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101.
Werner?Schneider Frank?MatternEmail author PuJun?Wang Cai?Li 《International Journal of Earth Sciences》2003,92(2):228-254
The ophiolite-bearing Bangong-Nujiang zone (BNZ) traversing central Tibet from east to west separates the Qiangtang block in the north from the Lhasa block in the south. Their stratigraphic development indicates that both blocks once formed a continuous continental platform until the Late Triassic. Following Late Paleozoic-Triassic rifting, ocean crust formed between both blocks during the Late Triassic creating the Dongqiao-Naqu basin (DNB) among other basins (Yu et al. 1991). The analysis of the rift flank sequences reveals that rifting was dominated by transtension. The basin was shortened by post-Mid-Cretaceous transpression. Thus, the overall basin evolution represents a Reading cycle despite some active margin processes which gave this cycle a special imprint. Major basin parts were preserved despite transpressional shortening suggesting that the eastern BNZ represents a remnant basin. Our understanding of the DNB solves the prior problem of viewing the BNZ as a Mid-Late Jurassic collisional suture although typical collision-related deformation, thickening, mountain building, as well as related molasse formation are lacking. Our model also explains the scattered linear ophiolite distribution by local transpression of remnant oceanic basin floor without having to consider problematic long range ophiolite thrusting. 相似文献
102.
Sabine Hilt Elisabeth M. Gross Michael Hupfer Harald Morscheid Jens Mhlmann Arnulf Melzer Jens Poltz Stefan Sandrock Eva-Maria Scharf Susanne Schneider Klaus van de Weyer 《Limnologica》2006,36(3):155-171
One of the most serious problems caused by eutrophication of shallow lakes is the disappearance of submerged macrophytes and the switch to a turbid, phytoplankton-dominated state. The reduction of external nutrient loads often does not result in a change back to the macrophyte-dominated state because stabilising mechanisms that cause resilience may delay a response. Additional internal lake restoration measures may therefore be needed to decrease the concentration of total phosphorus and increase water clarity. The re-establishment of submerged macrophytes required for a long-term stability of clear water conditions, however, may still fail, or mass developments of tall-growing species may cause nuisance for recreational use. Both cases are often not taken into account when restoration measures are planned in Germany, and existing schemes to reduce eutrophication consider the topic inadequately. Here we develop a step-by-step guideline to assess the chances of submerged macrophyte re-establishment in shallow lakes. We reviewed and rated the existing literature and case studies with special regard on (1) the impact of different internal lake restoration methods on the development of submerged macrophytes, (2) methods for the assessment of natural re-establishment, (3) requirements and methods for artificial support of submerged macrophyte development and (4) management options of macrophyte species diversity and abundance in Germany. This guideline is intended to help lake managers aiming to restore shallow lakes in Germany to critically asses and predict the potential development of submerged vegetation, taking into account the complex factors and interrelations that determine their occurrence, abundance and diversity. 相似文献
103.
Tom Magna Uwe Wiechert Timothy L. Grove Alex N. Halliday 《Earth and Planetary Science Letters》2006,250(3-4):428-443
We present lithium (Li) abundances and isotope compositions for a suite of anhydrous olivine tholeiites (HAOTs) and hydrous basalt-andesitic (BA) lavas from the Mt. Shasta and Medicine Lake regions, California. The values of δ7Li vary from + 0.9‰ to + 6.4‰ and correlate inversely with distance from the trench. These data are consistent with continuous isotope fractionation of Li during dehydration of the subducted oceanic lithosphere, an interpretation corroborated by uniformly high pre-eruptive H2O contents in basaltic andesites accompanied by high Li, Rb, Sr, Ba and Pb abundances. The subduction-derived component that was added to these hydrous magmas is shown to be very similar beneath both Mt. Shasta and Medicine Lake volcanoes despite characteristically distinct Li isotope compositions in the magmas themselves. More evolved andesites and dacites from Mt. Shasta have δ7Li from + 2.8 to + 6.9‰ which is identical with the range obtained for HAOTs and BA lavas from Mt. Shasta. Therefore, Li isotopes do not provide evidence for any other crustal component admixed to Mt. Shasta andesites or dacites during magmatic differentiation and magma mixing in the crust. 相似文献
104.
