全文获取类型
收费全文 | 2996篇 |
免费 | 142篇 |
国内免费 | 32篇 |
专业分类
测绘学 | 69篇 |
大气科学 | 237篇 |
地球物理 | 745篇 |
地质学 | 990篇 |
海洋学 | 267篇 |
天文学 | 467篇 |
综合类 | 14篇 |
自然地理 | 381篇 |
出版年
2024年 | 5篇 |
2023年 | 6篇 |
2022年 | 8篇 |
2021年 | 55篇 |
2020年 | 50篇 |
2019年 | 55篇 |
2018年 | 66篇 |
2017年 | 68篇 |
2016年 | 92篇 |
2015年 | 93篇 |
2014年 | 95篇 |
2013年 | 184篇 |
2012年 | 142篇 |
2011年 | 191篇 |
2010年 | 144篇 |
2009年 | 169篇 |
2008年 | 160篇 |
2007年 | 163篇 |
2006年 | 145篇 |
2005年 | 126篇 |
2004年 | 126篇 |
2003年 | 111篇 |
2002年 | 100篇 |
2001年 | 73篇 |
2000年 | 68篇 |
1999年 | 59篇 |
1998年 | 54篇 |
1997年 | 37篇 |
1996年 | 43篇 |
1995年 | 29篇 |
1994年 | 34篇 |
1993年 | 32篇 |
1992年 | 28篇 |
1991年 | 27篇 |
1990年 | 25篇 |
1989年 | 23篇 |
1988年 | 25篇 |
1987年 | 30篇 |
1986年 | 25篇 |
1985年 | 31篇 |
1984年 | 30篇 |
1983年 | 21篇 |
1982年 | 26篇 |
1981年 | 23篇 |
1980年 | 19篇 |
1979年 | 16篇 |
1978年 | 7篇 |
1977年 | 8篇 |
1975年 | 4篇 |
1974年 | 5篇 |
排序方式: 共有3170条查询结果,搜索用时 15 毫秒
41.
Mark A. Engle Fraser Goff David G. Jewett Gregory J. Reller Joel B. Bauman 《Hydrogeology Journal》2008,16(3):559-573
Boron, chloride, sulfate, δD, δ18O, and 3H concentrations in surface water and groundwater samples from the Sulphur Bank Mercury Mine (SBMM), California, USA were
used to examine geochemical processes and provide constraints on evaporation and groundwater flow. SBMM is an abandoned sulfur
and mercury mine with an underlying hydrothermal system, adjacent to Clear Lake, California. Results for non-3H tracers (i.e., boron, chloride, sulfate, δD, and δ18O) identify contributions from six water types at SBMM. Processes including evaporation, mixing, hydrothermal water input
and possible isotopic exchange with hydrothermal gases are also discerned. Tritium data indicate that hydrothermal waters
and other deep groundwaters are likely pre-bomb (before ~1952) in age while most other waters were recharged after ~1990.
A boron-based steady-state reservoir model of the Herman Impoundment pit lake indicates that 71–79% of its input is from meteoric
water with the remainder from hydrothermal contributions. Results for groundwater samples from six shallow wells over a 6–month
period for δD and δ18O suggests that water from Herman Impoundment is diluted another 3% to more than 40% by infiltrating meteoric water, as it
leaves the site. Results for this investigation show that environmental tracers are an effective tool to understand the SBMM
hydrogeologic regime.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
Résumé Les concentrations en bore, chlorure, sulfate, δD, δ18O, et 3H d’échantillons d’eaux de surface et souterraine prélevés dans le banc de soufre de la mine de mercure (SBMM en anglais) en Californie, USA, ont été utilisées pour étudier les processus géochimiques et pour fournir des contraintes à l’évaporation et à l’écoulement des eaux souterraines. La SBMM est une mine de soufre et de mercure abandonnée, adjacente au lac Clear en Californie et sous laquelle se trouve un système hydrothermal.Les résultats des traceurs autres que le tritium (bore, chlorure, sulfate, δD, et δ18O) ont permis d’identifier des contributions de six types d’eaux à SBMM. Des processus tels que l’évaporation, le mélange, l’entrée d’eau hydrothermale et de possibles échanges isotopiques avec des gaz hydrothermaux ont également été identifiés. Les données de tritium montrent que les eaux hydrothermales et d’autres eaux profondes sont probablement d’age antérieure à la bombe (avant ~1952), alors que la plupart des autres eaux sont issues de la recharge après ~1990. Un modèle de réservoir représentant le lac situé dans la partie Herman Impoundment de l’ancienne mine, en régime permanent et basé sur le bore, montre que 71–79% de l’eau provient des précipitations, le reste provenant de contributions hydrothermales. Les résultats de δD et δ18O pour des échantillons d’eau souterraine de six puits peu profonds sur une période de 6 mois suggèrent que l’eau de Hermann Impoundment est encore diluée entre 3% jusqu’à plus de 40% lorsqu’elle quitte le site du fait de l’infiltration d’eau météorique. Les résultats de cette étude montrent que les traceurs environnementaux constituent un outil efficace pour comprendre le régime hydrogéologique de la SBMM.
