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31.
Taniel Danelian Laurence Le Callonnec Jochen Erbacher David C. Mosher Mitchell J. Malone Debora Berti Karen L. Bice Helen Bostock Hans-Jürgen Brumsack Astrid Forster Felix Heidersdorf Jorijntje Henderiks Thomas J. Janecek Christopher Junium Ken MacLeod Philip A. Meyers Jörg H. Mutterlose Hiroshi Nishi Christine Glatz 《Comptes Rendus Geoscience》2005,337(6):609-616
Five sites located on a bathymetric transect of the distal Demerara Rise were studied by ODP Leg 207. Albian sediments of essentially terrigenous nature (clay, siltstone, sandstone) are the oldest drilled stratigraphic levels and form apparently the top of the synrift sequence. They are overlain by Cenomanian to Santonian finely laminated black shales, rich in organic matter of marine origin, which accumulated on a thermally subsiding ramp. Early Campanian hiatuses are thought to be the result of final disjunction of Demerara Rise (South America) from Africa and the onset of deep water communication between the two Atlantic basins (south and central). The overlying Uppermost Cretaceous–Oligocene chalk includes rich and diversified calcareous plankton assemblages, as well as two radiolarian-rich intervals (Late Campanian and Middle Eocene). A complex erosional surface developed during the Late Oligocene–Early Miocene. Sedimentation was impeded since then on the intermediate and deep sites of Demerara Rise, possibly due to the action of deep submarine currents. To cite this article: T. Danelian et al., C. R. Geoscience 337 (2005). 相似文献
32.
Enn Karro Ene Indermitte Astrid Saava Kadri Haamer Andres Marandi 《Environmental Geology》2006,50(3):389-396
A study was undertaken to examine the content and spatial distribution of fluoride in drinking water. Water samples (735) from public water systems covering all Estonian territory were analysed using SPADNS method. In order to specify the natural source of fluoride, the chemistry data from five aquifer systems utilised for water supply were included into the study. Fluoride concentrations in tap water, to a great extent, ranged from 0.01 to 6.95 mg/l. Drinking water in southern Estonia, where terrigenous Middle-Devonian aquifer system is exploited, has a fluoride concentration lower than recommended level (0.5 mg/l), thus promoting susceptibility to dental caries. The western part of the country is supplied by water with excess fluoride content (1.5–6.9 mg/l). Groundwater abstracted for drinking purposes originates from Ordovician and Silurian carbonate rocks. The content of fluoride in Silurian–Ordovician aquifer system is associated with the groundwater abstraction depth and the main controlling factors of dissolved fluoride are the pH value and the chemical type of water. 相似文献
33.
Astrid E. Schwarz 《Limnologica》2003,33(1):62-73
Most scientists tend to ignore the history of their own discipline, at best it occurs as a sort of ‘big names history’. History is thought to be useless or at least irrelevant for the own scientific work. In the following it is questioned, whether this is an indispensable assumption of productive and progressive science. Some propositions will be discussed, what a positive historical perspective could mean for the current limnological research. It is claimed that it could be not only interesting but even exciting for limnologists to deal with their own history. One important argument is that limnology not only has a history but also is history: the ‘discovery’ of new methods, theories, concepts and instruments depends on the forgoing. Consequently, scientific progress is related to the history and progress of a particular discipline. This means in same time that the historical reflection could be an important issue to a more complete ‘explanation’ and understanding of this progress.Another reason to overcome the traditional separation into actual and historical knowledge and acting could be the hybrid character of ecology. In ecology, and consequently also in limnology, several elements of opposite scientific methodologies are merged together. Therefore the discussions on the saprobial system, an ecosystem ‘lake’ and the ‘microbial loop’ differ fundamentally. Turned positively it means that limnology should be skilled in handling with different systems of knowledge. This skills should be accessible by approaching the own history in a selfconcious way. Of course the internal view alone is not sufficient to write a history of the discipline, which aims to demonstrate the movement of progress, the relation between science and society as well as science and technics. Its history must be studied from the perspective and with the methods of the history and philosophy of science. In doing so there should be a common objective for historians and scientists that is the amelioration of the science ‘limnology’. 相似文献
34.
