全文获取类型
收费全文 | 70101篇 |
免费 | 437篇 |
国内免费 | 1142篇 |
专业分类
测绘学 | 2360篇 |
大气科学 | 4320篇 |
地球物理 | 12924篇 |
地质学 | 30046篇 |
海洋学 | 5112篇 |
天文学 | 12180篇 |
综合类 | 2266篇 |
自然地理 | 2472篇 |
出版年
2022年 | 492篇 |
2021年 | 713篇 |
2020年 | 761篇 |
2019年 | 836篇 |
2018年 | 7454篇 |
2017年 | 6505篇 |
2016年 | 4778篇 |
2015年 | 895篇 |
2014年 | 1515篇 |
2013年 | 2084篇 |
2012年 | 2985篇 |
2011年 | 5613篇 |
2010年 | 4759篇 |
2009年 | 5251篇 |
2008年 | 4407篇 |
2007年 | 5378篇 |
2006年 | 1831篇 |
2005年 | 1313篇 |
2004年 | 1407篇 |
2003年 | 1422篇 |
2002年 | 1154篇 |
2001年 | 832篇 |
2000年 | 715篇 |
1999年 | 433篇 |
1998年 | 460篇 |
1997年 | 476篇 |
1996年 | 338篇 |
1995年 | 360篇 |
1994年 | 335篇 |
1993年 | 281篇 |
1992年 | 285篇 |
1991年 | 290篇 |
1990年 | 337篇 |
1989年 | 273篇 |
1988年 | 258篇 |
1987年 | 260篇 |
1986年 | 191篇 |
1985年 | 301篇 |
1984年 | 302篇 |
1983年 | 295篇 |
1982年 | 274篇 |
1981年 | 269篇 |
1980年 | 287篇 |
1979年 | 203篇 |
1978年 | 242篇 |
1977年 | 204篇 |
1976年 | 183篇 |
1975年 | 184篇 |
1974年 | 171篇 |
1973年 | 206篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
891.
Geochemical processes controlling the spatial distribution of selenium in soil and water,west central South Dakota,USA 总被引:8,自引:0,他引:8
Selenium (Se) is essential in the human diet, but has a low threshold for toxic concentration. It is recommended that nutrients
such as Se should be consumed through foods as part of a normal diet. Se concentrations in crops and meat depend on the amount
of labile Se in the soil where crops and forage are grown. Therefore, managing agriculture for optimal Se in grain crops and
forage requires an understanding of the distribution and mobility of Se. Elevated concentrations of Se occur in waters, soils,
and forage 120 km west of Pierre in west central South Dakota, USA. The research site lies in an elevated, dissected plain
where soils developed on gently dipping Pierre Shale. Soils were sampled along catena transects and waters collected from
soil, ponds, and shallow borings in areas of known elevated forage and crop Se. Soil extracts from saturated-paste extraction
and acid (aqua regia and hydrofluoric acid) extraction were analyzed. Selenate was the dominant Se species in both acid and
saturated-paste extracts; selenite and organic Se were below detection (<0.2 ppb) in the same soil extracts. On average, 98% of soil Se was not water-soluble. The distribution of total Se shows much less spatial variation than water-soluble
Se in the areas sampled. Se shows correlation with organic carbon in soils and waters, suggesting its association with organic
carbon. Ca shows some correlation to Se in acid extracts, but not in saturated-paste extracts or in waters. Total Se shows
no significant correlation to Na, Mg, and total S in the soils. Se in saturated-paste extracts and water samples shows good
correlations with Na, Mg, and SO4, suggesting that evaporitic Na–Mg–sulfate minerals may temporarily concentrate water-soluble Se in shallow soils. The dissolution
and precipitation of these Na–Mg–sulfate salts together with pH and oxidation–reduction conditions apparently control water-soluble
Se distribution and mobilization in shallow subsurface environments. 相似文献
892.
María Teresa Rodríguez-Salazar Ofelia Morton-Bermea Elizabeth Hernández-Álvarez Rufino Lozano Victor Tapia-Cruz 《Environmental Earth Sciences》2011,62(5):899-905
This research presents and discusses information concerning the spatial distribution of heavy metals (Pb, Cu, Zn, Ba, Co,
Cr, Ni and V) in the urban environments of Mexico City using geographical information system and statistical analysis. Superficial
soil samples (n = 146) were analyzed. The highest contamination indices were found in the north and center zone of the metropolitan area.
