全文获取类型
收费全文 | 1574篇 |
免费 | 86篇 |
国内免费 | 17篇 |
专业分类
测绘学 | 35篇 |
大气科学 | 116篇 |
地球物理 | 401篇 |
地质学 | 574篇 |
海洋学 | 153篇 |
天文学 | 196篇 |
综合类 | 3篇 |
自然地理 | 199篇 |
出版年
2023年 | 6篇 |
2021年 | 18篇 |
2020年 | 33篇 |
2019年 | 27篇 |
2018年 | 37篇 |
2017年 | 49篇 |
2016年 | 50篇 |
2015年 | 39篇 |
2014年 | 48篇 |
2013年 | 80篇 |
2012年 | 52篇 |
2011年 | 99篇 |
2010年 | 81篇 |
2009年 | 92篇 |
2008年 | 76篇 |
2007年 | 49篇 |
2006年 | 83篇 |
2005年 | 68篇 |
2004年 | 54篇 |
2003年 | 57篇 |
2002年 | 46篇 |
2001年 | 28篇 |
2000年 | 30篇 |
1999年 | 22篇 |
1998年 | 31篇 |
1997年 | 21篇 |
1996年 | 29篇 |
1995年 | 22篇 |
1994年 | 26篇 |
1993年 | 20篇 |
1992年 | 17篇 |
1991年 | 12篇 |
1990年 | 18篇 |
1989年 | 17篇 |
1988年 | 13篇 |
1987年 | 11篇 |
1986年 | 16篇 |
1985年 | 19篇 |
1984年 | 21篇 |
1983年 | 14篇 |
1982年 | 29篇 |
1981年 | 17篇 |
1980年 | 7篇 |
1979年 | 13篇 |
1978年 | 14篇 |
1977年 | 10篇 |
1976年 | 10篇 |
1975年 | 5篇 |
1974年 | 5篇 |
1973年 | 13篇 |
排序方式: 共有1677条查询结果,搜索用时 15 毫秒
991.
Mad S. Thomsen Thomas Wernberg Fernando Tuya Brian R. Silliman 《Estuarine, Coastal and Shelf Science》2010
Invasions by non-indigenous species (NIS) have been suggested to alter local, regional and global biota on unprecedented scales. To manage NIS, it is pivotal to identify whether a species is introduced or native, but even today the geographical origin of thousands of species worldwide remain uncertain. Most of these ‘cryptogenic species’ are inconspicuous and rare, but in a few instances, they can also be abundant and conspicuous species, with large impacts on community structure. The identification of cryptogenic species, and summarizing information on their most likely origin, is an important task in invasion biology, and can highlight the need for research and management. Here, we document that the gastropod Batillaria australis in the Swan River estuary (Perth, Western Australia) is a conspicuous species of uncertain origin. A literature review combined with new survey data revealed that all evidence point to a recent human-mediated transfer; for example, it is absent from the fossil record, was first collected in 1954, has a low parasite diversity, has increased its population size dramatically in recent times, is separated by >3000 km from conspecifics, has no long-distance dispersal mechanisms, and existing ocean currents run against a natural range extension. Surprisingly, despite political and scientific focus on NIS hardly any ecological data have been published on this species from Western Australia. We show that B. australis is highly abundant in both seagrass beds (424 ± 29 ind m−2) and on unvegetated sand flats (92 ± 22 ind m−2) being orders of magnitudes more abundant than any native gastropod in the Swan River. Experiments showed that high resistance to predation and environmental stress potentially explains its success. From our survey data, we calculated that >3.6 billion invasive snails today occupy the Swan River. This large snail populations support other organisms; for example, almost 1 billion macroalgae are found attached to living B. australis and >100 million hermit crabs occupy its empty shells. Given Battilaria’s high abundance, wide distribution, large size, persistent shells that support other organisms and bioturbating behavior, it seems inescapable that this potential invader has impacted the ecosystem functioning of the Swan River. We argue that the search for abundant species of uncertain origin should continue, and that these species generally should be treated with the same interest as high status invaders to mitigate impacts in already invaded systems and to avoid secondary spread into neighboring ecosystems. 相似文献
992.
