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871.
872.
We present numerical results of thermal-mechanical feedback in crustal quartz rheology and contrast this behavior to the vastly
different character of an olivine mantle. In the numerical experiments quartz is found to have a very strong tendency for
short-time-scale instabilities, while our numerical experiments show that olivine has a decisive tendency for a stable thermally
lubricated slip. At the same time, olivine can also go through a transitional period of creep bursts, which are physically
caused by multiple interacting ductile faults at various length and time scales which collocate quickly into a major shear
zone. Since olivine has this strong propensity to self organize in a large apparently stable fault system, it lacks the dynamics
of interacting ductile faults evident in other minerals. Quartz behaves totally different and keeps its jerky slip behavior
for prolonged deformation. An example is shown here in which a 30 × 50 km piece of a wet quarzitic crust is extended for about
2 Ma. The associated total displacement field clearly shows the unstable slipping events, which have a characteristic time
frame of one to several years, In contrast, olivine is very stable and has a much longer time scale for thermal instability
of 100 kyrs. 相似文献
873.
Vratislav Hurai Klaus Simon Uwe Wiechert Jochen Hoefs Patrik Konečný Monika Huraiová Jacques Pironon Jozef Lipka 《Contributions to Mineralogy and Petrology》1998,133(1-2):12-29
Globules of iron-dominated (59–69 wt% FeOtot) and titanium-dominated (43.5 wt% TiO2) oxide melts have been detected in igneous xenoliths from Pliocene-to-Pleistocene alkali basalts of the Western Carpathians.
Fluid inclusion and mineral composition data indicate immiscible separation of the high-iron-oxide melt (HIM) at magmatic
temperatures. The HIM separation occurred during clinopyroxene (augite) accumulation in an alkali trachybasalt and continued
during crystallization of amphibole (kaersutite) and K-feldspar (anorthoclase), the latter coexisting with trachyte and alkalic
rhyolite residual melts. Some HIM was also expelled from sub-alkalic rhyolite (70–77% SiO2), coexisting with An27–45 plagioclase and quartz in granitic (tonalite-trondhjemite) xenoliths. Oxygen fugacities during HIM separation range from
−1.4 to +0.6 log units around the QFM buffer. A close genetic relationship between HIM-hosted xenoliths and mantle-derived
basaltic magma is documented by mineral 18O values ranging from 4.9 to 5.9‰ V-SMOW. δD values of gabbroic kaersutite between −61 and −86‰ V-SMOW are in agreement with
a presumed primary magmatic water source. Most trace elements, except Li, Rb and Cs, have preferentially partitioned into
the HIM. The HIM/Si-melt partition coefficients for transition elements (Sc, V, Cr, Co, Ni) and base metals (Zn, Cu, Mo) are
between 2–160, resulting in extreme enrichment in the HIM. La and Ce also concentrate in the silicic melt, whereas Tb-Tm in
the HIM. Hence, the immiscible separation causes REE fractionation and produces residual silicic melt enriched in LREE and
depleted in HREE. The weak fractionation among Tb-Tm and Yb, Lu can be attributed to recurrent extraction of the HIM from
the magmatic system, while flat HREE chondrite-normalized patterns are interpreted to indicate no or little loss of the HIM.
Received: 30 September 1997 / Accepted: 23 March 1998 相似文献
874.
Klaus Kaiser 《Organic Geochemistry》1998,28(12):849-854
The purpose of the study was to evaluate the influence of polyvalent cations known to form complexes with natural organic substances on the operational fractionation of dissolved organic matter (DOM) using XAD-8 adsorber resin. Dissolved organic matter solutions from a forest floor were treated with increasing concentrations of polyvalent metal cations (Ca2+, Al3+, Fe3+) at different pH levels. Then the concentrations of total dissolved organic carbon (DOC) and the distribution between hydrophilic and hydrophobic DOC were determined. The concentrations of total DOC decreased slightly when the C/metal ratio was less than 10, especially for Al and Fe. Hydrophilic DOC increased and hydrophobic DOC decreased with increasing concentrations of metal cations. Effects increased in the order Ca<Al<Fe and were more pronounced at low DOC concentrations and high pH values. The reason for the reduction of the DOC concentrations seemed to be the formation of insoluble metal–DOM complexes, while soluble metal–DOM complexes may induce an alteration of the distribution between hydrophilic and hydrophobic DOC. Thus, the polyvalent cations and their concentration need to be considered when DOM fraction distributions, determined with XAD-8 resin, of different waters are compared, especially at low DOC contents and high pH. 相似文献
875.
Within the context of the European legislative (e.g. Water Framework Directive, Natura 2000 network) monitoring and assessment of wetland areas are of great significance. This research points out the relevance of spatially explicit habitat models as an essential part of a decision support system for lake shore management. The Great Reed Warbler (Acrocephalus arundinaceus L.) was selected as a key species of aquatic reed structures. Many results of investigation about breeding behaviour of this German Red List species are available. The factors of selecting a breeding range were modelled in a Geographic Information System (GIS) using indices that quantify the corresponding spatial characteristics of patches in the ecosystem (e.g. shape complexity or distance to other vegetation structures). A habitat suitability map was created for a nature conservation area at Lake Constance, Germany. The observed breeding places in the investigation area showed a great coincidence with the modelled habitat ranges. This modelling approach of integrating spatial indicators is an important basis to provide automated monitoring systems based on remote sensing methods. 相似文献
876.
877.
878.
879.
880.
The chironomid Corynocera ambigua (Tanytarsini) is commonly reported as a cold-stenothermal species living in shallow lakes in arctic and subarctic regions. In palaeoecological studies of temperate lakes, larval remains of C. ambigua are usually found in late-glacial sediments from the Allerød and Younger Dryas periods, and often in association with subfossil Chara oospores. During a surface sampling program of chironomid head-capsules in 41 Danish temperate lakes, C. ambigua was found to comprise 25% of the chironomid assemblages in two lakes, and was sporadically found in 8 other lakes (0.5-10%). A 70 cm palaeo-stratigraphy from the shallow (max depth 1.2 m) and eutrophic (total phosphorus = 150 g P l-1) Lake Stigsholm showed that C. ambigua has been abundant in the last 4-5 centuries. At a sediment level of 25 cm (~year 1925, 210Pb dating), C. ambigua began to decrease in frequency while Chironomus plumosus, Procladius sp., Cladotanytarsus gr. mancus and Tanytarsus spp. increased, suggesting an increased nutrient loading and an approach to eutrophic conditions. In 1995 C. ambigua was still very abundant in Lake Stigsholm but in early March 1997 no living larvae were found. An extremely heavy growth of Elodea, Enteromorpha and filamentous algae in the summers of 1995 and 1996, with following degradation in the fall, might have influenced the invertebrate population dynamics. No significant distinguishing characteristics were found for the lakes supporting C. ambigua. Its occurrence in warm (~20°C) Danish lakes brings into question the perception of the species as being cold-stenothermal. 相似文献