Astrid Besmehn 《Geochimica et cosmochimica acta》2003,67(24):4693-4703
We report results from NanoSIMS isotopic measurements on 37 presolar silicon carbide grains of type X which are believed to have formed in the ejecta of supernova explosions. Isotopic data were obtained for Si and Ca-Ti (all grains), C and N (two grains), and Ti (one grain). All X grains exhibit large enrichments in 28Si (up to 5× solar), in agreement with previously studied X grains. On a scale of 200 nm, the Si-isotopic ratios do not vary by more than the analytical uncertainties of several percent in all but one X grain. This implies that most X grains formed from well-mixed regions in supernova ejecta. X grain M9-68-3 is characterized by two regions with distinct Si- and Ti-isotopic signatures which may either represent two distinct grains or overgrowth of matter from two different mixtures in the supernova ejecta. Most of the Ca in the X grains is most likely contamination as indicated by close to normal 42Ca/40Ca ratios. Seven X grains show enhanced 44Ca/40Ca ratios of up to 6× the solar ratio. Spatial distributions of 44Ca excesses and Ti are positively correlated, giving strong support to the view that excesses in 44Ca are due to the decay of radioactive 44Ti. Inferred initial 44Ti/48Ti ratios are between 0.01 and 0.28 and are correlated with Si-isotopic ratios. Radiogenic 44Ca is widely distributed in six X grains. X grain M9-132-4 exhibits a pronounced heterogeneity in the distribution of radiogenic 44Ca and 48Ti as well as in 44Ti/48Ti, pointing to presence of a small Ti-rich subgrain or heterogeneous loss of Ca and Ti after grain formation. This grain has a unique Si-isotopic composition with 30Si/29Si = 2.2× the solar ratio and C- and N-isotopic compositions as typically observed in X grains. 相似文献
35.
Darije Maričić Bojan Vršnak Andrew L. Stanger Astrid M. Veronig Manuela Temmer Dragan Roša 《Solar physics》2007,241(1):99-112
We analyze the relationship between the acceleration of coronal mass ejections (CMEs) and the energy release in associated
flares, employing a sample of 22 events in which the CME kinematics were measured from the pre-eruption stage up to the post-acceleration
phase. The data show a distinct correlation between the duration of the acceleration phase and the duration of the associated
soft X-ray (SXR) burst rise, whereas the CME peak acceleration and velocity are related to the SXR peak flux. In the majority
of events the acceleration started earlier than the SXR burst, and it is usually prolonged after the SXR burst maximum. In
about one half of the events the acceleration phase is very closely synchronized with the fastest growth of the SXR burst.
An additional one quarter of the events may be still considered as relatively well-synchronized, whereas in the remaining
quarter of the events there is a considerable mismatch. The results are interpreted in terms of the feedback relationship
between the CME dynamics and the reconnection process in the wake of the CME. 相似文献
36.
Astrid Holstad 《Computational Geosciences》2000,4(2):103-139
We formulate mathematical and numerical models for multispecies, multi-phase and non-isothermal reactive fluid flow in porous
media focusing on the chemical reactions and the transport of solutes. Mass conservation and stability in the time integration
are emphasized. We use cell-centered finite volume differencing in space and an implicit Runge-Kutta method in time. Assuming
two space dimensions, we introduce flux approximation for arbitrarily shaped convex quadrilaterals. On equidistant and variable
sized rectangular grids we choose limited κ=
related schemes to approximate the advective flux and the central difference scheme for the diffusive flux. On non-rectangular
grids we recommend the VF9 scheme for the estimation of the diffusive flux. Our model exists as a code.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
37.
38.
Astrid N. Schwalb Mark S. Poos Josef D. Ackerman 《Aquatic Sciences - Research Across Boundaries》2011,73(2):223-231
Unionid mussels are highly imperiled and the survival of their local populations is linked to the availability and dispersal
potential of their host fish. We examined the displacement distance of logperch (Percina caprodes), which are obligate host fish for endangered snuffbox mussels (Epioblasma triquetra), to determine the dispersal potential by fish. Logperch in the Sydenham River, Ontario, Canada, were electrofished and marked
with visible implant elastomer on five sampling dates during the gravid period of E. triquetra. The majority of all recaptures (82%) of P. caprodes occurred within 30 m of their original capture location, with a mean displacement distance of 13 ± 3 m (mean ± standard error,
n = 28). These results were consistent with a review of movement studies of small benthic host fish (i.e., darters and sculpins),
which revealed average fish displacement distances of 37 ± 19 m (n = 14 species; range: 4–275 m). However, significantly greater movement distances were also found and the maximum displacement
distance increased significantly with the spatial extent of the study and with the duration of the study. These results indicate
that many P. caprodes remain in a small area, which could restrict the dispersal and (re)colonization potential of E. triquetra. Further studies are needed to determine the dispersal potential of mussels via host fish, which may be important for maintaining
the connectivity among unionid populations. 相似文献
39.
