全文获取类型
收费全文 | 3008篇 |
免费 | 407篇 |
国内免费 | 1026篇 |
专业分类
大气科学 | 28篇 |
地球物理 | 646篇 |
地质学 | 3245篇 |
海洋学 | 301篇 |
天文学 | 17篇 |
综合类 | 26篇 |
自然地理 | 178篇 |
出版年
2024年 | 38篇 |
2023年 | 83篇 |
2022年 | 116篇 |
2021年 | 163篇 |
2020年 | 172篇 |
2019年 | 196篇 |
2018年 | 174篇 |
2017年 | 185篇 |
2016年 | 173篇 |
2015年 | 159篇 |
2014年 | 179篇 |
2013年 | 217篇 |
2012年 | 198篇 |
2011年 | 154篇 |
2010年 | 120篇 |
2009年 | 199篇 |
2008年 | 290篇 |
2007年 | 229篇 |
2006年 | 201篇 |
2005年 | 174篇 |
2004年 | 180篇 |
2003年 | 110篇 |
2002年 | 104篇 |
2001年 | 91篇 |
2000年 | 103篇 |
1999年 | 70篇 |
1998年 | 75篇 |
1997年 | 65篇 |
1996年 | 43篇 |
1995年 | 26篇 |
1994年 | 55篇 |
1993年 | 25篇 |
1992年 | 15篇 |
1991年 | 5篇 |
1990年 | 13篇 |
1989年 | 7篇 |
1988年 | 9篇 |
1987年 | 2篇 |
1986年 | 7篇 |
1985年 | 5篇 |
1984年 | 5篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 3篇 |
1978年 | 1篇 |
排序方式: 共有4441条查询结果,搜索用时 15 毫秒
291.
Conservative tracers are necessary to obtain groundwater transport velocities at the field scale. Deuterated water is an
effective tracer for this purpose due to its similarity to water, chemical stability, non-reactivity, ease of handling and
sampling, relatively neutral buoyancy, and reasonable price. Reliable detection limits of 0.1 mg deuterium/L may be obtained
in field tests. A field example is presented in which deuterated water, bromide, and pentafluorobenzoic acid are used as groundwater
tracers. Deuterated water appeared to be transported conservatively, producing almost identical breakthrough curves as that
of other soluble tracers.
Electronic Publication 相似文献
292.
Fault-controlled dolomitization at Swan Hills Simonette oil field (Devonian), deep basin west-central Alberta, Canada 总被引:6,自引:0,他引:6
The partly dolomitized Swan Hills Formation (Middle‐Upper Devonian) in the Simonette oil field of west‐central Alberta underwent a complex diagenetic history, which occurred in environments ranging from near surface to deep (>2500 m) burial. Five petrographically and geochemically distinct dolomites that include both cementing and replacive varieties post‐date stylolites in limestones (depths >500 m). These include early planar varieties and later saddle dolomites. Fluid inclusion data from saddle dolomite cements (Th=137–190 °C) suggest that some precipitated at burial temperatures higher than the temperatures indicated by reflectance data (Tpeak=160 °C). Thus, at least some dolomitizing fluids were ‘hydrothermal’. Fluorescence microscopy identified three populations of primary hydrocarbon‐bearing fluid inclusions and confirms that saddle dolomitization overlapped with Upper Cretaceous oil migration. The source of early dolomitizing fluids probably was Devonian or Mississippian seawater that was mixed with a more 87Sr‐rich fluid. Fabric‐destructive and fabric‐preserving dolostones are over 35 m thick in the Swan Hills buildup and basal platform adjacent to faults, thinning to less than 10 cm thick in the buildup between 5 and 8 km away from the faults. This ‘plume‐like’ geometry suggests that early and late dolomitization events were fault controlled. Late diagenetic fluids were, in part, derived from the crystalline basement or Palaeozoic siliciclastic aquifers, based on 87Sr/86Sr values up to 0·7370 from saddle dolomite, calcite and sphalerite cements, and 206Pb/204Pb of 22·86 from galena samples. Flow of dolomitizing and mineralizing fluids occurred during burial greater than 500 m, both vertically along reactivated faults and laterally in the buildup along units that retained primary and/or secondary porosity. 相似文献
293.