Variability of Fram Strait sea ice export: causes, impacts and feedbacks in a coupled climate model 总被引:1,自引:2,他引:1
Analyses of a 500-year control integration of the global coupled atmosphere–sea ice–ocean model ECHAM5.0/MPI-OM show a high
variability in the ice export through Fram Strait on interannual to decadal timescales. This variability is mainly determined
by variations in the sea level pressure gradient across Fram Strait and thus geostrophic wind stress. Ice thickness anomalies,
formed at the Siberian coast and in the Chukchi Sea, propagate across the Arctic to Fram Strait and contribute to the variability
of the ice export on a timescale of about 9 years. Large anomalies of the ice export through Fram Strait cause fresh water
signals, which reach the Labrador Sea after 1–2 years and lead to significant changes in the deep convection. The associated
anomalies in ice cover and ocean heat release have a significant impact on air temperature in the Labrador Sea and on the
large-scale atmospheric circulation. This affects the sea ice transport and distribution in the Arctic again. Sensitivity
studies, simulating the effect of large ice exports through Fram Strait, show that the isolated effect of a prescribed ice/fresh
water anomaly is very important for the climate variability in the Labrador Sea. Thus, the ice export through Fram Strait
can be used for predictability of Labrador Sea climate up to 2 years in advance. 相似文献
105.
Leszek Marynowski Michał Zatoń Bernd R.T. Simoneit Angelika Otto Mariusz O. Jędrysek Cezary Grelowski Slawomir Kurkiewicz 《Applied Geochemistry》2007
The comprehensive biomarker characteristics from previously undescribed Middle Jurassic clays of Poland are presented. The molecular composition of the organic matter (OM) derived from clays of Aalenian to Callovian age has not changed significantly through time. High relative concentrations of many biomarkers typical for terrestrial material suggest a distinct dominance of OM derived from land plants. Increasing concentrations of C29-diaster-13(17)-enes towards the northern part of the basin indicate an increase in terrestrial input. This terrestrial material would have originated from the enhanced transport of organic matter from land situated at the northern bank of the basin, i.e., the Fennoscandian Shield. The organic matter was deposited in an oxic to suboxic environment, as indicated by relatively low concentrations of C33–C35 homohopanes, moderate to high Pr/Ph ratio values, an absence of compounds characteristic for anoxia and water column stratification, such as isorenieratane, aryl isoprenoids and gammacerane, as well as common benthic fauna and burrows. δ18O measurements from calcitic rostra of belemnites suggest that the mean value of the Middle Jurassic sea-water temperature of the Polish Basin was 13.1 °C. It is suggested that this mirrored the temperature of the lower water column because belemnites are considered here to be necto-benthic. The organic matter from the Middle Jurassic basin of Poland is immature. This is clearly indicated by a large concentration of biomarkers with the biogenic configurations, such as ββ-hopanes, hop-13(18)-enes, hop-17(21)-enes, diasterenes and sterenes. The identification of preserved, unaltered biomolecules like ferruginol, 6,7-dehydroferruginol and sugiol in Protopodocarpoxylon wood samples from these sediments present particularly strong evidence for the presence of immature OM in the Middle Jurassic sediments. Moreover, the occurrence of these polar diterpenoids is important due to the fact that they are definitely the oldest known natural products detected in geological samples. 相似文献
106.
