全文获取类型
收费全文 | 141篇 |
免费 | 10篇 |
国内免费 | 1篇 |
专业分类
测绘学 | 3篇 |
大气科学 | 9篇 |
地球物理 | 40篇 |
地质学 | 55篇 |
海洋学 | 15篇 |
天文学 | 13篇 |
自然地理 | 17篇 |
出版年
2022年 | 1篇 |
2021年 | 3篇 |
2020年 | 6篇 |
2019年 | 2篇 |
2018年 | 3篇 |
2017年 | 3篇 |
2016年 | 4篇 |
2015年 | 5篇 |
2014年 | 4篇 |
2013年 | 13篇 |
2012年 | 7篇 |
2011年 | 1篇 |
2010年 | 5篇 |
2009年 | 6篇 |
2008年 | 12篇 |
2007年 | 9篇 |
2006年 | 2篇 |
2005年 | 4篇 |
2004年 | 6篇 |
2003年 | 4篇 |
2002年 | 4篇 |
2001年 | 5篇 |
2000年 | 12篇 |
1999年 | 3篇 |
1998年 | 7篇 |
1997年 | 2篇 |
1996年 | 5篇 |
1995年 | 2篇 |
1993年 | 1篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1984年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有152条查询结果,搜索用时 15 毫秒
141.
By following a simple set of rules, a flow net can be manually constructed to obtain a graphical solution to the Laplace equation for simple two‐dimensional (2‐D) flow problems. With the advent of numerical solutions such as the finite difference and finite element methods, it is more common to generate a flow net automatically using the nodal head and flow values output by the computer program. Two methods have been published for automatically generating flow nets from finite element solutions: the stream‐function method and the particle‐tracking method. The stream‐function method works well for many cases, but it does not work for problems with holes in the mesh or internal sources or sinks. The particle‐tracking method works for all cases, but previously published algorithms that utilize this method do not result in the properly sized flow channels. A new approach is presented in this paper that is a hybrid of the stream‐function and particle‐tracking approaches. This method works for all cases and generates the properly sized flow channels. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
142.
Biopsy samples of blubber from adult male and female blue whales, and from female and young-of-the-year humpback whales were collected during the summers of 1992-1999 in the Gulf of St. Lawrence, Canada. In blue whales, concentrations of 25 PCB congeners, DDT and metabolites and several other organochlorine compounds were present at higher concentrations in the blubber of males relative to females; reflecting maternal transfer of these persistent contaminants from females into young. Sex-related differences in concentrations were not observed with less persistent contaminants, such as HCHs. In humpback whale samples, there were no significant differences in the concentrations of PCBs and organochlorine compounds in the blubber of females and calves. These data indicate that calves quickly bioaccumulate contaminants by transplacental and lactational routes to concentrations that are in equilibrium with females. In comparisons between contaminant concentrations and patterns in the blubber of female blue and humpback whales, there were no significant differences in concentrations, but the proportions of some PCB congeners, HCH isomers, and DDT and its metabolites were different in the two baleen whale species. These may reflect differences in the diet of the two species, since fish comprise a large part of the diet of humpback whales and blue whales feed exclusively on euphausiid crustaceans (i.e. krill). 相似文献
143.
Use of Cell-Specific PAM-Fluorometry to Characterize Host Shading in the Epiphytic Dinoflagellate Gambierdiscus toxicus 总被引:1,自引:0,他引:1
Abstract. Cell‐specific fluorescence characteristics were used to characterize the light tolerance of the toxic benthic dinoflagellate Gambierdiscus toxicus. The fluorescence parameter Fv : Fm was measured using pulse amplitude modulation (PAM) fluorometry on individual cells collected from foliose red algae growing in the sub‐tidal margin of South Water Cay, Belize. Samples were collected over several days during sunny and cloudy conditions and compared to samples incubated in situ. The data from individual cells were used to generate both Fv : Fm frequency histograms and averages. Maximum individual cell values of Fv : Fm reached 0.81 in pre‐dawn samples, a value near the theoretical maximum for PAM fluorometry. In field samples from macroalgal hosts, average Fv : Fm values declined only slightly during the day, but cells incubated in bottles under 47 % incident sunlight showed a significant mid‐day depression. In freshly collected samples, near‐maximum Fv : Fm values could be found in individual cells during the entire day; however, the frequency histograms indicated a greater range in Fv : Fm values during the afternoon than in the morning. In contrast, cultures of G. toxicus showed a tight distribution around a mean. Field samples showed a rapid recovery to near‐maximum Fv : Fm within 2 min when assayed using a standardized actinic light series. Similar results were obtained in laboratory cultures of G. toxicus grown at 73 µmol photons · m‐2 · s‐1, but not at 383 µmol photons · m‐2 · s‐1. These data provide empirical support for suggestions that G. toxicus exploits the three‐dimensional structure of the algal host thallus to minimize light exposure. This strategy permits G. toxicus, a high‐light intolerant species in culture, to thrive in shallow, well‐lit tropical seas. It may also partially explain the observed preference of G. toxicus for complex, foliose macroalgae as hosts. 相似文献
144.