M. Angeles García Del Cura Jose P. Calvo Salvador Ordóñez Blair F. Jones & Juan C. Cañaveras 《Sedimentology》2001,48(4):897-915
Upper Pliocene dolomites (‘white earth’) from La Roda, Spain, offer a good opportunity to evaluate the process of dolomite formation in lakes. The relatively young nature of the deposits could allow a link between dolomites precipitated in modern lake systems and those present in older lacustrine formations. The La Roda Mg‐carbonates (dolomite unit) occur as a 3·5‐ to 4‐m‐thick package of poorly indurated, white, massive dolomite beds with interbedded thin deposits of porous carbonate displaying root and desiccation traces as well as local lenticular gypsum moulds. The massive dolomite beds consist mainly of loosely packed 1‐ to 2‐μm‐sized aggregates of dolomite crystals exhibiting poorly developed faces, which usually results in a subrounded morphology of the crystals. Minute rhombs of dolomite are sparse within the aggregates. Both knobbly textures and clumps of spherical bodies covering the crystal surfaces indicate that bacteria were involved in the formation of the dolomites. In addition, aggregates of euhedral dolomite crystals are usually present in some more clayey (sepiolite) interbeds. The thin porous carbonate (mostly dolomite) beds exhibit both euhedral and subrounded, bacterially induced dolomite crystals. The carbonate is mainly Ca‐dolomite (51–54 mol% CaCO3), showing a low degree of ordering (degree of ordering ranges from 0·27 to 0·48). Calcite is present as a subordinate mineral in some samples. Sr, Mn and Fe contents show very low correlation coefficients with Mg/Ca ratios, whereas SiO2 and K contents are highly correlated. δ18O‐ and δ13C‐values in dolomites range from ?3·07‰ to 5·40‰ PDB (mean=0·06, σ=1·75) and from ?6·34‰ to ?0·39‰ PDB (mean=?3·55, σ=1·33) respectively. Samples containing significant amounts of both dolomite and calcite do not in general show significant enrichment or depletion in 18O and 13C between the two minerals. The correlation coefficient between δ18O and δ13C for dolomite is extremely low and negative (r=?0·05), whereas it is higher and positive (r=0·47) for calcite. The lacustrine dolomite deposit from La Roda is interpreted mainly as a result of primary precipitation of dolomite in a shallow, hydrologically closed perennial lake. The lake was supplied by highly saturated HCO3?/CO32? groundwater that leached dolomitic Mesozoic formations. Precipitation of dolomite from alkaline lake waters took place under a semi‐arid to arid climate. However, according to our isotopic data, strong evaporative conditions were not required for the formation of the La Roda dolomite. A significant contribution by bacteria to the formation of the dolomites is assumed in view of both petrographic and geochemical evidence. 相似文献
294.
Fluid-rock history of granulite facies humite-marbles from Ambasamudram, southern India. 总被引:1,自引:0,他引:1
An extensive humite‐bearing marble horizon within a supracrustal sequence at Ambasamudram, southern India, was studied using petrological and stable isotopic techniques to define its metamorphic history and fluid characteristics. At peak metamorphic temperatures of 775±73°C, based on calcite‐graphite carbon isotope thermometry, the mineral assemblages suggest layer‐by‐layer control of fluid compositions. Clinohumite + calcite‐bearing assemblages suggest XCO2 < 0.4 (at 700°C and 5 kbar), calcite + forsterite + K‐feldspar‐bearing assemblages suggest XCO2>0.9 (at 790°C); and local wollastonite + scapolite + grossular‐bearing zones formed at XCO2 of c. 0.3. Retrograde reaction textures such as scapolite + quartz symplectites after feldspar and calcite and replacement of dolomite + diopside or tremolite+dolomite after calcite+forsterite or calcite+clinohumite are indicative of retrogression under high XCO2 conditions. Calcite preserves late Proterozoic carbon and oxygen isotopic signatures and the marble lacks evidence for extensive retrograde fluid infiltration, while during prograde metamorphism the possible infiltration of aqueous fluids did not produce significant isotopic resetting. Isotopic zonation of calcite and graphite grains was likely produced by localized CO2 fluid infiltration during retrogression. Contrary to the widespread occurrence of humite‐marbles related to retrograde aqueous fluid infiltration, the Ambasamudram humite‐marbles record a prograde‐to‐peak metamorphic humite formation and retrogression under conditions of low XH2O. 相似文献
295.
