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101.
Mary R. Reid Jorge A. Vazquez Axel K. Schmitt 《Contributions to Mineralogy and Petrology》2011,161(2):293-311
Zircon has the outstanding capacity to record chronological, thermal, and chemical information, including the storage history
of zoned silicic magma reservoirs like the one responsible for the Bishop Tuff of eastern California, USA. Our novel ion microprobe
approach reveals that Bishop zircon rims with diverse chemical characteristics surround intermediate domains with broadly
similar compositions. The highest Y, REE, U, and Th concentrations tend to accompany the largest excesses in Y + REE3+:P beyond what can be explained by xenotime substitution in zircon. Apparent Ti-in-zircon temperatures of <720°C for zircon
rims are distinctly lower than most of the range in eruption temperatures, as estimated from FeTi-oxide equilibria and zircon
solubility at quench. While permissive of crystallization of zircon at near-solidus conditions, the low Ti-in-zircon temperatures
are probably better explained by sources of inaccuracy in the temperature estimates. After apparently nucleating from different
melts, zircons from across the Bishop Tuff compositional spectrum may have evolved to broadly similar chemical and thermal
conditions and therefore it is possible that there was no significant thermal gradient in the magma reservoir at some stage
in its evolution. There is also no compelling evidence for punctuated heat ± chemical influxes during the intermediate stages
of zircon growth. Judging by the zircon record, the main volume of the erupted magma evolved normally by secular cooling but
the latest erupted portion is characterized by a reversal in chemistry that appears to indicate perfusion of the magma reservoir
by—or zircon entrainment in—a less evolved melt from the one in which the zircons had previously resided. 相似文献
102.
Plant communities on semi-arid floodplains are ecologically important and support a diverse local and regional fauna and often pastoral economies. Water resource development may affect these communities and economies by decreasing water supply; determining the nature of these relationships is not straightforward because of the complex nature of plant responses to wetting and possible interactions with other drivers. We investigate the effects of reduced wetting on vegetation by examining spatial patterns in plant communities and above-ground herbaceous plant biomass across a flood frequency gradient, geomorphic settings and grazing exclosures. Community and biomass changes were also examined over time in relation to wetting events. The results demonstrate the importance of wetting on plant communities across timescales. At longer timescales, flood frequency influences community composition; at shorter timescales, wetting increase plant biomass and has a secondary influence on community composition. Plant biomass is also influenced in the short-term by grazing, but there is little influence of grazing on community composition. Soil nutrients do not vary systematically across the floodplain and have little influence on species distributions. We conclude that reduced water availability due to water resource development will result in reduced productivity in the short-term and community composition changes in the long-term. 相似文献
103.
Saleemul Huq Hannah Reid Mama Konate Atiq Rahman Youba Sokona Florence Crick 《Climate Policy》2013,13(1):25-43
Abstract The Least Developed Countries (LDCs) are a group of 49 of the world's poorest countries. They have contributed least to the emission of greenhouse gases (GHGs) but they are most vulnerable to the effects of climate change. This is due to their location in some of the most vulnerable regions of the world and their low capacities to adapt to these changes. Adaptation to climate change has become an important policy priority in the international negotiations on climate change in recent years. However, it has yet to become a major policy issue within developing countries, especially the LDCs. This article focuses on two LDCs, namely Bangladesh and Mali, where progress has been made regarding identifying potential adaptation options. For example, Bangladesh already has effective disaster response systems, and strategies to deal with reduced freshwater availability, and Mali has a well-developed programme for providing agro-hydro-meteorological assistance to communities in times of drought. However, much remains to be done in terms of mainstreaming adaptation to climate change within the national policymaking processes of these countries. Policymakers need targeting and, to facilitate this, scientific research must be translated into appropriate language and timescales. 相似文献
104.
Chromophoric Dissolved Organic Matter (CDOM) In Rainwater,Southeastern North Carolina,USA 总被引:3,自引:0,他引:3
Robert?J.?KieberEmail author Robert?F.?Whitehead Seth?N.?Reid Joan?D.?Willey Pamela?J.?Seaton 《Journal of Atmospheric Chemistry》2006,54(1):21-41
The abundance and optical characteristics of dissolved organic matter (DOM) were determined in 120 rain samples collected in Wilmington, North Carolina, USA, between February 21, 2002 and August 11, 2003. All rainwater samples contained chromophoric dissolved organic matter (CDOM) as well as fluorescent compounds. The absorbance spectra of CDOM in the samples decreased exponentially with wavelength with little or no measurable absorbance past 550 nm. Fluorescence excitation emission spectra (EEMS) of the precipitation revealed the presence of four major peaks indicating both terrestrial and marine influences. There was a strong positive correlation between total integrated fluorescence and the absorbance coefficient at 300 nm in rainwater samples, suggesting that these optical properties are directly interrelated and that the compounds responsible for absorbance may be the same as those responsible for fluorescence. Air-mass back-trajectory analysis indicated elevated CDOM levels in continentally influenced rainwater relative to marine dominated events implying that anthropogenic and/or terrestrial sources are important contributors to CDOM levels in precipitation. The presence of highly absorbing and fluorescing CDOM in rainwater has significant ramifications in atmospheric chemistry and may play a previously unrecognized role in the wavelength dependent spectral attenuation of solar radiation by atmospheric waters. 相似文献
105.
