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   1541篇
  免费   151篇
  国内免费   167篇
测绘学   40篇
大气科学   218篇
地球物理   351篇
地质学   608篇
海洋学   294篇
天文学   4篇
综合类   58篇
自然地理   286篇
  2024年   8篇
  2023年   17篇
  2022年   36篇
  2021年   42篇
  2020年   42篇
  2019年   46篇
  2018年   20篇
  2017年   88篇
  2016年   66篇
  2015年   78篇
  2014年   86篇
  2013年   80篇
  2012年   49篇
  2011年   90篇
  2010年   59篇
  2009年   116篇
  2008年   118篇
  2007年   91篇
  2006年   96篇
  2005年   73篇
  2004年   63篇
  2003年   44篇
  2002年   64篇
  2001年   39篇
  2000年   54篇
  1999年   48篇
  1998年   41篇
  1997年   33篇
  1996年   36篇
  1995年   38篇
  1994年   15篇
  1993年   20篇
  1992年   15篇
  1991年   13篇
  1990年   7篇
  1989年   10篇
  1988年   3篇
  1987年   3篇
  1985年   3篇
  1984年   3篇
  1983年   1篇
  1982年   2篇
  1980年   1篇
  1976年   1篇
  1972年   1篇
排序方式: 共有1859条查询结果,搜索用时 0 毫秒
801.
中国东南地区白垩纪红层通常被认为是陆相红色碎屑沉积物,是河流湖泊相沉积,但其古环境存在争论.本文以闽西晚白垩世红层为研究对象,利用环境磁学、粒度、地球化学、古土壤分析等方法,选取连城(LC)和冠豸山(GZS)两个剖面,分析探讨红层的古环境.结果显示:(1)闽西红层主要以细的粉砂颗粒为主,黏土和砂含量较少,表现为粉砂和砂互层,夹有薄层细粒砂砾层;样品磁化率偏低,主要载磁矿物为硬磁性矿物赤铁矿.(2)闽西红层有较高的风化程度,是暖湿气候下的中等风化程度;轻重稀土元素分异明显,Ce元素富集,Eu元素相对亏损,与上地壳(UCC)的分配模式相似,表明沉积物经历了充分混合,物质为混合沉积产物.(3)闽西红层虽然发育了一定的古土壤,但是成壤程度不强,没有明显的古土壤粘化层(Bt)和钙积层(Bk).由此推断,闽西红层沉积物在沉积前已在源区经过相当程度的风化过程,之后经过混合动力搬运沉积在盆地,沉积后风化成壤弱.可见,红层本身并不能直接反映沉积区环境,需结合古土壤发育特征判断沉积环境特征和变化.(4)闽西晚白垩世红层表现为相对干旱半干旱的古环境,红层中主要的着色矿物赤铁矿主要形成于源区,反映了地表透水性良好的干燥氧化条件,而不是"水成"环境.本文可为白垩纪红层古环境研究提供新的思路.  相似文献   
802.
Man-made polybrominated diphenyl ethers (PBDEs) used as flame retardants in various consumer products may be harmful to marine organisms. Larvae of some marine invertebrates, especially invasive species, can develop resistance to PBDEs through altered protein expression patterns or proteome plasticity. This is the first report of a proteomics approach to study BDE-47 induced molecular changes in the invasive limpet Crepidula onyx. Larvae of C. onyx were cultured for 5 days (hatching to metamorphosis) in the presence of BDE-47 (1 μg L−1). Using a 2-DE proteomics approach with triple quadrupole and high-resolution TOF-MS, we showed that BDE-47 altered the proteome structure but not the growth or metamorphosis of C. onyx larvae. We found eight significant differentially expressed proteins in response to BDE-47, deemed the protein expression signature, consisting of cytoskeletal, stress tolerance, metabolism and energy production related proteins. Our data suggest C. onyx larvae have adequate proteome plasticity to tolerate BDE-47 toxicity.  相似文献   
803.