Abstract: The Dajing Cu–Sn–Ag–Pb–Zn ore deposit, Inner Mongolia of China, is a fissure‐filling hydrothermal ore deposit that occurs within the Upper Permian Linxi group. No magmatic pluton and volcanic rocks outcrop on the surface of the deposit. Most of ore veins show clear‐cut boundary with country rocks. Wallrock alterations that include silicification, carbonation, chlori–tization, and sericitization are generally weak and occur in the close vicinity of ore veins. Mineralization is divided into three stages: (1) cassiterite–arsenopyrite–quartz stage, (2) sulfide stage, and (3) Pb–Zn–Ag–carbonate stage. These mineralization stages have distinct ranges of homogenization temperatures, 290–350C for Stage 1, 260–320C for Stage 2, and 150–250C for Stage 3. However, salinities for Stages 1, 2, and 3 overlap and range between 2.2 and 10.4 wt % NaCl equivalent. The dD values relative to V‐SMOW of inclusion water from quartz are lower than –88% and centered at –100 to –130%. The δ34S values relative to CDT of sulfide ore minerals and δ13C values relative to PDB of carbonate gangue minerals, vary from –0.3 to +2.6%, and from –7.0 to –2.9%, respectively. Integrated isotopic data point to two major contributions to the mineralizing fluid that include a dominant meteoric‐derived water and the other from hypogene magma for sulfur and carbon species. Analyses of inclusion gas and liquid compositions are performed. The H2O and CO2 are the two most abundant gaseous components, whereas SO42‐ and Cl‐, and Na+, Ca2+, and K+ are the major anions and cations, respectively. A linear trend is shown on the gaseous H2O versus CO2 plot. Phase separation is excluded as cause for the trend on the basis of isotope data and fluid inclusion microthermometry. In addition, a weak wallrock alteration does not support fluid‐rock interaction as an efficient mechanism. Hence, the linear H2O–CO2 trend is interpreted in terms of absorption or dilution of CO2–dominant magmatic vapor by meteoric‐derived water. Cooling effects resulting from dilution may have caused precipitation of ore minerals. Major and trace element compositions of regional granites show a high‐K calc–alkaline characteristics and an arc–affinity. Lead isotopic compositions of galena samples from the Dajing deposit exhibit elevated U/Pb and Th/Pb ratios. These characteristics indicate a common source of supra subduction zone mantle wedge for regional granites and metals from the Dajing deposit. 相似文献
296.
Radiogenic isotopes in fluid inclusions 总被引:7,自引:0,他引:7
Radiogenic isotopes studied in fluid inclusions are still a limited field, with great potential for expansion as analytical techniques improve. The main limitation for Sr, Ar and He isotope work is the very small number of radiogenic atoms produced in a typical fluid inclusion. The requirements to analysts are correspondingly high. Examples show that isotopic tracing on fluid inclusion fluids can be a decisive tool in solving geological problems. 相似文献
297.