Resumen Se han utilizado datos de concentraciones de boro, cloruros, sulfatos, δD, δ18O, y 3H en muestras de aguas superficiales y subterráneas procedentes de la Mina de Mercurio Sulphur Bank (SBMM), California, USA, para estudiar los procesos geoquímicos y caracterizar la evaporación y el flujo del agua subterránea. SBMM es una mina abandonada de azufre y mercurio con un sistema hidrotermal subyacente, cercano al Lago Clear, California.Los resultados de los trazadores que no son 3H (por ejemplo, boro, cloruros, sulfatos δD, y δ18O) identifican las contribuciones de seis tipos de agua en la SBMM. Se han identificado diversos procesos, como evaporación, mezcla, entrada de agua hidrotermal y la posibilidad de intercambios isotópicos con gases hidrotermales. Los datos de tritio indican que las aguas hidrotermales y otras aguas subterráneas profundas son probablemente pre-bombas (previas a ~1952) en edad mientras que la mayoría de las otras aguas se han recargado después de ~1990. Un modelo estacionario basado en el boro de la fosa del lago Herman Impoundment indica que el 71–79% de su entrada procede de agua meteórica con restos de contribución hidrotermal. Los resultados para las muestras de aguas subterráneas procedentes de seis pozos superficiales en un periodo de 6 meses para δD y δ18O sugieren que el agua procedente de Herman Impoundment se diluye en otro 3% hasta más del 40% debido a la infiltración de agua meteórica., que sale del sitio. Los resultados de esta investigación muestran que los trazadores ambientales son una herramienta efectiva para entender el régimen hidrogeológico de la SBMM.相似文献
42.
A Numerical Simulation of Residual Circulation in Tampa Bay. Part II: Lagrangian Residence Time 总被引:1,自引:0,他引:1
Lagrangian retention and flushing are examined by advecting neutrally buoyant point particles within a circulation field generated by a numerical ocean model of Tampa Bay. Large temporal variations in Lagrangian residence time are found under realistic changes in boundary conditions. Two 90-day time periods are examined. The first (P1) is characterized by low freshwater inflow and weak baroclinic circulation. The second (P2) has high freshwater inflow and strong baroclinic circulation. At the beginning of both time periods, 686,400 particles are released uniformly throughout the bay. Issues relating to particle distribution and flushing are examined at three different spatial scales: (1) at the scale of the entire bay, (2) the four major regions within the bay, and (3) at the scale of individual model grid cells. Two simple theoretical models for the particle number over time, N(t), are fit to the particle counts from the ocean model. The theoretical models are shown to represent N(t) reasonably well when considering the entire bay, allowing for straightforward calculation of baywide residence times: 156 days for P1 and 36 days for P2. However, the accuracy of these simple models decreases with decreasing spatial scale. This is likely due to the fact that particles may exit, reenter, or redistribute from one region to another in any sequence. The smaller the domain under consideration, the more this exchange process dominates. Therefore, definitions of residence time need to be modified for “non-local” situations. After choosing a reasonable definition, and removal of the tidal and synoptic signals, the residence times at each grid cell in P1 is found to vary spatially from a few days to 90 days, the limit of the calculation, with an average residence time of 53 days. For P2, the overall spatial pattern is more homogeneous, and the residence times have an average value of 26 days. 相似文献
43.
44.
The chemical potential of oxygen defined by the equilibrium:
相似文献
45.