The formal stereological transformation equation for particle sieve size distribution from measurements in lower dimensional spaces is applied to laser diffractometer measurements. The transformation function for iron ore particles is measured experimentally, and modeled. The solution is tested against the measured transformation function data as well as synthetic composite distributions of the original sample. The natural size distribution of a sample taken from a grinding circuit stream was measured by a combination of standard sieving and cyclosizer, and the result is compared to the transformed size distribution calculated from laser diffractometer measurements. The stereological transformation technique performed well in all cases. 相似文献
107.
108.
Uwe Mikolajewicz Matthias Gröger Ernst Maier-Reimer Guy Schurgers Miren Vizcaíno Arne M. E. Winguth 《Climate Dynamics》2007,28(6):599-633
A new complex earth system model consisting of an atmospheric general circulation model, an ocean general circulation model,
a three-dimensional ice sheet model, a marine biogeochemistry model, and a dynamic vegetation model was used to study the
long-term response to anthropogenic carbon emissions. The prescribed emissions follow estimates of past emissions for the
period 1751–2000 and standard IPCC emission scenarios up to the year 2100. After 2100, an exponential decrease of the emissions
was assumed. For each of the scenarios, a small ensemble of simulations was carried out. The North Atlantic overturning collapsed
in the high emission scenario (A2) simulations. In the low emission scenario (B1), only a temporary weakening of the deep
water formation in the North Atlantic is predicted. The moderate emission scenario (A1B) brings the system close to its bifurcation
point, with three out of five runs leading to a collapsed North Atlantic overturning circulation. The atmospheric moisture
transport predominantly contributes to the collapse of the deep water formation. In the simulations with collapsed deep water
formation in the North Atlantic a substantial cooling over parts of the North Atlantic is simulated. Anthropogenic climate
change substantially reduces the ability of land and ocean to sequester anthropogenic carbon. The simulated effect of a collapse
of the deep water formation in the North Atlantic on the atmospheric CO2 concentration turned out to be relatively small. The volume of the Greenland ice sheet is reduced, but its contribution to
global mean sea level is almost counterbalanced by the growth of the Antarctic ice sheet due to enhanced snowfall. The modifications
of the high latitude freshwater input due to the simulated changes in mass balance of the ice sheet are one order of magnitude
smaller than the changes due to atmospheric moisture transport. After the year 3000, the global mean surface temperature is
predicted to be almost constant due to the compensating effects of decreasing atmospheric CO2 concentrations due to oceanic uptake and delayed response to increasing atmospheric CO2 concentrations before. 相似文献
109.
Climatic impacts of historical wetland drainage in Switzerland 总被引:1,自引:0,他引:1
The effects of historical land-use and land-cover changes on the climate of the Swiss Plateau in the different seasons were
investigated. In the 19th century, a civil engineering project was initiated to reshape the lake and river system on the Swiss
Plateau in order to ban the frequent flooding during extreme weather events. The landscape modifications consisted primarily
of a conversion of wetlands with extended peat soils into a highly productive agricultural landscape. Historical maps (1800–1850)
served as a basis for the reconstruction of the past land use. The “Lokal-Modell” of the Consortium for Small-Scale Modelling
was used to conduct eight one-month long high-resolution simulations (1.5 × 1.5 km2) with present and past landscape conditions. The modified soil and surface properties led to distinctly altered energy and
moisture exchanges at the surface and as a consequence affected the local and regional climate. The climatic changes show
different characteristics and magnitudes in the cold (October – March) as compared to the warm season (April – September).
The landscape modifications led to an average daytime cooling between −0.12 °C (January) and −0.61 °C (April) and a night-time
warming of 0.19 °C−0.34 °C. The differences in the mean monthly temperatures show a warming of 0.1 °C−0.2 °C in the cold season
and a cooling of similar magnitude in most of the study area in the warm season. The modification of the radiation budget
and the surface energy balance distinctly affected the convective activity in the study area in the warm season, but had only
a weak effect on convectivity in the cold season. The cloud coverage in the warm season is therefore distinctly reduced compared
to the past. 相似文献
110.