全文获取类型
收费全文 | 75344篇 |
免费 | 1090篇 |
国内免费 | 523篇 |
专业分类
测绘学 | 1717篇 |
大气科学 | 4989篇 |
地球物理 | 15056篇 |
地质学 | 26695篇 |
海洋学 | 6596篇 |
天文学 | 17523篇 |
综合类 | 179篇 |
自然地理 | 4202篇 |
出版年
2021年 | 740篇 |
2020年 | 843篇 |
2019年 | 910篇 |
2018年 | 1807篇 |
2017年 | 1736篇 |
2016年 | 2024篇 |
2015年 | 1116篇 |
2014年 | 1978篇 |
2013年 | 3693篇 |
2012年 | 2236篇 |
2011年 | 2995篇 |
2010年 | 2686篇 |
2009年 | 3553篇 |
2008年 | 3068篇 |
2007年 | 3118篇 |
2006年 | 2913篇 |
2005年 | 2109篇 |
2004年 | 2120篇 |
2003年 | 2088篇 |
2002年 | 1925篇 |
2001年 | 1704篇 |
2000年 | 1595篇 |
1999年 | 1378篇 |
1998年 | 1425篇 |
1997年 | 1384篇 |
1996年 | 1170篇 |
1995年 | 1180篇 |
1994年 | 1030篇 |
1993年 | 922篇 |
1992年 | 851篇 |
1991年 | 868篇 |
1990年 | 955篇 |
1989年 | 831篇 |
1988年 | 764篇 |
1987年 | 932篇 |
1986年 | 845篇 |
1985年 | 1042篇 |
1984年 | 1157篇 |
1983年 | 1128篇 |
1982年 | 1003篇 |
1981年 | 977篇 |
1980年 | 866篇 |
1979年 | 807篇 |
1978年 | 804篇 |
1977年 | 778篇 |
1976年 | 706篇 |
1975年 | 720篇 |
1974年 | 686篇 |
1973年 | 745篇 |
1972年 | 466篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
931.
Sedimentary, palynologic and 14C analysis of marls and swamp‐peats, formed under fluctuating artesian spring conditions, provide climate and vegetation records from >52,000 to 0 yr BP at Mowbray, and 30,000 to 0 yr BP at Broadmeadows. Before about 65,000 yr BP conditions at Mowbray were relatively dry and the vegetation was Leptospermum shrubland. After 65,000 and before 55,000 yr BP moist conditions produced Cyperaceae swamps. Between 55,000 and 45,000 yr BP the climate was relatively dry, and between 45,000 and 35,000 yr BP relatively moist. Leptospermum shrubs were dominant in both periods. The climate was moist between 35,000 and 22,000 yr BP and sedge swamps formed. Between 22,000 and 11,000 yr BP the climate was relatively dry and grasses were important. Postglacial climate (11,000–0 yr BP) was warm and moist, and Melaleuca‐Leptospermum forest and shrubland flourished. The climatic changes suggested for north western Tasmania seem to compare broadly with changes suggested for Tasmanian and for other southern Australian sites, but the correlation is limited by imprecise dating. 相似文献
932.
Remote sensing is a geographic analysis tool capable of producing large quantities of data in the spectral, temporal and spatial domains. Techniques for automating the image analysis process would be advanced by the inclusion of artificial intelligence techniques in the design of image processing systems. The remote sensing application which show promise for successful implementation of artificial intelligence techniques are intelligent onboard processing, advanced database interrogation, and the automated analysis of multispectral imagery. 相似文献
933.
The organizational separation of management and other quaternary activities from production operations in American manufacturing has resulted in a divergent pattern of production and headquarter locations. We examine the external control relationships that arise from this separation between headquarter location and branch plant operations in nonmetropolitan Kentucky. Although corporate headquarters have diffused to the Sunbelt, no shift was found in the corporate control of branch plants in Kentucky. Nonmetropolitan manufacturing in Kentucky was oriented toward the metropolitan centers of the traditional manufacturing belt rather than the newer corporate centers of the South. Also control relationships are more likely to be local among smaller factories. Large plants having more autonomy in production or producing finished goods were more likely to be owned by a distant firm located in a large SMSA. 相似文献
934.
935.
936.
