全文获取类型
收费全文 | 62854篇 |
免费 | 597篇 |
国内免费 | 413篇 |
专业分类
测绘学 | 1835篇 |
大气科学 | 4563篇 |
地球物理 | 11862篇 |
地质学 | 24140篇 |
海洋学 | 4945篇 |
天文学 | 13544篇 |
综合类 | 245篇 |
自然地理 | 2730篇 |
出版年
2021年 | 328篇 |
2020年 | 363篇 |
2019年 | 449篇 |
2018年 | 3765篇 |
2017年 | 3487篇 |
2016年 | 2658篇 |
2015年 | 774篇 |
2014年 | 1139篇 |
2013年 | 2012篇 |
2012年 | 2223篇 |
2011年 | 4128篇 |
2010年 | 3791篇 |
2009年 | 4424篇 |
2008年 | 3645篇 |
2007年 | 4222篇 |
2006年 | 1728篇 |
2005年 | 1744篇 |
2004年 | 1631篇 |
2003年 | 1598篇 |
2002年 | 1471篇 |
2001年 | 1112篇 |
2000年 | 1049篇 |
1999年 | 893篇 |
1998年 | 884篇 |
1997年 | 888篇 |
1996年 | 711篇 |
1995年 | 666篇 |
1994年 | 625篇 |
1993年 | 543篇 |
1992年 | 475篇 |
1991年 | 486篇 |
1990年 | 466篇 |
1989年 | 485篇 |
1988年 | 434篇 |
1987年 | 503篇 |
1986年 | 456篇 |
1985年 | 535篇 |
1984年 | 658篇 |
1983年 | 568篇 |
1982年 | 557篇 |
1981年 | 515篇 |
1980年 | 457篇 |
1979年 | 432篇 |
1978年 | 445篇 |
1977年 | 381篇 |
1976年 | 344篇 |
1975年 | 349篇 |
1974年 | 324篇 |
1973年 | 362篇 |
1972年 | 260篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
871.
Discrimination diagrams have been developed that source Egyptian basaltic artefacts using whole‐rock major element geochemistry. These include K2O versus SiO2, TiO2 and P2O5 against MgO/Fe2O3t (total Fe as Fe2O3), and a discriminant analysis diagram using SiO2, Fe2O3t, CaO, and MnO. A complementary set of diagrams uses easily obtained trace element data (Nb/Y versus Zr/Nb; Zr [ppm] versus Rb/Sr; TiO2 [wt % volatile free] versus V; and Cr [ppm] versus Zr/Y) to determine the bedrock sources. These diagrams have been applied to seven First Dynasty basalt vessels (Abydos), two Fourth Dynasty basalt paving stones (Khufu's funerary temple, Giza), and two Fifth Dynasty paving stones (Sahure's complex, Abu Sir). They show that the bedrock source for all the artefacts was the Haddadin flow in northern Egypt. Multidimensional scaling and cluster analysis applied to the whole‐rock data (major elements and trace elements together) and previously published mineral fingerprinting studies confirm these results. Comparing mineral versus whole‐rock fingerprinting techniques, a major advantage of the former is the small sample size required (0.001 g compared to ≥ 0.1 g). Analytical costs are similar for both methods assuming that a comparison (bedrock) database can be assembled from the literature. For most archaeological problems, a whole‐rock bedrock database is more likely to exist than a mineral database, and whole‐rock analyses on artefacts will generally be easier to obtain than mineral analyses. Whole‐rock fingerprinting may be more sensitive than mineral‐based fingerprinting. Thus, if sample quantity is not an issue, whole‐rock analysis may have a slight cost, convenience, and technical advantage over mineral‐based methods. Our results also emphasize that the Egyptians cherished their Haddadin basalt flow and used it extensively and exclusively for manufacturing basalt vessels and paving stones for at least 600 years (∼3150 B.C. to 2500 B.C., approximate ages of the vessels and Abu Sir paving stones, respectively). © 2001 John Wiley & Sons, Inc. 相似文献
872.
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
873.
Significance of Europium anomalies in silicate melts and crystal-melt equilibria: a re-evaluation 总被引:1,自引:0,他引:1
Although europium speciation in silicate melts partly reflects prevailing oxygen fugacities, melt composition and structure play the major role in determining Eu2+/ Eu3+ ratios and europium partitioning into mineral phases. Experimental evidence by different investigators on the magnitude of the compositional effect on Eu2+/Eu3+ provides consistent results only if account is taken of the oxygen buffer system employed in the experiments. The medium-dependent reduction of europium can be understood in terms of the preferential stabilization of Eu2+ by a strong aluminosilicate complex in the melt phase, and to a much lesser degree by metasilicate complexes. The stability of these complexes increases as the field strength of the associated cation species decreases. Hence Eu2+-aluminosilicate complexes are preferentially stabilized relative to trivalent lanthanides in melts of appropriate composition and their presence minimizes the enthalpy of the melt. The influence of these complexes is particularly pronounced in melts with a high feldspathic component and a strongly polymerized structure. Their petrogenetic influence is best documented through the behaviour of europium relative to the other lanthanides during anhydrous anatexis in high-grade metamorphic terrains and in the anomalous europium partition coefficients of phenocryst phases which formed during the crystallization of highly silicic magmas. 相似文献
874.
