全文获取类型
收费全文 | 1641篇 |
免费 | 72篇 |
国内免费 | 26篇 |
专业分类
测绘学 | 36篇 |
大气科学 | 165篇 |
地球物理 | 432篇 |
地质学 | 563篇 |
海洋学 | 147篇 |
天文学 | 233篇 |
综合类 | 9篇 |
自然地理 | 154篇 |
出版年
2023年 | 10篇 |
2022年 | 9篇 |
2021年 | 16篇 |
2020年 | 29篇 |
2019年 | 35篇 |
2018年 | 41篇 |
2017年 | 61篇 |
2016年 | 47篇 |
2015年 | 51篇 |
2014年 | 54篇 |
2013年 | 100篇 |
2012年 | 49篇 |
2011年 | 91篇 |
2010年 | 72篇 |
2009年 | 100篇 |
2008年 | 94篇 |
2007年 | 98篇 |
2006年 | 70篇 |
2005年 | 62篇 |
2004年 | 63篇 |
2003年 | 48篇 |
2002年 | 41篇 |
2001年 | 36篇 |
2000年 | 32篇 |
1999年 | 36篇 |
1998年 | 28篇 |
1997年 | 26篇 |
1996年 | 20篇 |
1995年 | 32篇 |
1994年 | 20篇 |
1993年 | 17篇 |
1992年 | 10篇 |
1991年 | 15篇 |
1990年 | 14篇 |
1989年 | 7篇 |
1988年 | 12篇 |
1987年 | 10篇 |
1986年 | 10篇 |
1985年 | 19篇 |
1984年 | 13篇 |
1983年 | 23篇 |
1982年 | 15篇 |
1981年 | 11篇 |
1980年 | 11篇 |
1979年 | 8篇 |
1978年 | 10篇 |
1977年 | 9篇 |
1976年 | 12篇 |
1974年 | 7篇 |
1973年 | 10篇 |
排序方式: 共有1739条查询结果,搜索用时 171 毫秒
951.
952.
953.
Bacterial iron oxide reduction in a terrigenous sediment-impacted tropical shallow marine carbonate system, north Jamaica 总被引:1,自引:0,他引:1
Kevin G. Taylor Christopher T. Perry Anthony M. Greenaway Philip G. Machent 《Marine Chemistry》2007,107(4):449-463
Discovery Bay, a carbonate-dominated embayment in north Jamaica, has been subject to inputs for 40 years of iron-rich bauxite sediment associated with the local mining and transport of processed bauxite. As such, this site is an ideal natural laboratory to study the records and impacts of iron oxide inputs upon geochemical, diagenetic, and microbial processes in tropical carbonate sediments.Total Fe contents in sites in the bay not receiving bauxite inputs are negligible and porewater Ca2+, SO42− and Cl− indicate that bacterial sulphate reduction is an important process. In contrast, surface sediments receiving bauxite inputs contain significant total Fe, from 44 μmol/g in shallow (5 m water depth) sites to 110 μmol/g in deeper (20 m water depth) sites. Up-core increases in total Fe record increased temporal inputs into the bay. Within these Fe-rich sediments porewater data shows the presence of FeII released by bacterial FeIII reduction. There is no direct evidence for significant bacterial sulphate reduction in these sediments. Iron oxides within all bauxite-impacted sediments display a high potential reducibility, from 40% to 80% of the total Fe present as dithionite-extractable FeIII. Experimental analysis of the potential susceptibility to, and rates of, bacterial FeIII reduction, utilising Discovery Bay sediment and Shewanella putrefaciens CN32 (a known FeIII-reducer) has confirmed the high bacterial reducibility of iron oxides within the sediment. Up to 75% of initial dithionite-extractable FeIII in the sediments was reduced over 15 days.The presence of iron oxides within the Discovery Bay shallow marine carbonate systems has markedly altered the chemical diagenetic processes taking place, with a shift from apparent dominance of bacterial sulphate reduction at non-impacted (Fe-poor) sites, to highly significant bacterial FeIII reduction in Fe-rich bauxite-impacted sediments. Given the perceived global increases in terrigenoclastic sediment inputs into tropical carbonate systems as a result of land-use and climate changes, coupled with the documented role that iron oxide reduction plays in nutrient and contaminant cycling in sediment systems, more research into the perturbation of early diagenesis by iron oxide inputs is required. 相似文献
954.
955.
