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1.
The elm decline of 5000 14C yr ago has been the most widely discussed phenomenon in post‐glacial vegetation history. This pan‐European reduction of elm populations, echoed in the decimation of elmwoods in Europe during the twentieth century, has attracted a series of interrelated hypotheses involving climate change, human activity, disease and soil deterioration. The elm bark beetle (Scolytus scolytus L.) is an essential component of disease explanations. We present evidence for the presence of the beetle over a prolonged period (ca. 7950–4910 yr BP [8800–5660 cal. yr BP]) from a lowland raised mire deposit in northeast Scotland, with its final appearance at this site, and the first and only appearance in another mire of a single scolytid find, around the time of the elm decline. The subfossil S. scolytus finds are not only the first from Scotland, but they also represent the most comprehensive sequence of finds anywhere. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
2.
For ecosystem modelling of the Boreal forest it is important to include processes associated with low soil temperature during spring‐early summer, as these affect the tree water uptake. The COUP model, a physically based SVAT model, was tested with 2 years of soil and snow physical measurements and sap flow measurements in a 70‐year‐old Scots pine stand in the boreal zone of northern Sweden. During the first year the extent and duration of soil frost was manipulated in the field. The model was successful in reproducing the timing of the soil warming after the snowmelt and frost thaw. A delayed soil warming, into the growing season, severely reduced the transpiration. We demonstrated the potential for considerable overestimation of transpiration by the model if the reduction of the trees' capacity to transpire due to low soil temperatures is not taken into account. We also demonstrated that the accumulated effect of aboveground conditions could be included when simulating the relationship between soil temperature and tree water uptake. This improved the estimated transpiration for the control plot and when soil warming was delayed into the growing season. The study illustrates the need of including antecedent conditions on root growth in the model in order to catch these effects on transpiration. The COUP model is a promising tool for predicting transpiration in high‐latitude stands. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
3.
High-resolution paleoenvironmental data from a peat profile with a small pollen source area are used to reconstruct the impacts of landnám on vegetation and soils at a Norse farm complex (∅2 at Tasiusaq) comprising two farms in the Eastern Settlement of Greenland. Analyses include the AMS 14C dating of plant macrofossil samples and the use of Bayesian radiocarbon calibration to construct improved age-depth models for Norse cultural horizons. The onset of a regional landnám may be indicated by the clearance of Betula pubescens woodland immediately prior to local settlement. The latter is dated to AD 950-1020 (2σ) and is characterised by possible burning of Betula glandulosa scrub to provide grassland pasture for domestic stock. Clearance and grazing resulted in accelerated levels of soil erosion at a westerly farm. This was followed by an easterly migration of settlement and agriculture. Site constraints prevent an assessment of the demise of the easterly farm, but pressures of overgrazing and land degradation may have been the major factors responsible for the abandonment of the earlier farm. 相似文献
4.
Donald J. Walter Douglas N. Lambert David C. Young Kevin P. Stephens 《Geo-Marine Letters》1997,17(4):260-267
Real-time trackline plots of surficial sediment acoustic impedance delineate several sedimentary facies off Garden Key in
the Dry Tortugas. The sea floor within a 6×6 km surveyed area consists of carbonate muds (silts), sands and shell, rock, and
live corals. The 4-kHz acoustic data supports this finding by providing a pictographic representation of the distribution
and structure of several sediment facies types. Plotting the gridded acoustic data with commercial mapping software (Surfer)
provides a three-dimensional (3D) perspective of the bottom topography with a color contour map of surficial sediment impedance
(upper 0.4 m) draped over the 3D surface. 相似文献
5.
Juvenile coho salmon, Onchorynchus kisutch, were exposed to 95mTc in both saltwater and freshwater to study the biokinetic behavior of this element in an anadromous fish. In freshwater, equilibrium whole-body concentration factors (CF5) ranged from 28 to 32 in two separate experiments, in sharp contrast to an equilibriumCF of < 5 in saltwater. While the uptake kinetics were unimodal in saltwater, in freshwater, biomodal uptake kinetics were observed.Technetium-95m retention was biphasic in freshwater depuration experiments, an initial rapid loss of 21% to 26% of the steady-state activity with an elimination half-life of 3 to 5 days was followed by a slower loss rate, with a whole-body elimination half-life ranging from 23 to 33 days. In saltwater, retention was monophasic with an elimination half-life of 5–16 days. Dramatic decreases in 95mTc body burdens when freshwater-labeled fish are transferred to Tc-labeled saltwater indicate that the changes in physiology accompanying saltwater acclimation lead to the loss of 95mTc. 相似文献
6.
7.
