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131.
Ultrahigh-temperature metamorphism in the Achankovil Zone: Implications for the correlation of crustal blocks in southern India 总被引:1,自引:3,他引:1
The Achankovil Zone of southern India, a NW–SE trending lineament of 8–10 km in width and > 100 km length, is a kinematically debated crustal feature, considered to mark the boundary between the Madurai Granulite Block in the north and the Trivandrum Granulite Block in the south. Both these crustal blocks show evidence for ultrahigh-temperature metamorphism during the Pan-African orogeny, although the exhumation styles are markedly different. The Achankovil Zone is characterized by discontinuous strands of cordierite-bearing gneiss with an assemblage of cordierite + garnet + quartz + plagioclase + spinel + ilmenite + magnetite ± orthopyroxene ± biotite ± K-feldspar ± sillimanite. The lithology preserves several peak and post-peak metamorphic assemblages including: (1) orthopyroxene + garnet, (2) perthite and/or anti-perthite, (3) cordierite ± orthopyroxene corona around garnet, and (4) cordierite + quartz symplectite after garnet. We estimate the peak metamorphic conditions of these rocks using orthopyroxene-bearing geothermobarometers and feldspar solvus which yield 8.5–9.5 kbar and 940–1040 °C, the highest P–T conditions so far recorded from the Achankovil Zone. The retrograde conditions were obtained from cordierite-bearing geothermobarometers at 3.5–4.5 kbar and 720 ± 60 °C. From orthopyroxene chemistry, we record a multistage exhumation history for these rocks, which is closely comparable with those reported in recent studies from the Madurai Granulite Block, but different from those documented from the Trivandrum Granulite Block. An evaluation of the petrologic and geochronologic data, together with the nature of exhumation paths leads us to propose that the Achankovil Zone is probably the southern flank of the Madurai Granulite Block, and not a unit of the Trivandrum Granulite Block as presently believed. Post-tectonic alkali granites that form an array of “suturing plutons” along the margin of the Madurai Granulite Block and within the Achankovil Zone, but are absent in the Trivandrum Granulite Block, suggest that the boundary between the Madurai Granulite Block and the Trivandrum Granulite Block might lie along the Tenmalai shear zone at the southern extremity of the Achankovil Zone. 相似文献
132.
Masao Nakada 《Physics of the Earth and Planetary Interiors》2006,154(2):113-147
The differential axial and equatorial rotations of both cores associated with the Quaternary glacial cycles were evaluated based on a realistic earth model in density and elastic structures. The rheological model is composed of compressible Maxwell viscoelastic mantle, inviscid outer core and incompressible Maxwell viscoelastic inner core. The present study is, however, preliminary because I assume a rigid rotation for the fluid outer core. In models with no frictional torques at the boundaries of the outer core, the maximum magnitude of the predicted axial rotations of the outer and inner cores amounts to ∼2° year−1 and ∼1° year−1, respectively, but that for the secular equatorial rotations of both cores is ∼0.0001° at most. However, oscillating parts with a period of ∼225 years are predicted in the equatorial rotations for both cores. Then, I evaluated the differential rotations by adopting a time-dependent electromagnetic (EM) torque as a possible coupling mechanism at the core-mantle boundary (CMB) and inner core boundary (ICB). In a realistic radial magnetic field at the CMB estimated from surface magnetic field, the axial and equatorial rotations couple through frictional torques at the CMB, although these rotations decouple for dipole magnetic field model. The differential rotations were evaluated for conductivity models with a conductance of 108 S of the lowermost mantle inferred from studies of nutation and precession of the Earth and decadal variations of length of day (LOD). The secular parts of equatorial rotations are less sensitive to these parameters, but the magnitude for the axial rotations is much smaller than for frictionless model. These models, however, produce oscillating parts in the equatorial rotations of both cores and also in the axial rotations of the whole Earth and outer and inner cores. These oscillations are sensitive to both the magnitude of radial magnetic field at the CMB and the conductivity structure. No sharp isolated spectral peaks are predicted for models with a thin conductive layer (∼200 m) at the bottom of the mantle. In models with a conductive layer of ∼100 km thickness, however, sharp spectral peaks are predicted at periods of ∼225 and ∼25 years for equatorial and axial rotations, respectively, although these depend on the strength of radial magnetic field at the CMB. While the present study is preliminary in modelling the fluid outer core and coupling mechanism at the CMB, the predicted axial rotations of the whole Earth may be important in explaining the observed LOD through interaction between the equatorial and axial rotations. 相似文献
133.
