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201.
Françoise Vimeux Robert Gallaire Georg Hoffmann 《Earth and Planetary Science Letters》2005,240(2):205-220
Controversy has surrounded the interpretation of the water isotopic composition (δD or δ18O) in tropical and subtropical ice cores in South America. Although recent modeling studies using AGCM have provided useful constraints at interannual time scales, no direct calibration based on modern observations has been achieved. In the context of the recent ice core drilling at Nevado Illimani (16°39′S-67°47′W) in Bolivia, we examine the climatic controls on the modern isotopic composition of precipitation in the Zongo Valley, located on the northeast side of the Cordillera Real, at about 55 km from Nevado Illimani. Monthly precipitation samples were collected from September 1999 to August 2004 at various altitudes along this valley. First we examine the local and regional controls on the common δD signal measured along this valley. We show that (1) local temperature has definitely no control on δD variations, and (2) local rainout is a poor factor to explain δD variations. We thus seek regional controls upstream the Valley potentially affecting air masses distillation. Based on backtrajectory calculations and using satellite data (TRMM precipitation, NOAA OLR) and direct observations of precipitation (IAEA/GNIP), we show that moisture transport history and the degree of rainout upstream are more important factors explaining seasonal δD variations. Analysis of a 92-yr simulation from the ECHAM-4 model (T30 version) implemented with water stable isotopes confirms our observations at seasonal time scale and emphasize the role of air masses distillation upstream as a prominent factor controlling interannual δD variations. Lastly, we focus on the isotopic depletion along the valley when air masses are lifted up. Our results suggest that, if the temperature gradient between the base and the top of the Andes was higher by a few degrees during the Last Glacial Maximum (LGM), less than 10% of the glacial to interglacial isotopic variation recorded in the Illimani ice core could be accounted for by this temperature change. It implies that the rest of the variation would originate from wetter conditions along air masses trajectory during LGM. 相似文献
202.
New trends in marine chemical ecology 总被引:1,自引:0,他引:1
A. Ianora M. Boersma R. Casotti A. Fontana J. Harder F. Hoffmann H. Pavia P. Potin S. A. Poulet G. Toth 《Estuaries and Coasts》2006,29(4):531-551
This essay is the outcome of a colloquium convened in November 2005 at the Benthos Laboratory of the Stazione Zoologica Anton
Dohrn in Ischia, Italy, on chemical ecology and the role of secondary metabolites in the structuring and functioning of marine
biodiversity. The participants of the workshop are part of the European Network of Excellence MarBEF (Marine Biodiversity
and Ecosystem Function), a consortium of 56 European marine institutes to integrate and disseminate knowledge and expertise
on marine biodiversity. Here we review some of the new trends and emerging topics in marine chemical ecology. The first section
deals with microbial chemical interactions. Microbes communicate with each other using diffusible molecules such as N-acylhomoserine
lactones (AHL). These are regulators in cell-density-dependent gene regulation (quorum sensing) controlling microbial processes.
In chemical interactions with higher organisms, microbes can act either as harmful pathogens that are repelled by the host’s
chemical defense or as beneficial symbionts. These symbionts are sometimes the true producers of the host’s secondary metabolites
that have defensive and protective functions for their hosts. We also describe how allelochemicals can shape phytoplankton
communities by regulating competition for available resources, and also interactions among individuals of the same species.
Compounds such as the diatom-derived unsaturated aldehydes have been demonstrated to act as info chemicals, and they possibly
function as a diffusible bloom-termination signal that triggers an active cell death and bloom termination at sea. The same
molecules have also been shown to interfere with the reproductive capacity of grazing animals deterring future generations
of potential predators. Such compounds differ from those that act as feeding deterrents since they do not target the predator
but its offspring. Many of the neurotoxins produced by dinoflagellates act as feeding deterrents, and laboratory experiments
have shown that ingestion of these algae by some microzooplankton and macrozooplankton can cause acute, responses such as
death, incapacitation, altered swimming behavior, and reduced fecundity and egg-hatching success. These effects may rarely
occur in nature because of low individual grazing rates on dinoflagellate cells and grazing on other food sources such as
microflagellates and diatoms. We also consider the nutritional component of marine plant-herbivore interactions, especially
in the plankton, and the information available on the effects of growing conditions of algae on the production of toxic metabolites.
Species producing saxitoxins seem to consistently produce the highest amounts of toxins (on a per cell basis) in the exponential
phase of growth, and there is a decrease in their production under nitrogen, but not under phosphorus stress, where the production
actually increases. We try to explain the circumstances under which organisms defend themselves chemically and argue that
the most likely explanatory model for the production of secondary metabolites used for defense in planktonic organisms is
the carbon nutrient balance hypothesis, which predicts that most algae produce their toxins mainly under conditions where
carbon is in excess and nitrogen (or other nutrients) is limiting. We also discuss chemically mediated macroalgal-herbivore
interactions in the benthos and the large variation in concentration of seaweed defense metabolites at different spatial and
temporal scales. Seaweeds have been shown to produce a large variety of secondary metabolites with highly variable chemical
structures such as terpenoids, acetogenins, amino acid derivates, and polyphenols. Many of these compounds probably have multiple
simultaneous functions for the seaweeds and can act as allelopathic, antimicrobial, and antifouling or ultraviolet-screening
agents, as well as herbivore deterrents. We also provide examples of interactions between marine benthic invertebrates, especially
sponges, molluscs, and cnidarians, that are mediated by specific secondary metabolites and discuss the role of these in shaping
benthic communities. 相似文献
203.