Oxygen and sulfur isotope systematics of sulfate produced by bacterial and abiotic oxidation of pyrite 总被引:4,自引:0,他引:4
Nurgul Balci Wayne C. Shanks III Kevin W. Mandernack 《Geochimica et cosmochimica acta》2007,71(15):3796-3811
To better understand reaction pathways of pyrite oxidation and biogeochemical controls on δ18O and δ34S values of the generated sulfate in acid mine drainage (AMD) and other natural environments, we conducted a series of pyrite oxidation experiments in the laboratory. Our biological and abiotic experiments were conducted under aerobic conditions by using O2 as an oxidizing agent and under anaerobic conditions by using dissolved Fe(III)aq as an oxidant with varying δ18OH2O values in the presence and absence of Acidithiobacillus ferrooxidans. In addition, aerobic biological experiments were designed as short- and long-term experiments where the final pH was controlled at ∼2.7 and 2.2, respectively. Due to the slower kinetics of abiotic sulfide oxidation, the aerobic abiotic experiments were only conducted as long term with a final pH of ∼2.7. The δ34SSO4 values from both the biological and abiotic anaerobic experiments indicated a small but significant sulfur isotope fractionation (∼−0.7‰) in contrast to no significant fractionation observed from any of the aerobic experiments. Relative percentages of the incorporation of water-derived oxygen and dissolved oxygen (O2) to sulfate were estimated, in addition to the oxygen isotope fractionation between sulfate and water, and dissolved oxygen. As expected, during the biological and abiotic anaerobic experiments all of the sulfate oxygen was derived from water. The percentage incorporation of water-derived oxygen into sulfate during the oxidation experiments by O2 varied with longer incubation and lower pH, but not due to the presence or absence of bacteria. These percentages were estimated as 85%, 92% and 87% from the short-term biological, long-term biological and abiotic control experiments, respectively. An oxygen isotope fractionation effect between sulfate and water (ε18OSO4-H2O) of ∼3.5‰ was determined for the anaerobic (biological and abiotic) experiments. This measured value was then used to estimate the oxygen isotope fractionation effects between sulfate and dissolved oxygen in the aerobic experiments which were −10.0‰, −10.8‰, and −9.8‰ for the short-term biological, long-term biological and abiotic control experiments, respectively. Based on the similarity between δ18OSO4 values in the biological and abiotic experiments, it is suggested that δ18OSO4 values cannot be used to distinguish biological and abiotic mechanisms of pyrite oxidation. The results presented here suggest that Fe(III)aq is the primary oxidant for pyrite at pH < 3, even in the presence of dissolved oxygen, and that the main oxygen source of sulfate is water-oxygen under both aerobic and anaerobic conditions. 相似文献
8.
Abutment behavior significantly influences the seismic response of certain bridge structures. Specifically in the case of short bridges with relatively stiff superstructures typical of highway overpasses, embankment mobilization and inelastic behavior of the soil material under high shear deformation levels dominate the response of the bridge and its column bents. This paper investigates the sensitivity of bridge seismic response with respect to three different abutment modeling approaches. The abutment modeling approaches are based on three increasing levels of complexity that attempt to capture the critical components and modes of abutment response without the need to generate continuum models of the embankment, approach, and abutment foundations. Six existing reinforced concrete bridge structures, typical of Ordinary Bridges in California, are selected for the analysis. Nonlinear models of the bridges are developed in OpenSees. Three abutment model types of increasing complexity are developed for each bridge, denoted as roller, simplified, and spring abutments. The roller model contains only single-point constraints. The spring model contains discrete representations of backfill, bearing pad, shear key, and back wall behavior. The simplified model is a compromise between the efficient roller model and the comprehensive spring model. Modal, pushover, and nonlinear dynamic time history analyses are conducted for the six bridges using the three abutment models for each bridge. Comparisons of the analysis results show major differences in mode shapes and periods, ultimate base shear strength, as well as peak displacements of the column top obtained due to dynamic excitation. The adequacy of the three abutment models used in the study to realistically represent all major resistance mechanisms and components of the abutments, including an accurate estimation of their mass, stiffness, and nonlinear hysteretic behavior, is evaluated. Recommendations for abutment modeling are made. 相似文献
9.
Kevin Fong 《Earth, Moon, and Planets》2004,94(3-4):169-176
Human space exploration is not considered a strategic priority in the United Kingdom at present. However the UK would benefit from participating in human spaceflight, for both scientific and social reasons. From the point of view of medical science there are many parallels between the physiology of spaceflight and terrestrial disease processes, and studies of the response of astronauts to long-duration spaceflight can therefore help in the development of therapeutic strategies on Earth. On the social side, human spaceflight is an attractive vehicle for stimulating the interest of young people in science and engineering, something that must be of value for an aspiring ‘knowledge-based’ economy. 相似文献
10.
Klaus J. Stetzenbach Irene M. Farnham Vernon F. Hodge Kevin H. Johannesson 《水文研究》1999,13(17):2655-2673
Groundwater samples were collected from 11 springs in Ash Meadows National Wildlife Refuge in southern Nevada and seven springs from Death Valley National Park in eastern California. Concentrations of the major cations (Ca, Mg, Na and K) and 45 trace elements were determined in these groundwater samples. The resultant data were subjected to evaluation via the multivariate statistical technique principal components analysis (PCA), to investigate the chemical relationships between the Ash Meadows and Death Valley spring waters, to evaluate whether the results of the PCA support those of previous hydrogeological and isotopic studies and to determine if PCA can be used to help delineate potential groundwater flow patterns based on the chemical compositions of groundwaters. The results of the PCA indicated that groundwaters from the regional Paleozoic carbonate aquifers (all of the Ash Meadows springs and four springs from the Furnace Creek region of Death Valley) exhibited strong statistical associations, whereas other Death Valley groundwaters were chemically different. The results of the PCA support earlier studies, where potentiometric head levels, δ18O and δD, geological relationships and rare earth element data were used to evaluate groundwater flow, which suggest groundwater flows from Ash Meadows to the Furnace Creek springs in Death Valley. The PCA suggests that Furnace Creek groundwaters are moderately concentrated Ash Meadows groundwater, reflecting longer aquifer residence times for the Furnace Creek groundwaters. Moreover, PCA indicates that groundwater may flow from springs in the region surrounding Scotty's Castle in Death Valley National Park, to a spring discharging on the valley floor. The study indicates that PCA may provide rapid and relatively cost‐effective methods to assess possible groundwater flow regimes in systems that have not been previously investigated. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献