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   1033篇
  免费   97篇
  国内免费   319篇
测绘学   11篇
大气科学   402篇
地球物理   120篇
地质学   640篇
海洋学   61篇
天文学   16篇
综合类   18篇
自然地理   181篇
  2024年   8篇
  2023年   30篇
  2022年   51篇
  2021年   58篇
  2020年   52篇
  2019年   64篇
  2018年   65篇
  2017年   53篇
  2016年   62篇
  2015年   51篇
  2014年   76篇
  2013年   132篇
  2012年   48篇
  2011年   55篇
  2010年   35篇
  2009年   51篇
  2008年   60篇
  2007年   52篇
  2006年   42篇
  2005年   40篇
  2004年   36篇
  2003年   37篇
  2002年   32篇
  2001年   38篇
  2000年   30篇
  1999年   21篇
  1998年   34篇
  1997年   23篇
  1996年   21篇
  1995年   14篇
  1994年   18篇
  1993年   14篇
  1992年   8篇
  1991年   4篇
  1990年   6篇
  1989年   5篇
  1988年   5篇
  1987年   7篇
  1986年   1篇
  1985年   5篇
  1984年   2篇
  1983年   2篇
  1982年   1篇
排序方式: 共有1449条查询结果,搜索用时 15 毫秒
141.
Growing up and cleaning up: The environmental Kuznets curve redux   总被引:1,自引:0,他引:1  
Borrowing from the Kuznets curve literature, researchers have coined the term “environmental Kuznets curve” or EKC to characterize the relationship between pollution levels and income: pollution levels will increase with income but some threshold of income will eventually be reached, beyond which pollution levels will decrease. The link between the original Kuznets curve, which posited a similar relationship between income and inequality, and its pollution-concerned offspring lies primarily with the shape of both curves (an upside-down U) and the central role played by income change. Although the EKC literature has burgeoned over the past several years, few concrete conclusions have been drawn, the main themes of the literature have remained constant, and no consensus has been reached regarding the existence of an environmental Kuznets curve. EKC research has used a variety of types of data and a range of geographical units to examine the effects of income levels on pollution. Changes in pollution levels might also be at least partly explained by countries’ position in the demographic transition and their general population structure, however little research has included this important aspect in the analysis. In addition, few analyses confine themselves to an evaluation for one country of the long-term relationship between income and pollution. Using United States CO2 emissions as well as demographic, employment, trade and energy price data, this paper seeks to highlight the potential impact of population and economic structure in explaining the relationship between income and pollution levels.  相似文献   
142.
粤西大降坪黄铁矿床硫、铅同位素组成初步研究   总被引:18,自引:0,他引:18       下载免费PDF全文
大降坪黄铁矿床是产于云开隆起中段偏北震旦系海相碎屑岩-细碎屑岩中的层状硫化物矿床,矿床成因和海底喷气热水沉积作用有关,局部经受了后期热液的叠加改造。黄铁矿的δ~(34)S值在-25.55‰—+21.07‰之间,与矿石及黄铁矿中的有机碳含量呈反比。从条带状细粒黄铁矿到块状粗粒黄铁矿,亦即从南到北,硫同位素组成从富轻硫变为富重硫。条带状矿石的铅同位素组成与矿体围岩一致,为富放射成因上地壳源铅,块状矿石铅同位素组成较均一,三组铅同位素比值较条带状矿石略低,为原始沉积的矿石铅与热液带来基底混合岩铅的混合铅。  相似文献   
143.
The rate of the reversible homogeneous disproportionation of polysulfides was studied by following the optical absorbance of polysulfide solutions in a continuous plug flow reactor equipped with an on-line photometric detector. In order to avoid heterogeneous slow reactions involving sulfur colloids or precipitate, the reaction was initiated by an abrupt pH change from an undersaturated solution containing predominantly tetrasulfide species to a pH where pentasulfide is the dominant species. The disproportionation was found to follow first order reversible reaction dynamics. At environmentally relevant conditions the characteristic time of the disproportionation reaction is of the order of 10 s. This characteristic time implies that necessary conditions for speciation of the different polysulfide species by chromatography or another separation and subsequent quantification scheme should be of the order of 1 s.  相似文献   
144.
