全文获取类型
收费全文 | 7693篇 |
免费 | 652篇 |
国内免费 | 177篇 |
专业分类
测绘学 | 327篇 |
大气科学 | 825篇 |
地球物理 | 2759篇 |
地质学 | 2957篇 |
海洋学 | 476篇 |
天文学 | 558篇 |
综合类 | 196篇 |
自然地理 | 424篇 |
出版年
2022年 | 20篇 |
2021年 | 39篇 |
2020年 | 63篇 |
2019年 | 50篇 |
2018年 | 501篇 |
2017年 | 431篇 |
2016年 | 352篇 |
2015年 | 227篇 |
2014年 | 195篇 |
2013年 | 254篇 |
2012年 | 742篇 |
2011年 | 542篇 |
2010年 | 240篇 |
2009年 | 294篇 |
2008年 | 255篇 |
2007年 | 203篇 |
2006年 | 222篇 |
2005年 | 897篇 |
2004年 | 924篇 |
2003年 | 718篇 |
2002年 | 221篇 |
2001年 | 135篇 |
2000年 | 94篇 |
1999年 | 60篇 |
1998年 | 50篇 |
1997年 | 58篇 |
1996年 | 37篇 |
1995年 | 31篇 |
1994年 | 30篇 |
1993年 | 22篇 |
1992年 | 22篇 |
1991年 | 24篇 |
1990年 | 33篇 |
1989年 | 27篇 |
1988年 | 24篇 |
1987年 | 20篇 |
1986年 | 21篇 |
1985年 | 24篇 |
1984年 | 25篇 |
1983年 | 35篇 |
1982年 | 29篇 |
1981年 | 22篇 |
1980年 | 24篇 |
1979年 | 22篇 |
1978年 | 27篇 |
1977年 | 19篇 |
1976年 | 18篇 |
1975年 | 25篇 |
1974年 | 15篇 |
1973年 | 28篇 |
排序方式: 共有8522条查询结果,搜索用时 31 毫秒
991.
DONGSheng GANBuhong HAOXiaoli 《中国海洋大学学报(英文版)》2004,3(1):111-114
1 Introduction InChina ,coastalcitiesaremostlyregionaleconomicdevelopmentcenters.Becauseoftheirspecialgeogra phiclocations ,typhoonsandassociatedstormsurgescauseheavylossesoflivesandproperties.In 1 992 Qing daowasfloodedduetothecoincidenceoftheastrono m… 相似文献
992.
993.
Antonio?Acosta-VigilEmail author David?London George?B.?Morgan 《Contributions to Mineralogy and Petrology》2006,151(5):539-557
We have experimentally investigated the kinetics of melting of an aplitic leucogranite (quartz+sodic plagioclase of ≈Ab90+K-feldspar+traces of biotite) at 690, 740, and 800°C, all at 200 MPa H2O. Leucogranite cylinders, 3.5 mm in diameter and 7 mm in length, were run in the presence of excess H2O using cold-seal pressure vessels for 11–2,925 h. At 690 and 740°C and any experimental time, and 800°C and short run times, silicate glass (melt at run conditions) occurs as interconnected films along most of the mineral boundaries and in fractures, with the predominant volume occurring along quartz/feldspars boundaries and quartz/plagioclase/K-feldspar triple junctions. Glass film thickness is roughly constant throughout a given experimental charge and increases with experimental temperature and run duration. The results indicate that H2O-saturated partial melting of a quartzo-feldspathic protolith will produce an interconnected melt phase even at very low degrees (<5 vol%) of partial melting. Crystal grain boundaries are therefore completely occluded with melt films even at the lowest degrees of partial melting, resulting in a change in the mechanism of mass transport through the rock from advection of aqueous vapor to diffusion through silicate melt. At 690 and 740°C the compositions of glasses are homogeneous and (at both temperatures) close to, but not on, the H2O-saturated 200 MPa haplogranite eutectic; glass compositions do not change with run duration. At 800°C glasses are heterogeneous and plot away from the minimum, although their molar ratios ASI (=mol Al2O3/CaO+Na2O+K2O) and Al/Na are constant throughout the entire charge at any experimental time. Glass compositions within individual 800°C experiments form linear trends in (wt%) normative quartz–albite–orthoclase space. The linear trends are oriented perpendicular to the 200 MPa H2O haplogranite cotectic line, reflecting nearly constant albite/orthoclase ratio versus variable quartz/feldspar ratio, and have endpoints between the 800°C isotherms on the quartz and feldspar liquidus surfaces. With increasing experimental duration the trends migrate from the potassic side of the minimum toward the bulk rock composition located on the sodic side, due to more rapid (and complete) dissolution of K-feldspar relative to plagioclase. The results indicate that partial melting at or slightly above the solidus (690–740°C) is interface reaction-controlled, and produces disequilibrium melts of near-minimum composition that persist metastably for up to at least 3 months. Relict feldspars show no change in composition or texture, and equilibration between melt and feldspars might take from a few to tens of millions of years. Partial melting at temperatures well above the solidus (800°C) produces heterogeneous, disequilibrium liquids whose compositions are determined by the diffusive transport properties of the melt and local equilibrium with neighboring mineral phases. Feldspars recrystallize and change composition rapidly. Partial melting and equilibration between liquids and feldspars might take from a few to tens of years (H2O-saturated conditions) at these temperatures well above the solidus. 相似文献
994.