Anil?DeoEmail author Kevin?J.?E.?Walsh Alexandre?Peltier 《Theoretical and Applied Climatology》2017,127(3-4):711-724
A deficit in precipitation may impact greatly on soil moisture, snowpack, streamflow, groundwater and reservoir storage. Among the several approaches available to investigate this phenomenon, one of the most applied is the analysis of dry spells. In this study, a non-homogeneous Poisson model has been applied to a set of high-quality daily rainfall series, recorded in southern Italy (Calabria region) during the period 1981–2010, for the stochastic analysis of dry spells. Firstly, some statistical details of the Poisson models were presented. Then, the proposed model has been applied to the analysis of long dry spells. In particular, a Monte Carlo technique was performed to reproduce the characteristics of the process. As a result, the main characteristics of the long dry spells have shown patterns clearly related to some geographical features of the study area, such as elevation and latitude. The results obtained from the stochastic modelling of the long dry spells proved that the proposed model is useful for the probability evaluation of drought, thus improving environmental planning and management. 相似文献
956.
957.
Decadal atmosphere-ocean variations in the Pacific 总被引:67,自引:7,他引:60
Considerable evidence has emerged of a substantial decade-long change in the north Pacific atmosphere and ocean lasting from about 1976 to 1988. Observed significant changes in the atmospheric circulation throughout the troposphere revealed a deeper and eastward shifted Aleutian low pressure system in the winter half year which advected warmer and moister air along the west coast of North America and into Alaska and colder air over the north Pacific. Consequently, there were increases in temperatures and sea surface temperatures (SSTs) along the west coast of North America and Alaska but decreases in SSTs over the central north Pacific, as well as changes in coastal rainfall and streamflow, and decreases in sea ice in the Bering Sea. Associated changes occurred in the surface wind stress, and, by inference, in the Sverdrup transport in the north Pacific Ocean. Changes in the monthly mean flow were accompanied by a southward shift in the storm tracks and associated synoptic eddy activity and in the surface ocean sensible and latent heat fluxes. In addition to the changes in the physical environment, the deeper Aleutian low increased the nutrient supply as seen through increases in total chlorophyll in the water column, phytoplankton and zooplankton. These changes, along with the altered ocean currents and temperatures, changed the migration patterns and increased the stock of many fish species. A north Pacific (NP) index is defined to measure the decadal variations, and the temporal variability of the index is explored on daily, annual, interannual and decadal time scales. The dominant atmosphere-ocean relation in the north Pacific is one where atmospheric changes lead SSTs by one to two months. However, strong ties are revealed with events in the tropical Pacific, with changes in tropical Pacific SSTs leading SSTs in the north Pacific by three months. Changes in the storm tracks in the north Pacific help to reinforce and maintain the anomalous circulation in the upper troposphere. A hypothesis is put forward outlining the tropical and extratropical realtionships which stresses the role of tropical forcing but with important feed-backs in the extratropics that serve to emphasize the decadal relative to interannual time scales. The Pacific decadal timescale variations are linked to recent changes in the frequency and intensity of El Niño versus La Nina events but whether climate change associated with global warming is a factor is an open question.The National Center for Atmospheric Research is sponsored by the National Science Foundation 相似文献
958.
959.
Kevin G. Speer G. Siedler L. Talley 《Deep Sea Research Part I: Oceanographic Research Papers》1995,42(11-12)
Recent measurements indicate the transatlantic extent of the Namib Col Current at depths of 1300–3000 m near Lat. 22°S in the South Atlantic Ocean. This current forms a continuous circulation structure from the Namib Col on the Walvis Ridge to the western trough, though its characteristic change as deep water with varying properties enters and leaves the current owing to a meridional flow component. Transport estimates from hydrographic sections on the Walvis Ridge and at 15°W near the crest of the Mid-Atlantic Ridge indicate a strength of about 3 × 106 m3 s−1. The current is part of a larger-scale eastward flow at Lon. 25°W; transport estimates across the salinity maximum core there show a similar strength. Associated with this high-salinity high-oxygen current is a basin-wide front in these properties of varying intensity (weaker in the east) marking the transition to deep water whose North Atlantic characteristics have been partly erased by mixing with Circumpolar Deep Water in the southwest South Atlantic. The water which finally crosses the Walvis Ridge is supplied both by the eastward flow of this (diluted) North Atlantic Deep Water and by a general southeastward interior flow from the northern Angola Basin. Evidence suggests that this deep water continues south in the eastern Cape Basin, leaving the South Atlantic near the African continent. 相似文献
960.
Jason L. Blum Iris Knoebl Patrick Larkin Kevin J. Kroll Nancy D. Denslow 《Marine environmental research》2004,58(2-5):565
In this study male largemouth bass (LMB) were exposed to the naturally occurring androgens, dihydrotestosterone (DHT) or 11-ketotestosterone (11-KT) in order to identify genes that are differentially regulated by these steroid hormones. Using subtractive hybridization on livers of fish treated with DHT against vehicle control, many novel LMB genes were cloned. These genes were added to our gene library and arrayed. Six genes were up-regulated and five were down-regulated by both androgens. But, each androgen also regulated specific genes. One gene that was identified as a potential androgen marker was spermidine-spermine-N1-acetyltransferase that was up-regulated by both androgens. Determining which genes are responsive to natural androgens will help to identify biochemical pathways that are impacted. 相似文献