基于消费责任制的碳排放核算及全球环境压力   总被引:3,自引:0,他引:3  
钟章奇  姜磊  何凌云  王铮  柏玲 《地理学报》2018,73(3):442-459
因区域间贸易而转移的碳排放对于全球各个国家或者地区的碳排放核算及其减排责任划分具有重要而深远的影响。通过构建多区域投入产出分析模型和基于扩展的STIRPAT模型,本文核算了全球39个主要国家基于消费责任制的碳排放,并在此基础上深入探讨了全球环境压力的影响因素问题。研究发现:首先,从以净流出为主的中国和俄罗斯来看,尽管贸易给这些地区带来了大量的资源,但随之会产生严重的区域生态环境问题。而对于美国和欧盟等地区来说,这些国家或者地区通过全球贸易规避了大量的碳减排责任。此外,在全球贸易中,区域净流出的贸易隐含碳排放越小,基于消费责任制核算的区域碳排放量就越大,故而在全球减排目标分配中承担的减排任务也需相应地增加。全球贸易隐含碳排放净流出量较大的地区主要位于亚洲和东欧等地,而净流出量最小的地区主要是以西欧和北美等高度发达的经济体为主。其次,就基于消费责任制核算下的全球环境压力而言,人口因素和富裕程度是导致全球环境压力不断增加的两个重要因素,而提高生产技术水平以及逐渐提高清洁能源在总能源消费中的比重是推动全球节能减排、缓解全球环境压力的有效途径。  相似文献   
145.
A numerical simulation model is presented in this paper,which comprises the processes of crop growth,soil organic carbon decomposition,and methane emissions in agroecosystems.Simulation results show that the model can simulate the main process of methane emissions well,and the correlation coefficient between the simulated values and observed data is 0.79 with 239 samples,which passed a significance test of 0.01.The average error of methane emission simulation in whole growth period is about 15%.Numerical analysis of the model indicates that the average temperature during rice growth period has much impacts on methane emissions,and the basic trend of interannual methane emissions is similar to that of average temperature.The amount of methane emissions reduces about 34.93%,when the fertilizer is used instead of manure in single rice paddy.  相似文献   
146.
Sometime during AD 1452, according to new evidence, a large-magnitude, initially phreatomagmatic eruption, destroyed the island of Kuwae (16.83°S, 168.54°E), located in the present-day Republic of Vanuatu. It created a 12×6-km submarine caldera composed of two adjacent basins. Based on estimates of caldera volume, between 30 and 60 km3 DRE of dacite magma was ejected as pyroclastic flow and fall deposits during this event. Annual layers of ice dating from the period AD 1450–1460 contain acidity peaks representing fallout of sulfuric acid onto both the Greenland and Antarctic ice caps. These acidity peaks have been attributed by others to the sedimentation of H2SO4 aerosols that originated from sulfur degassing during the Kuwae eruption. Improved dating techniques and new data from nineteen ice cores reveal a single acidity peak attributed to Kuwae lasting from 1453 to 1457. In this study, we present new electron microprobe analyses of the S, Cl, and F contents of matrix glasses and glass inclusions in phenocrysts from tephra ejected during the Kuwae eruption. We establish that the Kuwae event did indeed yield a large release of sulfur gases. From our glass inclusion data and analysis, we calculate that the total atmospheric aerosol loading from the 1452 Kuwae eruption was ≫100 Tg H2SO4. Much of the volatile mass released during the eruption was probably contained in a separate, volatile-rich, fluid phase within the pre-eruptive Kuwae magma body. Comparing the volatile release of the Kuwae eruption with other large-magnitude eruptions, places Kuwae as the greatest sulfuric acid aerosol producer in the last seven centuries, larger even than sulfur emissions from the eruption of Tambora (Indonesia) in 1815, and possibly Laki (Iceland) in 1783. The severe and unusual climatic effects reported in the mid- to late-1450s were likely caused by the Kuwae eruption.  相似文献   
147.
目前中国的碳转移排放测算方法有很多,虽然方法不尽相同,但总的结论和计算出的变化趋势类似。造成碳转移量测算结果产生较大差异的原因主要有两方面,一是碳排放强度的测算模型不同,二是对于中国加工贸易转移碳排放的认识不同。测算避免转移排放量时,如果不考虑加工贸易的影响,在某些年份(如2002年)甚至会得到"中国是碳转移的净进口国...  相似文献   
148.