Cristina?PerinelliEmail author Pietro?Armienti Luigi?Dallai 《Contributions to Mineralogy and Petrology》2006,151(3):245-266
Peridotite xenoliths found in Cenozoic alkali basalts of northern Victoria Land, Antarctica, vary from fertile spinel-lherzolite
to harzburgite. They often contain glass-bearing pockets formed after primary pyroxenes and spinel. Few samples are composite
and consist of depleted spinel lherzolite crosscut by amphibole veins and/or lherzolite in contact with poikilitic wehrlite.
Peridotite xenoliths are characterized by negative Al2O3–Mg# and TiO2–Mg# covariations of clino- and orthopyroxenes, low to intermediate HREE concentrations in clinopyroxene, negative Cr–Al trend
in spinel, suggesting variable degrees of partial melting. Metasomatic overprint is evidenced by trace element enrichment
in clinopyroxene and sporadic increase of Ti–Fetot. Preferential Nb, Zr, Sr enrichments in clinopyroxene associated with high Ti–Fetot contents constrain the metasomatic agent to be an alkaline basic melt. In composite xenoliths, clinopyroxene REE contents
increase next to the veins suggesting metasomatic diffusion of incompatible element. Oxygen isotope data indicate disequilibrium
conditions among clinopyroxene, olivine and orthopyroxene. The highest δ18O values are observed in minerals of the amphibole-bearing xenolith. The δ18Ocpx correlations with clinopyroxene modal abundance and geochemical parameters (e.g. Mg# and Cr#) suggest a possible influence
of partial melting on oxygen isotope composition. Thermobarometric estimates define a geotherm of 80°C/GPa for the refractory
lithosphere of NVL, in a pressure range between 1 and 2.5 GPa. Clinopyroxene microlites of melt pockets provide P–T data close
to the anhydrous peridotite solidus and confirm that they originated from heating and decompression during transport in the
host magma. All these geothermometric data constrain the mantle potential temperature to values of 1250–1350°C, consistent
with the occurrence of mantle decompressional melting in a transtensive tectonic regime for the Ross Sea region. 相似文献
995.
Evaluation of an ensemble of Arctic regional climate models: spatiotemporal fields during the SHEBA year 总被引:1,自引:1,他引:0
A.?RinkeEmail author K.?Dethloff J.?J.?Cassano J.?H.?Christensen J.?A.?Curry P.?Du E.?Girard J.-E.?Haugen D.?Jacob C.?G.?Jones M.?K?ltzow R.?Laprise A.H.?Lynch S.?Pfeifer M.?C.?Serreze M.?J.?Shaw M.?Tjernstr?m K.?Wyser M.??agar 《Climate Dynamics》2006,26(5):459-472
Simulations of eight different regional climate models (RCMs) have been performed for the period September 1997–September
1998, which coincides with the Surface Heat Budget of the Arctic Ocean (SHEBA) project period. Each of the models employed
approximately the same domain covering the western Arctic, the same horizontal resolution of 50 km, and the same boundary
forcing. The models differ in their vertical resolution as well as in the treatments of dynamics and physical parameterizations.
Both the common features and differences of the simulated spatiotemporal patterns of geopotential, temperature, cloud cover,
and long-/shortwave downward radiation between the individual model simulations are investigated. With this work, we quantify
the scatter among the models and therefore the magnitude of disagreement and unreliability of current Arctic RCM simulations.
