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排序方式: 共有271条查询结果,搜索用时 250 毫秒
81.
Claudia Hellmann Carola Winkelmann Susanne Worischka Jürgen Benndorf 《Limnologica》2011,41(3):256-265
Many predation experiments in streams are carried out in enclosures. Hence, the relevance of their results to predict population dynamics is often unclear due to the relatively small spatial and temporal scale of the experiments. To enhance the transferability of experimental results on the ecosystem scale the impact of fish predators on a prey population was observed in a reach scale approach over 2 years in a natural stream. A 400-m reach inhabited by the small benthivorous fishes gudgeon (Gobio gobio) and stone loach (Barbatula barbatula) was compared with a fishless reference reach.It was shown that fish predation may affect the population of the grazing mayfly Rhithrogena semicolorata on the ecosystem scale. Although the larvae grew slower in the fish reach than in the fishless reach, the adults reached the same size and fecundity because they emerged 2–3 weeks later. By this compensation, the prey species avoided a reduction of their individual fecundity. On the other hand, the extended exposure to the fish predators resulted in an enhanced mortality and a reduced density of adult mayflies. Thus, there was obviously a trade-off between maximising fecundity and minimising mortality from fish predation.The observed differences were almost certainly caused be fish predation and not by natural differences of the reaches. This was concluded from results gained after eliminating all benthivorous fish from the former fish reach.With the help of scenario analyses based on our empirical data and simple model assumptions we could demonstrate that compensating the potential loss in fecundity by extending development time led to higher average fitness of the prey population than emerging at an earlier fixed time to avoid additional predation losses. Therefore, we concluded that this strategy was adaptive in the presence of benthivorous fish. 相似文献
82.
Horst ZiemannClaus-Jürgen Schulz 《Limnologica》2011,41(2):90-95
Salinisation of running waters is a severe problem in many parts of the world. Monitoring and management of such waters require ecological methods which consider the hydrochemical effects of salinisation on the aquatic communities in order to set targets to protect habitats and biodiversity. Several bioassays have been developed for this purpose and are surveyed here. They are based on the salt sensitivity of the following groups of organisms: diatoms, ciliates and macroinvertebrates. In this paper experiences gained so far are also considered as well as practical applications originating from this research. 相似文献
83.
Marjorie Schulz David Stonestrom Guntram Von Kiparski Corey Lawrence Carrie Masiello Art White John Fitzpatrick 《Applied Geochemistry》2011
Concentrations of CO2 in soil atmosphere and CO2 efflux were measured across a marine terrace soil chronosequence near Santa Cruz, California. Soil development, specifically the formation of an argillic horizon, has created a two-tier soil gas profile in the older terrace soils. The soil above the argillic horizon has seasonal variations in soil CO2 associated with plant respiration. The older soils with dense argillic horizons maintain a year round ~1%CO2 below the argillic horizon. The CO2efflux during the growing season is higher on the older terraces. 相似文献
84.
85.
H.-M. Schulz H. W. Hagemann M. Wolf A. Brammertz H.-J. Einbrodt 《Environmental Geology》1997,30(1-2):72-80
The fibrogenic and cytotoxic potential of coal mine dust is independent of the amount of quartz and other inorganic parameters.
Results of coal petrographical and organic geochemical investigations of coals and coal mine dust from the Ruhr and Ibbenbüren
Region of Germany demonstrate variations of organic dust amount possibly influencing these noxious properties. Coal mine dust
of high rank coals is characterized by a pronounced fibrogenic risk. This risk, independent of the quantity of quartz, is
probably based on shape variations of different coal macerals. With increasing coalification of the corresponding seam, the
vitrinite is enriched in its dust; however, lower concentrations have been determined for inertinite. Vitrinite shows constant
shapes and sizes independent of the rank of coal. Inertinite particles with elongated to fibrous shapes tend to larger sizes
with increasing coalification. Strikingly, coal mine dust from miners' lungs with high degrees of coal mine workers' pneumoconiosis
(CWP) is enriched in inertinite. In contrast, high cytotoxicities in cell tests are known for coal mine dust from low coalified
coals. High concentrations of phenolic compounds can be extracted by dichloromethane from low coalified coal mine dust. These
compounds, which are characterized by a high water solubility and therefore high bioavailability, explain the high cytotoxicities
of coal mine dust. Contamination of dust by diesel emissions in the coal mine can act as additionally supporting parameters
for extended cytotoxicities.