Tracy L. Johnson 《Pure and Applied Geophysics》1978,116(4-5):954-963
Summary Laboratory measurements of rupture and particle velocity are in surprisingly good agreement with seismic values, providing further evidence that stick-slip friction is a suitable mechanism for shallow earthquakes. A simple theory is developed to explain the linear relationship observed between average particle velocity and stress drop for stick-slip events. Both stick-slip ruptures and cracks in brittle material commonly propagate at velocities roughly comparable to theS wave velocity of the material. Rupture normally begins relatively slowly and accelerates to a steady velocity in a few centimeters. Observations suggest that stick-slip ruptures can propagate atS wave speeds or occasionally greater and that cracks in pre-stressed glass can also propagate faster than theS waves. Fracture and thus rupture velocity of intact rock specimens is greatly influenced by the inhomogeneous structure of rock. Fracture may be modeled by coalescence of many cracks rather than growth of a single crack.Lamont-Doherty Geological Observatory Contribution No. 2627. 相似文献
145.
Caroline R. McFarlin J. Stephen Brewer Tracy L. Buck Steven C. Pennings 《Estuaries and Coasts》2008,31(2):313-325
We examined the response of a salt marsh food web to increases in nutrients at 19 coastal sites in Georgia. Fertilization
increased the nitrogen content of the two dominant plants, Spartina alterniflora and Juncus roemerianus, indicating that added nutrients were available to and taken up by both species. Fertilization increased Spartina cover, height, and biomass and Juncus height, but led to decreases in Juncus cover and biomass. Fertilization increased abundances of herbivores (grasshoppers) and herbivore damage, but had little effect
on decomposers (fungi), and no effect on detritivores (snails). In the laboratory, herbivores and detritivores did not show
a feeding preference for fertilized versus control plants of either species, nor did detritivores grow more rapidly on fertilized
versus control plants, suggesting that changes in herbivore abundance in the field were driven more by plant size or appearance
than by plant nutritional quality. Community patterns in control plots varied predictably among sites (i.e., 17 of 20 regression
models examining variation in biological variables across sites were significant), but variation in the effects of fertilization
across sites could not be easily predicted (i.e., only 6 of 20 models were significant). Natural variation among sites was
typically similar or greater than impacts of fertilization when both were assessed using the coefficient of variation. Overall,
these results suggest that eutrophication of salt marshes is likely to have stronger impacts on plants and herbivores than
on decomposers and detritivores, and that impacts at any particular site might be hard to distinguish from natural variation
among sites. 相似文献
146.
147.
Amanda M. Oehlert Peter K. Swart Gregor P. Eberli Samantha Evans Tracy D. Frank 《Sedimentology》2019,66(1):241-261
This study investigates the covariation between carbonate and organic δ13C values in a proximal to distal transect of four outcrops in the Madison Limestone in the Western United States Rockies, combined with δ34S values of carbonate associated sulphate, the concentration of acid‐insoluble material and measurements of total organic carbon. These new geochemical datasets not only allow for an evaluation of carbon isotope covariance during one of the largest perturbations to the global carbon cycle over the past 550 Myr, but also constrain the cause of the excursion in carbonate δ13C values. The results support the hypothesis that a period of anoxia did not play a role in generating the positive carbonate δ13C values, but rather favour interpretations by previous workers that the proliferation of land plants destabilized the Carboniferous carbon cycle, setting the stage for a significant change in the carbonate δ13C values of contemporaneous marine carbonates. These results also demonstrate that one of the largest perturbations to the global carbon cycle did not produce synchronous variations in carbonate and organic δ13C values, emphasizing the importance of local depositional controls on carbon isotope covariance in the geological record in both modern and ancient environments. 相似文献
148.