M. Knab V. Hoffmann E. Petrovský A. Kapička N. Jordanova E. Appel 《Environmental Geology》2006,49(4):527-535
Measuring magnetic susceptibility is a method which is used to estimate the amount of magnetic particles in soils, sediments
or dusts. Changes in magnetic susceptibility can be due to various reasons: input from different sources of sediments, e.g.
from different soils or rocks, atmospheric fallout of anthropogenic dusts containing magnetic particles produced by fossil
fuel combustion, steel production or road traffic. In the case of river sediments, input from the catchment is of primary
significance. The main aim of this investigation was to test the potential of magnetic susceptibility screening in identifying
the effect and significance of anthropogenic activities in an area with complex geological conditions. We investigated the
magnetic susceptibility of riverbed sediments of the largest river of the Czech Republic, the Moldau river. Besides that,
the magnetic signal of nearby topsoils as well as of outcropping bedrocks in the vicinity of the river was examined. In the
upper 300 km of the river, the magnetic enhancement of the river sediments can be linked to anthropogenic activities. Positive
correlations were found in the river sediments between the contents of Cu and Zn and magnetic susceptibility, while Fe, Mn
and Ni did not show a correlation with magnetic susceptibility. However, the major geogenic magnetic anomaly in the area around
the Slapy dam has made it impossible to unambiguously interpret the magnetic signal in terms of anthropogenic impact in the
last 80 km downstream. 相似文献
204.
Dr. Christoph Hoffmann 《Contributions to Mineralogy and Petrology》1970,27(4):283-320
In the southern Apennin (= northern part of the region dealt with) and the Coasta Chain (= southern part) there are metabasalts wich are classified in the northern part as:
- Glaucophane rocks of the albite-lawsonite-glaucophane-subfacies with the assemblage glaucophane + pumpellyite + lawsonite ±albite ±aragonite ±muscovite (7 rock analyses, 8 mineral analyses). These rocks are conceived as relics of an older burial metamorphism.
- Rocks with pumpellyite and chlorite or also chlorite alone, that are interpreted as reaction rims between the metastable glaucophane rocks and the country rock (phyllites, quartzites). The assemblages pumpellyite + chlorite and chlorite alone are to be found (2 rock analyses and 2 mineral analyses).
- Rocks with lawsonite and/or epidote belong to the same mineral facies as the country rock: a facies similar to the greenschist facies (called “lawsonite-albite-chlorite-subfacies”) which is characterized by the assemblages lawsonite + albite + chlorite ±calcite and also epidote ±lawsonite + albite + chlorite ± muscovite. These types are attributed to a younger dynamo-metamorphism (2 rock analyses).
205.
Thomas Hoffmann 《Mineralium Deposita》1969,4(3):260-274
Several small barite deposits of Devonian age are known in the Monts de Cabrières region, Montagne Noire (southern France). A field and laboratory investigation of these stratabound deposits showed their possible diagenetic origin and a limited economic value.
...es ist zwar nicht sinnlos, aber doch etwas unlogisch, wenn man von vornherein nicht versucht, eine Lagerstätte in ihre Umgebung auf die wahrscheinlichste Art einzugliedern, sondern sie unbedingt einem ganz fremden, zunächst nicht von selbst verständlichen Bildungsvorgang zuordnen will. H. Schneiderhöhn (1954) 相似文献
Zusammenfassung De petits gisements de barytine du Dévonien dans les Monts de Cabrières (Montagne Noire, France) ont été étudié en détail sur le terrain et au laboratoire. Les observations ont apporté des critères pour une explication génétique de ces gisements «stratiformes». Ils indiquent que les concentrations minérales se sont effectuées par un processus de sécrétion au cours de la diagenèse. L'importance économique de ces gisements est limitée.
...es ist zwar nicht sinnlos, aber doch etwas unlogisch, wenn man von vornherein nicht versucht, eine Lagerstätte in ihre Umgebung auf die wahrscheinlichste Art einzugliedern, sondern sie unbedingt einem ganz fremden, zunächst nicht von selbst verständlichen Bildungsvorgang zuordnen will. H. Schneiderhöhn (1954) 相似文献
206.
Elevated concentrations of S(IV) and formaldehyde were observed in fog- and cloudwater at sites in California. The highest concentrations (up to 3 mM S(IV) and 0.7 mM CH2O) were measured at Bakersfield, during a prolonged period of repeated fog. In Bakersfield [S(IV)] generally exceeded [CH2O], while in the Los Angeles area the reverse was observed. The lowest concentrations of both species were observed at marine and high altitude sites away from local emissions. Equilibrium computations indicate that high concentrations of S(IV) cannot be achieved without the formation of S(IV)-RCHO adducts. 相似文献
207.