首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   90篇
  免费   4篇
  国内免费   1篇
大气科学   9篇
地球物理   17篇
地质学   32篇
海洋学   14篇
天文学   20篇
自然地理   3篇
  2023年   2篇
  2021年   1篇
  2020年   1篇
  2018年   3篇
  2017年   1篇
  2016年   2篇
  2015年   2篇
  2014年   3篇
  2013年   5篇
  2012年   2篇
  2011年   5篇
  2010年   2篇
  2009年   4篇
  2008年   1篇
  2007年   5篇
  2006年   5篇
  2005年   4篇
  2004年   2篇
  2003年   2篇
  2002年   1篇
  2001年   4篇
  2000年   1篇
  1999年   1篇
  1998年   2篇
  1997年   3篇
  1996年   2篇
  1995年   1篇
  1994年   2篇
  1993年   3篇
  1992年   2篇
  1991年   3篇
  1990年   4篇
  1989年   1篇
  1988年   2篇
  1987年   1篇
  1986年   2篇
  1985年   2篇
  1984年   1篇
  1983年   1篇
  1979年   2篇
  1977年   1篇
  1975年   1篇
排序方式: 共有95条查询结果,搜索用时 46 毫秒
1.
The surface distribution of M stars is studied by differentiating them according to whether they show a circumstellar dust shell (CS) or not. Analysis shows that galactic latitudinal and longitudinal distributions are not determined by spectral subclasses alone. The study also indicates that M-type stars with CS have higher luminosities in the K band than those without CS. The M stars used in the study are obtained from theTwo-Micron Sky-Survey Catalogue (IRC) which is a most unbiased sample with respect to the interstellar extinction. The CS feature is identified by the ratio of flux densities at 12 and 25 m in the IRAS point source catalog.  相似文献   
2.
Archeological evidence of Pacific salmon in Hokkaido is reviewed and compared with results from western North America. Salmon remains have been found at 24 sites in Hokkaido from the Early Jomon Period to the Ainu Period (6000–100 years ago). Fish remains at three archeological sites in the Kushiro River basin indicated that Pacific salmon (Oncorhynchus spp.) were distributed and utilized from 6000 years ago. The present Kushiro Wetland was formerly covered with seawater and called the Paleo Kushiro Bay 5000–6000 years ago. Based on the molluscan fossil fauna, seawater temperature at Paleo Kushiro Bay was about 5°C warmer than at present. Warmer conditions for salmon in Kushiro 5000–6000 years ago corresponded with the poor conditions for salmon in the Columbia River basin 6000–7000 years ago. If the future global warming is similar to the conditions that prevailed 5000–6000 years ago, the southern limit of salmon distribution will shift northwards and the salmon production will decrease. However, they will not disappear from either Hokkaido or southwestern North America.  相似文献   
3.
 Amphiboles were synthesized from bulk compositions prepared along the join Ca1.8Mg5.2Si8O22(OH)2–Ca1.8Mg3Ga4Si6O22(OH)2 hydrothermally at 750–850 °C and 1.0–1.8 GPa, and along the join Ca2Mg5Si8O22F2–Ca2Mg3Ga4Si6O22F2, anhydrously at 1000 °C and 0.7 GPa to document how closely the tschermak-type substitution is obeyed in these analogues of aluminous amphiboles. Electron-microprobe analyses and Rietveld X-ray diffraction structure refinements were performed to determine cation site occupancies. The extent of Ga substitution was found to be limited in both joins, but with the fluorine series having about twice the Ga content (0.6 atoms per formula unit, apfu) of the hydroxyl-series amphiboles (0.3 apfu). The tschermak-type substitution was followed very closely in the hydroxyl series with essentially equal partitioning of Ga between tetrahedral and octahedral sites. The fluorine-series amphiboles deviated significantly from the tschermak-type substitution and, instead, appeared to follow a substitution that is close to a Ca-pargasite substitution of the type: [6]Ga3++2[4]Ga3++1/2[A] Ca2+ = [6]Mg2++2[4]Si4++1/2[A]□. Infrared spectroscopy revealed an inverse correlation between the intensity of the OH-stretching bands and the Ga content for the hydroxyl- and fluorine-series amphiboles. The direct correlation between the Ga and F content and inverse relationship between the Ga and OH content may be a general phenomenon present in other minerals and suggests, for example, that high F contents in titanite are controlled by the Al content of the host rock and that there may be similar direct Al–F correlations in tschermakitic amphiboles. Evidence for the possibility that Al (Ga) might substitute onto only a subset of the tetrahedral sites in tschermakitic amphiboles was sought but not observed in this study. Received: 5 March 2001 / Accepted: 31 July 2001  相似文献   
4.
