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541.
Ulrike Hoff Oleg Dirksen Veronika Dirksen Ulrike Herzschuh Hans-Wolfgang Hubberten Hanno Meyer Christel van den Bogaard Bernhard Diekmann 《Journal of Paleolimnology》2012,47(4):549-560
Fossil diatom assemblages in a sediment core from a small lake in Central Kamchatka (Russia) were used to reconstruct palaeoenvironmental
conditions of the late Holocene. The waterbody may be a kettle lake that formed on a moraine of the Two-Yurts Lake Valley,
located on the eastern slope of the Central Kamchatka Mountain Chain. At present, it is a seepage lake with no surficial outflow.
Fossil diatom assemblages show an almost constant ratio between planktonic and periphytic forms throughout the record. Downcore
variations in the relative abundances of diatom species enabled division of the core into four diatom assemblage zones, mainly
related to changes in abundances of Aulacoseira subarctica, Stephanodiscus minutulus, and Discostella pseudostelligera and several benthic species. Associated variations in the composition and content of organic matter are consistent with the
diatom stratigraphy. The oldest recovered sediments date to about 3220 BC. They lie below a sedimentation hiatus and likely
include reworked deposits from nearby Two-Yurts Lake. The initial lake stage between 870 and 400 BC was characterized by acidic
shallow-water conditions. Between 400 BC and AD 1400, lacustrine conditions were established, with highest contributions from
planktonic diatoms. The interval between AD 1400 and 1900 might reflect summer cooling during the Little Ice Age, indicated
by diatoms that prefer strong turbulence, nutrient recycling and cooler summer conditions. The timing of palaeolimnological
changes generally fits the pattern of neoglacial cooling during the late Holocene on Kamchatka and in the neighbouring Sea
of Okhotsk, mainly driven by the prevailing modes of regional atmospheric circulation. 相似文献
542.
The expansivity of supercooled diopside liquid has been determined using techniques of container-based dilatometry. Two thermal
strategies have been employed, one in which the sample is brought to volumetric equilibrium by long-duration dwells at low
temperatures (817 °C) and one in which scanning dilatometry of the sample has been performed at somewhat higher temperatures
(890–913 °C). The results of both experiments yield a supercooled liquid expansivity for diopside liquid in the temperature
range of 817–913 °C of 84.4 ± 2.8 × 10−4 cm3/mol K. The expansivity is 65% higher than that obtained for diopside melt obtained at superliquidus temperatures using the
double bob Archimedean method. Combined fitting of the new low temperature, volume–temperature data from the present study
and the superliquidus data from the literature has been performed. The combined fit yields the following equations for the
volume–temperature relationship of diopside liquid (T=temperature in °C):
The standard error of the fit using both equations reproduces the volume–temperature data for diopside liquid within experimental
error. This result reconciles the disparate values of expansivity measured at low temperatures in the supercooled state and
at superliquidus temperatures and confirms the temperature-dependence of the expansivity of diopside liquid. Comparison with
previous low temperature estimates of melt volume and expansivity are discussed in light of these new results.
Received: 18 November 1999 / Accepted: 24 January 2000 相似文献
543.
