全文获取类型
收费全文 | 3345篇 |
免费 | 142篇 |
国内免费 | 10篇 |
专业分类
测绘学 | 126篇 |
大气科学 | 221篇 |
地球物理 | 1188篇 |
地质学 | 1066篇 |
海洋学 | 205篇 |
天文学 | 486篇 |
综合类 | 20篇 |
自然地理 | 185篇 |
出版年
2022年 | 25篇 |
2021年 | 50篇 |
2020年 | 49篇 |
2019年 | 52篇 |
2018年 | 305篇 |
2017年 | 199篇 |
2016年 | 164篇 |
2015年 | 93篇 |
2014年 | 137篇 |
2013年 | 164篇 |
2012年 | 75篇 |
2011年 | 155篇 |
2010年 | 156篇 |
2009年 | 172篇 |
2008年 | 130篇 |
2007年 | 106篇 |
2006年 | 116篇 |
2005年 | 89篇 |
2004年 | 88篇 |
2003年 | 86篇 |
2002年 | 73篇 |
2001年 | 52篇 |
2000年 | 50篇 |
1999年 | 33篇 |
1998年 | 46篇 |
1997年 | 36篇 |
1996年 | 53篇 |
1995年 | 37篇 |
1994年 | 33篇 |
1993年 | 41篇 |
1992年 | 31篇 |
1991年 | 21篇 |
1990年 | 26篇 |
1989年 | 32篇 |
1988年 | 26篇 |
1987年 | 29篇 |
1986年 | 29篇 |
1985年 | 27篇 |
1984年 | 23篇 |
1983年 | 28篇 |
1982年 | 25篇 |
1981年 | 21篇 |
1980年 | 24篇 |
1979年 | 23篇 |
1978年 | 27篇 |
1977年 | 25篇 |
1976年 | 19篇 |
1975年 | 18篇 |
1973年 | 17篇 |
1971年 | 21篇 |
排序方式: 共有3497条查询结果,搜索用时 390 毫秒
681.
Population growth and loss of arable land 总被引:1,自引:0,他引:1
Bo R. Ds 《Global Environmental Change》2002,12(4)
I discuss the loss of cropland in developing countries in connection with the ongoing land conversion caused by the growing population and socio-economic development, resulting in an increased demand for housing, industry, infrastructure, etc.Based on assumptions about the required space per capita for other purposes than agriculture, the portion of this area that is removed from presently used cropland, and the quality of the available land reserves, the required demand for land reserves has been calculated.The main conclusions are that during the next three decades (i) the loss of cropland is likely to be within the range 30–60 Mha, (ii) the reserve land utilized will be about 100–200 Mha, and (iii) the reserve land still in use after 30 years, about 50–100 Mha. 相似文献
682.
Guitart C García-Flor N Dachs J Bayona JM Albaigés J 《Marine pollution bulletin》2004,48(9-10):961-968
The sea surface microlayer (SML) may play an important role on the transport and fate of persistent organic pollutants in the marine environment. In order to evaluate the appropriateness of a number of sampling devices for the analysis of 14 parent polycyclic aromatic hydrocarbons (3-5 aromatic rings), marine SML waters were sampled using a glass plate, a rotating drum/roller, a metal screen and a surface slick sampler. The underlying waters were also sampled for the determination of the corresponding enrichment factors (EF = [C](microlayer)/[C](underlying water)). The EFs were phase dependent, ranging from 1 to 3 for the dissolved phase and between 4 and 7 for the particulate phase. In order to better assess the performance of the different sampling methods, in terms of phase partitioning, the truly dissolved and colloidal phases were also estimated. Generally, no significant differences were found for the enrichment factors provided by the different methods, due to the observed large variability in concentrations that can be attributed to small-scale coastal processes. However, the metal screen is recommended as the most efficient sampling method for the study of PAHs taking into account the amount of water collected versus time. 相似文献
683.
D. Utz A. Hanslmeier A. Veronig O. Kühner R. Muller J. Jurčák B. Lemmerer 《Solar physics》2013,284(2):363-378
Small-scale magnetic fields can be observed on the Sun in high-resolution G-band filtergrams as magnetic bright points (MBPs). We study Hinode/Solar Optical Telescope (SOT) longitude and latitude scans of the quiet solar surface taken in the G-band in order to characterise the centre-to-limb dependence of MBP properties (size and intensity). We find that the MBP’s sizes increase and their intensities decrease from the solar centre towards the limb. The size distribution can be fitted using a log–normal function. The natural logarithm of the mean (μ parameter) of this function follows a second-order polynomial and the generalised standard deviation (σ parameter) follows a fourth-order polynomial or equally well (within statistical errors) a sine function. The brightness decrease of the features is smaller than one would expect from the normal solar centre-to-limb variation; that is to say, the ratio of a MBP’s brightness to the mean intensity of the image increases towards the limb. The centre-to-limb variations of the intensities of the MBPs and the quiet-Sun field can be fitted by a second-order polynomial. The detailed physical process that results in an increase of a MBP’s brightness and size from Sun centre to the limb is not yet understood and has to be studied in more detail in the future. 相似文献
684.