在数据库技术的支持下,应用灰关联分析方法对2001年10月至2002年7月粤东柘林湾海域9个调查站点海水样品的理化、生物实测数据进行数据分析,从磷酸盐(PO4-P)和硅酸盐(SiO3-Si)等12项理化、生物因子中找出影响中肋骨条藻(Skeletonema costatum)细胞密度的关键因子。结果表明,浮游动物的个体数、铁(Fe)、水温和浊度是影响柘林湾中肋骨条藻细胞密度的关键因子。  相似文献   
804.
This paper provides a synthesis of the EU project MedVeg addressing the fate of nutrients released from fish farming in the Mediterranean with particular focus on the endemic seagrass Posidonia oceanica habitat. The objectives were to identify the main drivers of seagrass decline linked to fish farming and to provide sensitive indicators of environmental change, which can be used for monitoring purposes. The sedimentation of waste particles in the farm vicinities emerges as the main driver of benthic deterioration, such as accumulation of organic matter, sediment anoxia as well as seagrass decline. The effects of fish farming on P. oceanica meadows are diverse and complex and detected through various metrics and indicators. A safety distance of 400 m is suggested for management of P. oceanica near fish farms followed by establishment of permanent seagrass plots revisited annually for monitoring the health of the meadows.  相似文献   
805.
黄磷铁矿、磷钠铍石、磷方沸石、簇磷铁矿、磷锰钠石、水磷铈石和磷灰石是非硅元素的孔道结构矿物。[PO_4]四面体和[MO_n]多面体共用角顶构成杂多面体格架。形成一维或多堆沿一定方向分布的孔道结构,孔道内由水分子和其它金属或非金属离子占据。孔道结构的磷酸盐矿物具有类似沸石矿物的分子筛效应,可与其它有害元素进行离子交换、孔道过滤和表面吸附等功能,是另一类重要的环境矿物材料。  相似文献   
806.
1804号台风“艾云尼”龙卷分析   总被引:8,自引:5,他引:8  
2018年6月8日,在1804号台风“艾云尼”螺旋雨带中发生了两次陆龙卷天气,分别袭击了广州市南沙区横沥镇和佛山市南海区大沥镇。利用广州CINRAD/SA多普勒天气雷达、佛山CINRAD/XD多普勒天气雷达、5 min间隔的地面自动气象站和MICAPS等资料,研究了两次陆龙卷的天气背景、环境参数和龙卷风暴中尺度结构特征。结果表明:广州南沙龙卷为台风环流外围龙卷,位于台风中心的东北象限,强度为EF3级;佛山南海龙卷为台风环流内部龙卷,位于台风中心的东侧,强度为EF1级。龙卷均发生在中低空强东南急流在珠江口附近上下叠加和高层辐散的有利大尺度环流背景下。环境条件表现为较强的低层风垂直切变和较大的风暴相对螺旋度(SRH)、较小的对流有效位能(CAPE)和对流抑制能量(CIN)、极低的抬升凝结高度(LCL);地面存在中尺度辐合线和小尺度涡旋。广州S波段雷达探测到两次龙卷母风暴的低层钩状回波和入流缺口回波特征及低层中等强度中气旋,龙卷出现在钩状回波顶端、中气旋中心附近。佛山X波段双偏振雷达清晰地探测到佛山南海区大沥龙卷的微型超级单体和龙卷碎片特征(TDS)。   相似文献   
807.
Rural places are important centers of environmental and social transformation. Landholders are not only affected by socio-environmental changes, but they are influencing futures related to climate change, food security, freshwater, biodiversity, and social and economic development. Much environmental land use work understands individual landholders as rational actors, portrayed through the economic lens of “producer.” These approaches generally focus on present capacities and limitations as the principal factors contributing to land use, and the individual farm as the reference unit for decisions. Our research takes steps to expand conceptualizations of rural landholders as active and knowledgeable in envisioning, managing, and shaping environmental futures. We design and test a new approach using ecological mental maps and future imaginaries to understand land use practices through a case study in the cocoa-producing and Atlantic Forest region of Southern Bahia, Brazil. The integrated socio-perceptual (ISP) approach combines qualitative and quantitative methodologies to honor the depth of landholders’ experiences and perspectives and to allow broader regional relationships and insights to emerge. We demonstrate that the ISP approach—through 49 surveys—has the capacity to capture variation and identify patterns in ecological mental maps and future imaginaries in a population, approximate the relevant spatial scales underlying these factors, and identify relationships between these and land uses. In this context, the types of narratives landholders hold about the region’s future are associated with current forest land use on their properties. We discuss potential applications of the ISP approach for land use study and practice.  相似文献   
808.
