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   226篇
  免费   22篇
  国内免费   38篇
地球物理   25篇
地质学   104篇
海洋学   62篇
综合类   22篇
自然地理   73篇
  2024年   2篇
  2023年   4篇
  2022年   4篇
  2021年   4篇
  2020年   3篇
  2018年   8篇
  2017年   3篇
  2016年   11篇
  2015年   6篇
  2014年   8篇
  2013年   8篇
  2012年   4篇
  2011年   9篇
  2010年   10篇
  2009年   8篇
  2008年   19篇
  2007年   12篇
  2006年   20篇
  2005年   12篇
  2004年   7篇
  2003年   17篇
  2002年   12篇
  2001年   7篇
  2000年   6篇
  1999年   10篇
  1998年   7篇
  1997年   13篇
  1996年   5篇
  1995年   6篇
  1994年   9篇
  1993年   5篇
  1992年   6篇
  1991年   4篇
  1990年   5篇
  1989年   4篇
  1988年   4篇
  1987年   1篇
  1985年   1篇
  1983年   1篇
  1981年   1篇
排序方式: 共有286条查询结果,搜索用时 265 毫秒
271.
Grande Riviere beach, on the island of Trinidad, supports the largest nesting population of leatherback turtles in the Caribbean region. Throughout the nesting season, nests are naturally disturbed by newly nesting females, resulting in egg breakage and loss of some nest viability. This environment is ideal for the growth and proliferation of microorganisms. The range of bacterial flora present in beach sand and egg shells was examined, with emphasis on bacteria that may pose a threat to public and animal health. The extent to which the bacterial load and genera on the beach changed throughout the season was also assessed. Twenty‐five genera were identified, with Pseudomonas spp. found to be the most predominant environmental bacteria. Four genera identified possess zoonotic potential, while five additional genera are known to be of public and animal health significance. Distinct shifts in the density and distribution of bacteria were observed along the beach from early to peak nesting season. Shifts were seen across heavily traversed zones, thus highlighting the potential exposure threats posed to beach visitors and animals alike. Further studies aimed at speciating this population of bacteria, as well as isolating potential fungal pathogens may mitigate this threat. Identification of bacterial agents that are specifically pathogenic to leatherback turtles, turtle eggs, hatchlings and those who may interact with these animals will serve to enhance and guide efforts to better conserve this species and protect the health of all who visit this ecologically significant site.  相似文献   
272.
A sedimentary sequence from the Mediterranean coastal basin of Lago di Massaciuccoli (Tuscany, Italy) was analyzed for diatoms, covering two periods over the past ca. 7,000 years. The site was selected because it is situated in a sensitive position at the limit between Mediterranean and Central European climates and biomes. Our focus is on the impact of accelerated human activity during the recent past (water uptake in the catchment, sand extraction, wastewater discharge) and on a phase of evident change between 6,600 and 5,400 cal. BP. The diatom record suggests fresh-water conditions and rather high lake levels until ca. 6,000 years ago. The subsequent shift towards brackish conditions peaked at around 5,500 cal. BP. We relate this shift to a pervasive change towards a drier climate that has been observed elsewhere in the Mediterranean and Northern African regions, and stands in contrast to the shift towards a cooler and more humid climate in the nearby Alps (200–350 km distant) and in central Europe. Pollen and charcoal records from a previous study on the same sedimentary sequence were used to gain additional insights about the causes of the changes in the diatom assemblages and apply numerical methods to search for common trends and correlations.  相似文献   
273.
The siliceous sinter deposits of El Tatio geothermal field in northern Chile have been examined petrographically and mineralogically. These sinters consist of amorphous silica (opal-A) deposited around hot springs and geysers from nearly neutral, silica-saturated, sodium chloride waters. Water cooling and evaporation to dryness are the main processes that control the opal-A deposition in both subaqueous and subaerial settings, in close spatial relation to microbial communities. All fingerprints of organisms observed in the studied sinter samples represent microbes and suggest that the microbial community is moderately diverse (cyanobacteria, green bacteria, and diatoms). The most important ecological parameter is the temperature gradient, which is closely related to the observed depositional settings: 1) Geyser setting: water temperature = 70–86 °C (boiling point at El Tatio: 4200 m a.s.l.); coarse laminated sinter macrostructure with rapid local variations; biota comprises non-photosynthetic hyperthermophilic bacteria. 2) Splash areas around geysers: water temperature = 60–75 °C; laminated spicule and column macrostructure, locally forming cupolas (< 30 cm); predominant Synechococcus-like cyanobacteria. 3) Hot spring setting: water temperature = 40–60 °C; laminated spicules and columns and subspherical oncoids characterize the sinter macrostructure; filamentous cyanobacteria Phormidium and diatoms (e.g., Synedra sp.) are the most characteristic microbes. 4) Discharge environments: water temperature = 20–40 °C; sinter composed of laminated spicules and oncoids of varied shape; cyanobacterial mats of Phormidium and Calothrix and diatoms (e.g., Synedra sp.) are abundant. El Tatio is a natural laboratory of great interest because the sedimentary macrostructures and microtextures reflect the geological and biological processes involved in the primary deposition and early diagenesis of siliceous sinters.  相似文献   
274.
