全文获取类型
收费全文 | 66182篇 |
免费 | 1121篇 |
国内免费 | 493篇 |
专业分类
测绘学 | 1634篇 |
大气科学 | 5272篇 |
地球物理 | 13722篇 |
地质学 | 21403篇 |
海洋学 | 5735篇 |
天文学 | 15214篇 |
综合类 | 133篇 |
自然地理 | 4683篇 |
出版年
2020年 | 473篇 |
2019年 | 495篇 |
2018年 | 933篇 |
2017年 | 916篇 |
2016年 | 1362篇 |
2015年 | 1011篇 |
2014年 | 1413篇 |
2013年 | 3235篇 |
2012年 | 1490篇 |
2011年 | 2272篇 |
2010年 | 1943篇 |
2009年 | 2917篇 |
2008年 | 2660篇 |
2007年 | 2391篇 |
2006年 | 2455篇 |
2005年 | 2131篇 |
2004年 | 2234篇 |
2003年 | 2059篇 |
2002年 | 1963篇 |
2001年 | 1774篇 |
2000年 | 1747篇 |
1999年 | 1504篇 |
1998年 | 1491篇 |
1997年 | 1480篇 |
1996年 | 1273篇 |
1995年 | 1208篇 |
1994年 | 1090篇 |
1993年 | 994篇 |
1992年 | 943篇 |
1991年 | 800篇 |
1990年 | 1008篇 |
1989年 | 848篇 |
1988年 | 752篇 |
1987年 | 926篇 |
1986年 | 818篇 |
1985年 | 1020篇 |
1984年 | 1182篇 |
1983年 | 1123篇 |
1982年 | 1017篇 |
1981年 | 976篇 |
1980年 | 834篇 |
1979年 | 815篇 |
1978年 | 867篇 |
1977年 | 787篇 |
1976年 | 749篇 |
1975年 | 695篇 |
1974年 | 703篇 |
1973年 | 708篇 |
1972年 | 440篇 |
1971年 | 384篇 |
排序方式: 共有10000条查询结果,搜索用时 12 毫秒
981.
Andrew J. Mason Gideon M. Henderson Anton Vaks 《Geostandards and Geoanalytical Research》2013,37(3):261-275
A new method for the simultaneous recovery of U, Th and Pb from ca. 0.5 g calcium carbonate samples for the purpose of U‐(Th)‐Pb geochronometry is presented. The protocol employs ion‐exchange chromatography. Standard anion exchange resin (AG 1‐X8 100–200 mesh) was used as the static phase, and 90% acetic acid was used as the mobile phase to elute the unwanted matrix components; dilute nitric acid was used to elute the U, Th and Pb. Blanks of 1.8 pg Th, 6.4 pg Pb and 8.4 pg U were obtained. The protocol was evaluated by determining the isotopic composition of U‐Th‐Pb separates obtained from an in‐house reference material (prepared from a natural speleothem) by MC‐ICP‐MS. An independently dated speleothem was also reanalysed. Based on these tests, the extraction protocol had an acceptable blank and produced a Pb separate sufficiently free of matrix‐induced instrumental biases to be appropriate for U‐Th‐Pb chronology. 相似文献
982.
Environmental tracers as indicators of karst conduits in groundwater in South Dakota,USA 总被引:2,自引:1,他引:2
Environmental tracers sampled from the carbonate Madison aquifer on the eastern flank of the Black Hills, South Dakota, USA indicated the approximate locations of four major karst conduits. Contamination issues are a major concern because these conduits are characterized by direct connections to sinking streams, high groundwater velocities, and proximity to public water supplies. Objectives of the study were to estimate approximate conduit locations and assess possible anthropogenic influences associated with conduits. Anomalies of young groundwater based on chlorofluorocarbons (CFCs), tritium, and electrical conductivity (EC) indicated fast moving, focused flow and thus the likely presence of conduits. δ18O was useful for determining sources of recharge for each conduit, and nitrate was a useful tracer for assessing flow paths for anthropogenic influences. Two of the four conduits terminate at or near a large spring complex. CFC apparent ages ranged from 15 years near conduits to >50 years in other areas. Nitrate-N concentrations >0.4 mg/L in groundwater were associated with each of the four conduits compared with concentrations ranging from <0.1 to 0.4 mg/L in other areas. These higher nitrate-N concentrations probably do not result from sinking streams but rather from other areas of infiltration. 相似文献
983.
An automated strategy for calculation of phase diagram sections and retrieval of rock properties as a function of physical conditions 总被引:9,自引:0,他引:9
We formulate an algorithm for the calculation of stable phase relations of a system with constrained bulk composition as a function of its environmental variables. The basis of this algorithm is the approximate representation of the free energy composition surfaces of solution phases by inscribed polyhedra. This representation leads to discretization of high variance phase fields into a continuous mesh of smaller polygonal fields within which the composition and physical properties of the phases are uniquely determined. The resulting phase diagram sections are useful for understanding the phase relations of complex metamorphic systems and for applications in which it is necessary to establish the variations in rock properties such as density, seismic velocities and volatile‐content through a metamorphic cycle. The algorithm has been implemented within a computer program that is general with respect to both the choice of variables and the number of components and phases possible in a system, and is independent of the structure of the equations of state used to describe the phases of the system. 相似文献
984.