Beach Profiles surveys and gale climate data were utilised to assess medium timescale beach rotation at four beaches located along the shores of a crenulated embayment within Swansea Bay, Southwest Wales. The proposed Tidal Lagoon is located within this Bay. Results identified a 7 year (1998–2005) record of cyclic summer/winter rotation and a 14 year (1999–2013) record of annual rotation within the subaerial zone on all four assessed beaches. In the absence of headlands to trap sediment it is asserted that the driving force for beach rotation is the presence of Swansea Dockland/Tawe dredged channel complex, Port Talbot Harbour and the Neath dredged channel which form surrogate headlands essentially creating four separate beach systems through restricting sediment by-pass. Seasonal averaged wind and wave variables showed differing correlation with volume changes and cross-correlation results showed that volume variation lagged behind forcing variables by up to six months (i.e. the resolution of the data). This was confirmed by the annually averaged results which showed only subtle correlation. Here volume change in most cases lagged forcing variables by less than one year. Based on correlations, wind direction variability follows closely with volume changes but wind speed, wave period and height are generally opposite. Initial results suggest that the proposed Swansea Bay Tidal Lagoon, located between sediment cells and surrogate headlands, would have little negative effect on subaerial coastal processes. These datasets will be used as a benchmark for monitoring prior to, during and post construction, with results being used to update and inform subsequent strategies.  相似文献   
809.
Windsor–Essex County is a major cross-border truck and transportation route, with significant localized industrialization as well as rural and farming areas. Magnetic property measurements (in-field and laboratory susceptibility, frequency-dependent susceptibility, hysteresis properties, thermomagnetic and thermosusceptibility curves, anhysteretic and isothermal magnetizations) were made in order to determine the potential for using such variables to distinguish between natural and anthropogenic pollutants. In-field magnetic susceptibility measured on 324 soil sampling sites on a 0.5–2 km grid spacing through Windsor–Essex County ranged from 3.7 × 10− 6 to 305.2 × 10− 6 SI (average 36.2 ± 35.8 × 10− 6 SI), and showed that high magnetic susceptibility values were obtained on soil sampling sites in and around the cities/towns of Windsor, Harrow, Olinda and Oakland and near the beaches of Point Pelee National Park (PPNP) and Deerbrook, whereas lower susceptibility values were observed in near the towns of Lakeshore and Essex. On this grid spacing, Highway 401 (the major truck route) did not show anomalous susceptibility values; however, closer (1–3 m) sampling on other roads did show anomalously high values, suggesting that the coarser grid spacing may have missed anomalies. Laboratory measurements indicated that the dominant magnetic mineral in the Windsor–Essex County soils is magnetite; however, the grain size is variable. Pseudo-single domain (PSD)–multidomain (MD) magnetite is generally found on beaches and in PPNP, whereas single domain (SD)–PSD magnetite has been found near the City of Windsor and other towns. While certain correlations exist between some anthropogenic activities and the measured magnetic susceptibility and magnetic property values, no overall correlation can be made. A variety of geologic and anthropogenic factors must be considered when interpreting the origin of the magnetic signal in a particular area.  相似文献   
810.
长距离地铁隧道的运营期间会对地下水环境造成一定影响。本文以北京市M8二期北段为例,进行该类问题的分析研究,首次建立了科学的评价方法体系。根据本文的方法体系,在一定渗流计算的基础上,获得了北京市M8二期北段对渗流场阻隔作用的量化评价,并在此基础上做了进一步的环境及工程影响评价,评价结论为保证地下水环境和该新线建设节省造价和工期提供了科学依据。同时,本文工作也为今后地铁建设中类似问题的评价与研究提供方法上的借鉴与参考。  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