Materials of previous studies and new original geological and paleobotanic data are used to substantiate synchronism (late Eocene) of the Khasan and Nazimovskaya formations, the key stratigraphic units of the Paleogene in Primor’e. Coal-bearing sediments of the units rest upon early-middle Eocene volcanics (Kraskino rhyolite tuffs, Zaisanovka basaltic andesites) being overlain by the lower Fatash Subformation of the lower Oligocene with flora of the Kraskino type. The Ust’-Davydovskaya Formation of the Rechnoi Peninsula (outskirts of Vladivostok) is the most probable analogue of the Khasan (=Nazimovskaya) Formation.  相似文献   
275.
The relationship between surface sediment diatom assemblages and measured limnological variables in fifty eight lake samples from the south Bolivian Altiplano was examined by constructing a diatom-water chemistry dataset. Analysis of this dataset by canonical correspondence analysis revealed that salinity and ionic concentration accounted for a significant amount of the variation in the distribution of the diatom assemblages. Two methods weighted-averaging regression and calibration, and by-class mean percentage table were used to established a transfer function for future reconstruction of past lake water salinity and ionic concentration in the southern Bolivian Altiplano. Weighted-averaging regression and calibration with inverse deshrinking provided a better model for the water chemistry reconstructions in this region.  相似文献   
276.
漂浮海藻与海藻区系的关系   总被引:1,自引:0,他引:1  
区系是海藻生物地理研究的基本单位,是以一定时间、空间范围内的全部海藻为对象划分的生物地理单元,具有相对稳定的温度性质、物种组成和边界范围,且区系之间具有较明显的差异。然而,长期以来海藻的区系研究仅以定生的底栖海藻为研究对象,对于漂浮海藻则没有明确的定位。近年来世界各地漂浮海藻造成的藻华频发,在一些海域漂浮海藻已成为周期性存在,同时分子技术的发展为海藻生物地理学研究提供了有力的支撑,为追溯、比较漂浮海藻及其与定生海藻的关系提供了可靠的参考。漂浮海藻对于认识全球变化和人类活动对海藻分布、适应与进化的影响具有重要研究意义。本文结合漂浮海藻的特点和海藻生物地理学的基本研究方法探讨漂浮海藻与海藻区系的关系。  相似文献   
277.
吉林南部杉松岗植物群及其地质时代   总被引:1,自引:2,他引:1  
吉林南部杉松岗盆地早侏罗世植物群主要由松柏类、银杏类、真蕨类和茨康类组成,迄今已发现22属37种。其典型特征是锥叶蕨(Coniopteris)与准苏铁果(Cycadocarpidium)共生,显示了欧亚大陆早侏罗世植物群早期组合的特征,其地质时代似为早侏罗世早期。这一发现对确定杉松岗盆地含煤地层的地质时代和建立我国北方侏罗纪植物群序列具有重要的科学价值,对探讨吉林南部的地质发展史也具有重要的意义。  相似文献   
278.
藏南舌羊齿植物群的再研究   总被引:3,自引:0,他引:3       下载免费PDF全文
采自西藏南部定日县和定结县的舌羊齿植物群已由徐仁进行过研究(1973,1974,1976)。考虑当时文献资料的欠缺和近年来南半球舌羊齿类植物研究的新进展,作者重新审查了全部化石,对某些属种作了修正,并相应地对含该植物化石的地层时代也提出了新认识。  相似文献   
279.
通过2 400 m2样地调查,研究福建南平溪后安曹下76年生杉木丰产林群落山坡和山洼部位的植物区系组成、群落外貌、多样性指数等群落生态特征,结果表明:老龄杉木群落物种组成丰富,共有维管束植物71科,119属,168种;蕨类植物种类比例大;种子植物属的分布区类型以泛热带成分占的比例最高(山坡和山洼部分分别为27.59%和34.55%),热带性分布成分占优势(山坡和山洼部分分别为70.11%和74.55%),体现了群落的中亚热带性质;各层次的多样性表现为:灌木层(包括幼苗幼树)>草本层>乔木层;坡位因子对老龄杉木群落生态特征产生一定的影响.  相似文献   
280.
祁连山林区青海云杉群落种子植物区系分析   总被引:2,自引:0,他引:2  
根据祁连山林区青海云杉群落16块样地256个样方的调查结果,对青海云杉群落的植物种类和区系成分的多样性进行了研究,结果表明:青海云杉群落共有维管束植物96种,隶属25科52属;科的分布区类型以世界分布科为主,大部分主产温带;属的分布类型以温带性质居多,共31属59种,占总属数的62.26%,表明了青海云杉群落植物区系的温带性质;随海拔高度的变化,群落中不同林型的种数、属数和科数多样性表现出不同的变化规律,科和属的多样性最大值出现在山杨-青海云杉混交林中,种的多样性最大值出现在圆柏-青海云杉混交林中.  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