全文获取类型
收费全文 | 910篇 |
免费 | 17篇 |
国内免费 | 4篇 |
专业分类
测绘学 | 26篇 |
大气科学 | 29篇 |
地球物理 | 161篇 |
地质学 | 277篇 |
海洋学 | 87篇 |
天文学 | 233篇 |
综合类 | 2篇 |
自然地理 | 116篇 |
出版年
2021年 | 5篇 |
2020年 | 10篇 |
2019年 | 6篇 |
2018年 | 8篇 |
2017年 | 12篇 |
2016年 | 15篇 |
2015年 | 21篇 |
2014年 | 16篇 |
2013年 | 38篇 |
2012年 | 26篇 |
2011年 | 41篇 |
2010年 | 30篇 |
2009年 | 27篇 |
2008年 | 40篇 |
2007年 | 28篇 |
2006年 | 35篇 |
2005年 | 36篇 |
2004年 | 48篇 |
2003年 | 28篇 |
2002年 | 32篇 |
2001年 | 23篇 |
2000年 | 23篇 |
1999年 | 18篇 |
1998年 | 27篇 |
1997年 | 23篇 |
1996年 | 11篇 |
1995年 | 16篇 |
1994年 | 13篇 |
1993年 | 8篇 |
1992年 | 13篇 |
1991年 | 11篇 |
1990年 | 12篇 |
1989年 | 10篇 |
1988年 | 10篇 |
1987年 | 9篇 |
1986年 | 11篇 |
1985年 | 15篇 |
1984年 | 13篇 |
1983年 | 17篇 |
1982年 | 10篇 |
1981年 | 18篇 |
1980年 | 12篇 |
1979年 | 10篇 |
1978年 | 13篇 |
1977年 | 9篇 |
1975年 | 12篇 |
1974年 | 10篇 |
1973年 | 11篇 |
1972年 | 6篇 |
1971年 | 7篇 |
排序方式: 共有931条查询结果,搜索用时 859 毫秒
371.
R. G. D. Davidson‐Arnott B. O. Bauer I. J. Walker P. A. Hesp J. Ollerhead C. Chapman 《地球表面变化过程与地形》2012,37(11):1227-1241
Wind flow and sand transport intensity were measured on the seaward slope of a vegetated foredune during a 16 h storm using an array of sonic anemometers and Wenglor laser particle counters. The foredune had a compound seaward slope with a wave‐cut scarp about 0.5 m high separating the upper vegetated portion from the lower dune ramp, which was bare of vegetation. Wind direction veered from obliquely offshore at the start of the event to obliquely onshore during the storm peak and finally to directly onshore during the final 2 h as wind speed dropped to below threshold. Sand transport was initially inhibited by a brief period of rain at the start of the event but as the surface dried and wind speed increased sand transport was initiated over the entire seaward slope. Transport intensity was quite variable both temporally and spatially on the upper slope as a result of fluctuating wind speed and direction, but overall magnitudes were similar over the whole length. Ten‐minute average transport intensity correlates strongly with mean wind speed measured at the dune crest, and there is also strong correlation between instantaneous wind speed and transport intensity measured at the same locations when the data are smoothed with a 10 s running mean. Transport on the beach for onshore winds is decoupled from that on the seaward slope above the small scarp when the wind angle is highly oblique, but for wind angles <45° from shore perpendicular some sand is transported onto the lower slope. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
372.
A Bayesian approach to palaeoecological environmental reconstruction deriving from the unimodal responses generally exhibited
by organisms to an environmental gradient is described. The approach uses Bayesian model selection to calculate a collection
of probability-weighted, species-specific response curves (SRCs) for each taxon within a training set, with an explicit treatment
for zero abundances. These SRCs are used to reconstruct the environmental variable from sub-fossilised assemblages. The approach
enables a substantial increase in computational efficiency (several orders of magnitude) over existing Bayesian methodologies.
The model is developed from the Surface Water Acidification Programme (SWAP) training set and is demonstrated to exhibit comparable
predictive power to existing Weighted Averaging and Maximum Likelihood methodologies, though with improvements in bias; the
additional explanatory power of the Bayesian approach lies in an explicit calculation of uncertainty for each individual reconstruction.
The model is applied to reconstruct the Holocene acidification history of the Round Loch of Glenhead, including a reconstruction
of recent recovery derived from sediment trap data.
The Bayesian reconstructions display similar trends to conventional (Weighted Averaging Partial Least Squares) reconstructions
but provide a better reconstruction of extreme pH and are more sensitive to small changes in diatom assemblages. The validity
of the posteriors as an apparently meaningful representation of assemblage-specific uncertainty and the high computational
efficiency of the approach open up the possibility of highly constrained multiproxy reconstructions. 相似文献
373.
