全文获取类型
收费全文 | 1575篇 |
免费 | 86篇 |
国内免费 | 17篇 |
专业分类
测绘学 | 35篇 |
大气科学 | 116篇 |
地球物理 | 400篇 |
地质学 | 572篇 |
海洋学 | 153篇 |
天文学 | 196篇 |
综合类 | 3篇 |
自然地理 | 203篇 |
出版年
2023年 | 6篇 |
2021年 | 18篇 |
2020年 | 33篇 |
2019年 | 27篇 |
2018年 | 37篇 |
2017年 | 49篇 |
2016年 | 49篇 |
2015年 | 39篇 |
2014年 | 48篇 |
2013年 | 79篇 |
2012年 | 52篇 |
2011年 | 99篇 |
2010年 | 81篇 |
2009年 | 92篇 |
2008年 | 76篇 |
2007年 | 49篇 |
2006年 | 83篇 |
2005年 | 68篇 |
2004年 | 54篇 |
2003年 | 57篇 |
2002年 | 46篇 |
2001年 | 28篇 |
2000年 | 30篇 |
1999年 | 22篇 |
1998年 | 31篇 |
1997年 | 21篇 |
1996年 | 29篇 |
1995年 | 22篇 |
1994年 | 26篇 |
1993年 | 20篇 |
1992年 | 17篇 |
1991年 | 12篇 |
1990年 | 18篇 |
1989年 | 17篇 |
1988年 | 13篇 |
1987年 | 12篇 |
1986年 | 16篇 |
1985年 | 19篇 |
1984年 | 21篇 |
1983年 | 14篇 |
1982年 | 29篇 |
1981年 | 17篇 |
1980年 | 7篇 |
1979年 | 13篇 |
1978年 | 13篇 |
1977年 | 10篇 |
1976年 | 11篇 |
1975年 | 6篇 |
1974年 | 5篇 |
1973年 | 13篇 |
排序方式: 共有1678条查询结果,搜索用时 640 毫秒
61.
Formation of silica oncoids around geysers and hot springs at El Tatio, northern Chile 总被引:2,自引:0,他引:2
Siliceous oncoids, up to 4 cm in diameter, are common on the laterally extensive sinter aprons that surround the spectacular geysers and hot springs at El Tatio in northern Chile. Many of these complex oncoids developed close to geyser and spring vents that discharge boiling water. Internally the oncoids, which are composed of precipitated amorphous silica, are formed of complex arrays of spicules and concentric laminae as well as detrital volcanic grains. Spicular growth is dominant in most examples. The formation and growth of the spicules and concentric laminae were mediated by a microbial community which included filamentous microbes, mucus, and possibly bacteria. The microbes and mucus were silicified by replacement and encrustation. In some laminae the filamentous microbes lay parallel to the growth surface; in other laminae most filaments forming the thin mats were suberect. Amorphous silica precipitated between the filaments occluded porosity and commonly disguised the microbial fabric. The oncoids grew on the proximal sinter aprons around the geyser vents and hot spring pools. Most growth took place subaerially with the silica delivered to the precipitation sites by splashing water from the geysers and/or periodic shallow flooding of the discharge aprons. Unlike silica oncoids at other geothermal sites, vertical growth of oncoids that formed in some rimstone pools was not limited by water depth. 相似文献
62.
Brian Windley 《Geophysical Journal International》1997,129(1):219-219
63.
64.
65.
66.
Recent attention given to the concept of vertical integration and disintegration of production processes has identified the existence of a continuum, with small, flexibly specialized producers at one end and large, mass production units at the other end. Firms along the continuum constantly strive for the optimum combination of economies of scale and scope. This paper attempts to identify the organizational characteristics of high technology firms in one industrial complex of the U.S. Manufacturing Belt. A principal components analysis is performed on a number of theoretically relevant variables for a sample of high technology establishments in Northeast Ohio. The results indicate that the region contains a variety of organizational forms ranging from isolated workshops to large, vertically integrated assembly plants. These findings conform to expectations derived from recent conceptualizations in the literature. Additional empirical research will further enhance understanding of the processes that shape the organization of production in territorial complexes. 相似文献
67.