In contrast, the surrounding rural fields show a lower impact grade. The higher concentrations of Pb, Cu, Zn and Ba were observed
as being related to high vehicular traffic, nevertheless other elements such as Co, Cr, Ni and V do not show anthropogenic
influence and their content can be attributed to the parental rock. The results are compared with previous surveys carried
out in 2003 in order to evaluate temporal deposition trends. No changes were found on reported concentrations except for Cu
and Zn, whose concentration has increased in later years. The results suggest that spatial distribution analysis and results
in comparison with previous studies could be useful for the management and sustainable development of the metropolitan area
of Mexico City. 相似文献
893.
B. Panteleit K. Hamer R. Kringel W. Kessels H. D. Schulz 《Environmental Earth Sciences》2011,62(1):77-91
Geochemical processes, occurring in a stable transition zone between saltwater and freshwater, were simulated in a 2D, multi-layer
flow chamber experiment. Mixing, calcite dissolution, and oxidative degradation of organic matter were identified as the main
controlling factors. The results of the chamber experiment were compared to field data and verified by thermodynamic modeling.
Similarity in most ion distributions suggests the general applicability of the experimental method. Differences in the redox
conditions between field and experiment were reflected by the oxidants involved in the mineralization of organic carbon; while
field data show evidence of sulfate reduction, the presence of oxygen in the laboratory experiment resulted in the reoxidation
of sulfides. 相似文献
894.
Parameter estimation for a karst aquifer with unknown thickness using the genetic algorithm method 总被引:3,自引:2,他引:3
The combination of ecological fragility and agricultural activity in the loess hilly–gully regions of western China has received
broad environmental concerns. In this region, rainfall and soil moisture can fatally influence crop production under dry land
farming. In this study, field experiments were conducted, from March 2001 to September 2005, to demonstrate the variation
of soil moisture and fertilizer contents at different depths in slope and terraced lands, and to evaluate the ecological impacts
and economic benefits in the terraced land of Loess Plateau. The results of both field test and Grey model (GM) calculation
show that the terraced land, as compared to the sloping land, in the agricultural area of the Loess Plateau tends to store
and retain much water, promoting more favorable interactions between water and fertilizer. During the months from March to
June of the year with less rainfall, the water supply for crop growth is mainly derived from the deep storage of soil moisture
accumulated from July to September of the previous year. The field experiments indicate that the crop yield of the 3-year-old
terraced lands was 27% higher than that of the sloping lands with slopes greater than 10°, and that the crop yield can increase
by 27.07 to 52.78% in the following cultivation years. In particular, potato was found to be more drought-resistant than winter
wheat, thus it is more suitable for the arid and semi-arid Loess Plateau regions. 相似文献
895.
Ice-core samples from Upper Fremont Glacier (UFG), Wyoming, were used as proxy records for the chemical composition of atmospheric
deposition. Results of analysis of the ice-core samples for stable isotopes of nitrogen (δ15N, ) and sulfur (δ34S, ), as well as and deposition rates from the late-1940s thru the early-1990s, were used to enhance and extend existing National Atmospheric
Deposition Program/National Trends Network (NADP/NTN) data in western Wyoming. The most enriched δ34S value in the UFG ice-core samples coincided with snow deposited during the 1980 eruption of Mt. St. Helens, Washington.
The remaining δ34S values were similar to the isotopic composition of coal from southern Wyoming. The δ15N values in ice-core samples representing a similar period of snow deposition were negative, ranging from -5.9 to -3.2 ‰ and
all fall within the δ15N values expected from vehicle emissions. Ice-core nitrate and sulfate deposition data reflect the sharply increasing U.S.
emissions data from 1950 to the mid-1970s. 相似文献
896.
