全文获取类型
收费全文 | 1115篇 |
免费 | 49篇 |
国内免费 | 9篇 |
专业分类
测绘学 | 63篇 |
大气科学 | 82篇 |
地球物理 | 269篇 |
地质学 | 394篇 |
海洋学 | 62篇 |
天文学 | 183篇 |
综合类 | 11篇 |
自然地理 | 109篇 |
出版年
2023年 | 6篇 |
2022年 | 13篇 |
2021年 | 20篇 |
2020年 | 23篇 |
2019年 | 24篇 |
2018年 | 37篇 |
2017年 | 25篇 |
2016年 | 44篇 |
2015年 | 36篇 |
2014年 | 46篇 |
2013年 | 80篇 |
2012年 | 47篇 |
2011年 | 49篇 |
2010年 | 46篇 |
2009年 | 66篇 |
2008年 | 43篇 |
2007年 | 53篇 |
2006年 | 49篇 |
2005年 | 43篇 |
2004年 | 37篇 |
2003年 | 41篇 |
2002年 | 41篇 |
2001年 | 23篇 |
2000年 | 22篇 |
1999年 | 15篇 |
1998年 | 12篇 |
1997年 | 10篇 |
1996年 | 16篇 |
1995年 | 13篇 |
1994年 | 7篇 |
1993年 | 9篇 |
1992年 | 5篇 |
1991年 | 8篇 |
1990年 | 9篇 |
1989年 | 5篇 |
1988年 | 6篇 |
1987年 | 9篇 |
1984年 | 9篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 8篇 |
1980年 | 8篇 |
1979年 | 10篇 |
1978年 | 12篇 |
1977年 | 7篇 |
1975年 | 10篇 |
1974年 | 6篇 |
1973年 | 11篇 |
1972年 | 6篇 |
1970年 | 7篇 |
排序方式: 共有1173条查询结果,搜索用时 109 毫秒
1.
Daniel P. Glavin Jason P. Dworkin Andrew Aubrey Oliver Botta James H. Doty Zita Martins Jeffrey L. Bada 《Meteoritics & planetary science》2006,41(6):889-902
Abstract— Amino acid analyses of the Antarctic CM2 chondrites Allan Hills (ALH) 83100 and Lewis Cliff (LEW) 90500 using liquid chromatography‐time of flight‐mass spectrometry (LC‐ToF‐MS) coupled with UV fluorescence detection revealed that these carbonaceous meteorites contain a suite of indigenous amino acids not present in Antarctic ice. Several amino acids were detected in ALH 83100, including glycine, alanine, β‐alanine, γ‐amino‐n‐butyric acid (γ‐ABA), and α‐aminoisobutyric acid (AIB) with concentrations ranging from 250 to 340 parts per billion (ppb). In contrast to ALH 83100, the CM2 meteorites LEW 90500 and Murchison had a much higher total abundance of these amino acids (440–3200 ppb). In addition, ALH 83100 was found to have lower abundances of the α‐dialkyl amino acids AIB and isovaline than LEW 90500 and Murchison. There are three possible explanations for the depleted amino acid content in ALH 83100: 1) amino acid leaching from ALH 83100 during exposure to Antarctic ice meltwater, 2) a higher degree of aqueous alteration on the ALH 83100 parent body, or 3) ALH 83100 originated on a chemically distinct parent body from the other two CM2 meteorites. The high relative abundance of ?‐amino‐n‐caproic acid (EACA) in the ALH 83100 meteorite as well as the Antarctic ice indicates that Nylon‐6 contamination from the Antarctic sample storage bags may have occurred during collection. 相似文献
2.
3.
4.
5.
6.