Even with the relatively constrained experimental design we notice a considerable scatter among the different RCMs. We found
the largest across-model scatter in the 2 m temperature over land, in the surface radiation fluxes, and in the cloud cover
which implies a reduced confidence level for these variables.
An erratum to this article can be found at 相似文献
996.
AGUIRRE-SALADO Carlos Arturo TREVIO-GARZA Eduardo Javier AGUIRRE-CALDERóN Oscar Alberto JIMéNEZ-PéREZ Javier GONZáLEZ-TAGLE Marco Aurelio VALDEZ-LAZALDE José René MIRANDA-ARAGóN Liliana AGUIRRE-SALADO Alejandro Iván 《地理学报(英文版)》2012,22(4):669-680
Spatially-explicit estimation of aboveground biomass(AGB) plays an important role to generate action policies focused in climate change mitigation,since carbon(C) retained in the biomass is vital for regulating Earth’s temperature.This work estimates AGB using both chlorophyll(red,near infrared) and moisture(middle infrared) based normalized vegetation indices constructed with MCD43A4 MODerate-resolution Imaging Spectroradiometer(MODIS) and MOD44B vegetation continuous fields(VCF) data.The study area is located in San Luis Potosí,Mexico,a region that comprises a part of the upper limit of the intertropical zone.AGB estimations were made using both individual tree data from the National Forest Inventory of Mexico and allometric equations reported in scientific literature.Linear and nonlinear(expo-nential) models were fitted to find their predictive potential when using satellite spectral data as explanatory variables.Highly-significant correlations(p = 0.01) were found between all the explaining variables tested.NDVI62,linked to chlorophyll content and moisture stress,showed the highest correlation.The best model(nonlinear) showed an index of fit(Pseudo-r2) equal to 0.77 and a root mean square error equal to 26.00 Mg/ha using NDVI62 and VCF as explanatory variables.Validation correlation coefficients were similar for both models:lin-ear(r = 0.87**) and nonlinear(r = 0.86**). 相似文献
997.
The importance of the landscape surrounding a protected area for sustaining its values is frequently discussed in conservation literature. Studies on the interactions of urbanisation and nature conservation at the global scale suggest that protected nature attracts urbanisation, and that this in turn might negatively impact the area. However, studies specifically addressing such land use dynamics at city scale are largely missing. In this study we examine the change in proportion of built up area in two zones (500 m and 1000 m) surrounding 15 urban nature reserves in southern Sweden. By using comprehensive maps from the last 50 years, we compared the zones to the overall urbanisation in the cities to reveal discrepancies in land use surrounding the nature reserves. We found that the amount of built up area in the buffer zones surrounding nature reserves followed the same trend as the corresponding cities and this relation was stable over time, although the positive relationship was not significant. The establishment of nature reserves had no detectable effect on surrounding land use, however two distinguished groups of reserves were identified with either more or less built up area in buffers zones compared to cities. These differences were related to specific local drivers such as land ownership, land use history and nature reserve location. In contrast to earlier studies at global scale, our study did not show that nature reserves attract urbanisation. Instead, we stress that the great variety of contextual factors at the city scale makes quantitative analysis of this kind extremely difficult. However, a general neglect from planning and nature conservation agencies to recognise nature reserves’ dependence on the surrounding landscape configuration could be detrimental to sustain their values in the long-term. Hence we suggest that zones surrounding nature-protected areas should be planned and managed according to local land use history and current landscape conditions to enable and enhance necessary cross-boundary interactions. 相似文献
998.
Gabriella Sch?ll-Barna Attila Demény Gábor Serlegi Szilvia Fábián Pál Sümegi István Fórizs Bernadett Bajnóczi 《Journal of Paleolimnology》2012,47(1):87-100
Geochemical records of bivalve shells have been increasingly studied in the last decade to obtain information on climate conditions.
In this paper we present stable isotope compositions of living and prehistoric shells of freshwater mussels (Unionidae) and
their relationships with climate conditions in a shallow lake environment of Lake Balaton, West-Central Hungary. Physical
conditions and stable oxygen isotope compositions of lake water samples were monitored where living bivalves were collected.