Nagayoshi Katsuta Masao Takano Shin-ichi Kawakami Shoji Togami Hitoshi Fukusawa Mineo Kumazawa Yoshinori Yasuda 《Journal of Paleolimnology》2007,37(2):259-271
Event-related sedimentary layers, which are deposited occasionally due to volcanic eruptions, earthquakes or heavy rains,
are often contained in the rhythmical sequences of lacustrine and marine sediments. We have developed an analytical method
for separating the sedimentary rhythms and the event layers identified using the scanning X-ray analytical microscope (SXAM)
and obtained sequential profiles of seven elements Al, Si, K, Ca, Ti, Mn, and Fe in the lacustrine sediment from Lake Suigetsu,
Japan. Two types of event layers could be detected from the elemental composition of 33 layers of sediment: three known volcanic
ash layers and 30 clay layers containing 12 turbidites. The recurrence interval of the latter, which may potentially be initiated
and archived by locally important earthquakes, is estimated to be an average of 640 ± 160 years by using Sompi event analysis
(SEA) based on an autoregressive (AR) model. After removing those portions that represented event layers from the elemental
profiles, we obtained event-removed (ER) temporal profiles based on the tephrochronology of the three volcanic ash layers.
The ER temporal profiles of manganese and iron, probably representing the siderite content, showed a millennial-scale variation
in the Holocene that corresponded well with ice-rafting events in the North Atlantic. 相似文献
134.
Hope A. Ishii 《Meteoritics & planetary science》2019,54(1):202-219
Comet 81P/Wild 2 dust, the first comet sample of known provenance, was widely expected to resemble anhydrous chondritic porous (CP) interplanetary dust particles (IDPs). GEMS, distinctly characteristic of CP IDPs, have yet to be unambiguously identified in the Stardust mission samples despite claims of likely candidates. One such candidate is Stardust impact track 57 “Febo” in aerogel, which contains fine‐grained objects texturally and compositionally similar to GEMS. Their position adjacent the terminal particle suggests that they may be indigenous, fine‐grained, cometary material, like that in CP IDPs, shielded by the terminal particle from damage during deceleration from hypervelocity. Dark‐field imaging and multidetector energy‐dispersive X‐ray mapping were used to compare GEMS‐like‐objects in the Febo terminal particle with GEMS in an anhydrous, chondritic IDP. GEMS in the IDP are within 3× CI (solar) abundances for major and minor elements. In the Febo GEMS‐like objects, Mg and Ca are systematically and strongly depleted relative to CI; S and Fe are somewhat enriched; and Au, a known aerogel contaminant, is present, consistent with ablation, melting, abrasion, and mixing of the SiOx aerogel with crystalline Fe‐sulfide and minor enstatite, high‐Ni sulfide, and augite identified by elemental mapping in the terminal particle. Thus, GEMS‐like objects in “caches” of fine‐grained debris abutting terminal particles are most likely deceleration debris packed in place during particle transit through the aerogel. 相似文献
135.
The Hill Forests in Bangladesh have been depleted and degraded in volume, area, and quantity, thus requiring urgent forest
protection by identifying the causes of forest loss. This study seeks to identify the fundamental causes of the shrinking
forest cover in the Hill Forests in Bangladesh. The results indicate four sets of causes: (i) indigenous forest dwellers,
having their own types of problems; (ii) migrants, who, because of problems in their places of origin, have decided to move
to the forests; (iii) the timber industry, which, legal or not, are cutting too many trees; and (iv) the government through
its Forest Department which is not able or willing to implement suitable policies to regulate the cutting trees and to prevent
illegal cutting. Because it is a time consuming task to mitigate the first and second sets of factors, we recommend to involve
forest dwellers in forestry practices as much as possible and taking necessary steps to alleviate the third and fourth sets
and thereby reduce the rate of forest depletion. Accordingly, a number of strategies that should be adopted to halt the loss
of remaining forest cover are discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
136.
