全文获取类型
收费全文 | 740篇 |
免费 | 38篇 |
国内免费 | 11篇 |
专业分类
测绘学 | 19篇 |
大气科学 | 46篇 |
地球物理 | 238篇 |
地质学 | 277篇 |
海洋学 | 82篇 |
天文学 | 83篇 |
综合类 | 7篇 |
自然地理 | 37篇 |
出版年
2023年 | 4篇 |
2022年 | 11篇 |
2021年 | 17篇 |
2020年 | 28篇 |
2019年 | 19篇 |
2018年 | 37篇 |
2017年 | 25篇 |
2016年 | 42篇 |
2015年 | 29篇 |
2014年 | 46篇 |
2013年 | 57篇 |
2012年 | 27篇 |
2011年 | 52篇 |
2010年 | 45篇 |
2009年 | 56篇 |
2008年 | 52篇 |
2007年 | 27篇 |
2006年 | 31篇 |
2005年 | 24篇 |
2004年 | 19篇 |
2003年 | 16篇 |
2002年 | 18篇 |
2001年 | 11篇 |
2000年 | 6篇 |
1999年 | 12篇 |
1998年 | 11篇 |
1996年 | 6篇 |
1995年 | 4篇 |
1994年 | 6篇 |
1993年 | 2篇 |
1992年 | 7篇 |
1991年 | 3篇 |
1990年 | 4篇 |
1989年 | 4篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1978年 | 4篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1974年 | 5篇 |
1973年 | 2篇 |
1970年 | 1篇 |
排序方式: 共有789条查询结果,搜索用时 20 毫秒
31.
Hydrocarbon distributions and stable isotope ratios of carbonates (δ13Ccar, δ18Ocar), kerogen (δ13Cker), extractable organic matter (δ13CEOM) and individual hydrocarbons of Liassic black shale samples from a prograde metamorphic sequence in the Swiss Alps were used to identify the major organic reactions with increasing metamorphic grade. The studied samples range from the diagenetic zone (<100°C) to amphibolite facies (∼550°C). The samples within the diagenetic zones (<100 and 150°C) are characterized by the dominance of C<20n-alkanes, suggesting an origin related with marine and/or bacterial inputs. The metamorphic samples (200 to 550°C) have distributions significantly dominated by C12 and C13n-alkanes, C14, C16 and C18n-alkylcyclopentanes and to a lesser extend C15, C17 and C21n-alkylcyclohexanes. The progressive 13C-enrichment (up to 3.9‰) with metamorphism of the C>17n-alkanes suggests the occurrence of cracking reactions of high molecular weight compounds. The isotopically heavier (up to 5.6‰) C<17n-alkanes in metamorphic samples are likely originated by thermal degradation of long-chain homologous with preferential release of isotopically light C1 and C2 radicals. The dominance of specific even C-number n-alkylcyclopentanes suggests an origin related to direct cyclization mechanism (without decarboxylation step) of algal or bacterial fatty acids occurring in reducing aqueous metamorphic fluid conditions. The regular increase of the concentrations of n-alkylcycloalkanes vs. C>13n-alkanes with metamorphism suggests progressive thermal release of kerogen-linked fatty acid precursors and degradation of n-alkanes. Changes of the steroid and terpenoid distributions are clearly related to increasing metamorphic temperatures. The absence of 18α(H)-22,29,30-trisnorneohopane (Ts), the occurrence of 17β(H)-trisnorhopane, 17β(H), 21α(H)-hopanes in the C29 to C31 range and 5α(H),14α(H),17α(H)-20R C27, C29 steranes in the low diagenetic samples (<100°C) are characteristic of immature bitumens. The higher thermal stress within the upper diagenetic zone (150°C) is marked by the presence of Ts, the disappearance of 17β(H)-trisnorhopane and thermodynamic equilibrium of the 22S/(22S + 22R) homohopane ratios. The increase of the ααα-sterane 20S/(20S + 20R) and 20R ββ/(ββ + αα) ratios (from 0.0 to 0.55 and from 0.0 to 0.40, respectively) in the upper diagenetic zone indicates the occurrence of isomerization reactions already at <150°C. However, the isomerization at C-20 (R → S) reaches thermodynamic equilibrium values already at the upper diagenesis (∼150°C) whereas the epimerisation at C-14 and C-17 (αα → ββ) arrives to constant values in the lower anchizone (∼200°C). The ratios Ts vs. 17α(H)-22,29,30-trisnorneohopane [(Ts/(Ts + Tm)] and 18α(H)-30-norneohopane (C29Ts) vs. 17α(H),21β(H)-30-norhopane [C29Ts/(C29Ts + C29)] increase until the medium anchizone (200 to 250°C) from 0.0 to 0.96 and from 0.0 to 0.44, respectively. An opposite trend towards lower values is observed in the higher metamorphic samples.The occurrence of specific hydrocarbons (e.g., n-alkylcyclopentanes, cadalene, hydrogenated aromatic compounds) in metamorphic samples points to kerogen degradation reactions most probably occurring in the presence of water and under reducing conditions. The changes of hydrocarbon distributions and carbon isotopic compositions of n-alkanes related to metamorphism suggest that the organic geochemistry may help to evaluate the lowest grades of prograde metamorphism. 相似文献
32.