Pollution vulnerability of the Quaternary aquifer near Cairo, Egypt, as indicated by isotopes and hydrochemistry 总被引:2,自引:1,他引:2
The present study was conducted to delineate the pollution vulnerability of the Quaternary aquifer in two areas, Imbaba and
Shobra El-Khima, near Cairo, Egypt. Environmental isotopes combined with hydrochemistry were used for this purpose. The groundwater
in the Imbaba area (average total dissolved solids about 900 mg/L; sodium/chloride, sulfate, and bicarbonate water types)
is more mineralized than groundwater in the Shobra El-Khima area (average total dissolved solids 500 mg/L; calcium and sodium/bicarbonate
water type). A high nitrate content and significant mineralization in the groundwater are probably due to contamination of
recharge to the aquifer by irrigation drainage, deteriorated sewage networks, and septic tanks. The deuterium and oxygen-18
compositions of the groundwater are depleted compared to Nile River water, which is the main source of aquifer recharge. This
less isotopically enriched water probably represents older Nile water recharge that flooded the region before construction
of the Aswan High Dam in 1963, or it is a mixture of a young water and originally deposited paleowater that was in deeper
horizons at a time of cooler and more humid climate. Intensive pumping has moved the paleowater higher in the aquifer. Groundwater
in the Shobra El-Khima area has higher residence time, based on the tritium concentration, than groundwater in the Imbaba
area. The percentage of the isotopically depleted water equals 75% in the Shobra El-Khima and 35% in Imbaba, and the thickness
of the clay cap above the aquifer is 38 m in Shobra El-Khima and 20 m in Imbaba. These factors are indicative of the rate
of recharge to the aquifer and were used to evaluate the pollution vulnerability in the two areas.
Electronic Publication 相似文献
298.
Stuart A. Weinstein 《Pure and Applied Geophysics》1996,146(3-4):551-572
This study presents the results of numerical simulations of a model for lithospheremantle coupling in a terrestrial type planet. To first order, a geologically active terrestrial type planet may consist of a metallic core, silicate mantle and lithosphere, with the lithosphere being rheologically different from the mantle. Therefore we have developed a numerical model consisting of a thin non-Newtonian fluid hoop that is dynamically coupled to a thick Newtonian fluid cylindrical annulus. Thus the rheological dichotomy between mantle and lithosphere is built into the model. Time-dependent calculations show the existence of at least two regimes of behaviors. In one regime, the behavior of the hoop switches between periods characterized by low or high speeds, in response to changes in convective vigor and planform. This regime may apply to the planet Venus where the available evidence indicates that prior to 500 myr ago, the planet was resurfaced on a time scale of <100 myr. Since that time, large-scale tectonic activity on Venus has been sharply curtailed. In the other regime, which is more like plate tectonics on Earth, the hoop speeds rise and fall on short time scales. 相似文献
299.
H. C. Duthie J-R Yang T. W. D. Edwards B. B. Wolfe B. G. Warner 《Journal of Paleolimnology》1996,15(1):79-97
Limnological changes in Hamilton Harbour, Lake Ontario, over the Holocene were investigated by using proxy evidence from diatoms and other siliceous microfossils in a radiometrically dated sediment core (HH26comp), together with environmental data derived from sediment pollen and oxygen and carbon isotope analyses. The evidence demonstrates that the site of Hamilton Harbour has changed over the past 8300 y from a shallow, separate waterbody, to a deep embayment of Lake Ontario. The earliest evidence, from 8300 BP to 7000 BP, is of a mesotrophic pond of moderate alkalinity, warmer than present, and probably with an extensive marginal wetland. An initial transitory connection with the rising water level of Lake Ontario was established at c. 7000 BP, possibly via a deep outlet channel. This connection is 2000 y earlier then previously estimated. Permanent confluence with Lake Ontario was established at c. 6200 BP, causing a decline in inferred trophic level and water temperatures. Microfossils reach a minimum at 4400 BP coincident with the Nipissing Flood. Decreased mixing of Lake Ontario water from about 4000 BP following the Nipissing Flood highstand is evidenced in isotopic and diatom data. Three isolated shifts in the diatom spectrum at c. 4900 BP, 4500 BP, and 3500 BP may be associated with extreme turbidity or storm deposit events. Between 3200 BP and 280 BP, Hamilton Harbour was evidently a moderately alkaline embayment of Lake Ontario, oligotrophic to mesotrophic, and relatively cooler than present. The final 280 y sedimentary record reveals the magnitude of anthropogenically induced changes to the harbour, including eutrophication and organic pollution. 相似文献
300.