Steven J. Jaret Brian L. Phillips David T. King Jr Tim D. Glotch Zia Rahman Shawn P. Wright 《Meteoritics & planetary science》2017,52(1):147-163
Coesite has been identified within ejected blocks of shocked basalt at Lonar crater, India. This is the first report of coesite from the Lonar crater. Coesite occurs within SiO2 glass as distinct ~30 μm spherical aggregates of “granular coesite” identifiable both with optical petrography and with micro‐Raman spectroscopy. The coesite+glass occurs only within former silica amygdules, which is also the first report of high‐pressure polymorphs forming from a shocked secondary mineral. Detailed petrography and NMR spectroscopy suggest that the coesite crystallized directly from a localized SiO2 melt, as the result of complex interactions between the shock wave and these vesicle fillings. 相似文献
993.
George W. Luther III Brian T. Glazer Shufen Ma Robert E. Trouwborst Tommy S. Moore Edouard Metzger Chareonkwan Kraiya Tim J. Waite Gregory Druschel Bjrn Sundby Martial Taillefert Donald B. Nuzzio Timothy M. Shank Brent L. Lewis Paul J. Brendel 《Marine Chemistry》2008,108(3-4):221-235
Solid-state voltammetric (micro)electrodes have been used in a variety of environments to study biogeochemical processes. Here we show the wealth of information that has been obtained in the study of sediments, microbial mats, cultures and the water column including hydrothermal vents. Voltammetric analyzers have been developed to function with operator guidance and in unattended mode for temporal studies with an in situ electrochemical analyzer (ISEA). The electrodes can detect the presence (or absence) of a host of redox species and trace metals simultaneously. The multi-species capacity of the voltammetric electrode can be used to examine complex heterogeneous environments such as the root zone of salt marsh sediments. The data obtained with these systems clearly show that O2 and Mn2+ profiles in marine sedimentary porewaters and in microbial biofilms on metal surfaces rarely overlap indicating that O2 is not a direct oxidant for Mn2+. This lack of overlap was suggested originally by Joris Gieskes' group. In waters emanating from hydrothermal vents, Fe2+, H2S and soluble molecular FeS clusters (FeSaq) are detected indicating that the reactants for the pyrite formation reaction are H2S and soluble molecular FeS clusters. Using the ISEA with electrodes at fixed positions, data collected continuously over three days near a Riftia pachyptila tubeworm field generally show that O2 and H2S anti-correlate and that H2S and temperature generally correlate. Unlike sedimentary environments, the data clearly show that Riftia live in areas where both O2 and H2S co-exist so that its endosymbiont bacteria can perform chemosynthesis. However, physical mixing of diffuse flow vent waters with oceanic bottom waters above or to the side of the tubeworm field can dampen these correlations or even reverse them. Voltammetry is a powerful technique because it provides chemical speciation data (e.g.; oxidation state and different elemental compounds/ions) as well as quantitative data. Because (micro)organisms occupy environmental niches due to the system's chemistry, it is necessary to know chemical speciation. Voltammetric methods allow us to study how chemistry drives biology and how biology can affect chemistry for its own benefit. 相似文献
994.
Determining Effects of Area Burned and Fire Severity on Carbon Cycling and Emissions in Siberia 总被引:14,自引:0,他引:14
Susan G. Conard Anatoly I. Sukhinin Brian J. Stocks Donald R. Cahoon Eduard P. Davidenko Galina A. Ivanova 《Climatic change》2002,55(1-2):197-211
The Russian boreal forest contains about 25% of the global terrestrial biomass, and even a higher percentage of the carbon stored in litter and soils. Fire burns large areas annually, much of it in low-severity surface fires – but data on fire area and impacts or extent of varying fire severity are poor. Changes in land use, cover, and disturbance patterns such as those predicted by global climate change models, have the potential to greatly alter current fire regimes in boreal forests and to significantly impact global carbon budgets. The extent and global importance of fires in the boreal zone have often been greatly underestimated. For the 1998 fire season we estimate from remote sensing data that about 13.3 million ha burned in Siberia. This is about 5 times higher than estimates from the Russian Aerial Forest Protection Service (Avialesookhrana) for the same period. We estimate that fires in the Russian boreal forest in 1998 constituted some 14–20% of average annual global carbon emissions from forest fires. Average annual emissions from boreal zone forests may be equivalent to 23–39% of regional fossil fuel emissions in Canada and Russia, respectively. But the lack of accurate data and models introduces large potential errors into these estimates. Improved monitoring and understanding of the landscape extent and severity of fires and effects of fire on carbon storage, air chemistry, vegetation dynamics and structure, and forest health and productivity are essential to provide inputs into global and regional models of carbon cycling and atmospheric chemistry. 相似文献
995.
