全文获取类型
收费全文 | 61939篇 |
免费 | 912篇 |
国内免费 | 569篇 |
专业分类
测绘学 | 1493篇 |
大气科学 | 4499篇 |
地球物理 | 12559篇 |
地质学 | 21648篇 |
海洋学 | 5475篇 |
天文学 | 13715篇 |
综合类 | 108篇 |
自然地理 | 3923篇 |
出版年
2021年 | 494篇 |
2020年 | 512篇 |
2019年 | 560篇 |
2018年 | 1190篇 |
2017年 | 1062篇 |
2016年 | 1430篇 |
2015年 | 860篇 |
2014年 | 1343篇 |
2013年 | 3049篇 |
2012年 | 1542篇 |
2011年 | 2242篇 |
2010年 | 1987篇 |
2009年 | 2654篇 |
2008年 | 2297篇 |
2007年 | 2337篇 |
2006年 | 2229篇 |
2005年 | 1886篇 |
2004年 | 1867篇 |
2003年 | 1795篇 |
2002年 | 1763篇 |
2001年 | 1500篇 |
2000年 | 1444篇 |
1999年 | 1255篇 |
1998年 | 1234篇 |
1997年 | 1271篇 |
1996年 | 1039篇 |
1995年 | 1033篇 |
1994年 | 967篇 |
1993年 | 893篇 |
1992年 | 835篇 |
1991年 | 770篇 |
1990年 | 823篇 |
1989年 | 745篇 |
1988年 | 757篇 |
1987年 | 867篇 |
1986年 | 749篇 |
1985年 | 996篇 |
1984年 | 1081篇 |
1983年 | 1021篇 |
1982年 | 894篇 |
1981年 | 927篇 |
1980年 | 838篇 |
1979年 | 791篇 |
1978年 | 772篇 |
1977年 | 726篇 |
1976年 | 702篇 |
1975年 | 663篇 |
1974年 | 686篇 |
1973年 | 698篇 |
1972年 | 415篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
A simple sand trap is used to measure swash and backwash bedload transport rates on intertidal profiles. Data from sixty-eight beach experiments are used to calculate a mean value of 12.78 kg m?4s?2 for the calibration coefficient in the Bagnold beach equation. 相似文献
942.
The riverine mud that escapes retention in the estuaries and enters the Gulf of Papua appears to be transported southeastward, across depth contours, by the prevailing currents in a series of wind-driven events. The mud deposits to the southeast of the rivers, at the mid-shelf region within a depth range of 40–60 m. Mud transported farther eastward is carried down the continental rise. Coarser riverine sediment (silt and sand) is deposited closer inshore. On the outer shelf (depth >60 m) relict carbonate debris dominates. The area of mud has maximum rates of pelagic and benthic productivity in the gulf. 相似文献
943.
944.
Like any other engineering structure, the dynamic aspects of semisubmersible offshore platforms require serious consideration. The free vibrations of the semisubmersible structures have been investigated in the present work, in which the effect of the variation of the length, draft and hull spacings on the natural frequencies and mode shapes has been studied and the nature of the variations and their reasons have been discussed. 相似文献
945.
Brent L. Lewis Pamela D. Holt Steven W. Taylor Steven W. Wilhelm Charles G. Trick Alison Butler George W. Luther III 《Marine Chemistry》1995,50(1-4)
Thermodynamic stability constants have been estimated for the complexation of iron(III) with catecholate-type siderophores isolated from the marine bacterium Alteromonas luteoviolacea and from the marine cyanobacterium Synechococcus sp. PCC 7002. Stability constants were determined utilizing the “chelate scale” of Taylor et al. (1994). The scale is based upon a linear relationship between the reduction potentials and the pH-independent thermodynamic stability constants for known iron(III) complexes. Log K values for the alterobactin B ferric iron complex are 43.6 ± 1.5 at pH 8.2 and 37.6 ± 1.2 at pH 6, consistent with a shift from bis-catecholate to monosalicylate/monocatecholate iron coordination with decreasing pH. Synechococcus isolates PCC 7002 Nos. 1 and 3 formed iron(III) complexes with stability constants of approximately 38.1 ± 1.2 and 42.3 ± 1.5, respectively. The binding strengths of the iron(III) complexes examined in this study are quite high, suggesting that catecholate siderophores may play a role in the solubilization and biological uptake of iron in the marine environment. 相似文献
946.
