全文获取类型
收费全文 | 77266篇 |
免费 | 1013篇 |
国内免费 | 628篇 |
专业分类
测绘学 | 1994篇 |
大气科学 | 6176篇 |
地球物理 | 14817篇 |
地质学 | 30369篇 |
海洋学 | 5994篇 |
天文学 | 15356篇 |
综合类 | 265篇 |
自然地理 | 3936篇 |
出版年
2021年 | 396篇 |
2020年 | 462篇 |
2019年 | 487篇 |
2018年 | 3838篇 |
2017年 | 3648篇 |
2016年 | 2862篇 |
2015年 | 966篇 |
2014年 | 1420篇 |
2013年 | 2803篇 |
2012年 | 2538篇 |
2011年 | 4558篇 |
2010年 | 4150篇 |
2009年 | 4753篇 |
2008年 | 3902篇 |
2007年 | 4397篇 |
2006年 | 2075篇 |
2005年 | 2044篇 |
2004年 | 1905篇 |
2003年 | 1928篇 |
2002年 | 1733篇 |
2001年 | 1385篇 |
2000年 | 1398篇 |
1999年 | 1119篇 |
1998年 | 1079篇 |
1997年 | 1157篇 |
1996年 | 974篇 |
1995年 | 927篇 |
1994年 | 891篇 |
1993年 | 741篇 |
1992年 | 761篇 |
1991年 | 729篇 |
1990年 | 744篇 |
1989年 | 706篇 |
1988年 | 682篇 |
1987年 | 777篇 |
1986年 | 714篇 |
1985年 | 840篇 |
1984年 | 925篇 |
1983年 | 958篇 |
1982年 | 867篇 |
1981年 | 819篇 |
1980年 | 813篇 |
1979年 | 725篇 |
1978年 | 720篇 |
1977年 | 663篇 |
1976年 | 597篇 |
1975年 | 587篇 |
1974年 | 641篇 |
1973年 | 614篇 |
1972年 | 390篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Hueiwang Anna C. Jeng Andrew J. Englande Reda M. Bakeer Henry B. Bradford 《Estuarine, Coastal and Shelf Science》2005,63(4):513-526
Urban stormwater runoff could contribute to the deterioration of water quality of a receiving water body. In this research, field studies and laboratory experiments were conducted to assess the microbial contamination resulting from urban stormwater runoff into the Lake Pontchartrain estuary. Fecal coliform, Escherichia coli and enterococci were used as indicator organisms. The specific objectives of the research were to examine the distribution of the indicator organisms in different environmental elements (water column, suspended particles and sediment) and to further investigate the mechanisms related to their fate. Results of the research indicated satisfactory water quality at the study sites during dry weather periods. However, a significant increase was observed in the concentrations of the indicator organisms in the water columns and sediment at specific study sites following a given stormwater event. Three to seven days were needed for the elevated indicator organisms to return back to their background levels in the water column and sediment, respectively. The mechanism of sedimentation contributed to a reduction in the microbial concentration in the water column, as the indicator organisms were found to attach to the suspended particles in the stormwater. The percentage of fecal coliform, E. coli and enterococci attached to the suspended particles was found to be within the range of 9.8–27.5%, 21.8–30.4%, and 8.4–11.5% of the total indicator organisms in the stormwater loaded into the estuary, respectively. About 75–80% of the total indicator organisms remained free-floating for some distance in the water column before dying off. 相似文献
992.
We have sampled particles of native aluminium (Al°) in two sediment cores from the Central Indian Basin (CIB). The cores are geographically separated but are located at the base of two seamounts. The native Al° particles occurring as grains and spherules, have an average Al content of 95% and are associated with volcanogenic–hydrothermal material. Morphologically and compositionally, the specimens are similar to those reported from the East Pacific Rise. After ruling out several processes for the presence of the native Al°, we hypothesize that during progressive melting of magma, a basaltic magma is produced which has high contents of reductants such as methane and hydrogen, and a low oxygen fugacity. During the upward migration of such magma, reduction to metallic aluminium and the formation of native Al° particles takes place. 相似文献
993.
An unsteady wave driver for narrowbanded waves: modeling nearshore circulation driven by wave groups
In this paper, we derive an unsteady refraction–diffraction model for narrowbanded water waves for use in computing coupled wave–current motion in the nearshore. The end result is a variable coefficient, nonlinear Schrödinger-type wave driver (describing the envelope of narrow-banded incident waves) coupled to forced nonlinear shallow water equations (describing steady or unsteady mean flows driven by the short-wave field). Comparisons with experimental data show that good accuracy can be obtained for cases of nonbreaking wave transformation. Numerical simulations show that the interaction of wave groups with longshore topographic nonuniformities generates strong edge wave resonances, providing a generating mechanism for low-order edge waves. 相似文献
994.