Robert S. Webb Jonathan T. Overpeck David M. Anderson Bruce A. Bauer Mildred K. England Wendy S. Gross E. Ann Meyers Michael M. Worobec 《Journal of Paleolimnology》1993,9(1):69-75
Conclusion The World Data Center-A for Paleoclimatology, located in the NOAA/NGDC Paleoclimatology Program, is committed to providing the scientific community with easy access to all paleoenvironmental data. Efforts to make archived data readily available include international coordination of data acquisition, management, and distribution, sponsoring workshops and data cooperatives to facilitate the compilation of important data sets, development of a browse and visualization software package (PaleoVu), and dispersal of archived data on magnetic media or over ANONYMOUS FTP/INTERNET. The program publishes a semi-annual newsletter that highlights latest developments and accomplishments in the area of paleoenvironmental data for global change research. Contributions to the newsletter are welcome from researchers describing their efforts to coordinate the free flow of paleoclimate data throughout the international scientific community.For information on the program or to be added to the mailing list contact Mrs Mildred England (phone: 303-497-6227; Fax: 303 497-6513; e-mail: MKE@mail.ngdc.noaa.gov), NOAA National Geophysical Data Center, Paleoclimatology Program/World Data Center-A for Paleoclimatology, 325 Broadway, E/GC Boulder, CO 80303 USA 相似文献
937.
Late Quaternary palaeolimnology of a tropical marl lake: Wallywash Great Pond,Jamaica 总被引:1,自引:0,他引:1
F. A. Street-Perrott P. E. Hales R. A. Perrott J. C. Fontes V. R. Switsur A. Pearson 《Journal of Paleolimnology》1993,9(1):3-22
Wallywash Great Pond (17° 57 N, 77° 48 W, 7 m a.s.l.) is the largest perennial lake in Jamaica. It occupies a fault trough within the karstic White Limestone. The Great Pond is a hardwater lake with a pH of 8.2–8.6 and an alkalinity of 3.6–3.9 meq 1–1. Its chemistry is strongly influenced by the spring discharge from the limestone. The lake water is subject to degassing, evaporation and bicarbonate assimilation by submerged plants and algae, resulting in marl precipitation. A 9.23 m core (WGP2), taken from a water depth of 2.8 m, was analysed for magnetic susceptibility, loss-on-ignition, carbonate content, mole % MgCO3 in calcite, and stable isotopes in the fine carbonate fraction. The chronology is based on ten14C and four U/Th dates. Four main sediment types alternate in the core: marl; organic, calcareous mud; organic mud or peat; and earthy, brown, calcareous mud. The marls represent periods of wet/warm climate during sea-level highstands and the organic deposits, shallower, swampy conditions. In contrast, the brown, calcareous muds were laid down when the lake was dry or ephemeral. The last interglacial (120 000- 106 000 yr BP) is represented by three distinct marl units. After a dry interval, stable, wet/warm conditions set in from 106 000 to 93 000 yr BP. A dry/cool climate prevailed between 93 000 and at least 9500 yr BP. Three subsequent cycles of alternating wet and dry conditions culminated in flooding of the basin by the Black River during the late Holocene. These recent events cannot be accurately dated by14C due to significant and temporally-variable inputs of dead carbon from the springs. 相似文献
938.
939.
Remnants of the Last Interglacial shoreline occur at Middle Lagoon on the far south coast of New South Wales. Relict beach sediments can be traced to a height of at least +4.8 m and are indicative of a former mean sea level of about +3 m. Thermoluminescence (TL) ages of 126 ± 13 ka and 114 ± 15 ka were determined for beach and aeolian facies respectively. Sands in the lower part of an exposure on the adjacent Gillards Beach gave TL ages of 108 ± 13 ka, but sands in the upper part of that exposure gave an age of 19.9 ± 3.5 ka. This chronological evidence of a stratigraphic unconformity in what was initially taken as pedogenic differentiation at Gillards Beach is supported by contrasting electron traps and colour centres in crystal lattices of quartz grains in these two samples. No tectonic displacement is apparent. This site provides the first evidence of the Last Interglacial sea level for 1000 km along the coast between Gippsland and Newcastle. 相似文献
940.
Chemical weathering is commonly assumed to be negligible in the denudation of alpine landscapes. Spot samples of dissolved ions from alpine streams on the Blanca Massif had minimum average concentrations in 1978 of 35.53 ppm (range: 14.9–75.6 ppm) and in 1979, 20.23 ppm (range: 7.35–35.0 ppm). Correlations between the variation in the concentration of dissolved ions (silica, pH, and minimum ion concentration) and the elevations and drainage-basin areas of sample sites were significant at the 0.05 level. 相似文献