Astrid Schnetzer Stefanie D. MoorthiPeter D. Countway Rebecca J. GastIlana C. Gilg David A. Caron 《Deep Sea Research Part I: Oceanographic Research Papers》2011,58(1):16-26
Protistan community structure was examined from 6 depths (1.5, 20, 42, 150, 500, 880 m) at a coastal ocean site in the San Pedro Channel, California. A total of 856 partial length 18S rDNA protistan sequences from the six clone libraries were analyzed to characterize diversity present at each depth. The sequences were grouped into a total of 259 Operational Taxonomic Units (OTUs) that were inferred using an automated OTU calling program that formed OTUs with approximately species-level distinction (95% sequence similarity). Most OTUs (194 out of 259) were observed at only one specific depth, and only two were present in clone libraries from all depths. OTUs were obtained from 21 major protistan taxonomic groups determined by their closest BLAST matches to identified protists in the NCBI database. Approximately 74% of the detected OTUs belonged to the Chromalveolates, with Group II alveolates making up the largest single group. Protistan assemblages at euphotic depths (1.5, 20 and 42 m) were characterized by the presence of clades that contained phototrophic species (stramenopiles, chlorophytes and haptophytes) as well as consumers (especially ciliates). Assemblages in the lower water column (150, 500 and 800 m) were distinct from communities at shallow depths because of strong contributions from taxa belonging to euglenozoans, acantharians, polycystines and Taxopodida (Sticholonche spp. and close relatives). Species richness (Chao I estimate) and diversity (Shannon index) were highest within the euphotic zone and at 150 m, and lowest for protistan assemblages located in the oxygen minimum zone (500 and 880 m). Multivariate analyses (Bray-Curtis coefficient) confirmed that protistan assemblage composition differed significantly when samples were grouped into shallow (≤150 m) and deep water assemblages (≥150 m). 相似文献
40.
Astrid Holzheid Marina V. Charykova Vladimir G. Krivovichev Brendan Ledwig Elena L. Fokina Ksenia L. Poroshina Natalia V. Platonova Vladislav V. Gurzhiy 《Chemie der Erde / Geochemistry》2018,78(2):228-240
Any progress in our understanding of low-temperature mineral assemblages and of quantitative physico-chemical modeling of stability conditions of mineral phases, especially those containing toxic elements like selenium, strongly depends on the knowledge of structural and thermodynamic properties of coexisting mineral phases. Interrelation of crystal chemistry/structure and thermodynamic properties of selenium-containing minerals is not systematically studied so far and thus any essential generalization might be difficult, inaccurate or even impossible and erroneous. Disagreement even exists regarding the crystal chemistry of some natural and synthetic selenium-containing phases. Hence, a systematic study was performed by synthesizing ferric selenite hydrates and subsequent thermal analysis to examine the thermal stability of synthetic analogues of the natural hydrous ferric selenite mandarinoite and its dehydration and dissociation to unravel controversial issues regarding the crystal chemistry. Dehydration of synthesized analogues of mandarinoite starts at 56–87?°C and ends at 226–237?°C. The dehydration happens in two stages and two possible schemes of dehydration exist: (a) mandarinoite loses three molecules of water in the first stage of the dehydration (up to 180?°C) and the remaining two molecules of water will be lost in the second stage (>180?°C) or (b) four molecules of water will be lost in the first stage up to 180?°C and the last molecule of water will be lost at a temperature above 180?°C. Based on XRD measurements and thermal analyses we were able to deduce Fe2(SeO3)3·(6-x)H2O (x?=?0.0–1.0) as formula of the hydrous ferric selenite mandarinoite. The total amount of water apparently affects the crystallinity, and possibly the stability of crystals: the less the x value, the higher crystallinity could be expected. 相似文献