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161.
Allen P. Nutman Vickie C. Bennett Clark R. L. Friend Marc D. Norman 《Contributions to Mineralogy and Petrology》1999,137(4):364-388
In the Itsaq Gneiss Complex south of the Isua supracrustal belt (West Greenland) some areas of early Archaean tonalite and
quartz-diorite are non-gneissic, free of pegmatite veins, and in rarer cases are undeformed with relict igneous textures and
hence were little modified by heterogeneous ductile deformation under amphibolite facies conditions in several Archaean events.
Such well-preserved early Archaean rocks are extremely rare. Tonalites are high Al, and have bulk compositions close to experimental
liquids. Trace element abundances and modelling suggest that they probably originated as melts derived from basaltic compositions
at sufficiently high pressures to require residual garnet + amphibolites ± clinopyroxene in the source. The major element
characteristics of the quartz-diorites suggest these were derived from more mafic magmas than the tonalites, and underwent
either igneous differentiation or mixing with crustal material. As in modern arc magmas, high relative abundances of Sr, Ba,
Pb, and alkali elements cannot be generated simply from a basaltic source formed by large degrees of melting of a depleted
mantle. This may indicate an important role for fluids interacting with mafic rocks in generating the earliest preserved continental
crust. The high Ba/Th, Ba/Nb, La/Nb and low Nb/Th, Ce/Pb, and Rb/Cs ratios of these tonalites are also observed in modern
arc magmas. SHRIMP U-Pb zircon geochronology was undertaken on seven tonalites, one quartz-diorite, a thin pegmatitic vein
and a thin diorite dyke. Cathodoluminescence images show the zircon populations of the quartz-diorite and tonalites are dominated
by single-component oscillatory-zoned prismatic grains, which gave ages of 3806 ± 5 to 3818 ± 8 Ma (2σ) (quartz-diorite and
5 tonalites) and 3795 ± 3 Ma (1 tonalite). Dating of recrystallised domains cutting oscillatory-zoned zircon indicates disturbance
as early as 3800–3780 Ma. There are rare ca. 3600 Ma and 3800–3780 Ma (very high U and low Th/U) ≤ 20 μm wide partial overgrowths
on the prismatic grains. Given likely Zr-undersaturation of precursor melts and evidence of zircon recrystallisation and metamorphic
regrowth as early as 3800–3780 Ma, the age determinations on the prismatic oscillatory-zoned zircon populations give the igneous
crystallisation age of the tonalite and quartz-diorite protoliths. When the coherency of the geochemistry is considered, these
samples represent the best preserved suites of ca. 3800 Ma felsic igneous rocks yet documented.
Received: 1 December 1998 / Accepted: 23 July 1999 相似文献
162.
The formation or generation of hopanes are important processes during both the natural heating of organic-rich sediments and laboratory pyrolysis experiments. Molecular maturity parameters as well as the amounts (ng/g rock) of the C31 hopanes and C30–C32 hopanoic acids were quantified in a Jurassic silty shale horizon (Isle of Skye, Scotland) as a function of distance from an igneous intrusion. The maturity profiles of the homohopanes and the hopanoic acids are comparable. There is also a correlation between the decreasing amounts of C30–C32 hopanoic acids and concomitant increases in C29–C31 hopanes suggesting that free hopanoic acids could be one potential source of hopanes in this particular horizon. Other possible sources could include hopanoic acids that are bound into the macromolecular fraction. 相似文献
163.
macroscale processes that perturb general groundwater chemistry and therefore mineral–water equilibria; and microscale interactions, where attached organisms locally perturb mineral–water equilibria, potentially releasing limiting trace nutrients
from the dissolving mineral.
In the contaminated unconfined glacio-fluvial aquifer near Bemidji, Minnesota, USA, carbonate chemistry is influenced primarily
at the macroscale. Under oxic conditions, respiration by native aerobic heterotrophs produces excess carbon dioxide that promotes
calcite and dolomite dissolution. Aerobic microorganisms do not colonize dolomite surfaces and few occur on calcite. Within
the anoxic groundwater, calcite overgrowths form on uncolonized calcite cleavage surfaces, possibly due to the consumption
of acidity by dissimilatory iron-reducing bacteria. As molecular oxygen concentration increases downgradient of the oil pool,
aerobes again dominate and residual hydrocarbons and ferrous iron are oxidized, resulting in macroscale carbonate-mineral
dissolution and iron precipitation.