Quantitative analysis of variations in morphological types of charcoal were undertaken in sediment cores from three lakes
on the Interior Plateau (BC, Canada) over the period AD 1919–2000. Seven distinct morphological types of charcoal were identified
based on particle shape and structural features and were compared with seasonal precipitation and recorded area burned within
20 km-radius of study lakes. Fragile-type charcoal fragments, termed type M, displayed significant relationships to recorded
area burned in sediment cores from Prosser (r
2 = 0.5; p = 0.0001) and Opatcho (r
2 = 0.2; p = 0.02) lakes. However, nonsignificant correlations (p > 0.05) were found between total charcoal and area burned. Robust and highly elongated morphotypes C and F were correlated
to recorded spring precipitation (r
2 = 0.5; p = 0.002) in Opatcho Lake. Charcoal from a sediment core from Big Lake, the lake with the largest watershed, was significantly
but inversely related to past fires (r
2 = 0.44; p = 0.0003), suggesting important contributions from secondary transportation and deposition. Models were developed to infer
relative area burned and precipitation for the study lakes. Our results suggest that charcoal morphotypes are related to the
biogeoclimatic and lake watershed characteristics. This study also suggest that charcoal morphotypes can provide insights
on past fire and climate, which was not possible based on traditional analysis of total charcoal. 相似文献
996.
Jaromír Ulrych Lukáš Ackerman Kadosa Balogh Ernst Hegner Emil Jelínek Zoltan Pécskay Antonín Přichystal Brian G.J. Upton Jiří Zimák Radana Foltýnová 《Chemie der Erde / Geochemistry》2013
The Plio-Pleistocene volcanic rocks of the Bohemian Massif comprise a compositional spectrum involving two series: an older basanitic series (6.0–0.8 Ma) and a younger, melilititic series (1.0–0.26 Ma). The former consists of relatively undifferentiated basaltic rocks, slightly silica-undersaturated, with Mg# ranging from 62 to almost primitive mantle-type values of 74. The major and trace element characteristics correspond to those of primitive intra-plate alkaline volcanic rocks from a common sub-lithospheric mantle source (European Asthenospheric Reservoir – EAR) including positive Nb, and negative K and Pb anomalies. 87Sr/86Sr ratios of 0.7032–0.7034 and 143Nd/144Nd of 0.51285–0.51288 indicate a moderately depleted mantle source as for other mafic rocks of the central European volcanic province with signs of HIMU-like characteristics commonly attributed to recycling of subducted oceanic crust in the upper mantle during the Variscan orogeny. The melilititic series is characterized by higher degrees of silica-undersaturation, and high Mg# of 68–72 values, compatible with primitive-mantle-derived compositions. The high OIB-like Ce/Pb (19–47) and Nb/U (32–53) ratios indicate that assimilation of crustal material was negligible. In both series, concentrations of incompatible elements are mildly elevated and 87Sr/86Sr ratios (0.7034–0.7036) and 143Nd/144Nd ratios (0.51285–0.51288) overlap. Variations in incompatible element concentrations and isotopic compositions in the basanitic series and melilititic series can be explained by a lower degree of mantle melting for the latter with preferential melting of enriched mantle domains. The Sr and Nd isotopic compositions of both rock series are similar to those of the EAR. Minor differences in geochemical characteristics between the two series may be attributed to: (i) to different settings with respect to crust and lithospheric mantle conditions in (a) Western Bohemia (WB) and (b) Northeastern Bohemia (NEB) and the Northern Moravia and Silesia (NMS) areas, (ii) a modally metasomatized mantle lithosphere in WB in contrast to cryptically metasomatized domains in the NEB and NMS, (iii) different degrees of partial melting with very low degrees in WB but higher degrees in NEB and NMS. The geochemical and isotopic similarity between the Plio-Pleistocene volcanic rocks and those of the late Cretaceous and Cenozoic (79–6 Ma) suggests that their magmas came from compositionally similar mantle sources, that underwent low degrees of melting over an interval of ∼80 Ma. The Oligocene to Miocene basanitic series that accompanied the Plio-Pleistoicene basanitic series in the NMS region indicate that they shared a common mantle source. There is no geochemical evidence for thermal erosion of the lithospheric mantle or significant changes in mantle compositions within the time of a weak thermal perturbation in the asthenospheric mantle. These perturbations were caused by a dispersed mantle plume or passively upwelling asthenosphere in zones of lithospheric thinning. 相似文献
997.