Martin?ZimmerEmail author Steven?C.?Pennings Tracy?L.?Buck Thomas?H.?Carefoot 《Estuaries and Coasts》2004,27(5):753-769
Most primary production of angiosperms in coastal salt marshes enters the detritivore food web; studies of this link have
predominantly focused on one plant species (Spartina alterniflora) and one detritivore species (Littoraria irrorata). In mesocosm experiments, we studied the rates and pattern of decomposition of litter derived from four plant species common
in southeastern United States coastal salt marshes and marsh-fringing terrestrial habitats. Crustanceans and gastropods were
selected as detritivores feeding on, and affecting degradation of, the litter of two monocotyledons and two dicotyledons.
Despite interspecific similarities in consumption, detritivores exhibited species-specific effects on litter chemistry and
on the activity of litter-colonizing microbiota. The chemical composition of feces depended upon both the litter type and
the detritivores’ species-specific digestive capabilities. Growth rates and survival of detritivores differed among litter
species. Different salt marsh detritivores are likely to have different effects on decomposition processes in the salt marsh
and cannot be regarded as functionally redundant nor can the litter of different plant species be regard ed as redundant as
food for marsh detritivores. 相似文献
149.
Tracy?N.?WiegnerEmail author Sybil?P.?Seitzinger Denise?L.?Breitburg James?G.?Sanders 《Estuaries and Coasts》2003,26(2):352-364
Responses of autotrophic and heterotrophic processes to nutrients and trace elements were examined in a series of experimental
estuarine food webs of increasing trophic complexity using twenty 1-m3 mesocosms. Nutrients (nitrogen and phosphorus) and trace elements (a mix of arsenic, copper, cadmium) were added alone and
in combination during four experimental runs spanning from spring 1997 to spring 1998. Diel changes in dissolved oxygen were
used to examine whole system gross primary production (WS-GPP), respiration (WS-RESP), and net ecosystem metabolism (NEM).
Nutrient and trace element additions had the greatest effect on WS-GPP, WS-RESP, and NEM; trophic complexity did not significantly
affect any of these parameters (p>0.3). Effects of trophic complexity were detected in nutrient tanks where bivalves significantly
(p=0.03) reduced WS-GPP. Nutrient additions significantly enhanced WS-GPP and to a lesser extent WS-RESP during most mesocosm
runs. The system shifted from net heterotrophy (−17.2±1.8 mmol C m−3 d−1) in the controls to net autotrophy (29.1±7.6 mmol C m−3 d−1) in the nutrient tanks. The addition of trace elements alone did not affect WS-GPP and WS-RESP to the same extent as nutrients,
and their effects were more variable. Additions of trace elements alone consistently made the system more net heterotrophic
(−24.9±1.4 mmol C m−3 d−1) than the controls. When trace elements were added in combination with nutrients, the nutrient-enriched system became less
autotrophic (1.6±3.1 mmol C m−3 d−1). The effects of trace elements on NEM occurred primarily through reductions in WS-GPP rather than increases in WS-RESP.
Our results suggest that autotrophic and heterotrophic processes respond differently to these stressors. 相似文献
150.
Using a large database of residential short-term radon measurements in New Hampshire, this study evaluated the ability of expert-assigned bedrock radon potential for predicting residential radon concentration. First, each bedrock type was assigned a radon potential level by a geologist familiar with the local geology. Then, using residential radon measurements, a continuous surface of radon concentration was generated through a kriging process. The mean residential radon concentration within the spatial extent of each bedrock type was then calculated based on that surface. The Spearman Rank Correlation Coefficient was calculated between the two ranks of the bedrock types, one based on the expert-assigned potential level and the other based on the mean residential concentration. A strong correlation between the rank correlation and the area of the bedrock type was found. When only the 15 largest bedrock types were used, the Spearman Correlation Coefficient reached 0.6. Geological knowledge is concluded to be useful in predicting and mapping residential radon concentration, but the prediction should be interpreted with caution, especially for areas in which the underlying bedrocks are highly localized. 相似文献