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   43865篇
  免费   3951篇
  国内免费   5694篇
测绘学   2712篇
大气科学   6002篇
地球物理   9842篇
地质学   21200篇
海洋学   3924篇
天文学   2449篇
综合类   3896篇
自然地理   3485篇
  2024年   116篇
  2023年   407篇
  2022年   917篇
  2021年   1073篇
  2020年   865篇
  2019年   933篇
  2018年   5552篇
  2017年   4780篇
  2016年   3491篇
  2015年   1227篇
  2014年   1246篇
  2013年   1289篇
  2012年   2121篇
  2011年   3795篇
  2010年   3189篇
  2009年   3378篇
  2008年   2789篇
  2007年   3282篇
  2006年   836篇
  2005年   915篇
  2004年   939篇
  2003年   996篇
  2002年   886篇
  2001年   658篇
  2000年   743篇
  1999年   992篇
  1998年   815篇
  1997年   779篇
  1996年   749篇
  1995年   664篇
  1994年   579篇
  1993年   498篇
  1992年   416篇
  1991年   288篇
  1990年   232篇
  1989年   193篇
  1988年   181篇
  1987年   103篇
  1986年   111篇
  1985年   70篇
  1984年   60篇
  1983年   58篇
  1982年   57篇
  1981年   62篇
  1980年   59篇
  1979年   34篇
  1978年   14篇
  1976年   12篇
  1974年   9篇
  1958年   16篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
961.
The nature and style of emplacement of Continental Flood Basalt (CFB) lava flows has been a matter of great interest as well as considerable controversy in the recent past. However, even a cursory review of published literature reveals that the Columbia River Basalt Group (CRBG) and Hawaiian volcanoes provide most of the data relevant to this topic. It is interesting to note, however, that the CRBG lava flows and their palaeotopographic control is atypical of other CFB provinces in the world. In this paper, we first present a short overview of important studies pertaining to the emplacement of flood basalt flows. We then briefly review the morphology of lava flows from the Deccan Volcanic Province (DVP) and the Columbia-Oregon Plateau flood basalts. The review underscores the existence of significant variations in lava flow morphology between different provinces, and even within the same province. It is quite likely that there were more than one way of emplacing the voluminous and extensive CFB lava flows. We argue that the establishment of general models of emplacement must be based on a comprehensive documentation of lava flow morphology from all CFB provinces.  相似文献   
962.
Women in many rural areas of the US often engage in small-scale businesses as one of several avenues for contributing to household incomes. In those geographical areas that contain a significant minority population, many of the women's businesses display the cultural diversity of the regions in which they live. Likewise, women's roles as economic providers as well as wives and mothers are reflected in the manner in which they operate their businesses. Candida Brush suggests that women's strategies for operating their businesses are highly integrative because women's business decisions are intertwined with familial responsibilities, household economic demands and desires to contribute to their communities. Hispanic and Native American women in the Four Corners region of southwestern US and African American women in central Virginia participated in interviews in which they responded to numerous short answer and open-ended questions about their businesses and decision-making strategies. From these interviews an image emerges of rural minority business women who are operating their businesses at the nexus of family obligations, economic necessities, cultural ties and with a commitment for serving their communities. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
963.
964.
“Three-component” method consists of three clase-connected aspects: geological anomaly,diversity of mineralization and mineral deposit spectrum. All these three concepts are not new separately, but it is a new approach to combine these three aspects in one single concept for quantitative mineral resources prediction and assessment and it is also the first time to conduct a more detailed study in each aspect. Investigation and clarification of geological anomalies, diversity of mineralization and spectrum of mineral deposits are realized by digitization and quantification of ore forming controlling factors, oreexisting symbols or marks, characteristics of mineralization and regulation of ore-genesis and laws of distribution. These procedures lead to construction of a “digital model“ for mineral resources prediction andassessment.  相似文献   
965.