The chemical composition of surface water in the photic zone of the Precambrian ocean is almost exclusively known from studies of stromatolitic carbonates, while banded iron formations (IFs) have provided information on the composition of deeper waters. Here we discuss the trace element and Nd isotope geochemistry of very shallow-water IF from the Pongola Supergroup, South Africa, to gain a better understanding of solute sources to Mesoarchean shallow coastal seawater. The Pongola Supergroup formed on the stable margin of the Kaapvaal craton ∼2.9 Ga ago and contains banded iron formations (IFs) that represent the oldest documented Superior-type iron formations. The IFs are near-shore, pure chemical sediments, and shale-normalized rare earth and yttrium distributions (REYSN) exhibit positive LaSN, GdSN, and YSN anomalies, which are typical features of marine waters throughout the Archean and Proterozoic. The marine origin of these samples is further supported by super-chondritic Y/Ho ratios (average Y/Ho = 42). Relative to older Isua IFs (3.7 Ga) from Greenland, and younger Kuruman IFs (2.5 Ga) also from South Africa, the Pongola IFs are depleted in heavy rare earth elements (HREE), and appear to record variations in solute fluxes related to sea level rise and fall. Sm-Nd isotopes were used to identify potential sediment and solute sources within pongola shales and IFs. The ?Nd(t) for Pongola shales ranges from −2.7 to −4.2, and ?Nd(t) values for the coeval iron-formation samples (range −1.9 to −4.3) are generally indistinguishable from those of the shales, although two IF samples display ?Nd(t) as low as −8.1 and −10.9. The similarity in Nd isotope signatures between the shale and iron-formation suggests that mantle-derived REY were not a significant Nd source within the Pongola depositional environment, though the presence of positive Eu anomalies in the IF samples indicates that high-T hydrothermal input did contribute to their REY signature. Isotopic mass balance calculations indicate that most (?72%) of the Nd in these seawater precipitates was derived from continental sources. If previous models of Fe-Nd distributions in Archean IFs are applied, then the Pongola IFs suggest that continental fluxes of Fe to Archean seawater were significantly greater than are generally considered. 相似文献
998.
Data for the major element oxides and seventeen trace elements (Ba, Co, Cu, Ga, La, Mo, Nb, Ni, Pb, Rb, Sb, Sn, Sr, U, Y, Zn and Zr) have been determined by X-ray fluorescence spectrometry for the CANMET ore standards MA-1, TLG-1, BH-1, CT-1, OKA-1, PR-1, HV-1, MP-2 and TAN-1. 相似文献
999.
通过薄片分析、扫描电镜及氩离子抛光技术,对渝东南下古生界含气页岩中的孔隙类型及特征进行了分类观察、描述,结合有机碳含量、X衍射、岩石物性、氮气吸附及甲烷等温吸附实验测试,分析了页岩气储层中各类孔隙的储气贡献,探讨这些孔隙对页岩中烃类运移的影响。研究认为,有机质孔隙、颗粒内孔、粒间孔隙及微裂隙是页岩气储层孔隙的4种类型,有机质表面的分散状孔隙多为nm级,石英碎屑可形成粒内或粒间孔隙,黄铁矿颗粒内部小晶体排列分布产生孔隙空间,黏土絮状沉淀形成片粒状结构孔隙,微裂隙普遍发育在页岩基质中,可达mm级。不同类型的孔隙能够为页岩气的赋存提供不同尺度的储集空间,页岩吸附含气量与表征孔隙特征的参数(TOC、BET比表面积、BJH总孔体积)具有显著的正相关关系。微裂隙与粒间孔隙对页岩中烃类的运移最有利。 相似文献
1000.
Aditya Choukulkar Ronald Calhoun Brian Billings James Doyle 《Boundary-Layer Meteorology》2012,144(3):359-378
A study of an interesting meteorological episode over the Owens Valley, California, USA during the Terrain-Induced Rotor EXperiment was conducted using a recently adapted statistical interpolation method to retrieve wind-velocity vectors from Doppler lidar data. This vector retrieval method has been adapted from radar data assimilation techniques. Results show that the method allows better preservation of local variations in the flow field than other techniques. In addition, a high resolution Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS®) run is used to understand the large-scale flow within the valley and compared with lidar retrievals. Observations from 1030 UTC to 1230 UTC (0230 local time to 0430 local time) on March 27, 2006 are presented. Lidar observations show complex and uncharacteristic flows such as sudden bursts of westerly cross-valley wind mixing with the dominant up-valley wind. Model results from COAMPS and other in-situ instrumentation are used to corroborate and complement these observations. The optimal interpolation technique for Doppler lidar data vector retrieval appears well suited for scenarios with complex spatial variations in the flow field. 相似文献