Throughfall patterns in sugarcane and riparian forest: understanding the effect of sugarcane age and land use conversion 下载免费PDF全文
Robson Willians da Costa Silva Luiz Felippe Salemi Rafael Pires Fernandes Tatiana Morgan Berteli de Andrade Jorge Marcos de Moraes Plínio Barbosa de Camargo Luiz Antonio Martinelli 《水文研究》2016,30(15):2579-2589
Sugarcane is an annual crop with a dynamic canopy that changes over time mainly because of genetic adaptation. There is uncertainty about the temporal trends of throughfall (TF) in this important commercial crop. In the present paper, we used troughs to measure TF in a third and fourth ratoon and subsequently in a fourth and fifth ratoon. Additional measurements were carried out in an adjacent riparian forest. There were no significant differences between cycles of sugarcane, growth phases and riparian forest. The TF results for ratoon crop and riparian forest in 2011/2012 were 76% and 79.5% of gross rainfall, respectively, while in 2012/2013, they were 79% and 78%, respectively. However, TF was remarkably lower in the riparian forest relative to ratoon from the second half of the culm formation and elongation phase (280 days after harvest) until harvest. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
33.
34.
Short‐ and long‐term evapotranspiration rates at ecological restoration sites along a large river receiving rare flow events 下载免费PDF全文
Margaret Shanafield Hugo Gutiérrez‐Jurado Jesús Eliana Rodríguez‐Burgueño Jorge Ramírez‐Hernández Christopher J. Jarchow Pamela L. Nagler 《水文研究》2017,31(24):4328-4337
Many large rivers around the world no longer flow to their deltas, due to ever greater water withdrawals and diversions for human needs. However, the importance of riparian ecosystems is drawing increasing recognition, leading to the allocation of environmental flows to restore river processes. Accurate estimates of riparian plant evapotranspiration (ET) are needed to understand how the riverine system responds to these rare events and achieve the goals of environmental flows. In 2014, historic environmental flows were released into the Lower Colorado River at Morelos Dam (Mexico); this once perennial but now dry reach is the final stretch to the mighty Colorado River Delta. One of the primary goals was to supply native vegetation restoration sites along the reach with water to help seedlings establish and boost groundwater levels to foster the planted saplings. Patterns in ET before, during, and after the flows are useful for evaluating whether this goal was met and understanding the role that ET plays in this now ephemeral river system. Here, diurnal fluctuations in groundwater levels and Moderate Resolution Imaging Spectroradiometer (MODIS) data were used to compare estimates of ET specifically at 3 native vegetation restoration sites during 2014 planned flow events, and MODIS data were used to evaluate long‐term (2002–2016) ET responses to restoration efforts at these sites. Overall, ET was generally 0–10 mm d?1 across sites, and although daily ET values from groundwater data were highly variable, weekly averaged estimates were highly correlated with MODIS‐derived estimates at most sites. The influence of the 2014 flow events was not immediately apparent in the results, although the process of clearing vegetation and planting native vegetation at the restoration sites was clearly visible in the results. 相似文献
35.
The present study examined the influence of climate change on the spread of West Nile virus (WNV) in Canada among American crows (Corvus brachyrhynchos) by first identifying the significant climatic and environmental determinants of positive WNV cases in American crow specimens from 2009 to 2013. Using this information, we projected climate change scenarios on the potential spread of WNV in American crow species in Canada for three time periods: 2015–2039, 2040–2069, and 2070–2099. Using bird specimen, meteorological and land-use data, the statistical association between positive WNV cases in American crows and the environment was assessed by means of a general linear mixed model. Statistical results revealed that temperature and precipitation were significantly related to positive cases of WNV in American crows. Thus, climate change projections of summer mean temperature averages were projected for the three time periods. Climate change scenarios were created and imported into Quantum Geographic Information System (QGIS) and an algorithm was applied using the raster calculator to spatially delineate current and future areas of risk. Spatial analyses revealed that increased warming in the near future may increase the latitudinal extension of WNV in American crows in Canada. 相似文献
36.