R.?AltherrEmail author H.-P.?Meyer A.?Holl F.?Volker C.?Alibert M.?T.?McCulloch V.?Majer 《Contributions to Mineralogy and Petrology》2004,147(1):58-73
In the East European Alpine belt, leucite-sanidine-phlogopite-olivine-bearing volcanic rocks of Late Cenozoic age occur at eight localities within the Vardar suture zone and at one locality in the Southern Carpathian fold-and-thrust belt. Most of these volcanics are characterized by high Mg# (66.6–78.6), high abundances of Ni (117–373 ppm) and Cr (144–445 ppm) as well as high primary K2O contents (5.63–7.01 %) and K2O/Na2O values (1.93–4.91). Rocks with more differentiated compositions are rare. A lamproite affinity of these rocks is apparent from their relatively low contents of Al2O3 (9.9–14.3 wt%) and CaO (6.2–8.3 wt%) in combination with high abundances of Rb (85–967 ppm), Ba (1,027–4,189 ppm), Th (18.9–76.5 ppm), Pb (19–54 ppm), Sr (774–1,712 ppm) and F (0.16–0.52 wt%), and the general lack of plagioclase. Although eruption of the magmas took place in post-collisional extensional settings, significant depletions of Nb and Ta relative to Th and La, low TiO2 contents (0.92–2.17 %), low ratios of Rb/Cs, K/Rb and Ce/Pb as well as high ratios of Ba/La and Ba/Th suggest close genetic relationships to subduction zone processes. Whereas Sr and Nd isotope ratios show relatively large variations (87Sr/86Sr = 0.7078–0.7105, 143Nd/144Nd = 0.51242–0.51215), Pb isotope ratios display a very restricted range with 206Pb/204Pb = 18.68–18.88 and variable but generally high 7/4 (11–18) and 8/4 (65–95) values. The observed petrographic, geochemical and isotopic characteristics are best explained by a genetic model involving preferential melting of phlogopite-rich veins in an originally depleted lithospheric mantle source, whereby the metasomatic enrichment of the mantle source is tentatively related to the addition of components from subducted sediments during consumption of Tethyan oceanic lithosphere.Editorial responsibility: J. Hoefs 相似文献
544.
545.
Piñon (Pinus edulis)-juniper (Juniperus monosperma)-ecosystems increased substantially in the western USA during the 20th century. Sustainability of these ecosystems primarily depends on soil quality and water availability. This study was undertaken with the objective of assessing the effect of tree species on soil physical quality in a semi-arid region in the western part of Sugarite Canyon, northeast of Raton, Colfax County, NM (37°56′32″N and 104°23′00″W) USA. Three cores and three bulk soil samples were obtained from the site under the canopy of three juniper, Gambel oak (Quercus gambelii) and piñon trees for 0–10 and 10–20 cm depths. These samples were analyzed for particle size distribution, soil bulk density (ρb), water stable aggregation (WSA), mean weight diameter (MWD) of aggregates, pH, electrical conductivity (EC) and soil organic carbon (SOC) and total nitrogen (TN) concentrations and stocks. Sand content was greater under juniper (48%) than oak (32%), whereas clay content followed the opposite trend. The ρb, WSA, MWD, pH and EC were similar under juniper, piñon, oak canopies for both depths. Estimated (from Philip and Green and Ampt infiltration models) and measured water infiltration parameters did not vary among these sites and were in accord with the values for ρb, WSA and MWD. The SOC concentrations and stocks were greater under oak (43.1 Mg ha−1 for 0–10 and 37.5 Mg ha−1 for 10–20 cm depths) than piñon (23.3 Mg ha−1 for 0–10 and 18.5 Mg ha−1 for 10–20 cm depths). The TN concentrations were greater under oak (3.4 g kg−1) than piñon (1.7 g kg−1) for the 0–10 cm depth only. Accumulation of detritus material under tree canopies reduced soil compaction and crusting caused by raindrop impact and increased SOC, and TN concentrations, and water infiltration. Coefficients of variation ranged from low to moderate for most soil properties except infiltration rate at 2.5 h, which was highly variable. Overall, soil quality for each site was good and soil aggregation, water infiltration and SOC concentrations were high, and soil ρb was low. 相似文献
546.
Kelley A. Crews‐Meyer 《The Professional geographer》2006,58(4):421-435
The growing overlap between geographic information science (GIScience) and landscape ecology for landscape characterization has led to increasingly sophisticated measures of landscape site and situation. Scale, both temporal and spatial, has been injected into methodologies to improve our ability to derive process from pattern with the hope of defining, monitoring, and modeling ecological landscape function. This article addresses an evolving methodology of longitudinal or panel analysis designed to test the relative importance of thematic versus structural landscape configuration as well as interannual versus intra‐annual change. Landscape typologies for both temporal signature and dominant structural trajectories are offered as guideposts for rethinking dynamic landscape characterization. A case study from the Peruvian Amazon is provided to illustrate interpretive advances available via panel methods that allow for disentangling inter‐ and intra‐annual shifts as well as separating changes in composition versus configuration for improved understanding of landscape dynamics and dynamism. 相似文献
547.
