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301.
Hydrogeology Journal - Ancient river channels or subglacial drainage networks infilled with younger sediments can include significant deposits of highly permeable sands and gravels. Despite being... 相似文献
302.
Artur?P.?DeditiusEmail author Martin?Reich Adam?C.?Simon Alexandra?Suvorova Jaayke?Knipping Malcolm?P.?Roberts Sergey?Rubanov Aaron?Dodd Martin?Saunders 《Contributions to Mineralogy and Petrology》2018,173(6):46
Magnetite from hydrothermal ore deposits can contain up to tens of thousands of parts per million (ppm) of elements such as Ti, Si, V, Al, Ca, Mg, Na, which tend to either structurally incorporate into growth and sector zones or form mineral micro- to nano-sized particles. Here, we report micro- to nano-structural and chemical data of hydrothermal magnetite from the Los Colorados iron oxide–apatite deposit in Chile, where magnetite displays both types of trace element incorporation. Three generations of magnetites (X–Z) were identified with concentrations of minor and trace elements that vary significantly: SiO2, from below detection limit (bdl) to 3.1 wt%; Al2O3, 0.3–2.3 wt%; CaO, bdl–0.9 wt%; MgO, 0.02–2.5 wt%; TiO2, 0.1–0.4 wt%; MnO, 0.04–0.2 wt%; Na2O, bdl–0.4 wt%; and K2O, bdl–0.4 wt%. An exception is V2O3, which is remarkably constant, ranging from 0.3 to 0.4 wt%. Six types of crystalline nanoparticles (NPs) were identified by means of transmission electron microscopy in the trace element-rich zones, which are each a few micrometres wide: (1) diopside, (2) clinoenstatite; (3) amphibole, (4) mica, (5) ulvöspinel, and (6) Ti-rich magnetite. In addition, Al-rich nanodomains, which contain 2–3 wt% of Al, occur within a single crystal of magnetite. The accumulation of NPs in the trace element-rich zones suggest that they form owing to supersaturation from a hydrothermal fluid, followed by entrapment during continuous growth of the magnetite surface. It is also concluded that mineral NPs promote exsolution of new phases from the mineral host, otherwise preserved as structurally bound trace elements. The presence of abundant mineral NPs in magnetite points to a complex incorporation of trace elements during growth, and provides a cautionary note on the interpretation of micron-scale chemical data of magnetite. 相似文献
303.
The Atmospheric Radiation Measurement (ARM) Program Southern Great Plains (SGP) site has a rich history of actively sensed cloud observations. Fourteen years (1997–2010) of observations from the Millimeter Cloud Radar (MMCR), Micropulse Lidar (MPL), and Belfort/Vaisala Ceilometers are used to understand how instrument selection and sampling impacts estimates of Cloud Fraction (CF) at this location. Although all instruments should be used in combination for the best estimates of CF, instrument downtime limits available samples and increases observational errors, demanding that users make sacrifices when calculating CF at longer intervals relevant to climate studies. Selection of MMCR or MMCR + MPL cloud masks changes very little in the overall understanding of total CF. Addition of the MPL increases the 14-year average CF by 9 %, mainly through an increase in optically thin high clouds year-round, and mid-level clouds during the summer months. Splitting the period into two equal 7-year periods reveals negligible change in MMCR + MPL CF. For the MMCR, however, CF deceases by 6.1 %. This sudden change in CF occurs around the time the radar was upgraded, suggesting that this decrease is tied to hardware sensitivity or scanning strategy changes. Users must be cognizant of this and other issues when calculating CF from the variety of observations available at the ARM SGP site. 相似文献
304.
