全文获取类型
收费全文 | 48672篇 |
免费 | 376篇 |
国内免费 | 1230篇 |
专业分类
测绘学 | 1915篇 |
大气科学 | 3665篇 |
地球物理 | 8880篇 |
地质学 | 22106篇 |
海洋学 | 2999篇 |
天文学 | 6584篇 |
综合类 | 2199篇 |
自然地理 | 1930篇 |
出版年
2023年 | 42篇 |
2022年 | 52篇 |
2021年 | 56篇 |
2020年 | 70篇 |
2019年 | 83篇 |
2018年 | 6925篇 |
2017年 | 6223篇 |
2016年 | 3918篇 |
2015年 | 443篇 |
2014年 | 353篇 |
2013年 | 435篇 |
2012年 | 2021篇 |
2011年 | 4985篇 |
2010年 | 4326篇 |
2009年 | 4641篇 |
2008年 | 3732篇 |
2007年 | 4719篇 |
2006年 | 428篇 |
2005年 | 985篇 |
2004年 | 993篇 |
2003年 | 1047篇 |
2002年 | 690篇 |
2001年 | 282篇 |
2000年 | 287篇 |
1999年 | 191篇 |
1998年 | 189篇 |
1997年 | 161篇 |
1996年 | 154篇 |
1995年 | 136篇 |
1994年 | 114篇 |
1993年 | 92篇 |
1992年 | 78篇 |
1991年 | 87篇 |
1990年 | 77篇 |
1989年 | 83篇 |
1988年 | 59篇 |
1987年 | 101篇 |
1986年 | 64篇 |
1985年 | 64篇 |
1984年 | 69篇 |
1983年 | 71篇 |
1982年 | 72篇 |
1981年 | 88篇 |
1980年 | 78篇 |
1979年 | 51篇 |
1977年 | 48篇 |
1976年 | 54篇 |
1975年 | 35篇 |
1974年 | 30篇 |
1973年 | 41篇 |
排序方式: 共有10000条查询结果,搜索用时 125 毫秒
991.
Tritium concentrations were measured in a survey of 24 lakes, 15 wetlands, and 133 groundwaters in the oil sands region of northeastern Alberta and compared with both recent precipitation and precipitation sampled during the 1960s tritium peak caused by atmospheric thermonuclear weapons testing. Water samples from lakes included a group of 14 thaw lakes that had higher runoff attributed to melting of permafrost in peat plateaus within their watersheds. While tritium in all lakes was found to be intermediate between recent and 1960s concentrations, the thaw lakes were found to be significantly enriched in tritium compared with other lakes, as were unfrozen wetlands characterized by a thick sequence of low‐hydraulic conductivity peat. The results provide further evidence of different water sources to the thaw lakes and may indicate that melting of modern permafrost in part formed since the 1950s is occurring in these systems. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
992.
X. Jia T. S. Zha J. N. Gong B. Wu Y. Q. Zhang S. G. Qin G. P. Chen W. Feng S. Kellomäki H. Peltola 《水文研究》2016,30(6):972-985
During the last decade, the widely distributed shrublands in northern China have shown significant signs of recovery from desertification, the result of widespread conservation practices. However, to support the current efforts in conservation, more knowledge is needed on surface energy partitioning and its biophysical controls. Using eddy‐covariance measurements made over a semi‐arid shrubland in northwest China in 2012, we examined how surface energy‐balance components vary on diurnal and seasonal scales, and how biophysical factors control bulk surface parameters and energy exchange. Sensible heat flux (H) exceeded latent heat flux (λE) during most of the year, resulting in an annual Bowen ratio (β, i.e. H/λE) of 2.0. λE exceeded H only in mid‐summer when frequent rainfall co‐occurred with the seasonal peak in leaf area index (LAI). Evapotranspiration reached a daily maximum of 3.3 mm day?1, and summed to 283 mm yr?1. The evaporative fraction (EF, i.e. λE/Rn), Priestley–Taylor coefficient (α), surface conductance (gs) and decoupling coefficient (Ω) were all positively correlated with soil water content (SWC) and LAI. The direct enhancement of λE by high vapour pressure deficit (VPD) was buffered by a concurrent suppression of gs. The gs played a direct role in controlling EF and α by mediating the effects of LAI, SWC and VPD. Our results highlight the importance of adaptive plant responses to water scarcity in regulating ecosystem energy partitioning, and suggest an important role for revegetation in the reversal of desertification in semi‐arid areas. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
993.