68.
69.
This paper reconstructs precipitation variability in the southern Canadian Cordillera over the past 3–400 years using dendroclimatologicaltechniques. Fifty-three total ring-width (RW) chronologies, 28 earlywood (EW) and 28 latewood (LW) chronologies were developed from open-grown, low-elevation stands of Pseudotsuga menziesii (Douglas-fir) and Pinusponderosa (ponderosa pine) across the southern Canadian Cordillera. RW, EW and LW chronologies from both species were used to develop 13 annual (prior July to current June) precipitation reconstructions across the region. The reconstructions range in length from 165 to 688 years, pass verification tests and capture 39–64% of the variancein the instrumental record. Coincident, prolonged intervals of dry conditions are estimated for the years: 1717–1732, 1839–1859, 1917–1941 and1968–1979. Shorter dry intervals are identified between 1581–1586, 1626–1630,1641–1653 1701–1708, 1756–1761, 1768–1772, 1793–1800,1868–1875, 1889–1897 and 1985–1989. The historic drought of the 1920–1930s was the longest but not the most intense across this area in the last 300 years. Wet conditions occur in the majority of reconstructions for the years: 1689–1700, 1750–55,1778–1789, 1800–1830, 1880–1890, 1898–1916 and 1942–1960. Thesedata, in conjunction with data from adjacent areas, are used to provide the first maps of decadal precipitation anomalies for the region between 1700 and 1990. 相似文献
70.
Michael Bau Brian Alexander John T Chesley Susan L Brantley 《Geochimica et cosmochimica acta》2004,68(6):1199-1216
To compare relative reaction rates of mineral dissolution in a mineralogically simple groundwater aquifer, we studied the controls on solute concentrations, Sr isotopes, and rare earth element and yttrium (REY) systematics in the Cape Cod aquifer. This aquifer comprises mostly carbonate-free Pleistocene sediments that are about 90% quartz with minor K-feldspar, plagioclase, glauconite, and Fe-oxides. Silica concentrations and pH in the groundwater increase systematically with increasing depth, while Sr isotopic ratios decrease. No clear relationship between 87Sr/86Sr and Sr concentration is observed. At all depths, the 87Sr/86Sr ratio of the groundwater is considerably lower than the Sr isotopic ratio of the bulk sediment or its K-feldspar component, but similar to that of a plagioclase-rich accessory separate obtained from the sediment. The Si-87Sr/86Sr-depth relationships are consistent with dissolution of accessory plagioclase. In addition, solutes such as Sr, Ca, and particularly K show concentration spikes superimposed on their respective general trends. The K-Sr-87Sr/86Sr systematics suggests that accessory glauconite is another major solute source to Cape Cod groundwater. Although the authigenic glauconite in the Cape Cod sediment is rich in Rb, it is low in in-grown radiogenic 87Sr because of its young Pleistocene age. The low 87Sr/86Sr ratios are consistent with equilibration of glauconite with seawater. The impact of glauconite is inferred to vary due to its variable abundance in the sediments. In the Cape Cod groundwater, the variation of REY concentrations with sampling depth resembles that of K and Rb, but differs from that of Ca and Sr. Shale-normalized REY patterns are light REY depleted, show negative Ce anomalies and super-chondritic Y/Ho ratios, but no Eu anomalies. REY input from feldspar, therefore, is insignificant compared to input from a K-Rb-bearing phase, inferred to be glauconite. These results emphasize that interpretation of groundwater chemistry, even in relatively simple aquifers, may be complicated by solute contributions from “exotic” accessory minerals such as glauconite. To detect such peculiarities, groundwater studies should combine the study of elemental concentration and isotopic composition of several solutes that show different geochemical behavior. 相似文献