Feldspars, in contrast, weather exclusively at the microscale near attached microorganisms, principally in the anoxic region
of the plume. Native organisms preferentially colonize feldspars that contain trace phosphorus as apatite inclusions, apparently
as a consequence of the low P concentration in the groundwater. These feldspars weather rapidly, whereas nearby feldspars
without trace P are uncolonized and unweathered. Feldspar dissolution is accompanied by the precipitation of secondary minerals,
sometimes on the bacterial cell wall itself.
These observations suggest a tightly linked biogeochemical system whereby microbial processes control mineral diagenesis at
many scales of interaction, and the mineralogy and mineral chemistry influence microbial ecology. Only the macroscale interaction,
however, is easily observable by standard geochemical methods, and documentation of the microscale interactions requires microscopic
examination of microorganisms on mineral surfaces and the locally intense diagenetic reactions that result.
Received, May 1999/Revised, October 1999/Accepted, October 1999 相似文献
164.
Early Mississippian sandy siltstones preserve rare vertebrate fossils in seasonal flooding episodes
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Carys E. Bennett Timothy I. Kearsey Sarah J. Davies David Millward Jennifer A. Clack Timothy R. Smithson John E. A. Marshall 《Sedimentology》2016,63(6):1677-1700
Flood‐generated sandy siltstones are under‐recognised deposits that preserve key vertebrate (actinopterygians, rhizodonts, and rarer lungfish, chondrichthyans and tetrapods), invertebrate and plant fossils. Recorded for the first time from the lower Mississippian Ballagan Formation of Scotland, more than 140 beds occur throughout a 490 m thick core succession characterised by fluvial sandstones, palaeosols, siltstones, dolostone ‘cementstones’ and gypsum from a coastal–alluvial plain setting. Sandy siltstones are described as a unique taphofacies of the Ballagan Formation (Scotland, UK); they are matrix‐supported siltstones with millimetre‐sized siltstone and very fine sandstone lithic clasts. Common bioclasts include plants and megaspores, fish, ostracods, eurypterids and bivalves. Fossils have a high degree of articulation compared with those found in other fossil‐bearing deposits, such as conglomerate lags at the base of fluvial channel sandstones. Bed thickness and distribution varies throughout the formation, with no stratigraphic trend. The matrix sediment and clasts are sourced from the reworking of floodplain sediments including desiccated surfaces and palaeosols. Secondary pedogenic modification affects 30% of the sandy siltstone beds and most (71%) overlie palaeosols or desiccation cracks. Sandy siltstones are interpreted as cohesive debris flow deposits that originated by the overbank flooding of rivers and due to localised floodplain sediment transport at times of high rainfall; their association with palaeosols and desiccation cracks indicates seasonally wet to dry cycles throughout the Tournaisian. Tetrapod and fish fossils derived from floodplain lakes and land surfaces are concentrated by local erosion and reworking, and are preserved by deposition into temporary lakes on the floodplain; their distribution indicates a local origin, with sediment transported across the floodplain in seasonal rainfall episodes. These deposits are significant new sites that can be explored for the preservation of rare non‐marine fossil material and provide unique insights into the evolution of early terrestrial ecosystems. 相似文献
165.
166.
Estimation of Biodegradation Rates Using Respiration Tests During In Situ Bioremediation of Weathered Diesel NAPL 总被引:2,自引:1,他引:2
G.B. Davis C.D. Johnston B.M. Patterson C. Barber M. Bennett 《Ground Water Monitoring & Remediation》1998,18(2):123-132
Respiration tests were carried out during a seven month bioremediation field trial to monitor biodegradation rates of weathered diesel non-aqueous phase liquid (NAPL) contaminating a shallow sand aquifer. Multiple depth monitoring of oxygen concentrations and air-filled porosity were carried out in nutrient amended and nonamended locations to assess the variability of degradation rate estimates calculated from respiration tests.
The field trial consisted of periodic addition of nutrients (nitrogen and phosphorus) and aeration of a 100 m2 trial plot. During the bioremediation trial, aeration was stopped periodically, and decreases in gaseous oxygen concentrations were logged semi-continuously using data loggers attached to recently developed in situ oxygen probes placed at multiple depths above and within a thin NAPL-contaminated zone. Oxygen usage rate coefficients were determined by fitting zero-and first-order rate equations to the oxygen concentration reduction curves, although only zero-order rates were used to calculate biodegradation rates. Air-filled porosity estimates were found to vary by up to a factor of two between sites and at different times.