Asynchronous Holocene climatic change across China   总被引:1,自引:0,他引:1  
A review of Holocene climatic variations in different parts of China shows that they were asynchronous. Proxy data from ice cores, pollen, loess, lacustrine sediments, and changes of sea and lake levels demonstrate that many warm and cold oscillations have occurred in China during the Holocene, including a most important climatic event known as the “Holocene optimum,” a milder and wetter period, and that the duration and amplitude of the optimum period, as well as its start and end times, differed in different parts of China. Uplift of the Tibetan plateau over the past millions of years led to the development of the monsoon climate and to complex atmospheric circulation over continental China during the Holocene. As a result, the Holocene optimum began and terminated earlier in high-altitude regions of western China than at lower elevations in eastern China, and the amplitude of the variations was lower in the east. This suggests that the western higher-altitude areas were more sensitive to climatic change than were the eastern lower-altitude areas. Holocene climatic records in the Dunde and Guliya ice cores do not correspond. Inverse δ18O variations between the two cores indicate that the effects of climate and atmospheric processes on the stable isotopes at the two sites differed. The correlation between the isotopic composition of carbonates in lake deposits in western China and climatic variations is similar to that in the ice cores. The climatic resolution in ice cores and lake sediments is higher than that in other media. The lack of precise correspondence of climatic records constructed on the basis of proxy data from different parts of China is a result of the different locations and elevations of the sampling sites, the different resolutions of the source material, and the varied climatic conditions within China. Further work is needed to confirm both the conclusions and the inferences presented here.  相似文献   
966.
One hundred and eighty lakes, ponds and reservoirs in Taiwan, and the offshore islands have been investigated since 1985. Effects such as warming/cooling, mixing, photosynthesis and respiration on pH have been identified. These effects were found to be less profound than those resulting from the geology. Since there is little rain or lake water pH data available prior to 1970, it was necessary to use proxy data in order to ascertain the history of lake acidification. In this study, we present data based on diatoms in a sediment core collected from a lake in the remote subalpine region of southern Taiwan. The acidity of this lake water was found to have increased since 1900. We measured the concentrations of 55 chemical species in lake water samples. In addition, concentrations of 26 chemical species were obtained from sediments. Conductivity, alkalinity (HCO3 ?), most other major and minor chemical species, as well as the non sea-salt SO4/Na ratio in lake water clearly decrease with elevation. Distribution of pH is similar, although the trend is less clear. Distribution of trace metals, however, shows a different trend as anthropogenic pollution, diagenetic reduction and lake acidification are at play.  相似文献   
967.
在首次对特色生物资源的内涵科学界定的基础上,探讨了特色生物资源开发的理论基础及战略意义,重点介绍了国家“十五”重大科技攻关项目——“攀西地区特色生物资源综合开发与示范”项目的设计思路、研究内容及实施路线,对项目实施的社会、生态与经济效益进行分析,认为该项目为我国特色生物资源开发提供了一些可借鉴的经验。  相似文献   
968.
攀西地区具有发展各类果树得天独厚的自然资源优势。根据攀西特色水果的特点和产业发展的条件,在大区域背景下,分析了攀西地区特色水果在国内市场的竞争优势,明确了攀西地区特色水果的市场定位:大力发展早熟枇杷、石榴和晚熟芒果及深加工,以品质的优良、上市时间的优势占领高端市场,把资源优势转化为产品优势和经济优势。在此基础上,提出了攀西地区特色水果发展中需要重视的问题。  相似文献   
969.
西南地区中药产业发展   总被引:1,自引:0,他引:1  
本文总结西南地区中药产业发展现状,分析西南地区中药产业中药材、生产加工和研发能力上的竞争优势,提出西南地区中药产业的发展对策。  相似文献   
970.
Fluid inclusion and scanning electron microscope‐cathodoluminescence evidence indicates focused hot, saline, diagenetic fluid flow within the Eastern Flank of the Britannia Field, offshore Scotland, UK. The fluid was sourced from the Andrew Salt Dome, 10 km to the east. The fluids, which promoted quartz cementation of the upper zones within the field, were up to ~30°C hotter and had salinities up to ~10 wt% NaCl equivalent higher than fluids from lower in the reservoir section. During diagenesis hot saline fluids migrated westwards as part of a radiating ‘diagenetic front’ from the Andrew Salt Dome. Structural dip associated with the Eastern Flank of the Fladen Ground Spur impeded the westward movement of the diagenetic fluid. The quartz cements from the upper and lower reservoir zones can be distinguished by morphology. In the upper zones the quartz cements have well‐developed macro‐crystalline zoning and heterogeneous luminescence across the grain. In the lower zones, the cements are much less developed, unzoned and very weakly luminescent. The diagenetic fluids were primarily focused into Zone 45 within the upper reservoir. Furthermore, within the Main Platform Area the most prolific producing zone is Zone 45, indicating the importance of this interval as a permeable flow unit during both diagenetic and production timescales. Within the Eastern Flank, the quartz overgrowths have a major impact on reservoir permeability and thus well productivity. The overgrowths are most extensive in the originally clean sandstones with low clay content. Clay in optimum volumes (5–10%) can inhibit nucleation of the damaging quartz overgrowths without having a detrimental effect on pore connectivity. These observations provide a predictive concept for use in the search for relative reservoir sweetspots within the degraded Eastern Flank.  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