Hance D Smith 《Marine Policy》1995,19(6)
The environmental management of shipping requires systematic consideration of the geography of shipping routes, ship types and cargoes, and environments which can form the basis of a classificatory approach to Environmental Impact Assessment (EIA). Beyond this, effective environmental management depends in the first instance upon sound information management including monitoring of the environment, surveillance of shipping and information technology, followed by information assessment including, in particular, risk and environmental impact. The general management dimension includes co-ordination of technical management and organisational aspects related to shipping. It is suggested that effective environmental management of shipping may depend upon a regionalisation of the marine environment for EIA purposes akin to that found in the loadline rules, together with rationalisation of the increasing number of special areas in existence or proposed for this purpose. EIA will have to become an integral part of the overall management system in the shipping industry. 相似文献
947.
P. Ziveri R.C. Thunell D. Rio 《Deep Sea Research Part I: Oceanographic Research Papers》1995,42(11-12)
Seasonal changes in coccolithophore cell densities in the San Pedro Basin, Southern California Bight, were investigated for the period October 1991 to September 1992. Coccolitho phore cell densities ranged from 6.3 × 104 coccospheres per liter in March 1992 to 0 cells per liter in November 1991. High coccolithophore concentrations occurred in late winter and spring, and low densities occurred in the summer and fall. The high coccolithophore densities during spring 1992 were associated with unusually low surface nutrient concentrations and a lack of upwelling, suggesting that the high densities were not part of a typical spring phytoplankton bloom in this region. We propose that the suppression of upwelling during spring 1992 may have been related to the prevailing ENSO conditions. Emiliania huxleyi type A dominated the total coccolithophore population throughout the year, Umbilicosphaera sibogae var. sibogae being the second most important contributor to the coccolithophore assemblages. 相似文献
948.
The Korea (Tsushima) Strait is an important seaway through which the warm Tsushima Current flows into the East Sea (Japan
Sea). A paleogeographic map constrained by a regional sea-level curve developed on the basis of a number of recent 14C radiocarbon dates suggests that the Korea Strait was not closed during the last glacial period. Rather, it was open as a
channel-like seaway linking the western North Pacific and the East Sea. Some fraction of the paleo-Tsushima Current inflow
presumably continued at that time through the Korea Strait. The activity of the paleo-Tsushima Current is evidenced by the
distribution pattern of river-derived lowstand deposits, consisting of a beach/shoreface complex and lowstand deltaic wedges.
Received: 16 April 1999 / Revision accepted: 25 February 2000 相似文献
949.
Vertical distribution of anthropogenic carbon content of the water (exDIC) in the Oyashio area just outside of the Kuroshio/Oyashio Interfrontal Zone (K/O Zone) was estimated by the simple 1-D advection-diffusion model calibrated by the distribution of chlorofluorocarbons (CFCs). The average concentration of exDIC for = 26.60–27.00 is multiplied by the volume transport of Oyashio water into the North Pacific Intermediate Water (NPIW) to estimate the annual transport of exDIC into NPIW through K/O Zone. The estimated transport of exDIC was 0.018–0.020 GtC/y, which corresponds to 15% of the whole total exDIC accumulation in the temperate North Pacific. A simple assessment using the NPIW 1-box model indicates that the current study explains at least 70% of the total annual transport of exDIC into NPIW, and that small exDIC sources for NPIW still exists in addition to K/O Zone. 相似文献
950.
Mercury concentrations in blue crabs (Callinectes sapidus) collected from an area of mercury-contaminated sediments in Lavaca Bay, TX, USA, are more than an order of magnitude greater than concentrations in penaeid shrimp from the same area. Laboratory feeding experiments using mercury-contaminated fish as food showed that both blue crabs and pink shrimp (Penaeus duorarum) could accumulate mercury concentrations comparable to those in their food in 28 days. Calculated mercury assimilation efficiencies averaged 76% for blue crabs and 72% for pink shrimp. Significant depuration of mercury by blue crabs was not observed during a subsequent 28-day period, but pink shrimp lost mercury at a rate of about 0.012 day-1. Model calculations predict biomagnification factors of mercury of about two to three at steady state for both species. The large difference in observed concentrations of mercury in field-collected blue crabs and penaeid shrimp does not result from differences in efficiency of mercury assimilation from their food or from differences in excretion rates. It is more likely the result of differences in residence times in the contaminated area and of differences in feeding habits. 相似文献