全文获取类型
收费全文 | 1831篇 |
免费 | 145篇 |
国内免费 | 11篇 |
专业分类
测绘学 | 53篇 |
大气科学 | 171篇 |
地球物理 | 589篇 |
地质学 | 708篇 |
海洋学 | 137篇 |
天文学 | 217篇 |
综合类 | 10篇 |
自然地理 | 102篇 |
出版年
2023年 | 6篇 |
2022年 | 17篇 |
2021年 | 42篇 |
2020年 | 49篇 |
2019年 | 38篇 |
2018年 | 46篇 |
2017年 | 71篇 |
2016年 | 110篇 |
2015年 | 101篇 |
2014年 | 124篇 |
2013年 | 187篇 |
2012年 | 147篇 |
2011年 | 89篇 |
2010年 | 93篇 |
2009年 | 77篇 |
2008年 | 64篇 |
2007年 | 35篇 |
2006年 | 70篇 |
2005年 | 45篇 |
2004年 | 35篇 |
2003年 | 38篇 |
2002年 | 49篇 |
2001年 | 33篇 |
2000年 | 16篇 |
1999年 | 17篇 |
1998年 | 19篇 |
1997年 | 17篇 |
1996年 | 13篇 |
1995年 | 20篇 |
1994年 | 18篇 |
1993年 | 15篇 |
1992年 | 9篇 |
1991年 | 20篇 |
1990年 | 10篇 |
1989年 | 9篇 |
1987年 | 9篇 |
1985年 | 12篇 |
1984年 | 17篇 |
1983年 | 15篇 |
1982年 | 15篇 |
1981年 | 15篇 |
1980年 | 14篇 |
1979年 | 15篇 |
1978年 | 8篇 |
1976年 | 8篇 |
1975年 | 11篇 |
1974年 | 12篇 |
1973年 | 11篇 |
1971年 | 7篇 |
1969年 | 7篇 |
排序方式: 共有1987条查询结果,搜索用时 31 毫秒
151.
Manuel R. Palacios-Fest Ana Luisa Carreño José R. Ortega-Ramírez Guillermo Alvarado-Valdéz 《Journal of Paleolimnology》2002,27(2):185-206
Paleoecology of Laguna Babícora, Chihuahua, Mexico was reconstructed using ostracode faunal assemblages and shell chemistry. The paleolimnological record is used to show the magnitude of paleoclimatic changes in the area from 25,000 years to the present.Faunal assemblages consist of four species of the genus Limnocythere: L. sappaensis, L. ceriotuberosa, L. bradburyi and L. platyforma, all associated with Candona caudata, Candona patzcuaro and Cypridopsis vidua. A paleosalinity index developed from these assemblages indicates that the lake's salinity fluctuated frequently from oligo- to meso-haline conditions during the last 25,000 years. This pattern and low salinity range are in good agreement with modern TDS (here used as an indicator of salinity) values recorded from 26 wells and one spring from the area (258–975 mg l–1). To estimate paleotemperature we examined the trace element content (Mg/Ca ratios) from individual valves of L. ceriotuberosa and L. platyforma, the two species most commonly recorded in Laguna Babícora.Shell Mg/Ca ratios of 204 specimens of these two species were used to estimate water temperature (Mg/Ca) by means of experimental standard coefficients. Our data show that paleowater temperature ranged from 5.6–21.3 °C (with 2 values ranging from 0.2–4.8 °C), which suggest a close correlation with atmospheric temperatures around the lake. These results are in good agreement with a modern mean winter temperature (3.5 °C) and mean summer temperature (20 °C) recorded in the area between 1970 and 1980. 相似文献
152.
Lars Håkanson 《Aquatic Sciences - Research Across Boundaries》2002,64(2):185-197
This work presents a scenario to demonstrate how a given lake (Lake Stora Kröntjärn, Sweden) would respond to changes in colour related to lumbering operations in its watershed. The questions posed are: How would lake colour values increase, for how long would such increases last and how would this influence the production and biomasses of key functional organisms in the lake? The work is based on a comprehensive lake ecosystem model, LakeWeb, which accounts for production, biomasses, predation, abiotic/biotic interactions of nine key functional groups of organisms, phytoplankton, bacterioplankton, two types of zooplankton (herbivorous and predatory), two types of fish (prey and predatory), as well as zoobenthos, macrophytes and benthic algae. The LakeWeb-model has been calibrated and critically tested using empirical data and regressions for many lakes and it can capture typical functional and structural patterns in lakes. To obtain results like these by traditional field-work in one or more lakes would be very demanding (in terms of money, persons and time). It has been shown that single tributary peaks in colour concentrations are often "drowned" in weekly mean lake values. The lumbering operations must be extensive and continue for a long time to cause significant increases in tributary colour values in order to significantly influence fundamental lake foodweb structures. This means that it is often realistic to assume that lumbering activities do not strongly influence lake foodweb structures. The LakeWeb-model with the given mass-balance model for lake colour can be a useful tool to assess situations when this might not be valid. 相似文献
153.
