全文获取类型
收费全文 | 7006篇 |
免费 | 753篇 |
国内免费 | 758篇 |
专业分类
测绘学 | 495篇 |
大气科学 | 767篇 |
地球物理 | 1814篇 |
地质学 | 3451篇 |
海洋学 | 523篇 |
天文学 | 642篇 |
综合类 | 349篇 |
自然地理 | 476篇 |
出版年
2023年 | 93篇 |
2022年 | 238篇 |
2021年 | 318篇 |
2020年 | 244篇 |
2019年 | 222篇 |
2018年 | 339篇 |
2017年 | 335篇 |
2016年 | 413篇 |
2015年 | 356篇 |
2014年 | 443篇 |
2013年 | 489篇 |
2012年 | 427篇 |
2011年 | 359篇 |
2010年 | 410篇 |
2009年 | 359篇 |
2008年 | 308篇 |
2007年 | 250篇 |
2006年 | 200篇 |
2005年 | 206篇 |
2004年 | 134篇 |
2003年 | 128篇 |
2002年 | 126篇 |
2001年 | 144篇 |
2000年 | 105篇 |
1999年 | 100篇 |
1998年 | 111篇 |
1997年 | 82篇 |
1996年 | 86篇 |
1995年 | 61篇 |
1994年 | 66篇 |
1993年 | 51篇 |
1992年 | 47篇 |
1991年 | 39篇 |
1990年 | 43篇 |
1989年 | 42篇 |
1988年 | 32篇 |
1987年 | 43篇 |
1986年 | 36篇 |
1985年 | 37篇 |
1984年 | 44篇 |
1983年 | 56篇 |
1982年 | 46篇 |
1981年 | 39篇 |
1980年 | 40篇 |
1979年 | 43篇 |
1978年 | 46篇 |
1977年 | 39篇 |
1975年 | 45篇 |
1974年 | 38篇 |
1973年 | 40篇 |
排序方式: 共有8517条查询结果,搜索用时 692 毫秒
21.
Helmut Kerndorff Stephan Kühn Thomas Minden Dagmar Orlikowski Thomas Struppe 《Environmental Geology》2008,55(2):291-301
The aim of this research project is to identify, characterize and quantify natural attenuation (NA) processes in groundwater
affected by emissions of abandoned waste disposal sites in Berlin-Kladow/Gatow, Germany. It is part of the funding priority
called KORA established by the Federal Ministry for Education and Research (BMBF) to explore the extent to which NA can be
used for remedial purposes for varied forms of soil and groundwater contamination. Information on the emission behaviour of
individual parameters is generated on the basis of hydrogeochemical comparison of 20 years old and new data. Using groundwater-modelling
and CFC-analysis, information on the transport and retention of pollutants in groundwater is compiled. The microbial colonization
of contaminated aquifers is characterized by molecular biological methods [polymerase chain reaction (PCR) and denaturing
gradient gel electrophoresis (DGGE)] to differentiate between contaminated and not contaminated zones. 相似文献
22.
23.
Jürgen Konzett Daniel J. Frost Alexander Proyer Peter Ulmer 《Contributions to Mineralogy and Petrology》2008,155(2):215-228
Experiments have been conducted in the P-T range 2.5–15 GPa and 850–1,500°C using bulk compositions in the systems SiO2–TiO2–Al2O3–Fe2O3–FeO–MnO–MgO–CaO–Na2O–K2O–P2O5 and SiO2–TiO2–Al2O3–MgO–CaO–Na2O to investigate the Ca-Eskola (CaEs Ca0.5□0.5AlSi2O6) content of clinopyroxene in eclogitic assemblages containing garnet + clinopyroxene + SiO2 ± TiO2 ± kyanite as a function of P, T, and bulk composition. The results show that CaEsss in clinopyroxene increases with increasing T and is strongly bulk composition dependent whereby high CaEs-contents are favoured by bulk compositions with high normative
anorthite and low diopside contents. In this study, a maximum of 18 mol% CaEsss was found at 6 GPa and 1,350°C in a kyanite-eclogite assemblage garnet + clinopyroxene + kyanite + rutile + coesite. By comparison,
no significant increase in CaEsss with increasing P could be observed. If the formation of oriented SiO2-rods frequently observed in eclogititc clinopyroxenes is due to the retrogressive breakdown of a CaEs-component then these
textures are a cooling rather than a decompression phenomenon and are most likely to be found in kyanite-bearing eclogites
cooled from temperatures ≥750°C. The presence of clinopyroxene with approx. 4 mol% CaEsss in an experiment conducted at 2.5 GPa/850°C confirms earlier suggestions based on field data that vacancy-rich clinopyroxenes
are not necessarily restricted to ultrahigh pressure metamorphic conditions.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
24.
