全文获取类型
收费全文 | 415篇 |
免费 | 15篇 |
国内免费 | 9篇 |
专业分类
测绘学 | 15篇 |
大气科学 | 36篇 |
地球物理 | 89篇 |
地质学 | 125篇 |
海洋学 | 39篇 |
天文学 | 111篇 |
综合类 | 1篇 |
自然地理 | 23篇 |
出版年
2022年 | 4篇 |
2020年 | 5篇 |
2019年 | 8篇 |
2018年 | 10篇 |
2017年 | 5篇 |
2016年 | 10篇 |
2015年 | 12篇 |
2014年 | 14篇 |
2013年 | 22篇 |
2012年 | 19篇 |
2011年 | 20篇 |
2010年 | 20篇 |
2009年 | 26篇 |
2008年 | 10篇 |
2007年 | 12篇 |
2006年 | 12篇 |
2005年 | 11篇 |
2004年 | 8篇 |
2003年 | 12篇 |
2002年 | 15篇 |
2001年 | 8篇 |
2000年 | 7篇 |
1999年 | 8篇 |
1998年 | 8篇 |
1997年 | 7篇 |
1996年 | 8篇 |
1995年 | 7篇 |
1994年 | 8篇 |
1993年 | 4篇 |
1992年 | 3篇 |
1991年 | 5篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1988年 | 7篇 |
1987年 | 10篇 |
1986年 | 4篇 |
1985年 | 7篇 |
1984年 | 5篇 |
1983年 | 7篇 |
1981年 | 6篇 |
1980年 | 4篇 |
1979年 | 7篇 |
1978年 | 3篇 |
1977年 | 9篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1973年 | 4篇 |
1972年 | 3篇 |
1971年 | 4篇 |
1970年 | 2篇 |
排序方式: 共有439条查询结果,搜索用时 644 毫秒
111.
The early Holocene final drainage of glacial Lake Minong is documented by 21 OSL ages on quartz sand from parabolic dunes and littoral terraces and one radiocarbon age from a lake sediment core adjacent to mapped paleoshorelines in interior eastern Upper Michigan. We employ a simple model wherein lake-level decline exposes unvegetated littoral sediment to deflation, resulting in dune building. Dunes formed subsequent to lake-level decline prior to stabilization by vegetation and provide minimum ages for lake-level decline. Optical ages range from 10.3 to 7.7 ka; 15 ages on dunes adjacent to the lowest Lake Minong shoreline suggest final water-level decline ∼ 9.1 ka. The clustering of optical ages from vertically separated dunes on both sides of the Nadoway-Gros Cap Barrier around 8.8 ka and a basal radiocarbon date behind the barrier (8120 ± 40 14C yr BP [9.1 cal ka BP]) support the hypothesis that the barrier was breached and the final lake-level drop to the Houghton Low occurred coincident with (1) high meltwater flux into the Superior basin and (2) an abrupt, negative shift in oxygen isotope values in Lake Huron. 相似文献
112.
Tomohiro Usui Matthew Sanborn Harry Y. McSween Jr. 《Geochimica et cosmochimica acta》2010,74(24):7283-7306
Shergottites sampled two distinct geochemical reservoirs on Mars. Basaltic and olivine-phyric shergottites individually sampled both geochemically enriched and depleted reservoirs, whereas lherzolitic shergottites are previously known only to exhibit a relatively limited intermediate geochemical signature that may have resulted from the mixing of the two geochemical end-member reservoirs. Here we show that recently discovered shergottites Robert Massif (RBT) 04261 and RBT 04262 are the first examples of lherzolitic shergottites originating from the enriched reservoir.RBT 04261 and RBT 04262, initially identified as olivine-phyric shergottites, are actually lherzolitic shergottites. Both meteorites exhibit nearly identical textures and mineral compositions, suggesting that they should be paired. Each consists of two distinct textures: poikilitic and non-poikilitic. The poikilitic areas are composed of pyroxene oikocrysts enclosing olivine grains; all pyroxene oikocrysts have pigeonite cores mantled by augite. The non-poikilitic areas are composed of olivine, pyroxene, maskelynite and minor amounts of merrillite, chromite and ilmenite. Olivine and pyroxene show the lowest Mg-number, and maskelynite has the lowest anorthite component among the lherzolitic shergottites. Moreover, the modal abundances of maskelynite in these two meteorites are distinctly higher than the other lherzolitic shergottites.The rare earth element (REE) budgets of RBT 04261 and RBT 04262 are dominated by merrillite. The slightly light rare earth element (LREE)-enriched pattern of this mineral is similar to that of merrillite in the geochemically enriched basaltic shergottites Shergotty and Zagami, and unlike the LREE-depleted pattern of merrillite in the other lherzolitic shergottites. The REE patterns of both high- and low-Ca pyroxenes are also similar to those in Shergotty and Zagami. The REE pattern of a melt calculated to be in equilibrium with the core of a pyroxene oikocryst is parallel to that of the RBT 04262 whole-rock as well as whole-rock compositions of other geochemically enriched basaltic shergottites. These observations imply that RBT 04262 sampled an enriched and oxidized reservoir similar to that sampled by some of the basaltic shergottites and are consistent with an oxidizing condition for the formation of RBT 04262 (log fO2 = QFM-1.6).The petrographic and geochemical observations presented here suggest that RBT 04261 and RBT 04262 represent the most evolved magma among the lherzolitic shergottites and that this magma originated from a geochemically enriched reservoir on Mars. Based on an evaluation of the relationship between petrographic, geochemical and chronological signatures for shergottites including RBT 04261 and RBT 04262, we propose that both geochemically enriched and depleted shergottites were ejected from the same launch site on Mars. 相似文献
113.