We present the discovery and spectroscopic identification of two very high proper-motion ancient white dwarf stars, found in a systematic proper-motion survey. Their kinematics and apparent magnitude clearly indicate that they are halo members, while their optical spectra are almost identical to the recently identified cool halo white dwarf WD 0346+246. Canonical stellar halo models predict a white dwarf volume density that is 2 orders of magnitude less than the rho approximately 7x10-4 M middle dot in circle pc-3 inferred from this survey. With the caveat that the sample size is very small, it appears that a significant fraction, approximately 10%, of the local dark matter halo is in the form of very old, cool, white dwarfs. 相似文献
154.
The distribution of some trace metals (Cu, Zn, Ni, Co,Fe, Mn) and of DOC over a particulate (> 1 m),a colloidal (size < 0.45 m and molecular weight > 10 kD) and an ultrafiltered fraction (< 10 kD)was determined at several sites on the Thur River,Switzerland, at various times of the year. Thecomplexation of Cu by strong ligands in theultrafiltrate and in the conventional filtrate (<0.45 m) was compared using a ligand-exchange/CSV method.The <0.45 m concentrations of Cu (from anaverage of 7 nM to 24 nM), Zn (<5–23 nM), Ni (5–13 nM),Co (1.5–3 nM) and Mn (7–92 nM)increased downstream. The major part of Cu, Zn, Niand Co usually occurred in the ultrafiltratefraction at all sites, whereas Fe and Mn were mostlyin the particulate fraction, under conditions of lowsuspended matter content (< 10 mg L-1) in theriver. The percentage of metal in the colloidalfraction, with respect to the 0.45-m filtrate,decreased in the order: Cu (median 11%) > Zn Ni(median 5–6%) > Mn Co (median < 5%). DOCalso consisted mostly of molecules in the < 10 kDrange.Cu was strongly complexed by natural organic ligandsin all filtrate and ultrafiltrate samples. A largepart of the strong Cu binding ligands consisted ofcompounds in the < 10 kD range, but colloidalligands with similar properties also occurred. Cu wasdistributed among the dissolved and the colloidalligands, roughly in proportion to organic carbon.The colloidal fraction (as defined here) did notincrease in its proportional amount downstream and wasonly of limited significance in transporting traceelements in the Thur River under low discharge conditions. 相似文献
155.
Low-frequency current fluctuations in the deep central equatorial Atlantic are analyzed using current meter measurements recorded
from November 1992 to November 1994. Current meters were located at about 14°W of longitude and 1° of latitude on both sides
of the equator between 1,700 m depth and the ocean bottom. At all sampling depths, the velocity fluctuations are dominantly
zonal and symmetrical with respect to the equator. At 1,700 and 2,000 m, the flow is dominated by annual period fluctuations,
at 3,000 m, the velocity field amplitude presents a minimum, and at 3,750 and 3,950 m, the flow is modulated by annual and
semiannual period variability. The annual signal exhibits an apparent upward phase propagation. When considering the phase
and the amplitude of the seasonal fluctuations, the data compare well with the outputs of a realistic numerical simulation
of the Atlantic Ocean. Together with a previous analysis of the model simulations, this supports the idea that the observed
annual fluctuations are due to wind-forced vertically propagating Kelvin and Rossby waves. Data and model do not provide deciding
evidences of the presence of semiannual equatorial waves deeper than 3,500 m depth in the central equatorial Atlantic Ocean. 相似文献
156.
The purpose of this study was to evaluate the global energy production potential of woody biomass from forestry for the year
2050 using a bottom-up analysis of key factors. Woody biomass from forestry was defined as all of the aboveground woody biomass
of trees, including all products made from woody biomass. This includes the harvesting, processing and use of woody biomass.
The projection was performed by comparing the future demand with the future supply of wood, based on existing databases, scenarios,
and outlook studies. Specific attention was paid to the impact of the underlying factors that determine this potential and
to the gaps and uncertainties in our current knowledge. Key variables included the demand for industrial roundwood and woodfuel,
the plantation establishment rates, and the various theoretical, technical, economical, and ecological limitations related
to the supply of wood from forests. Forests, as defined in this study, exclude forest plantations. Key uncertainties were
the supply of wood from trees outside forests, the future rates of deforestation, the consumption of woodfuel, and the theoretical,
technical, economical, or ecological wood production potentials of the forests. Based on a medium demand and medium plantation
scenario, the global theoretical potential of the surplus wood supply (i.e., after the demand for woodfuel and industrial roundwood is met) in 2050 was calculated
to be 6.1 Gm3 (71 EJ) and the technical potential to be 5.5 Gm3 (64 EJ). In practice, economical considerations further reduced the surplus wood supply from forests to 1.3 Gm3 year−1 (15 EJ year−1). When ecological criteria were also included, the demand for woodfuel and industrial roundwood exceeded the supply by 0.7 Gm3 year−1 (8 EJ year−1). The bioenergy potential from logging and processing residues and waste was estimated to be equivalent to 2.4 Gm3 year−1 (28 EJ year−1) wood, based on a medium demand scenario. These results indicate that forests can, in theory, become a major source of bioenergy,
and that the use of this bioenergy can, in theory, be realized without endangering the supply of industrial roundwood and
woodfuel and without further deforestation. Regional shortages in the supply of industrial roundwood and woodfuel can, however,
occur in some regions, e.g., South Asia and the Middle East and North Africa. 相似文献
157.
