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491.
Abigail K. Barker Paul M. Holm David W. Peate Joel A. Baker 《Contributions to Mineralogy and Petrology》2010,160(1):133-154
High-precision Pb isotope data and Sr–Nd–Hf isotope data are presented together with major and trace element data for samples spanning the 4.6 Ma history of volcanism at Santiago, in the southern Cape Verde islands. Pb isotope data confirm the positive Δ8/4 signature of the southern islands indicating that the north–south compositional heterogeneity in the Cape Verde archipelago has persisted for at least 4.6 Ma. The Santiago volcanics show distinct compositional differences between the old, intermediate and young volcanics, and suggest greater involvement of an enriched mantle (EM1)-like source over time. Isotopic variations in the Santiago volcanics indicate convergence towards a homogeneous EM1-like end-member and distinct temporal variations in the FOZO-like end-member. Santiago and Santo Antão (a northern island, Holm et al. 2006), show a simultaneous decrease in 208Pb/204Pb of the high 206Pb/204Pb FOZO-like source with time. Such systematic archipelago-wide variations in the FOZO-like component suggest that this component is more likely to be present as a coherent package of recycled ocean crust rather than as multiple small heterogeneities dispersed in the upwelling mantle. The temporal variations in 208Pb/204Pb reflect minor lateral variations in Th/U of this recycled ocean crust package entering the melting zone beneath the islands. The location of the EM1-like component is more equivocal. A shallow lithospheric location is possible, but this would require a coincidence between spatial compositional variations in the lithosphere (EM1 is spatially restricted to the southern islands) and flow lines in the upwelling mantle revealed by seismic anisotropy. Therefore, we favour a deeper asthenospheric mantle source for the EM1-like source. 相似文献
492.
Future development and climate change pose potentially serious threats to estuarine fish populations around the world. We
examined how habitat suitability for delta smelt (Hypomesus transpacificus), a state and federally protected species, might be affected by changes in outflow in the San Francisco Estuary due to future
development and climate change. Forty years of sampling data collected during fall from 1967 to 2008 were examined to define
abiotic habitat suitability for delta smelt as a function of salinity and water transparency, and to describe long-term trends
in habitat conditions. The annual habitat index we developed, which incorporated both quantity and quality of habitat, decreased
by 78% over the study period. Future habitat index values under seven different development and climate change scenarios,
representing a range of drier and wetter possibilities, were predicted using a model which related estuarine outflow to the
habitat index. The results suggested that each of the scenarios would generally lead to further declines in delta smelt habitat
across all water year types. Recovery targets for delta smelt will be difficult to attain if the modeled habitat conditions
are realized. 相似文献
493.
David J. Clifford Patrick G. Hatcher Robert E. Botto John V. Muntean Beverly Michels Ken B. Anderson 《Organic Geochemistry》1997,27(7-8)
Soluble polylabdanoids isolated by sequential solvent extraction have been characterized by liquid-state 13C- and 1H NMR and 13C-1H HMQC (heteronuclear correlation) NMR spectroscopy in addition to solid-state NMR and Py-GC-MS techniques. Two Holocene resins originating from Santander, Colombia and Mombasa, Kenya were analyzed. Soluble polymers were isolated by extraction with a 1:1 (v/v) methylene chloride-methanol mixture following sequential extractions with methylene chloride and methanol. The molecular weight of polymer extracts was shown by GPC analyses to exceed that of non-polymeric occluded terpenoids. Py-GC-MS, solid-state 13C CP/MAS and 13C cross-polarization/depolarization NMR spectroscopy results indicated that chemical compositions of soluble polymers isolated from immature resins are highly representative of the structure of corresponding insoluble polymers, i.e. polylabdatrienes. These data provide evidence for cross-linking or cyclization of side-chain olefinic carbons during or shortly after polymerization. Generally, the characterization of soluble resin polymers by liquid-state NMR spectroscopy has proven to be an excellent means for investigating the maturation mechanism of polylabdanoid resinites, and has potential for furthering the application of Class I resinites as geothermal indicators. 相似文献
494.
阿尔泰南缘古生代康布铁堡组火山岩系是许多铁矿、铜锌矿以及铅锌矿的赋矿围岩,但该赋矿地层目前还缺乏精确的同位素年代学资料。通过对变安山质凝灰岩和花岗岩LA-ICP-MS锆石U-Pb定年,获得阿尔泰南缘苏普特背斜花岗岩的年龄为412.7±0.78 Ma,安山质凝灰岩的年龄为411.2±3.4 Ma,表明阿尔泰南缘苏普特背斜内泥盆纪早期存在着强烈的火山活动,与区内广泛发育的火山岩属于同一事件的产物,具有相同的动力学背景。为研究阿尔泰南缘古生代构造演化及苏普特背斜异常区提供了重要的理论依据。 相似文献
495.
We performed spectral analyses on the ages of 89 well-dated major geological events of the last 260 Myr from the recent geologic literature.These events include times of marine and non-marine extinctions,major ocean-anoxic events,continental flood-basalt eruptions,sea-level fluctuations,global pulses of intraplate magmatism,and times of changes in seafloor-spreading rates and plate reorganizations.The aggregate of all 89 events shows ten clusters in the last 260 Myr,spaced at an average interval of~26.9 Myr,and Fourier analysis of the data yields a spectral peak at 27.5 Myr at the≥96%confidence level.A shorter period of~8.9 Myr may also be significant in modulating the timing of geologic events.Our results suggest that global geologic events are generally correlated,and seem to come in pulses with an underlying~27.5-Myr cycle.These cyclic pulses of tectonics and climate change may be the result of geophysical processes related to the dynamics of plate tectonics and mantle plumes,or might alterna-tively be paced by astronomical cycles associated with the Earth's motions in the Solar System and the Galaxy. 相似文献
496.