Andreas J. Scheib Manfred Birke Enrico Dinelli GEMAS Project Team 《Boreas: An International Journal of Quaternary Research》2014,43(1):175-192
Hafnium (Hf) and zirconium (Zr) concentrations measured in over 4100 agricultural soil samples from Europe were assessed with the focus on their relationship to the distribution of aeolian deposits, such as loess and coversands. Comparison of extractable (aqua regia; ICP‐MS) and total (XRFS) concentrations shows that only 1.0 to 1.7% of the total Hf and Zr is chemically extractable because of the resistant nature of their host minerals. Resistate minerals, such as zircon, are commonly found in the predominantly silty fraction of loess deposits. In this study a statistical analysis of total Hf and Zr soil data from areas with and without loess was carried out to derive threshold values of 10 mg kg?1 (Hf) and 318 mg kg?1 (Zr). These values were subsequently applied across the project area in an attempt to indicate the presence of aeolian deposits. The spatial distribution of above‐threshold concentrations suggests a more extensive and coherent loess belt across central and eastern Europe, providing additional evidence of loess across Brittany, Aquitaine and near the Vosges mountains in France as well as in the basins of central and northern Spain. Above‐threshold concentrations were also detected in loess regardless of its thickness, emphasising the importance of the abundance of zircon in the upper part of the soil profile rather than the actual thickness of the deposit. Soil data however, failed to indicate various loess facies within most of the Pannonian Basin, suggesting that this approach only works where deposits contain a sufficient amount of zircon. This may also explain why the extensive coversands across northern Germany and Poland were largely undetected. This study demonstrates that continental‐scale soil geochemical data can help identify and map the distribution of zircon‐rich loess and coversand, and subsequently enhance and improve current knowledge of the extent of these deposits. 相似文献
985.
986.
We presented a finite‐element‐based algorithm to simulate plane‐strain, straight hydraulic fractures in an impermeable elastic medium. The algorithm accounts for the nonlinear coupling between the fluid pressure and the crack opening and separately tracks the evolution of the crack tip and the fluid front. It therefore allows the existence of a fluid lag. The fluid front is advanced explicitly in time, but an implicit strategy is needed for the crack tip to guarantee the satisfaction of Griffith's criterion at each time step. We enforced the coupling between the fluid and the rock by simultaneously solving for the pressure field in the fluid and the crack opening at each time step. We provided verification of our algorithm by performing sample simulations and comparing them with two known similarity solutions. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
987.
988.
Tomoko Komada David J. Burdige Cédric Magen Huan-Lei Li Jeffrey Chanton 《Aquatic Geochemistry》2016,22(5-6):593-618
Geochemical and isotopic data for the uppermost 1.2 m of the sediments of the central Santa Monica Basin plain were examined to better understand organic matter deposition and recycling at this site. Isotopic signatures (Δ14C and δ13C) of methane (CH4) and dissolved inorganic carbon (DIC) indicate the occurrence of anaerobic oxidation of CH4 that is fueled by CH4 supplied from a relict reservoir that is decoupled from local organic carbon (Corg) degradation and methanogenesis. This finding was corroborated by a flux budget of pore-water solutes across the basal horizon of the profile. Together these results provide a plausible explanation for the anomalously low ratio between alkalinity production and sulfate consumption reported for these sediments over two decades ago. Shifts in Δ14C and δ13C signatures of Corg have previously been reported across the 20-cm depth horizon for this site and attributed to a transition from oxic to anoxic bottom water that occurred ~350 years BP. However, we show that this horizon also coincides with a boundary between the base of a hemipelagic mud section and the top of a turbidite interval, complicating the interpretation of organic geochemical data across this boundary. Radiocarbon signatures of DIC diffusing upward into surface sediments indicate that remineralization at depth is supported by relatively 14C-enriched Corg within the sedimentary matrix. While the exact nature of this Corg is unclear, possible sources are hemipelagic mud sections that were buried rapidly under thick turbidites, and 14C-rich moieties dispersed within Corg-poor turbidite sections. 相似文献
989.
990.
Little is known about the importance of salmarshes to juvenile and adult fishes in temperate Australia. We assessed diel and
feeding patterns of fish inhabiting saltmarsh in a sheltered embayment along the coast of Victoria, Australia, between October
2002 and May 2003. The saltmarsh flat was generally only inundated during low-pressure weather systems (barometric pressure
<1,013 hP). Fish were sampled over the saltmarsh flats using fyke and seine nets. A total of 2,047 individuals (10 species,
including juveniles and adults) were caught.Atherinasoma microstoma was most abundant (fyke [F], μ=1.6 fish h−1; seine [S], μ=28.2 fish shot−1), followed byFavonigobius, lateralis (F: 0.5; S: 0.6),Galaxias maculatus (F: 0.1),Heteroclinus adelaide (F:<0.1),Kestratherina esox (F: <0.1; S: 1.6),Leptatherina presbyteroides (F: <0.1; S: 7.1) andTetractenos glaber (F: 1). Commercial species, includingAldrichetta forsteri (F: <0.1; S: 3.2),Sillaginodes punctata (F: <0.1; S: 0.9), andRhombosolea tapirina (F: 0.4), were commonly sampled. Variability in species richness or fish abundance was not explained by water temperature,
salinity, depth, or barometric pressure. Significantly more species were sampled with the seine during nocturnal periods (p=0.002);
fish abundance did not vary between diel periods, nor did fish abundance and species richness in fyke net samples. Diets of
the most abundant species (A. microstoma, A. forsteri, andF. lateralis) were primarily composed of gammaridean amphipods and hemipteran insects. There was no correlation between fish diets and
the composition of benthic invertebrates as sampled at 3 different regions of the saltmarsh flat. The saltmarsh flats in our
study region are inhabited by several species normally associated with alternative habitat types such as seagrass, and the
patterns of habitat use observed appear to be partially attributed to foraging behavior. 相似文献