M. M. Walker 《新西兰海洋与淡水研究杂志》2013,47(4):393-397
Development, settlement, and subsequent metamorphosis on several substrata were observed in cultures of Evechinus chloroticus larvae. The period from fertilisation to metamorphosis ranged between 30 and 63 days for larvae collected from spawnings induced at 3 different times. Newly settled sea urchins collected from the field grew about 1 mm per month in aquaria. Correlation of the larval life span with the growth rate of newly settled sea urchins suggested that the earliest spawning could occur was mid‐November. This finding corroborates the results of previous histo‐logical studies of the gonads of adult sea urchins in the Hauraki Gulf, New Zealand. 相似文献
374.
In environments with shallow ground water elevation, small changes in the water table can cause significant variations in recharge and evapotranspiration fluxes. Particularly, where ground water is close to the soil surface, both recharge and evapotranspiration are regulated by a thin unsaturated zone and, for accuracy, must be represented using nonconstant and often nonlinear relationships. The most commonly used ground water flow model today, MODFLOW, was originally designed with a modular structure with independent packages representing recharge and evaporation processes. Systems with shallow ground water, however, may be better represented using either a recharge function that varies with ground water depth or a continuous recharge and evapotranspiration function that is dependent on depth to water table. In situations where the boundaries between recharging and nonrecharging cells change with time, such as near a seepage zone, a continuous ground water flux relationship allows recharge rates to change with depth rather than having to calculate them at each stress period. This research article describes the modification of the MODFLOW 2000 recharge and segmented evapotranspiration packages into a continuous recharge-discharge function that allows ground water flux to be represented as a continuous process, dependent on head. The modifications were then used to model long-term recharge and evapotranspiration processes on a saline, semiarid floodplain in order to understand spatial patterns of salinization, and an overview of this process is given. 相似文献
375.
376.
Rolling isolation systems (RISs) protect mission‐critical equipment and valuable property from earthquake hazards by decoupling the dynamic responses of vibration‐sensitive objects from horizontal floor motions. These responses involve the constrained rolling of steel balls between bowl‐shaped surfaces. The light damping of steel balls rolling between steel plates can be augmented by adhering thin rubber sheets to the plates, thereby increasing the rolling resistance and decreasing the displacement demand on the RIS. An assessment of the ability of lightly‐ and heavily‐damped RISs to mitigate the hazard of seismically induced failures requires high‐fidelity models that can adequately capture the systems' intrinsic nonlinear behavior. The simplified model presented in this paper is applicable to RISs with any potential energy function, is amenable to both lightly‐ and heavily‐damped RISs, and is validated through the successful prediction of peak responses for a wide range of disturbance frequencies and intensities. The validated model can therefore be used to compute the spectra of peak floor motions for which displacement demands equal capacity. These spectra are compared with representative floor motion spectra provided by the American Society of Civil Engineers 7–10. The damping provided by rolling between thin viscoelastic sheets increases the allowable floor motion intensity by a factor of 2–3, depending on the period of motion. Acceleration responses of isolation systems with damping supplied in this fashion do not grow with increased damping, even for short‐period excitations. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
377.
Bradley J. Peters Andrea Mundl‐Petermeier Mary F. Horan Richard W. Carlson Richard J. Walker 《Geostandards and Geoanalytical Research》2019,43(2):245-259
One requirement for isotope ratio measurement results with small measurement uncertainties is that the element of interest is effectively separated from the sample matrix. Efficient chemical separation of W from matrix components, especially Ti, can be challenging, particularly for large test portion masses (> 1 g). We present a new W separation procedure that takes advantage of the distinct complexation behaviour of Ti and W with citrate ligand in a moderately low pH, oxidising solution. This preparation procedure can reduce the Ti/W ratio of large (4–10 g) basaltic (i.e., high‐matrix) test portions by a factor of 105, relative to their original compositions, in a two‐step separation procedure. The procedure additionally provides a separate, well‐purified Mo fraction. We show that optimal separation requires precise selection of reagent concentrations and sample load. The procedure was employed to determine the μ182W composition of BHVO‐2 as ?6.7 ± 4.2 (2 standard deviation, 2s). The principles derived from this method may prove useful for chemical separation of other elements used for geochemical and cosmochemical applications given an appropriate selection of organic acid. Future successful applications of this method may reveal that the use of organic acids as procedural reagents is a currently under‐utilised tool for efficient chemical separation protocols. 相似文献
378.
正Climate change and its impacts have become topical issues of global news, scientific research and conferences. Environmental Geosciences incorporate the various disciplines of geosciences and their multifaceted interactions with life. Research discussions on the interaction of climate change, geosciences and environment may often be blur, and Schmidt-Thoméet al.(2010) stated that"Often past climate changes that can be deduced from geological records may help in understanding the speed of potential climate change effects, i.e. how quickly have sea levels changed, how drastic has nature reacted to ups and downs in temperature, etc. These analyses of past events help in giving outlooks on potential changes in our living environment. It is also of important to understand the magnitude and potential effects of extreme events, such as droughts and floods". 相似文献
379.