The altitude effect on the isotopic composition of precipitation and its application to paleoelevation reconstruction using authigenic or pedogenic minerals have been intensively studied. However, there are still no studies on variations in biomarker δD along altitude transects to investigate its potential as a paleoelevation indicator, although it has been observed that δD of higher plant lipid may record changes in precipitation δD (δDp). Here, we present δD values of higher plant-derived C27, C29, and C31n-alkanes from surface soil along the eastern slope of Mount Gongga, China with great changes in physical variables and vegetation over a range from 1000 to 4000 m above sea level. The weighted-mean δD values of these n-alkanes (δDwax) show significant linear correlations with predicted δDp values (R2 = 0.76) with an apparent isotopic enrichment (εwax-p) of −137 ± 9‰, indicating that soil δDwax values track overall δDp variation along the entire altitudinal transect. Leaf δDwax is also highly correlated with mountain altitude by a significant quadratic relationship (R2 = 0.80). Evapotranspiration is found declining with altitude, potentially lowering δDwax values at higher elevations. However, this evapotranspiration effect is believed to be largely compensated by the opposing effect of vegetation changes, resulting in less varied εwax-p values over the slope transect. This study therefore confirms the potential of using leaf δDwax to infer paleoelevations, and more generally, to infer the δD of precipitation. 相似文献
25.
M. El Ghorfi F. Melcher T. Oberthür A. E. Boukhari L. Maacha A. Maddi M. Mhaili 《Mineralogy and Petrology》2008,92(1-2):59-80
Summary The Neoproterozoic Bou Azzer ophiolite complex hosts numerous, small lenticular bodies of massive and disseminated chromite.
Metallurgical-grade high-Mg and high-Cr spinels (cores with 48–62 wt% Cr2O3) reveal complex alteration patterns of successive Cr and Mn enrichment and loss of Al towards the rims, while the Mg# ratios
[(Mg/(Mg + Fe2+)] remain almost constant. Concentration patterns of platinum-group elements are typical for ophiolitic chromitite poor in
sulfides, with predominance of the IPGE, variable Rh, and low Pt and Pd. The most abundant platinum-group mineral is Rh-bearing
laurite that occurs either included in spinel or in silicate matrix, whereas Os-Ir-Ru alloy is always included in spinel.
Laurite inclusions reveal complex intergrowth textures with Rh-Ru-Pt rich alloy, and with Rh-rich sulfide. Most laurites display
trends to sulfur-poor compositions leading to local formation of very fine-grained Ru-Os-Ir alloy phases. Ni-Co-Fe sulfides,
arsenides and sulfarsenides devoid of PGE are associated with the alteration of chromite. Textural position and chemical composition
of the base metal inclusions, as well as comparison of alteration features between chromite and accessory chromian spinel
in the Co-Ni-As ores of the Bou Azzer ophiolite indicate a close connection. It is suggested that hydrothermal fluids percolated
through the marginal zones of the ophiolite belt during greenschist facies metamorphism and deposited Ni-Co-Fe arsenides,
sulfarsenides and minor sulfides as accessories within altered chromitites, and also in structurally favourable zones as Ni-Co-As
ores.
Author’s address: Dr. Frank Melcher, Federal Institute for Geosciences and Natural Resources, Stilleweg 2, 30655 Hannover, Germany 相似文献
26.
National flood discharge mapping in Austria 总被引:5,自引:0,他引:5
This article presents the approach and the results of a study in which 30, 100 and 200 year return period flood discharges
were estimated for 26,000 km of Austrian streams. Three guiding principles were adopted: combination of automatic methods
and manual assessments by hydrologists to allow speedy processing and account for the local hydrological situation; combination
of various sources of information including flood peak samples, rainfall data, runoff coefficients and historical flood data;
and involvement of the Hydrographic Services to increase the accuracy and enhance the acceptance of results. The flood discharges
for ungauged catchments were estimated by the Top-kriging approach with manual adjustment to the local flood characteristics.
The adopted combination approach proved to be very efficient both in terms of the project time required and in terms of the
accuracy and acceptability of the estimated flood discharges of given return periods. 相似文献
27.