This paper presents a new analytical criterion for brittle failure of rocks and heavily over-consolidated soils. Griffith’s model of a randomly oriented defect under a biaxial stress state is used to keep the criterion simple. The Griffith’s criterion is improved because the maximum tensile strength is not evaluated at the boundary of the defect but at a certain distance from the boundary, known as half of the critical distance. This fracture criterion is known as the point method, and is part of the theory of critical distances, which is utilised in fracture mechanics. The proposed failure criterion has two parameters: the inherent tensile strength, σ 0, and the ratio of the half-length of the initial crack/flaw to the critical distance, a/L. These parameters are difficult to measure but they may be correlated with the uniaxial compressive and tensile strengths, σ c and σ t. The proposed criterion is able to reproduce the common range of strength ratios for rocks and heavily overconsolidated soils (σ c/σ t = 3–50) and the influence of several microstructural rock properties, such as texture and porosity. Good agreement with laboratory tests reported in the literature is found for tensile and low-confining stresses. 相似文献
37.
38.
39.
The study of human-environment relationships in mountain areas is important for both theoretical and practical reasons, as many mountain areas suffer similar problems, such as depopulation, unemployment and natural hazards. Medium mountains constitute a special case within mountains, because they are more populated but less attractive as tourist destinations than high mountains. In this context, the Apuseni Mts (Romania) are considered as a case study. In this paper, we apply GIS-based, quantitative methods to characterize the strength and dynamics of human-environment interactions, taking into consideration some environmental factors (elevation, relative height, slope, river distance, lithology, land cover, natural attractions) as well as historical population and recent tourism data. We found that population density has strong (r 2>0.8) relationships with all relief factors (elevation, relative height, slope, river distance), and that best-fit functions are nonlinear. We outlined the varying demographic scenarios by elevation zones and interpreted the historically switching sign of population change versus elevation relationship. We demonstrated that lithology also has an impact on the spatial distribution of population, although it is not independent from the relief effect. The land cover of the mainly cultural landscape is very strongly correlated with relief parameters (especially slope), which suggests good adaptation. We pointed out the dominance of karst objects in the natural tourism potential of the Apuseni Mts and also explored further components of real tourism (spas, heritage, towns). Finally, we concluded that the environmental settings investigated do in fact constrain the spatial framework of society, but socio-economic changes in history can be explained from the side of society, which conforms to the theory of cultural possibilism. 相似文献
40.
Zircon M127 – A Homogeneous Reference Material for SIMS U–Pb Geochronology Combined with Hafnium,Oxygen and,Potentially, Lithium Isotope Analysis 下载免费PDF全文
Lutz Nasdala Fernando Corfu John W. Valley Michael J. Spicuzza Fu‐Yuan Wu Qiu‐Li Li Yue‐Heng Yang Chris Fisher Carsten Münker Allen K. Kennedy Peter W. Reiners Andreas Kronz Michael Wiedenbeck Richard Wirth Chutimun Chanmuang Manuela Zeug Tamás Váczi Nicholas Norberg Tobias Häger Alfred Kröner Wolfgang Hofmeister 《Geostandards and Geoanalytical Research》2016,40(4):457-475
In this article, we document a detailed analytical characterisation of zircon M127, a homogeneous 12.7 carat gemstone from Ratnapura, Sri Lanka. Zircon M127 has TIMS‐determined mean U–Pb radiogenic isotopic ratios of 0.084743 ± 0.000027 for 206Pb/238U and 0.67676 ± 0.00023 for 207Pb/235U (weighted means, 2s uncertainties). Its 206Pb/238U age of 524.36 ± 0.16 Ma (95% confidence uncertainty) is concordant within the uncertainties of decay constants. The δ18O value (determined by laser fluorination) is 8.26 ± 0.06‰ VSMOW (2s), and the mean 176Hf/177Hf ratio (determined by solution ICP‐MS) is 0.282396 ± 0.000004 (2s). The SIMS‐determined δ7Li value is ?0.6 ± 0.9‰ (2s), with a mean mass fraction of 1.0 ± 0.1 μg g?1 Li (2s). Zircon M127 contains ~ 923 μg g?1 U. The moderate degree of radiation damage corresponds well with the time‐integrated self‐irradiation dose of 1.82 × 1018 alpha events per gram. This observation, and the (U–Th)/He age of 426 ± 7 Ma (2s), which is typical of unheated Sri Lankan zircon, enable us to exclude any thermal treatment. Zircon M127 is proposed as a reference material for the determination of zircon U–Pb ages by means of SIMS in combination with hafnium and stable isotope (oxygen and potentially also lithium) determination. 相似文献