Pt-Re-Os and Sm-Nd isotope and HSE and REE systematics of the 2.7 Ga Belingwe and Abitibi komatiites
High-precision Pt-Re-Os and Sm-Nd isotope and highly siderophile element (HSE) and rare earth element (REE) abundance data are reported for two 2.7 b.y. old komatiite lava flows, Tony’s flow (TN) from the Belingwe greenstone belt, Zimbabwe, and the PH-II flow (PH) from Munro Township in the Abitibi greenstone belt, Canada. The emplaced lavas are calculated to have contained ∼25% (TN) and ∼28% (PH) MgO. These lavas were derived from mantle sources characterized by strong depletions in highly incompatible lithophile trace elements, such as light REE (Ce/SmN = 0.64 ± 0.02 (TN) and 0.52 ± 0.01 (PH), ε143Nd(T) = +2.9 ± 0.2 in both sources). 190Pt-186Os and 187Re-187Os isochrons generated for each flow yield ages consistent with respective emplacement ages obtained using other chronometers. The calculated precise initial 186Os/188Os = 0.1198318 ± 3 (TN) and 0.1198316 ± 5 (PH) and 187Os/188Os = 0.10875 ± 17 (TN) and 0.10873 ± 15 (PH) require time-integrated 190Pt/188Os and 187Re/188Os of 0.00178 ± 11 and 0.407 ± 8 (TN) and 0.00174 ± 18 and 0.415 ± 5 (PH). These parameters, which by far represent the most precise and accurate estimates of time-integrated Pt/Os and Re/Os of the Archean mantle, are best matched by those of enstatite chondrites. The data also provide evidence for a remarkable similarity in the composition of the sources of these komatiites with respect to both REE and HSE. The calculated absolute HSE abundances in the TN and PH komatiite sources are within or slightly below the range of estimates for the terrestrial Primitive Upper Mantle (PUM). Assuming a chondritic composition of the bulk silicate Earth, the strong depletions in LREE, yet chondritic Re/Os in the komatiite sources are apparently problematic because early Earth processes capable of fractionating the LREE might also be expected to fractionate Re/Os. This apparent discrepancy could be reconciled via a two-stage model, whereby the moderate LREE depletion in the sources of the komatiites initially occurred within the first 100 Ma of Earth’s history as a result of either global magma ocean differentiation or extraction and subsequent long-term isolation of early crust, whereas HSE were largely added subsequently via late accretion. The komatiite formation, preceded by derivation of basaltic magmas, was a result of second-stage, large-degree dynamic melting in mantle plumes. 相似文献
380.
James M.D. Day Richard J. Walker Odette B. James Igor S. Puchtel 《Earth and Planetary Science Letters》2010,289(3-4):595-605
Coupled 187Os/188Os and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundance data are reported for pristine lunar crustal rocks 60025, 62255, 65315 (ferroan anorthosites, FAN) and 76535, 78235, 77215 and a norite clast in 15455 (magnesian-suite rocks, MGS). Osmium isotopes permit more refined discrimination than previously possible of samples that have been contaminated by meteoritic additions and the new results show that some rocks, previously identified as pristine, contain meteorite-derived HSE. Low HSE abundances in FAN and MGS rocks are consistent with derivation from a strongly HSE-depleted lunar mantle. At the time of formation, the lunar floatation crust, represented by FAN, had 1.4 ± 0.3 pg g? 1 Os, 1.5 ± 0.6 pg g? 1 Ir, 6.8 ± 2.7 pg g? 1 Ru, 16 ± 15 pg g? 1 Pt, 33 ± 30 pg g? 1 Pd and 0.29 ± 0.10 pg g? 1 Re (~ 0.00002 × CI) and Re/Os ratios that were modestly elevated (187Re/188Os = 0.6 to 1.7) relative to CI chondrites. MGS samples are, on average, characterised by more elevated HSE abundances (~ 0.00007 × CI) compared with FAN. This either reflects contrasting mantle-source HSE characteristics of FAN and MGS rocks, or different mantle–crust HSE fractionation behaviour during production of these lithologies. Previous studies of lunar impact-melt rocks have identified possible elevated Ru and Pd in lunar crustal target rocks. The new results provide no supporting evidence for such enrichments.If maximum estimates for HSE in the lunar mantle are compared with FAN and MGS averages, crust–mantle concentration ratios (D-values) must be ≤ 0.3. Such D-values are broadly similar to those estimated for partitioning between the terrestrial crust and upper mantle, with the notable exception of Re. Given the presumably completely different mode of origin for the primary lunar floatation crust and tertiary terrestrial continental crust, the potential similarities in crust–mantle HSE partitioning for the Earth and Moon are somewhat surprising. Low HSE abundances in the lunar crust, coupled with estimates of HSE concentrations in the lunar mantle implies there may be a ‘missing component’ of late-accreted materials (as much as 95%) to the Moon if the Earth/Moon mass-flux estimates are correct and terrestrial mantle HSE abundances were established by late accretion. 相似文献