Mineral compositions and abundances derived from visible/near-infrared (VIS/NIR or VNIR) spectra are used to classify asteroids, identify meteorite parent bodies, and understand the structure of the asteroid belt. Using a suite of 48 equilibrated (types 4-6) ordinary (H, L, and LL) chondrites containing orthopyroxene, clinopyroxene, and olivine, new relationships between spectra and mineralogy have been established. Contrary to previous suggestions, no meaningful correlation is observed between band parameters and cpx/(opx + cpx) ratios. We derive new calibrations for determining mineral abundances (ol/(ol + px)) and mafic silicate compositions (Fa in olivine, Fs in pyroxene) from VIS/NIR spectra. These calibrations confirm that band area ratio (BAR) is controlled by mineral abundances, while Band I center is controlled by mafic silicate compositions. Spectrally-derived mineralogical parameters correctly classify H, L and LL chondrites in ∼80% of cases, suggesting that these are robust relationships that can be applied to S(IV) asteroids with ordinary chondrites mineralogies. Comparison of asteroids and meteorites using these new mineralogical parameters has the advantage that H, L and LL chemical groups were originally defined on the basis of mafic silicate compositions. 相似文献
114.
Tomohiro USUI Harry Y. McSWEEN Jr. David W. MITTLEFEHLDT Thomas H. PRETTYMAN 《Meteoritics & planetary science》2010,45(7):1170-1190
Abstract– The Dawn spacecraft carries a gamma‐ray and neutron detector (GRaND), which will measure and map the abundances of selected elements on the surface of asteroid 4 Vesta. We compare the variability of moderately volatile/refractory incompatible element ratios (K/Th and K/Ti) in howardite, eucrite, and diogenite (HED) meteorites with those in other achondrite suites that represent asteroidal crusts, because these ratios may be accurately measured by GRaND and likely reflect initial chemical compositions of the HED parent body. The K/Th and K/Ti variations can differentiate HED meteorites from angrites and some unique eucrite‐like lithologies. The results suggest that K, Th, and Ti abundances determined from GRaND data could not only confirm that Vesta is the parent body of HED meteorites but might also allow recognition of as‐yet unsampled compositional terranes on Vesta. Besides the K‐Th‐Ti systematics study, we propose a new three‐component mixing model for interpretation of GRaND spectra, required because the spatial resolution of GRaND is coarser than the spectral (compositional) heterogeneity of Vesta’s surface. The mixing model uses abundances of K, Ti, Fe, and Mg that will be analyzed more accurately than other prospective GRaND‐analyzed elements. We examine propagated errors due to GRaND analytical uncertainties and intrinsic errors that stem from an assumption introduced into the mixing model. The error investigation suggests that the mixing model can adequately estimate not only the diogenite/eucrite mixing ratio but also the abundances of most major and minor elements within the GRaND propagated errors. 相似文献
115.
Gwendolynne Young Humberto Zavala Johanna Wandel Barry Smit Sonia Salas Elizabeth Jimenez Melitta Fiebig Roxana Espinoza Harry Diaz Jorge Cepeda 《Climatic change》2010,98(1-2):245-276
Livelihoods in drylands are already challenged by the demands of climate variability, and climate change is expected to have further implications for water resource availability in these regions. This paper characterizes the vulnerability of an irrigation-dependent agricultural community located in the Elqui River Basin of Northern Chile to water and climate-related conditions in light of climate change. The paper documents the exposures and sensitivities faced by the community in light of current water shortages, and identifies their ability to manage these exposures under a changing climate. The IPCC identifies potentially increased aridity in this region with climate change; furthermore, the Elqui River is fed by snowmelt and glaciers, and its flows will be affected by a warming climate. Community vulnerability occurs within a broader physical, economic, political and social context, and vulnerability in the community varies amongst occupations, resource uses and accessibility to water resources, making some more susceptible to changing conditions in the future. This case study highlights the need for adaptation to current land and water management practices to maintain livelihoods in the face of changes many people are not expecting. 相似文献
116.