NAPL degradation rates calculated from measured air-filled porosity and oxygen usage rate coefficients ranged up to 69 mg kg-1 day-1 . These rates are comparable to and higher than rates quoted in other studies, despite the high concentrations and weathered state of the NAPL at this test site. For nutrient-amended sites within the trial plot, estimates of NAPL degradation rates were two to three times higher than estimates from nonamended sites. Rates also increased with depth. 相似文献
The field trial consisted of periodic addition of nutrients (nitrogen and phosphorus) and aeration of a 100 m
NAPL degradation rates calculated from measured air-filled porosity and oxygen usage rate coefficients ranged up to 69 mg kg
167.
South Australian rainfall variability and climate extremes 总被引:1,自引:0,他引:1
Rainfall extremes over South Australia are connected with broad-scale atmospheric rearrangements associated with strong meridional
sea surface temperature (SST) gradients in the eastern Indian Ocean. Thirty-seven years of winter radiosonde data is used
to calculate a time series of precipitable water (PW) and convective available potential energy (CAPE) in the atmosphere.
Principle component analysis on the parameters of CAPE and PW identify key modes of variability that are spatially and seasonally
consistent with tropospheric processes over Australia. The correlation of the leading principle component of winter PW to
winter rainfall anomalies reveal the spatial structure of the northwest cloudband and fronts that cross the southern half
of the continent during winter. Similarly the second and third principle components, respectively, reveal the structures of
the less frequent northern and continental cloudbands with remarkable consistency. 850 hPa-level wind analysis shows that
during dry seasons, anomalous offshore flow over the northwest of Australia inhibits advection of moisture into the northwest,
while enhanced subsidence from stronger anticyclonic circulation over the southern half of the continent reduces CAPE. This
coincides with a southward shift of the subtropical ridge resulting in frontal systems passing well to the south of the continent,
thus producing less frequent interaction with moist air advected from the tropics. Wet winters are the reverse, where a weaker
meridional pressure gradient to the south of the continent allows rain-bearing fronts to reach lower latitudes. The analysis
of SSTs in the Indian Ocean indicate that anomalous warm (cool) waters in the southeast Indian Ocean coincide with a southward
(northward) shift in the subtropical ridge during dry (wet) seasons. 相似文献
168.
J. V. Buckle R. E. Hills H. Smith W. R. F. Dent G. Bell E. I. Curtis R. Dace H. Gibson S. F. Graves J. Leech † J. S. Richer R. Williamson ‡ S. Withington G. Yassin † R. Bennett P. Hastings I. Laidlaw J. F. Lightfoot T. Burgess P. E. Dewdney G. Hovey A. G. Willis R. Redman B. Wooff D. S. Berry B. Cavanagh G. R. Davis J. Dempsey P. Friberg T. Jenness R. Kackley N. P. Rees § R. Tilanus C. Walther W. Zwart T. M. Klapwijk M. Kroug T. Zijlstra 《Monthly notices of the Royal Astronomical Society》2009,399(2):1026-1043
This paper describes a new Heterodyne Array Receiver Program (HARP) and Auto-Correlation Spectral Imaging System (ACSIS) that have recently been installed and commissioned on the James Clerk Maxwell Telescope. The 16-element focal-plane array receiver, operating in the submillimetre from 325 to 375 GHz, offers high (three-dimensional) mapping speeds, along with significant improvements over single-detector counterparts in calibration and image quality. Receiver temperatures are ∼120 K across the whole band, and system temperatures of ∼300 K are reached routinely under good weather conditions. The system includes a single-sideband (SSB) filter so these are SSB values. Used in conjunction with ACSIS, the system can produce large-scale maps rapidly, in one or more frequency settings, at high spatial and spectral resolution. Fully sampled maps of size can be observed in under 1 h.