Despite the wealth of archaeological sites and excellent conditions for preservation, few phytolith investigations have been undertaken from the Arabian Gulf region. The results from the Sasanian and Islamic archaeological tell of Kush, Ras al-Khaimah, United Arab Emirates, are presented. Kush is situated just inside the Gulf on an important trade route. The occupation sequence dates from the 4th century A.D. until the 13th century A.D., recording the development of the site in the Sasanian period, followed by the arrival of Islam in the 7th century A.D. and the final abandonment of the site in the late 13th century when the nearby site of al-Mataf (Julfar) began to develop closer to the present day coastline. All the samples analyzed contained abundant phytoliths (short cells, elongated cells, and groups of elongated cells) of various types. They included date palm (Phoenix dactylifera), papillae (possibly from barley (Hordeum)), and hair cells possibly from species of canary grass (Phalaris spp.). Some researchers have suggested that groups of elongated cells may indicate the presence of irrigation in semiarid environments. The present results for this class of phytoliths appeared to imply that intensive irrigation was unlikely to have taken place around Kush.  相似文献   
5.
The Aral Sea, Lake Balkhash, and Lake Kamyslybas are closed lakes in Central Asia. They range from oligosaline to metasaline. The salinity of the Aral Sea has increased by more than 30 g L−1 since widespread irrigation began in its catchment area. Few studies of the phytoplankton have been conducted on these lakes since extensive irrigation started. The investigation reported here compares the flora of phytoplankton in these saline lakes. In the Small Aral Sea, phytoplankton density gradually decreased with increasing electrical conductivity (EC) (∼ salinity), but there was no such relation in Lake Balkhash and Lake Kamyslybas. In the Aral Sea, Dinophyceae and Bacillariophyceae were frequently observed in most areas of high EC value, and Cyanophyceae were most conspicuous in the area of medium and lower EC values. In Lake Balkhash, Cyanophyceae were most conspicuous, but Chlorophyceae were also noticeable. Most Cyanophyceae in Aral Sea formed filaments with heterocysts. The distinct characteristic of the phytoplankton of the Lake Balkhash was that all dominant species form colonies covered with a gelatinous film. Siliceousplankton diversity gradually decreased with increasing EC values in the Aral Sea and Lake Balkhash.  相似文献   
6.
7.
8.
Our analysis of the last three decades of retrospective data of vertical distributions and size composition of chlorophyll-a (Chl-a) over the western North Pacific has revealed significant changes of three indices related to Chl-a during summer season, as follows: (1) decreasing linear trend of the proportion of Chl-a in surface layer to that of the whole water column by 0.4 and 2.3% year−1 in the subtropical area along 137°E (STA137) during 1972 to 1997 and in the Kuroshio Extension area along 175°E (KEA175) during 1990 to 2001; (2) increasing linear trend of the depth of subsurface Chl-a maximum (DCM) by 0.4 and 2.6 m year−1 in STA137 and KEA175; and (3) decreasing linear trend of larger-size Chl-a (>3 μm) by 0.1 and 2.5% year−1 in STA137 and KEA175, respectively. Water density (σ θ ) at 75 m depth had also decreased by 0.006 and 0.05 year−1 in STA137 and KEA175, respectively. The ratio of biogenic opal to biogenic CaCO3 in the sinking flux decreased by 0.015 year−1 in the subtropical region from 1997 to 2005. These findings may indicate that the subsurface chlorophyll maximum is deepening and larger phytoplankton such as diatoms has been decreasing during the past decade, associated with the decreasing density of surface water caused by warming in the western North Pacific, especially in the summer.  相似文献   
9.
Two onboard observation campaigns were carried out in the western boundary region of the Philippine Sea in December 2006 and January 2008 during the 2006/07 El Niño and the 2007/08 La Niña to observe the North Equatorial Current (NEC), Mindanao Current (MC), and Kuroshio current system. The NEC and MC measured in late 2006 under El Niño conditions were stronger than those measured during early 2008 under La Niña conditions. The opposite was true for the current speed of the Kuroshio, which was stronger in early 2008 than in late 2006. The increase in dynamic height around 8°N, 130°E from December 2006 to January 2008 resulted in a weakening of the NEC and MC. Local wind variability in this region did not appear to contribute to changes in the current system.  相似文献   
10.
Meso- or submeso-scale features of the Antarctic sea ice are investigated using the MOS-1/1b MESSR Images (spatial resolution of approximately 50 m) received at Syowa Station. Particular attention is paid to the ice bands and ice streamers in coastal polynyas. In the Antarctic Ocean, ice bands can be often seen not only at the ice edge but also in the ice interior zone throughout the year and they extend for hundreds of kilometers in the latitudinal direction. It is found that the width and spacing of ice bands tend to decrease from winter to summer. The width of ice band is about 2–6 km in August and September, and 0.1–0.7 km in December. The spacing of ice bands is about 3–10 km in August and September, and 0.1–2 km in December. In coastal polynyas, ice streamers, which are composed of new ice, are sometimes observed. In general, the row of the streamers is spaced at 0.5–2 km with a width of 0.1–1.0 km.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号