全文获取类型
收费全文 | 59515篇 |
免费 | 751篇 |
国内免费 | 303篇 |
专业分类
测绘学 | 1415篇 |
大气科学 | 4031篇 |
地球物理 | 10929篇 |
地质学 | 24236篇 |
海洋学 | 4765篇 |
天文学 | 12058篇 |
综合类 | 236篇 |
自然地理 | 2899篇 |
出版年
2022年 | 258篇 |
2021年 | 440篇 |
2020年 | 499篇 |
2019年 | 578篇 |
2018年 | 3750篇 |
2017年 | 3517篇 |
2016年 | 2529篇 |
2015年 | 693篇 |
2014年 | 1062篇 |
2013年 | 1852篇 |
2012年 | 2204篇 |
2011年 | 4122篇 |
2010年 | 3740篇 |
2009年 | 4188篇 |
2008年 | 3457篇 |
2007年 | 4078篇 |
2006年 | 1602篇 |
2005年 | 1497篇 |
2004年 | 1426篇 |
2003年 | 1532篇 |
2002年 | 1244篇 |
2001年 | 894篇 |
2000年 | 832篇 |
1999年 | 723篇 |
1998年 | 715篇 |
1997年 | 723篇 |
1996年 | 588篇 |
1995年 | 572篇 |
1994年 | 501篇 |
1993年 | 453篇 |
1992年 | 410篇 |
1991年 | 424篇 |
1990年 | 437篇 |
1989年 | 391篇 |
1988年 | 369篇 |
1987年 | 400篇 |
1986年 | 416篇 |
1985年 | 509篇 |
1984年 | 545篇 |
1983年 | 542篇 |
1982年 | 496篇 |
1981年 | 466篇 |
1980年 | 449篇 |
1979年 | 408篇 |
1978年 | 375篇 |
1977年 | 384篇 |
1976年 | 348篇 |
1975年 | 353篇 |
1974年 | 340篇 |
1973年 | 370篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
R.E. Kopp 《Geochimica et cosmochimica acta》2003,67(17):3247-3256
The magnetites and sulfides located in the rims of carbonate globules in the Martian meteorite ALH84001 have been claimed as evidence of past life on Mars. Here, we consider the possibility that the rims were formed by dissolution and reprecipitation of the primary carbonate by the action of water. To estimate the rate of these solution-precipitation reactions, a kinetic model of magnesite-siderite carbonate dissolution was applied and used to examine the physicochemical conditions under which these rims might have formed. The results indicate that the formation of the rims could have taken place in < 50 yr of exposure to small amounts of aqueous fluids at ambient temperatures. Plausible conditions pertaining to reactions under a hypothetical ancient Martian atmosphere (1 bar CO2), the modern Martian atmosphere (8 mbar CO2), and the present terrestrial atmosphere (0.35 mbar CO2) were explored to constrain the site of the process. The results indicated that such reactions likely occurred under the latter two conditions. The possibility of Antarctic weathering must be entertained, which, if correct, would imply that the plausibly biogenic minerals (single-domain magnetite of characteristic morphology and sulfide) reported from the rims may be the products of terrestrial microbial activity. This model is discussed in terms of the available isotope data and found to be compatible with the formation of ALH84001 rims. Particularly, anticorrelated variations of radiocarbon with δ13C indicate that carbonate in ALH84001 was affected by solution-precipitation reactions immediately after its initial fall (∼13,000 yr ago) and then again during its recent exposure prior to collection. 相似文献
952.