DJ Karoly JA Cohen GA Meehl JFB Mitchell AH Oort RJ Stouffer RT Wetherald 《Climate Dynamics》1994,10(1-2):97-105
As an example of the technique of fingerprint detection of greenhouse climate change, a multivariate signal or fingerprint of the enhanced greenhouse effect is defined using the zonal mean atmospheric temperature change as a function of height and latitude between equilibrium climate model simulations with control and doubled CO2 concentrations. This signal is compared with observed atmospheric temperature variations over the period 1963 to 1988 from radiosonde-based global analyses. There is a significant increase of this greenhouse signal in the observational data over this period.These results must be treated with caution. Upper air data are available for a short period only, possibly too short to be able to resolve any real greenhouse climate change. The greenhouse fingerprint used in this study may not be unique to the enhanced greenhouse effect and may be due to other forcing mechanisms. However, it is shown that the patterns of atmospheric temperature change associated with uniform global increases of sea surface temperature, with El NinoSouthern Oscillation events and with decreases of stratospheric ozone concentrations individually are different from the greenhouse fingerprint used here. 相似文献
106.
Two Karroo dolerite sills display chemical and mineralogical variation compatible with cumulus enrichment. The Blaauwkrans sill is an olivine tholeiite and contains a central zone slightly enriched in olivine, plagioclase and clinopyroxene. The thicker Hangnest sill is a quartz tholeiite and shows evidence of crystal settling and has a lower zone enriched in cumulus orthopyroxene and plagioclase.The two sills differ quite markedly in their trace element compositions, with the Hangnest magma enriched by a factor of two in LIL elements (Rb, Ba, Nb, Zr, Y) relative to the Blaauwkrans magma. The Hangnest magma contained extremely low Ni contents (3–5 ppm), whereas the Blaauwkrans magma contained higher but more normal Ni (100–110ppm). Such contrasting trace element compositions preclude any simple genetic relationship between the two Karroo magmas but they may be related either through a common parent or are derivatives from separate parental magmas.South African Contribution No. 24 to the International Geodynamics Project 相似文献
107.
L. H. Bowen E. DeGrave D. A. Reid R. C. Graham S. B. Edinger 《Physics and Chemistry of Minerals》1989,16(7):697-703
Celadonite from the northwestern Mojave Desert area of California was examined by detailed Mössbauer spectroscopy at temperatures from 10 K to 400 K. In addition to the predominant Fe3+ doublet with isomer shift 0.4 mm s–1 and quadrupole splitting 0.4 mm s–1, another Fe3+ doublet with 0.4, 1.2 mm/s and two Fe2+ doublets with 1.1, 1.7, 2.7 mm s–1 at 300 K were distinguished. The minor Fe3+ component is ascribed to dehydroxylated surface sites. Most of the remaining Fe(90%) is M2 cis-OH octahedral in an ordered M+–M2+ array. However, about 10% is M1 trans-OH Fe2+. Isomer shift vs. T gives Debye temperatures of 570 K for Fe3+ in M2 and 380 K for both Fe2+ sites, indicating greater vibrational freedom for Fe2+. Quadrupole splitting vs. T for Fe2+ gives a valence electronic energy splitting of 760 cm–1 between the ground and first excited state for M2. The M1 sites have a more drastic variation in vs. T which indicates not only a lower first excited state but a rhombic distortion at these sites. A proposed explanation is a neighboring M2 site vacancy. The soil clay formed from this celadonite, which is mostly Fe-rich smectite, was also studied by Mössbauer spectroscopy. About half the Fe2+ has been oxidized in the clay, but the isomer shifts and quadrupole splittings are essentially the same as in the original celadonite. A texture orientation in the clay absorber was detected by measuring the absorber at 55° to the source radiation. This texture effect produces asymmetric doublets in the usual 90° measurement. 相似文献
108.
109.