The \(\mu _{O_2 } \) defined by the reaction 6 MnO+O2 =2 Mn3O4 has been determined from 917 to 1,433 K using electrochemical cells (with calcia-stabilized zirconta, CSZ) of the type: Steady emfs were achieved rapidly at all temperatures on both increasing and decreasing temperature, indicating that the MnO-Mn3O4 oxygen buffer equilibrates relatively easily. It therefore makes a useful alternative choice in experimental petrology to Fe2O3-Fe3O4 for buffering oxygen potentials at oxidized values. The results are (in J/mol, temperature in K, reference pressure 1 bar); \(\mu _{O_2 } \) (±200)=-563,241+1,761.758T-220.490T inT+0.101819T 2 with an uncertainty of ±200 J/mol. Third law analysis of these data, including a correction for the deviations in stoichiometry of MnO, impliesS 298.15 for Mn3O4 of 166.6 J/K · mol, which is 2.5 J/K · mol higher than the calorimetric determination of Robie and Hemingway (1985). The low value of the calorimetric entropy may be due to incomplete ordering of the magnetic spins. The third law value of Δ r H 298.15 0 is-450.09 kJ/mol, which is significantly different from the calorimetric value of-457.5±3.4 kJ/mol, calculated from Δ f H 298.15 0 of MnO and Mn3O4, implying a small error in one or both of these latter. 相似文献
46.
A new estimate of Pluto's mass within the range of possible masses considered in an earlier work has enabled us to refine our model of Pluto's interior. 相似文献
47.
Mark E. Hodson 《Geochimica et cosmochimica acta》2006,70(7):1655-1667
Laboratory determined mineral weathering rates need to be normalised to allow their extrapolation to natural systems. The principle normalisation terms used in the literature are mass, and geometric- and BET specific surface area (SSA). The purpose of this study was to determine how dissolution rates normalised to these terms vary with grain size. Different size fractions of anorthite and biotite ranging from 180-150 to 20-10 μm were dissolved in pH 3, HCl at 25 °C in flow through reactors under far from equilibrium conditions. Steady state dissolution rates after 5376 h (anorthite) and 4992 h (biotite) were calculated from Si concentrations and were normalised to initial- and final- mass and geometric-, geometric edge- (biotite), and BET SSA. For anorthite, rates normalised to initial- and final-BET SSA ranged from 0.33 to 2.77 × 10−10 molfeldspar m−2 s−1, rates normalised to initial- and final-geometric SSA ranged from 5.74 to 8.88 × 10−10 molfeldspar m−2 s−1 and rates normalised to initial- and final-mass ranged from 0.11 to 1.65 molfeldspar g−1 s−1. For biotite, rates normalised to initial- and final-BET SSA ranged from 1.02 to 2.03 × 10−12 molbiotite m−2 s−1, rates normalised to initial- and final-geometric SSA ranged from 3.26 to 16.21 × 10−12 molbiotite m−2 s−1, rates normalised to initial- and final-geometric edge SSA ranged from 59.46 to 111.32 × 10−12 molbiotite m−2 s−1 and rates normalised to initial- and final-mass ranged from 0.81 to 6.93 × 10−12 molbiotite g−1 s−1. For all normalising terms rates varied significantly (p ? 0.05) with grain size. The normalising terms which gave least variation in dissolution rate between grain sizes for anorthite were initial BET SSA and initial- and final-geometric SSA. This is consistent with: (1) dissolution being dominated by the slower dissolving but area dominant non-etched surfaces of the grains and, (2) the walls of etch pits and other dissolution features being relatively unreactive. These steady state normalised dissolution rates are likely to be constant with time. Normalisation to final BET SSA did not give constant ratios across grain size due to a non-uniform distribution of dissolution features. After dissolution coarser grains had a greater density of dissolution features with BET-measurable but unreactive wall surface area than the finer grains. The normalising term which gave the least variation in dissolution rates between grain sizes for biotite was initial BET SSA. Initial- and final-geometric edge SSA and final BET SSA gave the next least varied rates. The basal surfaces dissolved sufficiently rapidly to influence bulk dissolution rate and prevent geometric edge SSA normalised dissolution rates showing the least variation. Simple modelling indicated that biotite grain edges dissolved 71-132 times faster than basal surfaces. In this experiment, initial BET SSA best integrated the different areas and reactivities of the edge and basal surfaces of biotite. Steady state dissolution rates are likely to vary with time as dissolution alters the ratio of edge to basal surface area. Therefore they would be more properly termed pseudo-steady state rates, only appearing constant because the time period over which they were measured (1512 h) was less than the time period over which they would change significantly. 相似文献
48.