K. Pecher D. McCubbery E. Kneedler J. Bargar L. Cox B. Tonner 《Geochimica et cosmochimica acta》2003,67(6):1089-1098
We have applied Scanning Transmission Soft X-ray Microscopy (STXM) to investigate the charge state distribution of Mn in two kinds of Mn-biominerals, Mn nodules collected from Lake Michigan sediments and Mn precipitates formed by spores of a marine bacillus SG-1 under transport limited reaction conditions. A data analysis technique was developed, which allows for extraction of spatially resolved 2-d charge state maps of manganese on a submicron level. We find that the charge state of Mn dominates the spectral shape of L-edge spectra of environmentally important single oxidation state Mn minerals and that spectra of mixed oxidation state oxides can be modelled by a combination of appropriate single oxidation state reference spectra. Two-dimensional maps of charge state distributions clearly reveal domains of different oxidation states within single particles of Mn-micronodules. Spots of preferred accumulation of Mn(II) were found, which indicates biogenic precipitation of Mn(II)-species as a first step of nodule formation. The presence of Mn(III) in the studied sediment samples suggests the involvement of one-electron oxidation processes and reaction conditions which inhibit or slow down the disproportionation of Mn(III)-products. Under transport limited conditions, Mn oxidation products formed by spores of the marine bacillus SG-1 can vary from cell to cell. The presence of significant amounts of Mn(III) containing species points to the involvement of one-electron oxidation reactions as in the case of the micro-nodules. Our technique and the results obtained form a new basis for the mechanistic understanding of the formation of Mn biominerals in the environment. 相似文献
953.
Stephen E Grasby 《Geochimica et cosmochimica acta》2003,67(9):1659-1666
Vaterite, a rare hexagonal CaCO3 polymorph, was identified in precipitates forming at a supraglacial sulfur spring, in Borup Fiord Pass, northern Ellesmere Island, Canadian High Arctic. Vaterite occurs in a precipitate mound along with calcite, gypsum, and native sulfur. The unusual conditions of the site, including an extremely cold climate, supersaturated alkaline waters, and the presence of gypsum, mimics conditions used to grow vaterite in laboratory experiments. Stable isotope data suggest that vaterite may preferentially form during colder periods of the year. Vaterite found at the site is characterized by 2- to 10-μm rounded to spherical shaped particles (comprising smaller 0.5- to 2-μm spheres) as both individuals and in chainlike structures. The spherical habit of vaterite resembles carbonate structures that have been interpreted to be organic; however, δ13C values are indicative of an inorganic origin. The thick permafrost, and the extreme cold and dry environment make this site a good terrestrial analog for carbonate precipitates that might be expected at potential deposits associated with water discharge on other planetary bodies. 相似文献
954.
In the first known kinetic application of the technique, synchrotron 57Fe-Mössbauer spectroscopy was used to follow the rate of heterogeneous electron transfer between aqueous reagents and a solid phase containing Fe. The solid, a synthetic 57Fe-enriched Fe(III)-bearing pyroaurite-like phase having terephthalate (TA) in the interlayer [Mg3Fe(OH)8(TA)0.5 · 2H2O], was reduced by Na2S2O4 and then reoxidized by K2Cr2O7 by means of a novel flow-through cell. Synchrotron Mössbauer spectra were collected in the time domain at 30-s intervals. Integration of the intensity obtained during a selected time interval in the spectra allowed sensitive determination of Fe(II) content as a function of reaction time. Analysis of reaction end member specimens by both the synchrotron technique and conventional Mössbauer spectroscopy yielded comparable values for Mössbauer parameters such as center shift and Fe(II)/Fe(III) area ratios. Slight differences in quadrupole splitting values were observed, however. A reactive diffusion model was developed that fit the experimental Fe(II) kinetic data well and allowed the extraction of second-order rate constants for each reaction. Thus, in addition to rapidly collecting high quality Mössbauer data, the synchrotron technique seems well suited for aqueous rate experiments as a result of the penetrating power of 14.4 keV X-rays and high sensitivity to Fe valence state. 相似文献
955.
956.
957.