Comparisons between seasonal variations in ambient temperature, water composition and within-shell isotopic variations indicate
that the shells of Unio pictorum do reflect local changes at high resolution and thus can be used to study past conditions. Additionally, shells covering
the last two decades were gathered at several locations along the lake in order to determine spatial and temporal variations
in the shells’ isotopic compositions as a function of weather conditions. As an application, prehistoric shells collected
in archaeological excavations were analysed in order to study past environmental variations. Climate variations during the
Late Copper Age (5460–4870 cal. yr BP) have been assumed on the basis of geomorphological and archaeozoological observations
at the site Balatonkeresztúr-Réti-dűlő (south of Lake Balaton), that suggested increasing humidity as a cause of changes in
settlement location and domestic livestock husbandry. Stable carbon and oxygen isotope compositions of prehistoric bivalve
shells were analyzed from excavations representing five archaeological subphases (Boleráz subphase, 5460–5310 cal. yr BP;
two transitional subphases around 5310 cal. yr BP; Early Classic subphase, 5310–5060 cal. yr BP; Late Classic subphase, 5040–4870 cal.
yr BP). The analyses revealed significant negative C and O isotope shifts in the transitional subphases relative to the earlier
and later subphases. The isotopic variations indicate that the local climate became relatively wet and possibly cold around
5310 cal. yr BP, then it returned to drier (and likely warmer) conditions during the Classic subphases. This interpretation
is in agreement with previous studies on climate changes related to the “5.3 ky event” in the European continental area and
the North Atlantic Region, indicating an Atlantic influence in the Carpathian Basin. 相似文献
999.
Alexander M. Heyng Christoph Mayr Andreas Lücke Bernd Striewski Stefan Wasteg?rd Holger Wissel 《Journal of Paleolimnology》2012,48(2):351-366
Maar lakes in the Auckland Volcanic Field are important high-resolution archives of Holocene environmental change in the Southern Hemisphere mid-latitudes. Stable carbon and nitrogen isotope analyses were applied on bulk organic matter and the green alga Botryococcus from a sediment core from Lake Pupuke (Auckland, North Island, New Zealand) spanning the period since 7,165?cal.?year BP. The origin of organic matter was established using total-organic?Ccarbon-to-nitrogen ratios (TOC/TN) as well as organic carbon (??13COM) and nitrogen (??15N) isotope composition of potential modern sources. This approach demonstrated that the contribution of allochthonous organic matter to the lake sediment was negligible for most of the record. The sedimentary TOC/TN ratios that are higher than Redfield ratio (i.e. >7) are attributed to N-limiting conditions throughout the record. Variations of nitrogen and carbon isotopes during the last 7,165?years are interpreted as changes in the dominant processes in the lake. While epilimnetic primary productivity controlled isotope composition before 6,600?cal.?year BP, microbial processes, especially denitrification and methane oxidation, caused overall shifts of the ??15N and ??13C values since the Mid-Holocene. Comparisons with climate reconstructions from the Northern Island suggest that changes in the wind-induced lake overturn and a shift to more pronounced seasonality were the most likely causes for lake-internal changes since 6,600?cal.?year BP. 相似文献
1000.
Gunnar Luderer Valentina Bosetti Michael Jakob Marian Leimbach Jan C. Steckel Henri Waisman Ottmar Edenhofer 《Climatic change》2012,114(1):9-37
This paper synthesizes the results from the model intercomparison exercise among regionalized global energy-economy models conducted in the context of the RECIPE project. The economic adjustment effects of long-term climate policy are investigated based on the cross-comparison of the intertemporal optimization models ReMIND-R and WITCH as well as the recursive dynamic computable general equilibrium model IMACLIM-R. A number of robust findings emerge. If the international community takes immediate action to mitigate climate change, the costs of stabilizing atmospheric CO2 concentrations at 450?ppm (roughly 530?C550?ppm-e) discounted at 3% are estimated to be 1.4% or lower of global consumption over the twenty-first century. Second best settings with either a delay in climate policy or restrictions to the deployment of low-carbon technologies can result in substantial increases of mitigation costs. A delay of global climate policy until 2030 would render the 450?ppm target unachievable. Renewables and CCS are found to be the most critical mitigation technologies, and all models project a rapid switch of investments away from freely emitting energy conversion technologies towards renewables, CCS and nuclear. Concerning end use sectors, the models consistently show an almost full scale decarbonization of the electricity sector by the middle of the twenty-first century, while the decarbonization of non-electric energy demand, in particular in the transport sector remains incomplete in all mitigation scenarios. The results suggest that assumptions about low-carbon alternatives for non-electric energy demand are of key importance for the costs and achievability of very low stabilization scenarios. 相似文献