The scientific need for array receivers arises from the requirement for programmes to study samples of objects of statistically significant size, in large-scale unbiased surveys of galactic and extra-galactic regions. Along with morphological information, the new spectral imaging system can be used to study the physical and chemical properties of regions of interest. Its three-dimensional imaging capabilities are critical for research into turbulence and dynamics. In addition, HARP/ACSIS will provide highly complementary science programmes to wide-field continuum studies and produce the essential preparatory work for submillimetre interferometers such as the Submillimeter Array (SMA) and Atacama Large Millimeter/Submillimeter Array (ALMA). 相似文献
The scientific need for array receivers arises from the requirement for programmes to study samples of objects of statistically significant size, in large-scale unbiased surveys of galactic and extra-galactic regions. Along with morphological information, the new spectral imaging system can be used to study the physical and chemical properties of regions of interest. Its three-dimensional imaging capabilities are critical for research into turbulence and dynamics. In addition, HARP/ACSIS will provide highly complementary science programmes to wide-field continuum studies and produce the essential preparatory work for submillimetre interferometers such as the Submillimeter Array (SMA) and Atacama Large Millimeter/Submillimeter Array (ALMA). 相似文献
169.
Sarah A. Bennett Olivier Rouxel Katja Schmidt Dieter Garbe-Schnberg Peter J. Statham Christopher R. German 《Geochimica et cosmochimica acta》2009,73(19):5619-5634
Fe isotopes are a potential tool for tracing the biogeochemical redox cycle of Fe in the ocean. Specifically, it is hypothesized that Fe isotopes could enable estimation of the contributions from multiple Fe sources to the dissolved Fe budget, an issue that has received much attention in recent years. The first priority however, is to understand any Fe isotope fractionation processes that may occur as Fe enters the ocean, resulting in modification of original source compositions. In this study, we have investigated the Fe inputs from a basalt-hosted, deep-sea hydrothermal system and the fractionation processes that occur as the hot, chemically reduced and acidic vent fluids mix with cold, oxygen-rich seawater.The samples collected were both end-member vent fluids taken from hydrothermal chimneys, and rising buoyant plume samples collected directly above the same vents at 5°S, Mid-Atlantic Ridge. Our analyzes of these samples reveal that, for the particulate Fe species within the buoyant plume, 25% of the Fe is precipitated as Fe-sulfides. The isotope fractionation caused by the formation of these Fe-sulfides is δFe(II)–FeS = +0.60 ± 0.12‰.The source isotope composition for the buoyant plume samples collected above the Red Lion vents is calculated to be −0.29 ± 0.05‰. This is identical to the value measured in end-member vent fluids collected from the underlying “Tannenbaum” chimney. The resulting isotope compositions of the Fe-sulfide and Fe-oxyhydroxide species in this buoyant plume are −0.89 ± 0.11‰ and −0.19 ± 0.09‰, respectively. From mass balance calculations, we have been able to calculate the isotope composition of the dissolved Fe fraction, and hypothesize that the isotope composition of any stabilised dissolved Fe species exported to the surrounding ocean may be heavier than the original vent fluid. Such species would be expected to travel some distance from areas of hydrothermal venting and, hence, contribute to not only the dissolved Fe budget of the deep-ocean but also it’s dissolved Fe isotope signature. 相似文献
170.
Mark Williams Vincent Perrier Carys Bennett Thomas Hearing Christopher Stocker Tom Harvey 《Geology Today》2015,31(5):193-200
Ostracods are tiny crustacean arthropods just a few millimetres long, with a bivalved carapace made of calcium carbonate that covers the whole body, and into which the animal can retreat from the world outside. Because of their diminutive size they are largely overlooked as fossils, but they have a fascinating history. Silent witnesses to life in the seas since the time of trilobites, they have a fossil record extending back to the Early Ordovician, and possibly the Cambrian. Ostracods have survived nearly 500 million years of Earth history including the ‘big five’ mass extinctions of the Phanerozoic Eon; they are true survivors. They are almost perfectly adapted for the aquatic environments in which they live, and can be found from the ocean abyssal plains to damp leaf litter. The ostracod carapace is a triumph of biological engineering that has been re‐configured into myriad different morphologies according to environment. Streamlined and agile species plough through the ocean water column, sometimes reaching a ‘giant’ size of a centimetre in length, whilst their tinier sea bottom cousins make elaborately ornamented carapaces to withstand the pressures of living at the seabed, or shape their carapaces into forms that facilitate burrowing into sediment. Ostracods are key components of aquatic ecosystems. As primary consumers they are food for larger animals both in seabed and planktonic habitats, and they recycle much of the organic detritus produced by larger animals and plants. Delve into the history of ostracods and it is possible to find pioneers who triumphed in the plankton, early colonisers of terrestrial aquatic ecosystems, and ostracods that literally conquered the land. And in more recent times, ostracods have even hitched rides on rockets into space. 相似文献