Ronald Amundson Stephanie Ewing Brad Sutter Oliver Chadwick Michelle Walvoord 《Geochimica et cosmochimica acta》2008,72(15):3845-3864
Early (>3 Gy) wetter climate conditions on Mars have been proposed, and it is thus likely that pedogenic processes have occurred there at some point in the past. Soil and rock chemistry of the Martian landing sites were evaluated to test the hypothesis that in situ aqueous alteration and downward movement of solutes have been among the processes that have transformed these portions of the Mars regolith. A geochemical mass balance shows that Martian soils at three landing sites have lost significant quantities of major rock-forming elements and have gained elements that are likely present as soluble ions. The loss of elements is interpreted to have occurred during an earlier stage(s) of weathering that may have been accompanied by the downward transport of weathering products, and the salts are interpreted to be emplaced later in a drier Mars history. Chemical differences exist among the sites, indicating regional differences in soil composition. Shallow soil profile excavations at Gusev crater are consistent with late stage downward migration of salts, implying the presence of small amounts of liquid water even in relatively recent Martian history. While the mechanisms for chemical weathering and salt additions on Mars remain unclear, the soil chemistry appears to record a decline in leaching efficiency. A deep sedimentary exposure at Endurance crater contains complex depth profiles of SO4, Cl, and Br, trends generally consistent with downward aqueous transport accompanied by drying. While no model for the origin of Martian soils can be fully constrained with the currently available data, a pedogenic origin is consistent with observed Martian geology and geochemistry, and provides a testable hypothesis that can be evaluated with present and future data from the Mars surface. 相似文献
7.
The S and O isotopic composition of dissolved SO4, used as a tracer for SO4 sources, was applied to the water of the Llobregat River system (NE Spain). The survey was carried out at 30 sites where surface water was sampled on a monthly basis over a period of 2a. The concentration of dissolved SO4 varied from 20 to 1575 mg L−1. Sulphur isotopic compositions clustered in two populations: one – 93% of the samples – had positive values with a mode of +9‰; the other had negative values and a mode of −5‰. Data for δ18OSO4 showed a mean value of +11‰, with no bi-modal distribution, though lower values of δ18O corresponded to samples with negative δ34S. These values can not be explained solely by the contribution of bedrock SO4 sources: that is, sulphide oxidation and the weathering of outcrops of sulphates, though numerous chemical sediments exist in the basin. Even in a river with a high concentration of natural sources of dissolved SO4, such as the Llobregat River, the δ34S values suggest that dissolved SO4 is controlled by a complex mix of both natural and anthropogenic sources. The main anthropogenic sources in this basin are fertilizers, sewage, potash mine effluent and power plant emissions. Detailed river water sampling, together with the chemical and isotopic characterisation of the main anthropogenic inputs, allowed determination of the influence of redox processes, as well as identification of the contribution of natural and anthropogenic SO4 sources and detection of spatial variations and seasonal changes among these sources. For instance, in the Llobregat River the input of fertilisers is well marked seasonally. Minimum values of δ34S are reported during fertilization periods – from January to March – indicating a higher contribution of this source. The dual isotope approach, δ34S and δ18O, is useful to better constrain the sources of SO4. Moreover, in small-scale studies, where the inputs are well known and limited, the mixing models can be enhanced and the contribution of the different sources can be quantified to some extent. 相似文献
8.
9.
Genetic Relationships Among One Non-Endemic and Two Endemic Mediterranean Triplefin Blennies (Pisces, Blennioidei) 总被引:1,自引:0,他引:1
Gerard J. Geertjes Albert Kamping Wilke van Delden & John J. Videler 《Marine Ecology》2001,22(3):255-265
Abstract. Three triplefin blennies occur sympatrically in the Mediterranean Sea; Tripterygion tripteronotus and T. melanurus are endemic, whereas T. delaisi is also found in the Eastern Atlantic. Although very similar in morphology, ecology and behaviour, some striking differences exist among reproductive strategies. Several authors proposed hypotheses on the evolution of these species. In order to enhance insight into the genetic structure of this genus, an enzyme electrophoretic survey was performed. Samples of 40 to 45 specimens of each species were collected near Calvi (Corsica, France). Twenty-four enzyme loci were analysed, of which 17 appeared polymorphic. T. delaisi showed a high level of average observed heterozygosity (Ho ) of 0.140, as opposed to very low levels of 0.009 and 0.021 in T. tripteronotus and T. melanurus , respectively. Nei's genetic distance was 0.21 between T. delaisi and T. tripteronotus , 0.35 between T. delaisi and T. melanurus and 0.45 between T. tripteronotus and T. melanurus . We hypothesise that divergence of these species started before the Pleistocene and that the endemic species survived the Pleistocene glaciations in refugia within the Mediterranean. The possibility that T. tripteronotus and T. delaisi diverged through sympatric speciation is discussed. 相似文献
10.
Microzooplankton production in the oceans 总被引:7,自引:0,他引:7