This paper presents one of the richest and most complete vertebrate faunas of the late Middle Miocene (~12 Ma) of Central Europe. Up to now, sixty-two vertebrate taxa, comprising all major groups (fishes, amphibians, reptiles, birds, mammals), have been recorded. Based on sedimentological and palaeobiological evidences, this Fossillagerstätte is assumed to originate from a floodplain paleosol formed on top of a braided river sequence. The fauna points to a highly structured, somewhat vegetated landscape with a wide array of habitats (e.g., fluvial channels, sporadically moist floodplains, short-lived ponds, savannah-like open areas and screes). It was preserved due to a rapid drowning and the switch to a freshwater lake environment. Palaeoclimatological data, derived from pedogenic features as well as from biota, indicate an overall semi-arid, subtropical climate with distinct seasonality (mean annual precipitation 486 ± 252 mm, mean annual temperature ~15°C). This underlines the late Middle/early Late Miocene dry-spell in Central Europe. From taphonomical point of view, the irregularly distributed but roughly associated larger vertebrate remains refer to an in situ accumulation of the bone bed. Splintered bones, gnawing marks as well as rhizoconcretions and root corrosion structures record some pre- and post-burial modification of the taphocoenose. However, the findings of pellet remains argue for a very fast burial and thus to a low degree of time-averaging. For this reason, the fossil fauna reflects the original vertebrate community rather well and is a cornerstone for the understanding of late Middle Miocene terrestrial ecosystems in this region. Certainly, Gratkorn will be one of the key faunas for a high-resolution continental biostratigraphy and the comprehension of Europe’s faunal interchanges near the Middle/Late Miocene transition. 相似文献
875.
全球变化下地表反照率研究进展 总被引:12,自引:0,他引:12
地表反照率表征地球表面对太阳辐射的反射能力,决定着地表与大气之间辐射能量的分配过程,是影响地球气候系统的关键变量。在全球变化日益突出的今天,地表反照率与全球变化的相互影响机制已经成为地球科学研究领域的热点问题之一。地表反照率的细微变化,会影响到地气系统的能量收支平衡,进而引起区域以至全球气候变化。详细介绍地表反照率影响... 相似文献
876.
877.
Gennaro DicataldoWilliam P. Johnson David L. NaftzDonald F. Hayes William O. MoellmerTheron Miller 《Applied Geochemistry》2011,26(1):28-36
Diel (24-h) changes in Se and As concentrations in a freshwater wetland pond bordering the Great Salt Lake (GSL) were examined. Selenium concentrations (filtered and unfiltered) changed on a diel basis, i.e., were depleted during early morning and enriched during daytime over August 17-18. During the May 24-25, 2006 and September 29-30 diel studies, no significant 24-h trends were observed in Se concentrations compared to August, which showed daily maximums up to 59% greater than the daily minimum. Both filtered and unfiltered As concentrations also varied on a diel cycle, with increased concentrations during early morning and decreased concentrations during daytime. Filtered As concentrations increased 110% during the May 24-25, 2006 diel study. Selenium varied in phase with pH, dissolved O2 (DO), and water temperature (Tw) whereas As varied opposite to Se, pH, DO and Tw. Changes in pH, DO and Tw showed a direct linear correlation (r = 0.74, 0.75, and 0.55, respectively) to filtered Se. Also pH, DO and Tw were inversely correlated to filtered As concentration (r = −0.88, −0.87, and −0.84, respectively). Equilibrium geochemical speciation and sorption models were used to examine the potential oxidation state changes in Se and As, and sorption and desorption reactions corresponding to the observed 24-h variations in pe and pH. In this wetland it was postulated that diel Se variation was driven by sorption and desorption due to photosynthesis-induced changes in pH and redox conditions. Diel variations of As were hypothesized to be linked to pH-driven sorption and desorption as well as co-precipitation and co-dissolution with mineral phases of Mn. 相似文献
878.
The dominant factors affecting the development of weathering forms on the Prav?ice Rock Arch (PRA) (Bohemian Switzerland National Park, Czech Republic) are discussed, based on the in situ monitoring of weathering forms, as well as on laboratory studies of materials taken from this site. The in situ monitoring shows the progressive development of distinct weathering forms (delamination of case-hardened rock surfaces, cyclic efflorescence, and granular disintegration of exposed weakly cemented rock mass) in specific portions of the PRA, whose location is controlled by lithology, jointing/local hydrodynamics, and exposure to climatic factors. From the analysis of efflorescence, the dominance of sulphates is evident; particularly of gypsum and alums. However, the synergistic effects of the several salt species (that also include minor nitrates and chlorides) upon the mechanical disintegration of sandstone can be expected. 相似文献
879.
880.
B. P. Thomson 《Australian Journal of Earth Sciences》2013,60(1):203-228
The stratigraphical problem of defining the lower boundary of the Adelaide System is discussed in relation to the geology of several critical areas in the Adelaide Geosyncline and adjacent shelf‐platform. The Precambrian stratigraphical succession and geological history is outlined with the aid of Rb/Sr age‐determinations made by Dr W. Compston of the Australian National University. It is concluded that the lower boundary of the Adelaide System is related to the collapse of older basement positive areas on which a regional erosional surface had developed. This surface is defined by the Callanna Beds, the oldest deposits of Willouran age. Willouran sedimentation began some time between 1,340 m.y. and 1,490 m.y. ago. Erosion of the basement rocks probably occupied a major early part of this time interval. 相似文献