Formation of deep basins on continental crust in fold belts is often explained by stretching. This mechanism inevitably produces large deformations in the upper crust. No deformations typical of significant stretching were revealed in the predominant part of deep basins on continental crust in the Alpine Belt. This means that these basins were not produced by stretching. Most basins were formed during a short period of time of a few million years. The short duration of the subsidences eliminates thermal relaxation as the mechanism. The space and time relationships between the subsidence and orogeny and the profile of the basin floor exclude thrust loading as a cause of formation for practically all large basins. Gabbro to eclogite transformation is suggested as a mechanism of rapid subsidence. This occurs under the upwelling of hydrous asthenosphere at moderate temperature to the base of the crust. Eclogite sinking into the mantle results in a strong attenuation of the crust and lithosphere, which permits intense subsequent folding. The major part of deep basins in continental crust that formed by rapid subsidence was intensely shortened in the Alpine Belt. Significant crustal shortening did not spread over the cratonic lithosphere. 相似文献
897.
Two methods are compared for estimating the shape parameters of Pareto field-size (or pool-size) distributions for petroleum
resource assessment. Both methods assume mature exploration in which most of the larger fields have been discovered. Both
methods use the sizes of larger discovered fields to estimate the numbers and sizes of smaller fields: (1) the tail-truncated
method uses a plot of field size versus size rank, and (2) the log–geometric method uses data binned in field-size classes
and the ratios of adjacent bin counts. Simulation experiments were conducted using discovered oil and gas pool-size distributions
from four petroleum systems in Alberta, Canada and using Pareto distributions generated by Monte Carlo simulation. The estimates
of the shape parameters of the Pareto distributions, calculated by both the tail-truncated and log–geometric methods, generally
stabilize where discovered pool numbers are greater than 100. However, with fewer than 100 discoveries, these estimates can
vary greatly with each new discovery. The estimated shape parameters of the tail-truncated method are more stable and larger
than those of the log–geometric method where the number of discovered pools is more than 100. Both methods, however, tend
to underestimate the shape parameter. Monte Carlo simulation was also used to create sequences of discovered pool sizes by
sampling from a Pareto distribution with a discovery process model using a defined exploration efficiency (in order to show
how biased the sampling was in favor of larger fields being discovered first). A higher (more biased) exploration efficiency
gives better estimates of the Pareto shape parameters. 相似文献
898.
899.
900.
Preface: Proceedings of the 5th International Symposium on Lithographic Limestone and Plattenkalk 总被引:1,自引:0,他引:1
Daniel Marty Jean-Paul Billon-Bruyat Christian A. Meyer Loïc Costeur Basil Thüring 《Swiss Journal of Geoscience》2011,104(1):1-29
Magmatic rocks from the pre-Mesozoic basements of the Sambuco and Maggia nappes have been dated by U–Pb zircon ages with the LA-ICPMS technique. Several magmatic events have been identified in the Sambuco nappe. The mafic banded calc-alkaline suite of Scheggia is dated at 540 Ma, an age comparable to that of mafic rocks in the Austroalpine Silvretta nappe. The Sasso Nero peraluminous augengneiss has an age of 480–470 Ma, like many other “older orthogneisses” in Alpine basement units. It hosts a large proportion of inherited zircons, which were dated around 630 Ma, a Panafrican age indicating the Gondwanan affiliation of the Sambuco basement. The calc-alkaline Matorello pluton yielded ages around 300 Ma, similar to numerous Late Carboniferous intrusions in other basement units of the Lower Penninic (Monte Leone, Antigorio, Verampio) and Helvetic domains (Gotthard and other External Crystalline Massifs). Associated lamprophyric dykes are slightly younger (300–290 Ma), like similar dykes sampled in gneiss blocks included in the sedimentary cover of the underlying Antigorio nappe (290–285 Ma). The Cocco granodiorite and Rüscada leucogranite, both intruding the basement of the neighbouring Maggia nappe, yielded ages of ca. 300–310 Ma, identical within errors to the age of the Matorello pluton. They are significantly older than former age determinations. This age coincidence, coupled with remarkable petrologic similarities between the Cocco and Matorello granodiorites, strongly suggests paleogeographic proximity of the Sambuco and Maggia nappes in Late Carboniferous times. In recent publications these two nappes have been interpreted as belonging to distinct Mesozoic paleogeographic domains: “European” for Sambuco and “Briançonnais” for Maggia, separated by the “Valais” oceanic basin. In this case, the similarity of the Matorello and Cocco intrusions would demonstrate the absence of any significant transcurrent movement between these two continental domains. Alternatively, according to a more traditional view, Sambuco and Maggia might belong to a single large Alpine tectonic unit. 相似文献