Masao Nakada 《Geophysical Journal International》2000,143(1):230-238
Previous studies of the wander of the rotation pole associated with the Late Pleistocene glacial cycles indicate that the predicted polar wander speed is sensitive to the density jump at the 670 km discontinuity, the thickness of the elastic lithosphere, and the lower mantle viscosity. In particular, the M1 mode related to the density jump at 670 km depth has been shown to contribute a dominant portion of predicted polar wander speed for sufficiently small lower mantle viscosities. In this study, we examine the sensitivity of polar wander to variations in the viscosity of the viscoelastic lithosphere using simplified compressible Maxwell viscoelastic earth models. Model calculations for earth models with a viscoelastic lithosphere of finite viscosity indicate that the contribution of the M1 mode is similar to those associated with the density discontinuity at the core–mantle boundary (C0 mode) and the lithosphere (L0 mode). We speculate that this is due to the interaction between the M1 mode and the transient mode associated with the viscoelastic lithosphere, which reduces the magnitude of polar wander rates. Therefore, the M1 mode does not contribute a dominant portion of the predicted polar wander speed for earth models with a viscoelastic lithosphere of finite viscosity. In this case, predictions of polar wander speed as a function of lower mantle viscosity exhibit the qualitative form of an 'inverted parabola', as predicted for the J ˙2 curve. We caution, however, that these results are obtained for simplified earth models, and the results for seismological earth models such as PREM may be complicated by the interaction between the M1 mode and the large set of transient modes. 相似文献
137.
From an analysis of the current water resources and their development and utilization in arid north-west China, the authors
conclude the extent of channelled water accounts for only 56.0% of the total, exploitable surface-water resources of arid
north-west China. The utilization ratio of canal systems is 42%, and farmland use is 0.8. When the ground and surface waters
of river basins in the region are comprehensively developed, the channelled water will reach its climax, accounting for 80%
of the total, exploitable surface-water resources, which would constitute a 91% increase over current levels in surface-water
resource development. In the future, the utilization ratio of canal system as well as the utilization ratio of farmland water
will reach 0.9 with the help of scientific and technological advancements. The channelled water is the same as the comprehensive
development and utilization stages, but the total water use will be increased by 247×108 m3, and will reach 756.8×108 m3, accounting for 88.2% of the total, exploitable surface-water resources in arid north-west China. Also, the authors suggest
that the scientific and technological measures to increase the water-use ratio include improving management, strengthening
protection of water resources and the environment, and increasing studies of water saving techniques.
Received: 11 January 1999 · Accepted: 6 July 1999 相似文献
138.
Shocked chromites in fossil L chondrites: A Raman spectroscopy and transmission electron microscopy study
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Surya S. Rout Philipp R. Heck Nestor J. Zaluzec Takayuki Ishii Jianguo Wen Dean J. Miller Birger Schmitz 《Meteoritics & planetary science》2017,52(9):1776-1796
Chromites from Middle Ordovician fossil L chondrites and from matrix and shock‐melt veins in Catherwood, Tenham, and Coorara L chondrites were studied using Raman spectroscopy and TEM. Raman spectra of chromites from fossil L chondrites showed similarities with chromites from matrix and shock‐melt veins in the studied L chondrite falls and finds. Chromites from shock‐melt veins of L chondrites show polycrystallinity, while the chromite grains in fossil L chondrites are single crystals. In addition, chromites from shock‐melt veins in the studied L chondrites have high densities of planar fractures within the subgrains and many subgrains show intergrowths of chromite and xieite. Matrix chromite of Tenham has similar dislocation densities and planar fractures as a chromite from the fossil meteorite Golvsten 001 and higher dislocation densities than in chromite from the fossil meteorite Sextummen 003. Using this observation and knowing that the matrix of Tenham experienced 20–22 GPa and <1000° C, an upper limit for the P,T conditions of chromite from Golvsten 001 and Sextummen 003 can be estimated to be 20–22 GPa and 1000° C (shock stage S3–S6) and 20 GPa and 1000° C (S3–S5), respectively, and we conclude that the studied fossil meteorite chromites are from matrix. 相似文献
139.
140.
Isobe T Oshihoi T Hamada H Nakayama K Yamada TK Tajima Y Amano M Tanabe S 《Marine pollution bulletin》2011,63(5-12):564-571
Contamination status of brominated flame retardants (BFRs) and persistent organic pollutants (POPs) in blubber of finless porpoises (Neophocaena phocaenoides) stranded along the coasts of Seto Inland Sea and Omura Bay in Japan were investigated. Levels of PCBs, DDTs and CHLs were significantly higher than those of HCHs, HCB, PBDEs and HBCDs. Concentrations of PBDEs and HBCDs, as well as organochlorine compounds in males increased with body length (p<0.05). Among 14 PBDE congeners analyzed, BDE-47 was the predominant, which is similar to those generally reported in biota. PBDEs, HBCDs and PCBs showed no obvious temporal trend in concentrations during the study period, suggesting continuous environmental release of these chemicals. On the other hand, levels of DDT, CHLs and HCHs have decreased. Concentrations of PCBs in liver trematode infected individuals were significantly higher than those in not infected individuals, implying there could be a relationship between contaminant levels and parasitic infection. 相似文献