全文获取类型
收费全文 | 50385篇 |
免费 | 8518篇 |
国内免费 | 11746篇 |
专业分类
测绘学 | 4272篇 |
大气科学 | 5527篇 |
地球物理 | 10836篇 |
地质学 | 30142篇 |
海洋学 | 6324篇 |
天文学 | 2200篇 |
综合类 | 3609篇 |
自然地理 | 7739篇 |
出版年
2024年 | 281篇 |
2023年 | 741篇 |
2022年 | 1486篇 |
2021年 | 1657篇 |
2020年 | 1739篇 |
2019年 | 1995篇 |
2018年 | 1609篇 |
2017年 | 1844篇 |
2016年 | 2040篇 |
2015年 | 2203篇 |
2014年 | 2703篇 |
2013年 | 2645篇 |
2012年 | 2963篇 |
2011年 | 3170篇 |
2010年 | 2721篇 |
2009年 | 3189篇 |
2008年 | 3192篇 |
2007年 | 3502篇 |
2006年 | 3395篇 |
2005年 | 3011篇 |
2004年 | 2782篇 |
2003年 | 2682篇 |
2002年 | 2401篇 |
2001年 | 2121篇 |
2000年 | 1978篇 |
1999年 | 1834篇 |
1998年 | 1563篇 |
1997年 | 1460篇 |
1996年 | 1320篇 |
1995年 | 1126篇 |
1994年 | 1143篇 |
1993年 | 985篇 |
1992年 | 781篇 |
1991年 | 541篇 |
1990年 | 485篇 |
1989年 | 381篇 |
1988年 | 297篇 |
1987年 | 192篇 |
1986年 | 125篇 |
1985年 | 89篇 |
1984年 | 36篇 |
1983年 | 33篇 |
1982年 | 32篇 |
1981年 | 25篇 |
1980年 | 24篇 |
1979年 | 32篇 |
1978年 | 35篇 |
1977年 | 25篇 |
1975年 | 4篇 |
1954年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
142.
Abstract. The emergence of the Isthmus of Panama subdivided the amphi-American biota. In the present study, Pacific and Atlantic populations of four cognate pairs of crabs were used to discern whether exposure to different thermal regimes in habitats, in the putative absence of gene flow, has resulted in physiological divergence. Populations that potentially form a common genetic pool were also used; these were populations of the Atlantic Panama cognate that occur in Belize and Florida. Decreases in water temperature occur periodically in Pacific Panama and Florida, but not in Atlantic Panama or Belize. In this study, physiological divergence in oxygen uptake was assessed in response to repeated exposure to either control and decreased temperature or control and increased temperature. Results indicate that, in only some of the genera tested, exposure to decreases in habitat temperature has resulted in divergence. Partial support is found for the corollary that adaptation to an environment with periods of decreased temperature results in reduced compensation during exposure to elevated temperature. 相似文献
143.
热带气旋活动对中国夏季雨带类型与ENSO的响应关系 总被引:2,自引:0,他引:2
本文分析研究了热带气旋1)活动与中国夏季2)3组(包括8种)主要多雨带类型的响应关系。同时对热带气旋活动与夏季大陆气温和ENSO事件的始终时间的响应关系,也进行了探讨。分析表明,夏季中国的雨带分布特征对热带气旋活动有着十分重要的指示意义。热带气旋活动与中国大陆气温有着正相关关系,与ENSO事件的发生有着反相关关系 相似文献
144.
145.
146.
辽河盆地大民屯凹陷流体压力特征 总被引:1,自引:0,他引:1
大民屯凹陷是辽河断陷内4个下第三系凹陷之一。在综合利用钻井、试井及地震等资料的基础上,系统研究并论述了大民屯凹陷流体压力特征。基于57口井的声波测井资料,凹陷内泥岩压力特征可区分为正常压力、异常压力或强超压等类型;根据152口井391个点的压力测试数据,凹陷内产油层段的压力梯度多接近于1;利用公式法模拟计算了47条地震剖面的流体压力、剩余压力及压力系数的分布特征,凹陷内剖面压力系统自上而下一般由正常压力、弱超压和强超压3部分组成。此外,还根据流体压力演化的基本原理及钻井、岩性与试井等实际资料,模拟恢复了大民屯凹陷的压力演化史,其可划分为超压原始积累、超压部分释放及超压再积聚3个阶段。总体上,大民屯凹陷的超压强度低于渤海湾盆地其他地区的超压强度。 相似文献
147.
Following Bagnold's approach, a relationship between sediment transport and energy dissipation is developed. The major assumption made in the study is that the near bed velocity plays a dominant role in the process of sediment transport. A general relationship between energy dissipation and sediment transport is first proposed. Then the equations for total sediment transport are derived by introducing the appropriate expression of energy dissipation rate under different conditions, such as open channel flows, combination of wave and current, as well as longshore sediment transport. Within the flows investigated, the derived relationships are fairly consistent with the available data over a wide range of conditions. 相似文献
148.
The Ocean Drilling Program Leg 175 recovered a unique series of stratigraphically continuous sedimentary sections along the SW African margin, an area which is presently affected by active coastal upwelling. The accumulation rates of organic and inorganic carbon are a major component of this record. Four Leg 175 sites (1082, 1084, 1085, 1087) are chosen as part of a latitudinal transect from the present northern to southern boundaries of the Benguela Current upwelling system, to decipher the Pliocene–Pleistocene history of biogenic production and its relationship with global and local changes in oceanic circulation and climate. The pattern of CaCO3 and Corg mass accumulation rates (MARs) over 0.25-Myr intervals indicates that the evolution of carbon burial is highly variable between the northern and the southern Benguela regions, as well as between sites that have similar hydrological conditions. This, as well as the presence over most locations of high-amplitude, rapid changes of carbon burial, reflect the partitioning of biogenic production and patterns of sedimentation into local compartments over the Benguela margin. The combined mapping of CaCO3 and Corg MARs at the study locations suggests four distinct evolutionary periods, which are essentially linked with major steps in global climate change: the early Pliocene, the mid-Pliocene warm event, a late Pliocene intensification of northern hemisphere glaciation and the Pleistocene. The early Pliocene spatially heterogeneous patterns of carbon burial are thought to reflect the occurrence of mass-gravitational movements over the Benguela slope which resulted in disruption of the recorded biogenic production. This was followed (3.5–3 Ma) by an episode of peak carbonate accumulation over the whole margin and, subsequently, by the onset of Benguela provincialism into a northern and a southern sedimentary regime near 2 Ma. This mid and late Pliocene evolution is interpreted as a direct response to changes in the ventilation of bottom and intermediate waters, as well as to dynamics of the subtropical gyral circulation and associated wind stress. 相似文献
149.
150.