M. D. Roberts D. L. Reid J. A. Miller I. J. Basson M. Roberts D. Smith 《Mineralium Deposita》2007,42(3):271-292
The Merensky Reef of the Bushveld Complex occurs in its highest stratigraphic position as a heterogeneous, pegmatitic, feldspathic
melanorite bounded by two narrow chromitite stringers at the base of the Merensky Cyclic Unit (MCU). In the Swartklip Facies
of the Rustenburg Layered Suite, the occurrence of widespread thermal and mechanical erosion termed “potholing” has led to
the subdivision of the Merensky Reef into Normal Reef and Regional Pothole Reef sub-facies. The transition between the two
sub-facies occurs where the MCU transgresses the lower chromitite stringer of the Normal Merensky Reef and cuts down into
the underlying cumulate lithologies. In the Regional Pothole Reef at the Northam Platinum Mine, several economic reef types
are identified, where the Merensky Reef becomes conformable to cumulate layering, in particular, to the footwall marker (NP2
reef type) and the upper pseudoReef (P2 reef type). The Normal Merensky Reef, as well as the P2 and NP2 Reefs, contains economic
platinum group element (PGE) grades and includes the lower portion of the MCU melanorite and the Merensky Chromitite. Whole
rock geochemistry indicates that this package is compositionally identical in Normal, P2, and NP2 Reefs, suggesting that the
base of the MCU is a relatively homogeneous drape over both Normal and Regional Pothole Reef regions. However, the lower sections
of the three Reefs are variables depending on the depth of transgression of the MCU. In the Normal and P2 reef types, transgression
by the MCU was arrested within harzburgites, melanorites, and norites, resulting in coarse, pegmatitic textures in the immediate
footwall units. For the NP2 Reef, transgression by the MCU was arrested within leucocratic rocks and resulted in the formation
of troctolites below the Merensky Chromitite. These troctolites are characterised by a coupled relationship between olivine
and sulphides and by changes in major element chemistry and PGE contents relative to equivalent units in the footwall of the
Normal Reef. Along with micro-textural relationships, these features suggest that troctolization of leucocratic cumulates
in the NP2 Reef beneath the Merensky chromitite was a result of a reactive infiltration of a chromite-saturated melt and an
immiscible sulphide liquid from the overlying MCU, rather than a significant fluid flux from below. In all reef types, the
concentration of S defines symmetrical peaks centred on the Merensky Chromitite (and chromitites from pre-existing cyclic
units in Normal and P2 Reefs), whereas PGE concentrations define asymmetrical peaks with higher PGE contents in reconstituted
footwall rocks relative to the MCU melanorite. This signature is attributable to a magmatic model of PGE collection followed
by deposition towards the base of the MCU and within reconstituted footwall rocks. The continuity of the asymmetrical magmatic
PGE signature between the Normal Reef and Regional Pothole Reef sub-facies indicates that PGE mineralization inherent to the
Merensky magma occurred as a drape over a variably eroded and subsequent texturally and geochemically reworked or reconstituted
footwall. 相似文献
110.
Peter Momme Christian Tegner C. Kent Brooks Reid R. Keays 《Contributions to Mineralogy and Petrology》2006,151(1):88-100
Previously published platinum group element (PGE) and rare earth element data (REE) from a sample suite of the Palaeogene
flood basalts of the East Greenland rifted margin are used to approximate primary magma compositions by numerical models of
mantle melting. Both high-Ti and low-Ti basalts are found intercalated in the coastal section “the Sortebre Profile” in central
East Greenland, and the apparent lack of mixing between the two series indicates coexistence of two geographically separated
melting regions and plumbing systems during continental breakup above the Palaeogene Iceland plume. The lavas show little
or no sign of crustal contamination and the limited variation in La/Sm and Cu/Pd ratios can be interpreted to reflect mantle
source composition and melting processes. Numerical modelling indicate that the low-Ti series formed by F~20% melting in a columnar melting regime from a slightly depleted upper mantle source with a relatively normal S-content
(~180 ppm S). In contrast, the high-Ti series formed by much lower degrees of melting (F~6%) in a spreading-related, triangular melting regime from a relatively S-poor (~100 ppm S) source. The low-Ti suite was
S-undersaturated at the stage of melt segregation from a shallow mantle source due to the high degree of melting. In contrast,
the high-Ti suite probably formed from a S-poor source where some low degree melt batches were S-saturated at the stage of
deep segregation in distal parts of the triangular melting regime. This suite shows a geochemical high pressure garnet-signature
and adiabatic decompression could therefore have played a role in keeping the mantle-derived S in solution before Fe-enrichment
related to fractional crystallisation also increased the S-capacity of these melts.
An erratum to this article can be found at 相似文献