Li isotopes and trace elements as a petrogenetic tracer in zircon: insights from Archean TTGs and sanukitoids 总被引:1,自引:0,他引:1
Anne-Sophie Bouvier Takayuki Ushikubo Noriko T. Kita Aaron J. Cavosie Reinhard Kozdon John W. Valley 《Contributions to Mineralogy and Petrology》2012,163(5):745-768
We report δ7Li, Li abundance ([Li]), and other trace elements measured by ion probe in igneous zircons from TTG (tonalite, trondhjemite,
and granodiorite) and sanukitoid plutons from the Superior Province (Canada) in order to characterize Li in zircons from typical
Archean continental crust. These data are compared with detrital zircons from the Jack Hills (Western Australia) with U–Pb
ages greater than 3.9 Ga for which parent rock type is not known. Most of the TTG and sanukitoid zircon domains preserve typical
igneous REE patterns and CL zoning. [Li] ranges from 0.5 to 79 ppm, typical of [Li] in continental zircons. Atomic ratios
of (Y + REE)/(Li + P) average 1.0 ± 0.7 (2SD) for zircons with magmatic composition preserved, supporting the hypothesis that
Li is interstitial and charge compensates substitution of trivalent cations. This substitution results in a relatively slow
rate of Li diffusion. The δ7Li and trace element data constrain the genesis of TTGs and sanukitoids. [Li] in zircons from granitoids is significantly
higher than from zircons in primitive magmas in oceanic crust. TTG zircons have δ7Li (3 ± 8‰) and δ18O in the range of primitive mantle-derived magmas. Sanukitoid zircons have average δ7Li (7 ± 8‰) and δ18O higher than those of TTGs supporting genesis by melting of fluid-metasomatized mantle wedge. The Li systematics in sanukitoid
and TTG zircons indicate that high [Li] in pre-3.9-Ga Jack Hills detrital zircons is a primary igneous composition and suggests
the growth in proto-continental crust in magmas similar to Archean granitoids. 相似文献
305.
A hydrogeological study was conducted in northwestern New Brunswick, Canada, to improve the predictability of fracture-dominated groundwater flow within folded bedrock composed of fine-grained turbidites. Borehole televiewer logging and outcrop mapping, integrated with hydraulic packer tests revealed enhanced hydraulic conductivity associated with northeasterly striking bedding-plane fractures formed during folding and flexural slip. These fractures impart azimuthal anisotropy to the aquifer because of moderately dipping fold limbs. High-angle fractures form a well-developed non-stratabound network, comprising two open fracture sets striking NNE parallel to the current direction of principal stress, and WNW parallel to the direction of principal stress that dominated during the Acadian orogeny. The subset of fractures showing significant oxidation, deemed most important to the groundwater flow system, is dominated by bedding-plane and high-angle fractures striking near-parallel to the maximum principal stress direction, resulting in extensional opening and enhanced hydraulic conductivities. An equivalent porous media model, incorporating anisotropy and varying hydraulic conductivity with depth, indicates that horizontal flow dominates the aquifer with relatively minor exchange between different model layers. These findings have implications for understanding flow directions in the Black Brook Watershed and elsewhere in the Matapédia Basin where fractures formed under similar stress conditions. 相似文献
306.
Roger G. Young Aaron J. Quarterman Rebekah F. Eyles Robert A. Smith William B. Bowden 《新西兰海洋与淡水研究杂志》2013,47(4):803-825
We examined the effects of land use, geology, and longitudinal position within the river network on water quality and thermal regime at 23 sites within the Motueka River catchment. The concentrations of suspended solids, nitrate nitrogen, total nitrogen, Escherichia coli, and Campylobacter were higher at sites draining pastoral and horticultural land than in similar‐sized native or plantation forest streams. Average daily mean temperature and minimum temperature in summer and maximum winter temperature were higher in unshaded pastoral and horticultural streams than at native forest sites. Differences in water quality and thermal regime were also observed among sites with contrasting geology. Conductivity, pH, and minimum winter temperatures were highest at sites draining marble terrain. In contrast, longitudinal patterns in water quality and temperature regime along the 120‐km length of the river were relatively weak, although longitudinal patterns in amplitude of daily temperature fluctuation matched theoretical predictions. In this study, differences in land use appeared to have the strongest influence on most water quality and thermal variables examined. However, geology was an important factor explaining variation in certain variables (e.g., pH and conductivity). Longitudinal patterns in water quality and temperature were relatively weak and in many instances were linked with longitudinal patterns in land use and geology rather than catchment location alone. 相似文献
307.