Trimethylbenzene (TMB), as a constituent of gasoline, is often expected to be used as a conservative tracer in anaerobic BTEX-contaminated
groundwater site to correct for attenuation due to dispersion, dilution and sorption along a flow path. To evaluate the suitability
of using TMB as a tracer and to better understand biodegradability of TMB in contaminated groundwater by gasoline under anaerobic
conditions, laboratory microcosms were conducted with mixed nitrate/iron/sulfate electron-acceptor amendments, using aquifer
materials collected from Canadian Forces Base (CFB), Borden, Ontario, Canada. The results showed that under denitrifying conditions,
biodegradation of 1,3,5-TMB, 1,2,4-TMB and 1,2,3-TMB were relatively slow and after 204 days of incubation approximately 27,
24, and 16% of the initial concentrations, respectively, were degraded in the microcosms. Under sulfate-reducing conditions,
TMB isomers were recalcitrant. In contrast, significant biodegradation of TMB was observed under iron-reducing conditions.
1,3,5-TMB, 1,2,4-TMB and 1,2,3-TMB were degraded to 44, 47, and 24% of initial concentrations with first-order biodegradation
rate constants of 0.003, 0.006 and 0.013 d−1, respectively. This study indicates that TMB biodegradation is insignificant under nitrate and sulfate-reducing conditions
but significant under iron-reducing conditions. Therefore, the use of TMB as a tracer for interpreting removal of other biodegradable
gasoline constituents such as BTEX requires caution, especially in the presence of iron-reducing conditions. 相似文献
497.
Juan‐Carlos Carracedo Francisco J. Perez‐Torrado Alejandro Rodriguez‐Gonzalez Raphaël Paris Valentin R. Troll Abigail K. Barker 《Geology Today》2015,31(4):146-152
In recent months the media have drawn attention to the Cape Verde archipelago, with particular focus on the island of Fogo, the only island presently active and with an eruption that began on 23 November 2014, finally ceasing on 7 February 2015. The Monte Amarelo conical shield forms most of the 476 km2 almost circular island of Fogo. After attaining a critical elevation of about 3500 m, the Monte Amarelo shield volcano was decapitated by a giant landslide that formed a caldera‐like depression (Cha das Caldeiras), which was subsequently partially filled by basaltic nested volcanism. This younger eruptive activity culminated in the construction of the 2829 m‐high Pico do Fogo stratocone, apparently entirely made of layers of basaltic lapilli. Continued growth of the Pico do Fogo summit eruptions was interrupted in 1750, most likely after the stratocone reached a critical height. Since then, at least eight eruptions have taken place inside the landslide depression at the periphery of the Pico do Fogo cone, including the 2014–2015 eruptive event. Strong geological similarities with the Canary Islands, 1400 km to the north, have been frequently noted, probably as a consequence of a common process of origin and evolution associated with a mantle hot‐spot. These similarities are particularly evident when comparing Fogo with the Teide Volcanic Complex on Tenerife, where a lateral collapse of the Las Cañadas stratovolcano also formed a large depression (the Caldera de Las Cañadas), now partially filled with the 3718 m‐high Teide stratocone. However, important geological differences also exist and probably relate to the contrasting evolutionary stages of both islands. The Las Cañadas volcano on Tenerife formed at a late post‐erosional stage, with predominantly evolved (trachyte and phonolite) magmas, while at Fogo basaltic volcanism is still dominant. 相似文献
498.
Robert G. Hatfield Barbara A. Maher Jacqueline M. Pates Philip A. Barker 《Journal of Paleolimnology》2008,40(4):1143-1158
We examine sediment dynamics in an upland, temperate lake system, Lake Bassenthwaite (NW England), in the context of changing
climate and land use, using magnetic and physical core properties. Dating and analysis of the sedimentary records of nine
recovered cores identify spatially variable sedimentation rates across the deep lake basin. Mineral magnetic techniques, supported
by independent geochemical analyses, identify significant variations both in sediment source and flux over the last ∼2100 years.
Between ∼100 years BC and ∼1700 AD, sediment fluxes to the lake were low and dominated by material sourced from within the
River Derwent sub-catchment (providing 80% of the hydraulic load at the present day). Post-1700 AD, the lake sediments became
dominantly sourced from Newlands Beck (presently providing ∼10% of the lake’s hydraulic load). Three successive, major pulses
of erosion and increased sediment flux appear linked to specific activities within the catchment, specifically: mining activities
and associated deforestation in the mid-late nineteenth century; agricultural intensification in the mid-twentieth century
and, within the last decade, the additional possible impact of climate change. These results are important for all upland
areas as modifications in climate become progressively superimposed upon the effects of previous and/or ongoing anthropogenic
catchment disturbance. 相似文献
499.
Gustavo Indalecio Cancelo Juan Cruz Estrada Guillermo Fernandez Moroni Ken Treptow Ted Zmuda H. Thomas Diehl 《Experimental Astronomy》2012,34(1):13-29
Scientific CCDs designed in thick high resistivity silicon (Si) are excellent detectors for astronomy, high energy and nuclear physics, and instrumentation. Many applications can benefit from CCDs ultra low noise readout systems. The present work shows how sub electron noise CCD images can be achieved using digital signal processing techniques. These techniques allow 0.4 electrons of noise at readout bandwidths of up to 10?Kpixels per second while keeping the full CCD spatial resolution and signal dynamic range. 相似文献