995.
996.
Thomas F. Wever Hannelore M. Fiedler Gunther Fechner Friedrich Abegg Ingo H. Stender 《Geo-Marine Letters》1997,17(4):246-252
Between 2 and 6 February, 1995, a 25 km2 area at the Dry Tortugas (Florida Keys) was surveyed with a 100 kHz side-scan sonar system and 3.5-kHz subbottom profiler.
The side-scan system revealed a pattern of alternating high and low backscatter. The subbottom profiler showed areas with
no acoustic penetration between sediment troughs. The combination of both methods allowed delineation of the boundaries in
high-backscatter regions, and sediment samples allowed correlations between high backscatter and coarser-grained sediments. 相似文献
997.
An analysis of published and original data on the meiobenthos abundance in the depth interval from 100 to 9807 m (in total, 665 records, 445 of them obtained for depths exceeding 1000 m) revealed general regularities in its distribution. The influence of the sampling and data processing methods on the quantitative estimates of the meiobenthos abundance is considered to demonstrate changes in the proportions of the main meiobenthic taxa at different depths and to characterize latitudinal changes in the meiobenthos abundance. The dependence of the abundance of free-living nematodes, the most abundant group of metazoan meiobenthos, on trophic conditions is analyzed. No significant differences in the meiobenthos abundance in the samples obtained by box-and multicorers are established. It is shown that the share of nematodes in metazoan meiobenthos communities increases with the depth. In temperate latitudes, a distinct maximum in the population density confined to depths exceeding 1 km is observed. The quantitative distribution of the meiobenthos at the depths gradient is controlled by the bottom macrotopography and trophic conditions. 相似文献
998.
C. K. Paull W. Ussler H. G. Greene R. Keaten P. Mitts J. Barry 《Geo-Marine Letters》2003,22(4):227-232
A sediment gravity flow descended through the axis of Monterey Canyon on 20 December 2001 at 13:35 Pacific standard time.
The timing of this event is documented by a current-meter package which recorded an 11.9-dbar pressure increase in less than
10 min and was found 550 m down-canyon from its deployment site, buried completely within a >70-cm-thick gravity flow deposit.
This event is believed to have started in less than 290 m of water because an instrument at this location was also lost at
the same time. A 178-cm core collected after the event from the axis of the canyon at 1,297-m water depth contained fresh,
greenish, chlorophyll-rich organic material at 32-cm sub-bottom depth, suggesting the event extended to this water depth.
The only trigger identified for this mass movement event appears to be moderate sea and surf conditions. Thus, gravity flow
events of this magnitude do not require an exceptional triggering event. 相似文献
999.
C. K. Paull W. Ussler H. G. Greene R. Keaten P. Mitts J. Barry 《Geo-Marine Letters》2002,22(4):227-232
A sediment gravity flow descended through the axis of Monterey Canyon on 20 December 2001 at 13:35 Pacific standard time. The timing of this event is documented by a current-meter package which recorded an 11.9-dbar pressure increase in less than 10 min and was found 550 m down-canyon from its deployment site, buried completely within a >70-cm-thick gravity flow deposit. This event is believed to have started in less than 290 m of water because an instrument at this location was also lost at the same time. A 178-cm core collected after the event from the axis of the canyon at 1,297-m water depth contained fresh, greenish, chlorophyll-rich organic material at 32-cm sub-bottom depth, suggesting the event extended to this water depth. The only trigger identified for this mass movement event appears to be moderate sea and surf conditions. Thus, gravity flow events of this magnitude do not require an exceptional triggering event. 相似文献
1000.
Arnold H. Bouma James M. Coleman Charles E. Stelting Barry Kohl 《Geo-Marine Letters》1989,9(3):161-170
A conceptual sea-level-driven depositional model for individual fanlobes (channel-overbank systems) of the Mississippi Fan
does not permit direct application of the sequence stratigraphic principles of Vail and colleagues. Deep Sea Drilling Project
Leg 96 results suggest that, during initial relative lowering of sea level, the canyon and upper fan channel were formed;
excavated fine-grained slope sediments may have formed a debris flow deposit base for the fanlobe. Continued lowering produced
constructional channel-levee-overbank deposits. Rising relative sea level inhibited input of coarse clastics, and channel
depressions filled with muds. A blanket of (hemi)pelagics represents relative high sea level stand. 相似文献