In the present study, an attempt has been made to delineate and characterize the different geomorphic units of Tundiya river
catchment in a part of Lower Maharahstra Metamorphic Plateau, north-eastern part of Nagpur district, Maharashtra. The drainage,
contour and delineated geological units have been overlaid on IRS-ID LISS III satellite imagery (bands 2,3 and 4) in EASI/PACE
analysis system to delineate and characterize different geomorphological units and analysis of their processes based on the
field observations. The study area is basically of metamorphic in origin with different geological formations and is influenced
by the various fluvio-morphological processes. Based on the satellite data analysis, the distinct geomorphological units viz.,
table top summits, structural hills, subdued plateau, linear ridges, shallow, moderate and deeply buried foot slopes, shallow
valley fills and deep valley fills have been delineated and characterized. The information generated from satellite data in
the form of vector layers has been used in GIS to generate geological and geomorphological maps of the study area. The present
study demonstrates that IRS-ID LISS-III data in conjunction with geology, drainage and contour parameters to enable detailed
evaluation of different geomorphological units and analysis of their processes based on the field observations. The delineated
geomorphological units can be utilized for evaluation and management of natural resources and geo-environment on sustainable
basis at river catchment level. 相似文献
958.
The paper discusses the land damage assessment and change detection analysis with reference to a mineral bearing zone in Manjhi,
Manuni and Churan valleys. The area is located in environmentally sensitive and fragile region of Himalaya and constitutes
of nearly 400 small-scale mines of slate, which were operative since last one hundred years and are stopped by court of law
since 1995 on account of environment deterioration. The status of land degradation has been studied using IRS-1B satellite
data of 1988,1992 and 1995. The geo-coded data on 1:50,000 scale has been interpreted and an increase in land degradation
status was noticed. Finally, the management strategy for arresting the further land damage in a broader perspective is suggested. 相似文献
959.
Permeshwar S. Chauhan Mahesh C. Porwal Lalit Sharma Jay Devs.negi 《Journal of the Indian Society of Remote Sensing》2003,31(3):211-218
The review of study site have revealed the change in vegetation cover of Sal Dense to Sal Medium and Sal Open in 6 forest
Mosaics owing to biotic and abiotic conditions prevailing in the specific areas. Analysis carried out using thematic map derived
from aerial photograph of 1976 and satellite data of IRS 1C LISS III False Colour Composite (FCC) of March 1999 revealed the
cause for change in forest density classes. Deforestation, encroachment and agriculture have been identified as the underlying
causes, which have affected some specific locations to a marked extent. There has been a progressive and remarkable change
among vegetation classes from 1976 to 1999. It is evident from forest type and density map that Sal density has significantly
reduced from Sal Dense 65.61 % in 1976 to Sal Dense 11.12% in the year 1999 followed by Sal Open 11.18 % and Sal Medium 18.24
%. The overall change has been estimated to be 42.11% of the total forested area. 相似文献
960.
The Himalayas has one of the largest concentrations of glaciers outside the Polar Regions. Various reports suggest that significant number of mountain glaciers is shrinking due to climatic variations. Monitoring of these glaciers is important to assess future availability of water resources in the Himalayan region. However, Himalayan glaciers are normally difficult to monitor due to the rugged, mountainous terrain. Therefore, images of Indian Remote Sensing Satellite were used to monitor glaciers in the Baspa basin. Investigations have shown the presence of 30 glaciers in the basin, with areal extent of 167 km2. Out of these, 19 glaciers, with areal extent of 140 km2 were selected to estimate retreat. Investigation suggests that almost all glaciers are retreating in the study basin and overall 19% deglaciation has been observed from 1962 to 2001. In general, altitude distribution appears to have significant influence on glacial retreat. Glaciers located around 5000 m altitude range are showing 24% loss as compared to 14% by glaciers located in altitude range higher than 5400 m. In addition, mean altitude of glacier terminus is shifted upward by 88 m, i.e. from 4482 to 4570 m in last 39 years. The glacial volumes were estimated using regression relationship between area and depth. The investigations have suggested that 19.10 km3 of glacial water stored in the 19 glaciers in 1962, has been reduced to 14.71 km3 in 2001, respectively, an overall loss of 23 percent in a period between 1962 and 2001. These investigations suggest that all glaciers in the Baspa Basin are reducing and in long term, such reducing trend can create scarcity of water in the region. 相似文献