.
This paper briefly gives the Norwegian government's perspective and role in promoting developments in environmental technologies. These are necessary to create environmental improvements, and thus comply with environmental standards, against a background of increasing industrial demands and developments. The role of governments in promoting and increasing co-operation and collaboration with industry is discussed. For example, the European EUREKA programme aims to promote advanced technologies but against the present environmentally sound criteria.  相似文献   
19.
Feminist research methodologies have many advantages over more traditional positivist methodologies. Feminist research is differentiated from nonfeminist research in terms of its critiques of universality and objectivity and its emancipatory purpose. Drawing on my own research on the survival strategies of low-waged women workers in Worcester, Massachusetts, I argue that we need to examine more critically our feminist research methods in terms of the unequal power relationships on which the research process necessarily rests.  相似文献   
20.
Seventeen physical and chemical parameters were obtained from a hydroelectric reservoir located in a tropical region in the south of Brazil. Multivariate Principal Component Analysis (PCA) and Hierarchical Group Analysis (HGA) were used to identify the parameters discriminating the origin of water from the Tibagi and the Primeiro de Maio River, after it has passed the mixing region. The study was conducted during the dry and rainy seasons in July 2002 and February 2003 at three depths and three sampling sites located 0, 5, and 10 km away from the mixing region. The statistical methods showed to be appropriate for identifying the contribution of each tributary in the water mixing site of a complex water system. The most important discriminating parameter was the absorbance relation A(253 nm)/A(203 nm), followed by the concentrations of Fe(III), Mn(III), and Ni(II). An anthropogenic interference was found in the reservoir due to high Ni(II) and orthophosphate concentrations caused by a nearby town sewage discharge. The interference was more important during the dry periods due to the lower dispersion of the pollutants. Urgent initiatives should be taken from the state government to build treatment stations for the wastewater of the small cities around the Capivara hydroelectric reservoir to prevent the drinking water quality from deteriorating.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号