Craig L. Hebert Alan J. Kaufman Sarah C. Penniston-Dorland Aaron J. Martin 《Precambrian Research》2010
Radiometric constraints on mid-Ediacaran Period glaciation (Gaskiers) in Newfoundland narrowed the known temporal gap between widespread ice ages and the evolution of complex metazoans to several million years. To further evaluate this claim we studied an Ediacaran glacial diamictite at the base of the Fauquier Formation of northern Virginia, and discovered a conformable relationship between the post-glacial cap carbonate and overlying volcanic rocks of the Catoctin Formation. U/Pb zircon age constraints for the rift-related volcanic flows suggest initial emplacement around 571 million years ago. Application of the Catoctin age to the Fauquier succession indicates the occurrence of an ice age about 10 million years younger than the 582 Ma Gaskiers event, supporting the view of multiple Ediacaran Period glaciations. Furthermore, the age constraint from eastern Laurentia falls within radiometric uncertainty of fossiliferous strata in Avalonia, indicating that the Fauquier glaciation was coincident with early metazoan evolution. 相似文献
308.
Scott A. Rush Jill A. Olin Aaron T. Fisk Mark S. Woodrey Robert J. Cooper 《Estuaries and Coasts》2010,33(4):963-970
Much of North America’s tidal marsh habitat has been significantly altered by both natural and man-made processes. Thus, there
is a need to understand the trophic ecology of organisms endemic to these ecosystems. We applied carbon (δ
13C) and nitrogen (δ
15N) stable isotope analysis, along with isotope mixing models, to egg yolk, liver, and muscle tissues of clapper rails (Rallus longirostris) and their likely prey items. This analysis enabled us to explore variation in trophic niche and diet composition in this
important marsh bird in two northern Gulf of Mexico tidal marshes that are river and ocean-dominated. For the river-associated
estuary, δ
13C and δ
15N of egg yolks, liver, and pectoral muscle tissue samples provided evidence that clapper rails maintained a similar diet during
both the winter and the breeding season. A trophic link between C3 primary productivity and the clapper rail’s diet was also indicated as the δ
13C of clapper rail egg yolks related negatively with the aerial cover of C3 macrophytes. Clapper rails from the ocean-dominated estuary had a narrower trophic niche and appeared to be utilizing marine
resources, particularly, based on modeling of liver stable isotope values. Variation in stable isotope values between egg
yolk and liver/muscle in both systems suggests that endogenous resources are important for egg production in clapper rails.
These results demonstrate that diet composition, prey source, and niche width of clapper rails can vary significantly across
different estuaries and appear to be influenced by hydrological conditions. 相似文献
309.
Iron isotope compositions of carbonatites record melt generation, crystallization, and late-stage volatile-transport processes 总被引:2,自引:0,他引:2
Clark M. Johnson Keith Bell Brian L. Beard Aaron I. Shultis 《Mineralogy and Petrology》2010,98(1-4):91-110
Carbonatites define the largest range in Fe isotope compositions yet measured for igneous rocks, recording significant isotopic fractionations between carbonate, oxide, and silicate minerals during generation in the mantle and subsequent differentiation. In contrast to the relatively restricted range in δ56Fe values for mantle-derived basaltic magmas (δ56Fe?=?0.0?±?0.1‰), calcite from carbonatites have δ56Fe values between ?1.0 and +0.8‰, similar to the range defined by whole-rock samples of carbonatites. Based on expected carbonate-silicate fractionation factors at igneous or mantle temperatures, carbonatite magmas that have modestly negative δ56Fe values of ~ ?0.3‰ or lower can be explained by equilibrium with a silicate mantle. More negative δ56Fe values were probably produced by differentiation processes, including crystal fractionation and liquid immiscibility. Positive δ56Fe values for carbonatites are, however, unexpected, and such values seem to likely reflect interaction between low-Fe carbonates and Fe3+-rich fluids at igneous or near-igneous temperatures; the expected δ56Fe values for Fe2+-bearing fluids are too low to produced the observed positive δ56Fe values of some carbonatites, indicating that Fe isotopes may be a valuable tracer of redox conditions in carbonatite complexes. Further evidence for fluid-rock or fluid-magma interactions comes from the common occurrence of Fe isotope disequilibrium among carbonate, oxide, silicate, and sulfide minerals in the majority of the carbonatites studied. The common occurrence of Fe isotope disequilibrium among minerals in carbonatites may also indicate mixing of phenocyrsts from distinct magmas. Expulsion of Fe3+-rich brines into metasomatic aureols that surround carbonatite complexes are expected to produce high-δ56Fe fenites, but this has yet to be tested. 相似文献