An accumulation terrace close to the El'gygytgyn Impact Crater in northeastern Siberia contains stratigraphic and periglacial evidence of the paleoenvironmental and paleoclimatic history and permafrost dynamics during late Quaternary time. A succession of paleo active-layer deposits that mirror environmental changes records periods favorable for the establishment and growth of ice-wedge polygonal networks and sediment variations. These two elements of the periglacial landscape serve as complementary paleoenvironmental archives that can be traced back to ∼ 14,000 cal yr BP. The slope sediments and the ground ice contained therein have prominent relative maxima and minima in properties (grain size, total organic content, oxygen isotopes). They document a regional early Holocene thermal maximum at about 9000 cal yr BP, followed by a transition to slightly cooler conditions, and a subsequent transition to slightly warmer conditions after about 4000 cal yr BP. Results from sedimentary analysis resemble morphological and geochemical (oxygen and hydrogen isotopes) results from ice wedge studies, in which successive generations of ice-wedge polygonal networks record warmer winters in late Holocene time. Moreover, peaks of light soluble cation contents and quartz-grain surface textures reveal distinct traces of cryogenic weathering. We propose a conclusive sedimentation model illustrating terrace formation in a permafrost terrain. 相似文献
548.
549.
Scott D. Reynolds Scott D. Mildren Richard R. Hillis Jeremy J. Meyer 《Tectonophysics》2006,415(1-4):123-140
The magnitude of the in situ stresses in the Cooper–Eromanga Basins have been determined using an extensive petroleum exploration database from over 40 years of drilling. The magnitude of the vertical stress (Sv) was calculated based on density and velocity checkshot data in 24 wells. Upper and lower bound values of the vertical stress magnitude are approximated by Sv = (14.39 × Z)1.12 and Sv = (11.67 × Z)1.15 functions respectively (where Z is depth in km and Sv is in MPa). Leak-off test data from the two basins constrain the lower bound estimate for the minimum horizontal stress (Shmin) magnitude to 15.5 MPa/km. Closure pressures from a large number of minifrac tests indicate considerable scatter in the minimum horizontal stress magnitude, with values approaching the magnitude of the vertical stress in some areas. The magnitude of the maximum horizontal stress (SHmax) was constrained by the frictional limits to stress beyond which faulting occurs and by the presence of drilling-induced tensile fractures in some wells. The maximum horizontal stress magnitude can only be loosely constrained regionally using frictional limits, due to the variability of both the minimum horizontal stress and vertical stress estimates. However, the maximum horizontal stress and thus the full stress tensor can be better constrained at individual well locations, as demonstrated in Bulyeroo-1 and Dullingari North-8, where the necessary data (i.e. image logs, minifrac tests and density logs) are available. The stress magnitudes determined indicate a predominantly strike-slip fault stress regime (SHmax > Sv > Shmin) at a depth of between 1 and 3 km in the Cooper–Eromanga Basins. However, some areas of the basin are transitional between strike-slip and reverse fault stress regimes (SHmax > Sv ≈ Shmin). Large differential stresses in the Cooper–Eromanga Basins indicate a high upper crustal strength for the region, consistent with other intraplate regions. We propose that the in situ stress field in the Cooper–Eromanga Basins is a direct result of the complex interaction of tectonic stresses from the convergent plate boundaries surrounding the Indo-Australian plate that are transmitted into the center of the plate through a high-strength upper crust. 相似文献
550.
This paper presents the numerical simulation of a creeping slope in Upper Austria, using a visco-hypoplastic material law
which describes the mechanical behavior of cohesive soils allowing for viscous effects, i.e. creep and relaxation. The method
consists of: (1) determination of the parameters of the material law, based on laboratory tests on soil samples taken from
the slope; (2) simulation of the laboratory tests with an element test program in which the used material law was implemented,
in order to test whether the model holds for the soils studied; and (3) simulation of slope movements at different sections
along the slope, assuming an infinite slope. The simulation results fit well with the field measurements. This demonstrates
that despite strongly simplified boundary conditions and limited availability of subsurface data (e.g. density) the visco-hypoplastic
law is a promising tool for predicting creep movements. 相似文献