The role of hotter than ambient plume mantle in the formation of a rifted volcanic margin in the northern Arabian Sea is investigated using subsidence analysis of a drill site located on the seismically defined Somnath volcanic ridge. The ridge has experienced > 4 km of subsidence since 65 Ma and lies within oceanic lithosphere. We estimate crustal thickness to be 9.5–11.5 km. Curiously < 400 m of the thermal subsidence occurred prior to 37 Ma, when subsidence rates would normally be at a maximum. We reject the hypothesis that this was caused by increasing plume dynamic support after continental break-up because the size of the thermal anomalies required are unrealistic (> 600 °C), especially considering the rapid northward drift of India relative to the Deccan-Réunion hotspot. We suggest that this reflects very slow lithospheric growth, possibly caused by vigorous asthenospheric convection lasting > 28 m.y., and induced by the steep continent–ocean boundary. Post-rift slow subsidence is also recognized on volcanic margins in the NE Atlantic and SE Newfoundland and cannot be used as a unique indicator of plume mantle involvement in continental break-up. 相似文献
685.
Anthony Hildenbrand Pedro Madureira Fernando Ornelas Marques Inês Cruz Bernard Henry Pedro Silva 《Earth and Planetary Science Letters》2008,273(3-4):289-298
New K/Ar dating and geochemical analyses have been carried out on the WNW–ESE elongated oceanic island of S. Jorge to reconstruct the volcanic evolution of a linear ridge developed close to the Azores triple junction. We show that S. Jorge sub-aerial construction encompasses the last 1.3 Myr, a time interval far much longer than previously reported. The early development of the ridge involved a sub-aerial building phase exposed in the southeast end of the island and now constrained between 1.32 ± 0.02 and 1.21 ± 0.02 Ma. Basic lavas from this older stage are alkaline and enriched in incompatible elements, reflecting partial melting of an enriched mantle source. At least three differentiation cycles from alkaline basalts to mugearites are documented within this stage. The successive episodes of magma rising, storage and evolution suggest an intermittent re-opening of the magma feeding system, possibly due to recurrent tensional or trans-tensional tectonic events. Present data show a gap in sub-aerial volcanism before a second main ongoing building phase starting at about 750 ka. Sub-aerial construction of the S. Jorge ridge migrated progressively towards the west, but involved several overlapping volcanic episodes constrained along the main WNW–ESE structural axis of the island. Mafic magmas erupted during the second phase have been also generated by partial melting of an enriched mantle source. Trace element data suggest, however, variable and lower degrees of partial melting of a shallower mantle domain, which is interpreted as an increasing control of lithospheric deformation on the genesis and extraction of primitive melts during the last 750 kyr. The multi-stage development of the S. Jorge volcanic ridge over the last 1.3 Myr has most likely been greatly influenced by regional tectonics, controlled by deformation along the diffuse boundary between the Nubian and the Eurasian plates, and the increasing effect of sea-floor spreading at the Mid-Atlantic Ridge. 相似文献
686.
687.
Rodríguez-Ortega MJ Rodríguez-Ariza A Gómez-Ariza JL Muñoz-Serrano A López-Barea J 《Marine pollution bulletin》2009,58(1):64-327
Molecular biomarkers are among the most sensitive and earliest responses to pollutants. However, lack of detailed knowledge on variability of responses and their possible seasonal variation limit their use. In addition, the seasonality of biological processes modulates the response of organisms to pollutant stressors. Using multivariate statistics, we have studied the influence of environmental and biological factors on the response of a battery of molecular biomarkers in the clam Chamaelea gallina collected along the South-Spanish littoral. Multivariate discriminant analysis clearly distinguished biomarker response between clean and polluted areas, using heavy metals as indicator of pollution. Such differences disappeared when the dataset was normalised for metal content, thus indicating that pollution was the main significant cause of the changes observed between clean and polluted sites. In conclusion, this work shows that, when applying a complete biomarker panel, multivariate statistical tools can be used to discern pollutant- from non pollutant-related responses. 相似文献
688.
689.
690.
J. Guilln S. Soriano M. Demestre A. Falqus A. Palanques P. Puig 《Continental Shelf Research》2008,28(17):41
This work deals with the potential influence of benthic communities on the sediment dynamics of the coastal zone, and specifically with the modifications to bottom roughness caused by communities and their effects on wave propagation across the coastal profile. Time-series of video observations of the sea bottom on the Ebro delta coast were analysed in order to estimate the bottom roughness associated with physical and biological morphological components and the sediment reworking rates caused by epifaunal organisms. Biological roughness was mainly caused by ophiuroids and tanatocenosis of bivalve and gastropod shells, which changed their abundance during the study period. The total biological roughness (Kbio) ranged between 0.27 and 0.81 cm and represented a significant part (<20%) of the total form drag roughness. Flattening of ripples caused by bioturbation was observed under low-energy conditions. Surface sediment perturbation and bioturbation rates were also estimated. Based on these observations a wave propagation model was applied in order to carry out a sensitivity analysis of the significance of biological roughness on wave dissipation in the study area under different wave conditions. 相似文献