拜仁达坝和维拉斯托是近年来在内蒙古东部地区发现的2个大型银多金属矿床,文章对其开展了硫和铅同位素研究。结果表明,拜仁达坝矿床矿石中硫化物的δ34S值为-4.0‰~+1.6‰,维拉斯托矿床矿石中硫化物的δ34S值为-0.8‰~+2.0‰,与岩浆热液型矿床的硫同位素值接近,表明这2个矿床中的硫主要来自岩浆。拜仁达坝矿区43件金属硫化物的206Pb/204Pb值为18.333~18.515,207Pb/204Pb值为15.532~15.656,208Pb/204Pb值为38.057~38.610;维拉斯托矿区20件金属硫化物的206Pb/204Pb值为18.304~18.377,207Pb/204Pb值为15.520~15.610,208Pb/204Pb值为38.112~38.435。拜仁达坝东矿区矿石中的铅同位素组成与维拉斯托矿区相似,变化范围小,相对贫放射性铅同位素,并且均为混合铅。矿石中的铅可能来自围岩地层及深源岩浆。  相似文献   
149.
广东大降坪硫(铅锌)矿床位于与岩浆作用有关的大绀山多金属矿田的中部,主矿体为赋存在震旦纪变质岩中的层状、透镜状黄铁矿矿体,最近在两个不同产状的黄铁矿矿体下部又新发现了脉状及层状铅锌矿体。文章通过对铅锌矿体的年龄及硫同位素研究,探讨其与主矿体的成因关系,获得了脉状铅锌矿体中闪锌矿的Rb-Sr等时线年龄为(88.5±3.9)Ma,即晚白垩世,与上部黄铁矿矿体的年龄(约630 Ma)相差较大,而与整个大绀山多金属矿田的成矿作用时限一致。三种不同产状的矿体硫同位素组成差异明显:层状黄铁矿矿体富集硫的轻同位素(δ34S=-10.90‰~-25.55‰),且与围岩的硫同位素范围一致,说明硫来自生物的细菌还原硫;透镜状黄铁矿矿体δ34S组成范围较宽(-9.38‰~22.69‰),具多源性硫的特征;铅锌矿体的δ34S在-7.1‰~6.4‰之间变化,硫可能来自深源岩浆,并受围岩成分混染。多方面的证据表明,大降坪黄铁矿矿体下部的铅锌矿体形成于晚白垩世的岩浆热液成矿作用,透镜状黄铁矿矿体受到岩浆热液的叠加。  相似文献   
150.
A bathymetric survey of Kawah Ijen crater lake was conducted by acoustic sounding in 1996 to compare the lake morphology with those measured in 1922, 1925 and 1938, and to calculate the present lake volume. Even though the lake experienced several hydrothermal eruptions, the maximum depth became shallower (182 m) than before (200 m), resulting in a reduced lake volume (3.0×107 m3).Fifty-two major and minor constituents including rare earth elements and polythionates (PT) of the lake waters at various depths were determined by ICP-AES, ICP-MS and HPLC, respectively. These ions except for several volatile elements are taken up by lake fringe through congruent dissolution of pyroclastics of Kawah Ijen volcano. Most ions are homogeneously distributed throughout the lake, although PT showed a considerable vertical variation. Rare earth elements (REE) in the Kawah Ijen water as well as those from other hyper-acidic crater lakes show distribution patterns likely due to the three rock dissolution (preferential, congruent and residual) types, and their logarithmic concentrations linearly depend upon the pH values of the lake waters.Using the PT degradation kinetics data, production rates of PT, injection rates of SO2 and H2S into the lake were estimated to be 114, 86 and 30 tons/day, respectively. Also travel time of the spring water at the Banyupahit Riverhead from Kawah Ijen was estimated to be 600–1000 days through the consideration of decreasing rates of PT. Molten sulfur stocks containing Sn, Cu, Bi sulfides and Pb-barite exposed on the inner crater slope were presumed to be extinct molten sulfur pools at the former lake bottom. This was strongly supported by the barite precipitation temperature estimated through the consideration of the temperature dependence of Pb-chlorocomplex formation.  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