Sven Klimpel Markus Wilhelm Busch Tracey Sutton Harry Wilhelm Palm 《Deep Sea Research Part I: Oceanographic Research Papers》2010,57(4):596-603
Seven meso- and bathy-pelagic fish species from the Mid-Atlantic Ridge (MAR) were firstly studied for fish parasites and feeding ecology. With a total of seven parasite species, the 247 meso- and bathy-pelagic deep-sea fish specimens belonging to the families Melamphaidae (3 spp.), Myctophidae (3 spp.) and Stomiidae (1 sp.) revealed low parasite diversity. The genetically identified nematodes Anisakis simplex (s.s.) and Anisakis pegreffii from the body cavity, liver and muscles of Myctophum punctatum were the most abundant parasites, reaching a prevalence of 91.4% and mean intensity of 3.1 (1–14). Anisakis sp. (unidentified) infected Chauliodus sloani and Poromitra crassiceps. Bothriocephalidean and tetraphyllidean cestode larvae infected Benthosema glaciale, the latter also occurring in C. sloani and Scopelogadus beanii, at low prevalences. Adult parasites at low infection rates included the digenean Lethadena sp. (2.9%), and the two copepod species Sarcotretes scopeli (5.7%) and Tautochondria dolichoura (5.3–11.4%). The myctophid Lampanyctus macdonaldi and the melamphaid Scopelogadus mizolepis mizolepis were free of parasites. Analyses of the stomach contents revealed crustaceans, especially copepods and euphausiids for the myctophids and also amphipods for the melamphaids as predominant prey items. While all stomachs showing distinct content comprising often unidentified ‘tissue’ (possibly gelatinous zooplankton), only C. sloani preyed upon fish. Though this feeding habit would enable transfer of a variety of crustacean-transmitted parasites into the fish, the parasite fauna in the meso- and bathy-pelagic fish was species poor. All observed parasites showed low host specificity, demonstrating no distinct pattern of host–parasite co-evolution. The MAR is no barrier for the parasite distribution in the North Atlantic meso- and bathy-pelagial. 相似文献
117.
An evaluation of fluidized bed combustor ash as a source of alkalinity to treat toxic rock materials
The coal mining industry is in need of effective, locally available, low-cost ameliorants that can be used to treat the toxic materials generated by surface mining and coal cleaning plants. Ashes generated by fluidized bed combustors may locally fit these criteria. Samples of fly ash and bottom ash were acquired from facilities in West Virginia, Texas, and California and evaluated for their ability to generate alkalinity. The results show that FBC ash has the potential to produce solutions of high alkalinity depending on the CaO and CaCO3 contents. The relative content of CaO, and to a lesser extent Ca(OH)2, determines the rate at which alkalinity will be produced. The neutralization potential is the best parameter to evaluate the potential of an FBC ash to produce alkalinity. Because the composition of ashes from a facility vary over time and because no single parameter will accurately evaluate the rate at which alkalinity will be produced, the rate of alkaline production must be evaluated by controlled leach experiments. 相似文献
118.
ABSTRACTThe critical need for hydrological observations in support of water resources management, particularly during extreme events, has transformed traditional methods of hydrological data management. This transformation has given rise to a framework of e-monitoring the hydrological cycle, the aim of which is to improve understanding of the nature of water. New trends in data science, coupled with increasing technological evolution, make the new generation of data systems more agile and responsive to the needs and expectations for efficient and effective data sharing and service delivery. The WMO Hydrological Observing System was designed around the integration of observations, data exchange, research, data processing, modelling and forecasting, in such a way that societal needs for disaster risk reduction, improved sustainability of environmental resources, climate resilience and economic growth can be effectively met. With its implementation of conceptual functionalities for sustainable data management, the WHOS operational architecture is hydrology’s system for the future. 相似文献
119.
120.
Improvements to Griggs-type apparatus for mechanical testing at high pressures and temperatures 总被引:2,自引:0,他引:2
Tracy N. Tingle Harry W. Green Jr. Thomas E. Young Ted A. Koczynski 《Pure and Applied Geophysics》1993,141(2-4):523-543
New and improved techniques and apparatus for testing the mechanical properties of materials at high presures and temperatures are described. These include an improved Griggs-type deformation apparatus designed to operate to 5 GPa and associated servo-controlled hydraulic drive and electronics, the design of hydrostatic (molten alkali halide mixtures) pressure assemblies to measure flow stresses as low as a few MPa, the characterization of temperature gradients and friction in such assemblies, measurement of the melting curve of an alkali halide mixture used as a confining pressure medium, and the measurement of acoustic emissions. 相似文献