994.
995.
996.
997.
New P–T constraints on the Tamayen glaucophane‐bearing rocks,eastern Taiwan: Perple_X modelling results and geodynamic implications
下载免费PDF全文
![点击此处可从《Journal of Metamorphic Geology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
I. Baziotis C.‐H. Tsai W. G. Ernst B.‐M. Jahn Y. Iizuka 《Journal of Metamorphic Geology》2017,35(1):35-54
New pseudosection modelling was applied to better constrain the P–T conditions and evolution of glaucophane‐bearing rocks in the Tamayen block of the Yuli belt, recognized as the world's youngest known blueschist complex. Based on the predominant clinoamphibole, textural relationships, and mineral compositions, these glaucophane‐bearing high‐P rocks can be divided into four types. We focused on the three containing garnet. The chief phase assemblages are (in decreasing mode): amphibole + quartz + epidote + garnet + chlorite + rutile/titanite (Type‐I), phengite + amphibole + quartz + garnet + chlorite + epidote + titanite + biotite + magnetite (Type‐II), and amphibole + quartz + albite + epidote + garnet + rutile + hematite + titanite (Type‐III). Amphibole exhibits compositional zoning from core to rim as follows: glaucophane → pargasitic amphibole → actinolite (Type‐I), barroisite → Mg‐katophorite/taramite → Fe‐glaucophane (Type‐II), glaucophane → winchite (Type‐III). Using petrographic data, mineral compositions and Perple_X modelling (pseudosections and superimposed isopleths), peak P–T conditions were determined as 13 ± 1 kbar and 550 ± 40 °C for Type‐I, 10.5 ± 0.5 kbar and 560 ± 30 °C for Type‐II (thermal peak) and 11 ± 1 kbar and 530 ± 30 °C for Type‐III. The calculations yield higher pressures and temperatures than previously thought; the difference is ~1–6 kbar and 50–200 °C. The three rock types record similar P–T retrograde paths with clockwise trajectories; all rocks followed trajectories with substantial pressure decrease under near‐isothermal conditions (Type‐I and Type‐III), with the probable exception of Type‐II where decompression followed colder geotherms. The P–T paths suggest a tectonic environment in which the rocks were exhumed from maximum depths of ~45 km within a subduction channel along a relative cold geothermal gradient of ~11–14 °C km?1. 相似文献
998.
Ranganath Navalgund 《Journal of the Indian Society of Remote Sensing》2018,46(3):483-489
999.
1000.
L. Lespez Y. Le Drezen A. Garnier M. Rasse B. Eichhorn S. Ozainne A. Ballouche K. Neumann E. Huysecom 《Quaternary Science Reviews》2011,30(5-6):737-756
The Yamé river, in the Bandiagara Plateau, Dogon Country, Mali, is characterised by extensive alluvial sedimentary records, particularly in the 1 km long Ounjougou reach where Holocene floodplain pockets are inset in the Pleistocene formations. These alluvial records have been investigated via geomorphologic fieldwork and sedimentologic and micromorphologic analyses and are supported by 79 radiocarbon dates. The alluvial deposits of the valley floor correspond to a vertical accretion of 3–10 m. The reconstruction of fluvial style changes provides evidence of four main aggradation periods. From 11,500 to 8760 cal. BP, the alluvial architecture and grain-size parameters indicate a wandering river. This period included phases of pulsed high-energy floods and avulsion related to a northward shift of the summer monsoon to around 14°N after 11,500 cal. BP. From 7800 to 5300 cal. BP, a swampy floodplain environment with standing water pools within a Sudanian savanna/woodland mosaic corresponds to the culmination of the Holocene humid period. From 3800 cal. BP onwards, rhythmic sedimentation attests to an increase in the duration and/or intensity of the dry season, giving a precise date for the local termination of the Holocene Optimum period. During the last two millennia and for the first time during the Holocene, the alluvial formations are progressively restricted whereas the colluvial deposits increase, indicating strong soil erosion and redeposition within the watershed related to an increase in human impact. Four major periods are characterised by incision (I1: ante 11,500, I2: 8760–7800; I3: 6790–6500 cal. BP; I4; 2400–1700 cal. BP) pointing to dramatic changes in fluvial style. They result from high-energy flood flows during dry spells and confirm the capacity of the floodplain pocket in the upstream reach of the Sahelian belt to record rapid Holocene climatic change. 相似文献