Marcelo Chamecki René van Hout Charles Meneveau Marc B. Parlange 《Boundary-Layer Meteorology》2007,125(1):25-38
An expression for the vertical equilibrium concentration profile of heavy particles, including the effects of canopy on the
eddy diffusivity as well as corrections for atmospheric stability, is proposed. This expression is validated against measurements
of vertical concentration profiles of corn pollen above a corn field. The fitted theoretical profiles show that particle settling
is correctly accounted for. The sensitivity to variations in the turbulent Schmidt number, settling velocity and stability
corrections are explicitly characterized. The importance of independent measurements of the surface flux of pollen in future
experiments is noted. 相似文献
158.
The scaled-decomposed atmospheric water budget over North America is investigated through the analysis of 25 years of simulation
by the Canadian Regional Climate Model (CRCM) driven by the NCEP–NCAR reanalyses for the period 1975–1999. The time average
and time variability of the atmospheric water budget for the winter and summer seasons are decomposed into their large-scale
and small-scale components to identify the added value of the regional model. For the winter season, the intra-seasonal transient-eddy
variance is the main temporal variability. The large- and small-scale terms are of the same order of magnitude, and are large
over both coasts and weak over the continent. For the summer season, the time–mean atmospheric water budget is rather different
to that of winter, with maximum values over the south-eastern part of the continent. The summer intra-seasonal variance is
about twice stronger than in winter and also dominates the variability, but the inter-monthly variance is non-negligible and
can be in part associated to North American Monsoon System. Over the continent, the intra-seasonal climatological variance
is dominated by the variability of the small scales. The small scales, that is those scales that are only resolved in the
regional model but not in the reanalyses, contribute to the added value in a regional climate simulation. In the winter season,
the added value of the CRCM is large and dominated by oceanic forcing, while in summer, it is dominant (larger than the large
scales) and controlled mainly by convective processes. 相似文献
159.
Wolf-Rainer Abraham Alexandre José Macedo Luiz Humberto Gomes Flavio C. A. Tavares 《洁净——土壤、空气、水》2007,35(4):339-347
The load of pathogenic bacteria, their fate and their dangerousness in the Tietê River were assessed along 100 km starting from the city of São Paulo, Brazil. The data were compared with those from two German rivers. High loads of pathogens were found in the Tietê River near the city of São Paulo (Escherichia coli O157:H7, Shigella flexneri, and Shigella boydii), which were absent 30 km downstream of São Paulo. The antibiotic resistances observed in the Tietê river were rather low and decreased after the major input in São Paulo to significantly lower levels about 30 km downstream. While the Brazilian isolates were more susceptible for ampicillin than the German ones, the reverse was observed for gentamycin. For optimal control of infections in humans critical areas where these bacteria survived longer and their elimination mechanisms should be identified as well as the extent and the origin of antibiotic resistance should be determined. 相似文献
160.
André F. Lotter H. John B. Birks Wolfgang Hofmann Aldo Marchetto 《Journal of Paleolimnology》1998,19(4):443-463
Surface sediments from 68 small lakes in the Alps and 9 well-dated sediment core samples that cover a gradient of total phosphorus (TP) concentrations of 6 to 520 g TP l-1 were studied for diatom, chrysophyte cyst, cladocera, and chironomid assemblages. Inference models for mean circulation log10 TP were developed for diatoms, chironomids, and benthic cladocera using weighted-averaging partial least squares. After screening for outliers, the final transfer functions have coefficients of determination (r2, as assessed by cross-validation, of 0.79 (diatoms), 0.68 (chironomids), and 0.49 (benthic cladocera). Planktonic cladocera and chrysophytes show very weak relationships to TP and no TP inference models were developed for these biota. Diatoms showed the best relationship with TP, whereas the other biota all have large secondary gradients, suggesting that variables other than TP have a strong influence on their composition and abundance. Comparison with other diatom – TP inference models shows that our model has high predictive power and a low root mean squared error of prediction, as assessed by cross-validation. 相似文献