B. Bühn 《Mineralogy and Petrology》2008,92(3-4):453-470
Summary Two bimodal carbonatite complexes in Namibia of Cretaceous age are explored as to the presence and composition of a coexisting
carbonatitic fluid. The Kalkfeld and Ondurakorume complexes contain both Ca- and Mg/Fe-carbonatites, composed of calcite alone
or calcite with ferroan dolomite, fluorapatite and strontianite. The major element evolution in the bulk rocks from s?vites
to beforsites is due to crystallization of calcite and fluorapatite. All carbonatites show a negative Y anomaly in normalised
REE plots. Fractionation is accompanied by successively lower HREE contents between Tb and Yb, expressed by the ratios Nd/Ho
and Ho/Lu. The evolution of this downward-facing hump goes along with decreasing Y contents in bulk rocks and minerals. All
this requires an additional phase coexisting with the carbonate liquid during fractionation. Comparison between the bulk rocks
and the expelled fluid shows that the latter had preferentially accumulated the HREE and Y. Further evidence for this process
are hydrothermal, HREE, Y-rich fluorites in other carbonatite complexes which reflect the composition of the expelled fluid.
The high strength of fluoride complexes suggests that fluoride complexing in the carbonatitic fluid is the process responsible
for extracting HREE and Y from the carbonatite magma, leaving fractionated carbonatite rocks depleted in these elements. The
geochemical evolution of carbonatite magmas along fractionation has therefore to be considered in a melt-mineral-fluid system.
Correspondence: B. Bühn, Instituto de Geociências, Universidade de Brasilia, Campus Universitário Darcy Ribeiro, Brasilia
70910-900, Brazil 相似文献
28.
29.
在柴达木盆地北缘地区,分别选取有机碳含量很低和较高的侏罗系泥岩样品,对比分析了它们在生物标志物组成上的差异。结果发现,高有机质丰度泥岩的生标组成与我国西北地区侏罗纪煤系有机质的特征差异不大,相比而言,低有机质丰度泥岩的正烷烃以前主峰为特征,Pr/Ph比值在1.0左右,三环萜烷和伽马蜡烷丰度较高,并在部分样品中检出了25 降藿烷系列。结合泥岩的有机岩石学特征,认为这些差异可能反映了泥岩沉积环境和生烃母质的不同:高有机质丰度泥岩的有机显微组分以相对弱还原条件下的形态有机质为主,包括藻类体、孢子体和角质体等,而低有机质丰度泥岩的有机显微组分以相对强还原条件下的矿物沥青基质为主,其母质可能来源于低等显微菌藻类。进一步通过对比不同有机质丰度泥岩,以及区内原油生标组成之间的相互关系,讨论了研究区的油源问题。 相似文献
30.
Zusammenfassung Die Davoser Windverh?ltnisse werden in ihrem tages- und jahreszeitlichen Ablauf sowie in ihrer Abh?ngigkeit von verschiedenen
typischen Wetterlagen untersucht. Neben den ungest?rten Sch?nwetterlagen mit ihrem ausgepr?gten Talwind, der wie der Malojawind
talabw?rts weht, werden die F?hnstr?mung aus Südwest und die Nordoststr?mung bei Staulagen und Bise kurz besprochen. Gegenüber
dem benachbarten Dischmatal zeigt sich ein sp?terer Beginn des Talwindes bei Sch?nwetterlagen, w?hrend im Vergleich zu anderen
T?lern, z. B. zum Wallis, ein früheres und schw?cheres nachmitt?gliches Windmaximum auftritt.
Mit 6 Textabbildungen
Herrn Dr.W. M?rikofer zu seinem 70. Geburtstag gewidmet. 相似文献
Summary The wind at Davos is analysed in his diurnal and annual variation and in the depence on the different weather types. The fair and undisturbed weather shows a typical valley breeze during the day, but not in the normal direction towards the higher part of the valley, but downstream, similar to the Malojawind in the Engadine. The opposite wind direction, from southwest, characterises the foehn-situation. Northeasterly winds develop in the case of higher pressure north of the Alps (e. g. in situations of barrage). The valley-breeze begins later than in the adjacent Dischma Valley, but it is weaker and reaches the highest intensity earlier than in other valleys, as e. g. in the Valais.
Résumé Les vents de Davos sont analysés selon leurs variations diurnes et annuelles et selon des types de temps caractéristiques. La situation de beau temps montre un vent de vallée assez accentué, soufflant du haut en bas, donc dans une direction anormale, mais analogue au vent de la Maloja en Engadine. En cas de ?foehn? le vent souffle du sud-ouest tandis qu'un vent du nord-est se manifeste par situations de surpression au nord des Alpes. Le vent de vallée se met à souffler plus tard à Davos que dans la vallée de Dischma toute proche. Il y atteint par contre sa vitesse maximale plus t?t que les vents correspondants d'autres vallées p. ex. du Valais. Le maximum y est cependant moins élevé.
Mit 6 Textabbildungen
Herrn Dr.W. M?rikofer zu seinem 70. Geburtstag gewidmet. 相似文献