Manuela A. Fehr Mark Rehkämper Alex N. Halliday Maria Schönbächler Bodo Hattendorf 《Geochimica et cosmochimica acta》2006,70(13):3436-3448
Tellurium isotope data acquired by multiple-collector inductively coupled plasma-mass spectrometry (MC-ICPMS) are presented for sequential acid leachates of the carbonaceous chondrites Orgueil, Murchison, and Allende. Tellurium isotopes are produced by a broad range of nucleosynthetic pathways and they are therefore of particular interest given the isotopic anomalies previously identified for other elements in these meteorites. In addition, the data provide new constraints on the initial solar system abundance of the r-process nuclide 126Sn, which decays to 126Te with a half-life of 234,500 years. The 126Te/128Te ratios of all leachates were found to be identical, within uncertainty, despite variations in 124Sn/128Te of between about 0.002 and 1.4. The data define a 126Sn/124Sn ratio of <7.7 × 10−5 at the time of last isotopic closure, consistent with the value of <18 × 10−5 previously reported for bulk carbonaceous chondrites. How close this is to the initial 126Sn/124Sn ratio of the solar system depends on when the investigated samples last experienced redistribution of Sn and Te. No clear evidence is found for nucleosynthetic anomalies in the abundances of p-, s-, and r-process nuclides. The largest effect detected in this study is a small excess of the r-process nuclide 130Te in a nitric acid leachate of Murchison. This fraction displays an anomalous ε130Te of +3.5 ± 2.5. Although barely resolvable given the analytical uncertainties, this is consistent with the presence of a small excess r-process component or an s-process deficit. The general absence of anomalies contrasts with previous results obtained for K, Cr, Zr, Mo, and Ba isotopes in similar leachates, which display nucleosynthetic anomalies of up to 3.8%. The reason for this discrepancy is unclear but it may reflect volatility and more efficient mixing of Te in the solar nebula. 相似文献
49.
Beach seines are widely used to estimate the density and species richness of fishes in estuaries. We evaluated the causes
and extent of bias in estimates from seines using a series of field experiments in small estuaries in southern California,
USA. Seining in spatially paired areas that were either enclosed by block nets or not, revealed that seines used without block
nets und erestimated density by more than 4-fold and species richness by more than 2-fold relative to blocked areas. Seining
in paired blocked areas with seines of two lengths revealed that net length affected estimates of density, but not species
richness; a 7.6-m long seine produced 1.6-fold higher estimates of total density than did a 15.2-m long seine due to increased
catches of demersal fishes, but not midwater species. Paired sampling in blocked areas also revealed that many fishes initially
evaded capture by the seine. Estimates of density but not species richness were significantly higher in areas through which
a seine was swept 5 times compared to once. This was due to higher catches of demersal fishes but not midwater fishes in areas
seined 5 times. Repeated seining through blocked areas revealed that the vast majority (90% or more) of species and individuals
of midwater fishes were captured within the first 5 sweeps, compared to only about 50% of the individuals of demersal species.
A mark-recapture study in blocked areas revealed lower probabilities of capture for demersal species relative to midwater
species. 相似文献
50.
We evaluated the sampling characteristics of enclosure traps in estuaries in southern California, USA. Using enclosure traps
that sampled 0.25, 0.5, and 1-m2 footprints, we found that enclosure trap size significantly affected estimates of fish density and the precision of these
estimates. The highest estimates were produced by the 0.5-m2 trap and the lowest by the 0.25-m2 trap. Precision of the density estimates improved with increasing trap size, while the proportion of zero values in the data
sets decreased and estimates of species richness increased. The largest trap was difficult to use in the field and often did
not function properly; thus we concluded that intermediate enclosure trap sizes offered the best compromise between statistical
and logistical considerations. By examination of burrows in sediment cores taken in fished out enclosure traps, we found no
evidence to support the widely held view that burrow-dwelling fishes evaded capture by hiding in burrows. We also used mark-recapture
techniques to estimate recovery efficiency in 0.43-m2 enclosure traps. Recovery efficiency averaged 91% and did not differ significantly among estuaries or sampling stations within
estuaries. Based on extensive netting within enclosure traps, we determined that in areas with dense fish populations (>90
fish 0.43-m−2), netting could be ceased after the first sweep that captured no fish with only a trivial effect on the estimate of density.
In more sparsely populated areas, netting had to continue until 2–3 sweeps had captured no fish in order to obtain estimates
of density that were within 90% of the actual values present. Overall, we found enclosure traps to be effective tools for
sampling small, abundant fishes in shallow estuaries in southern California, but we recommend that care be taken when choosing
trap size and sampling (netting) effort within traps in order to optimize their sampling characteristics. 相似文献
|