Received: 18 August 1995 · Accepted: 15 October 1995 相似文献
86.
A simple model for superluminal double radio sources is presented. A relativistic electron-positron beam drives the expansion of an elongated magnetic loop configuration. This is treated using basic conservation laws. Relativistically expanding radio components on a scale of parsecs result for plausible physical parameters. 相似文献
87.
We investigated the kinematics and ionization mechanism of the innermost arc seconds of the nucleus of NGC 6764 (SBb), using spatially-resolved optical spectra. The observed radial variations of the emission line ratios cannot be explained by one single-ionizing agent, but are consistent with circumnuclear ionizeation by hot stars in addition to a central power-law source. We conclude that NGC 6764 is a Composite Galactic Nucleus (CGN).Visiting Astronomer, German-Spanish Astronomical Center, Calar Alto, operated by the Max-Planck-Institut für Astronomie in Heidelberg jointly with the Spanish National Commission for Astronomy. 相似文献
88.
Target rocks,impact glasses,and melt rocks from the Lonar crater,India: Highly siderophile element systematics and Sr‐Nd‐Os isotopic signatures 下载免费PDF全文
Toni Schulz Ambre Luguet Wencke Wegner David van Acken Christian Koeberl 《Meteoritics & planetary science》2016,51(7):1323-1339
The Lonar crater is a ~0.57‐Myr‐old impact structure located in the Deccan Traps of the Indian peninsula. It probably represents the best‐preserved impact structure hosted in continental flood basalts, providing unique opportunities to study processes of impact cratering in basaltic targets. Here we present highly siderophile element (HSE) abundances and Sr‐Nd and Os isotope data for target basalts and impactites (impact glasses and impact melt rocks) from the Lonar area. These tools may enable us to better constrain the interplay of a variety of impact‐related processes such as mixing, volatilization, and contamination. Strontium and Nd isotopic compositions of impactites confirm and extend earlier suggestions about the incorporation of ancient basement rocks in Lonar impactites. In the Re‐Os isochron plot, target basalts exhibit considerable scatter around a 65.6 Myr Re‐Os reference isochron, most likely reflecting weathering and/or magma replenishment processes. Most impactites plot at distinctly lower 187Re/188Os and 187Os/188Os ratios compared to the target rocks and exhibit up to two orders of magnitude higher abundances of Ir, Os, and Ru. Moreover, the impactites show near‐chondritic interelement ratios of HSE. We interpret our results in terms of an addition of up to 0.03% of a chondritc component to most impact glasses and impact melt rocks. The magnitude of the admixture is significantly lower than the earlier reported 12–20 wt% of extraterrestrial component for Lonar impact spherules, reflecting the typical difference in the distribution of projectile component between impact glass spherules and bulk impactites. 相似文献
89.
Siegesmund S. Oriolo S. Heinrichs T. Basei M. A. S. Nolte N. Hüttenrauch F. Schulz B. 《International Journal of Earth Sciences》2018,107(6):2293-2315
International Journal of Earth Sciences - New U–Pb and Lu–Hf detrital zircon data together with whole-rock geochemical and Sm–Nd data were obtained for paragneisses of the... 相似文献
90.
Cecilia Eriksson Harry Burton Stuart Fitch Martin Schulz John van den Hoff 《Marine pollution bulletin》2013,66(1-2):199-208
The worlds’ oceans contain a large but unknown amount of plastic debris. We made daily collections of marine debris stranded at two sub-Antarctic islands to establish (a) physical causes of strandings, and (b) a sampling protocol to better estimate the oceans’ plastic loading. Accumulation rates at some beaches were dependent on tide and onshore winds. Most of the 6389 items collected were plastic (Macquarie 95%, Heard 94%) and discarded or lost fishing gear comprised 22% of those plastic items. Stalked barnacles (Lepas spp.) were a regular attachment on Macquarie debris but not at Heard Island. The daily accumulation rate of plastic debris on Macquarie Island was an order of magnitude higher than that estimated from monthly surveys during the same 4 months in the previous 5 years. This finding suggests that estimates of the oceans’ plastic loading are an order of magnitude too low. 相似文献