全文获取类型
收费全文 | 5986篇 |
免费 | 148篇 |
国内免费 | 62篇 |
专业分类
测绘学 | 124篇 |
大气科学 | 504篇 |
地球物理 | 1375篇 |
地质学 | 1986篇 |
海洋学 | 592篇 |
天文学 | 957篇 |
综合类 | 14篇 |
自然地理 | 644篇 |
出版年
2021年 | 47篇 |
2020年 | 65篇 |
2019年 | 73篇 |
2018年 | 111篇 |
2017年 | 117篇 |
2016年 | 132篇 |
2015年 | 103篇 |
2014年 | 129篇 |
2013年 | 315篇 |
2012年 | 178篇 |
2011年 | 260篇 |
2010年 | 238篇 |
2009年 | 236篇 |
2008年 | 241篇 |
2007年 | 211篇 |
2006年 | 228篇 |
2005年 | 182篇 |
2004年 | 185篇 |
2003年 | 171篇 |
2002年 | 165篇 |
2001年 | 117篇 |
2000年 | 108篇 |
1999年 | 103篇 |
1998年 | 95篇 |
1997年 | 85篇 |
1996年 | 88篇 |
1995年 | 92篇 |
1994年 | 86篇 |
1993年 | 76篇 |
1992年 | 86篇 |
1991年 | 70篇 |
1990年 | 98篇 |
1989年 | 85篇 |
1988年 | 75篇 |
1987年 | 100篇 |
1986年 | 75篇 |
1985年 | 96篇 |
1984年 | 131篇 |
1983年 | 108篇 |
1982年 | 97篇 |
1981年 | 100篇 |
1980年 | 88篇 |
1979年 | 100篇 |
1978年 | 71篇 |
1977年 | 86篇 |
1976年 | 71篇 |
1975年 | 70篇 |
1974年 | 55篇 |
1973年 | 61篇 |
1972年 | 35篇 |
排序方式: 共有6196条查询结果,搜索用时 15 毫秒
71.
Norbert Schwarzenbeck Ron Erley Belinda S. Mc Swain Peter A. Wilderer Robert L. Irvine 《洁净——土壤、空气、水》2004,32(1):16-24
Aerobic granular sludge was successfully cultivated in a sequencing batch reactor (SBR) treating wastewater from the malting process with a high content of particulate organic matter. At an organic loading rate of 3.2 kg/(m3 d) CODtotal and an influent particle concentration of 0.95 g/L MLSS an average removal of 50% in CODtotal and 80% in CODdissolved could be achieved. A comparison of granular and flocculent sludge grown under the same operating conditions showed no significant difference in removal efficiency although granules exhibited a higher metabolic activity in terms of specific oxygen uptake rate (rO2, X). Two distinct mechanisms of particle removal were observed for granular sludge: during initial granule formation, particles were incorporated into the biofilm matrix. For mature granules, a high level of protozoa growth on the granule surface accounted for the ability to remove particulate COD. Combined evaluation of the development in MLSS content and sludge bed settling rate (i.e., mean derivative of the normalized sludge volume) was found to be an adequate method for monitoring the characteristic settling properties of a granulizing sludge bed. By means of this method, a distinct substrate gradient out of several operating conditions was concluded to have the biggest impact on the formation of aerobic granular sludge. 相似文献
72.
Slab behaviour and its surface expression: new insights from gravity modelling in the SE-Carpathians 总被引:1,自引:0,他引:1
We use lithosphere-scale gravity models to calculate gravity anomalies resulting from oceanic subduction, continental collision, slab steepening, delamination, and break-off. Local isostasy was assumed for determining vertical movements caused by mass changes related to these tectonic processes. Our results show that subduction is accompanied by basin subsidence on the upper plate caused by the heavy lithospheric root of the subducting slab. The basin evolution goes parallel with the slab evolution and shows considerable modifications when the processes at depth change (slab steepening, delamination, break-off). Characteristic gravity anomaly curves were acquired for the different tectonic scenarios. These curves together with other data (e.g. basin evolution on the upper and the lower plate) were used for the reconstruction of the tectonic evolution of the SE-Carpathians which includes Tertiary subduction and collision followed by slab steepening and delamination. 相似文献
73.
This article examines the recent progress of geography education as an academic subfield of geography and as an area of activity in K–12 education. Challenges to both endeavors are described. A restructuring of geography education's research program that focuses on spatial thinking and reasoning and merges applied and theoretical research is proposed. 相似文献
74.
75.
Robert W. Morton 《Marine Geology》1975,19(1):1-17
A series of sediment cores were obtained from the Whiting Basin southeast of Puerto Rico to investigate the factors affecting the velocity of sound in marine carbonate deposits. The cores indicated that the deposits in the Whiting Basin are similar to abyssal-plain deposits with lenticular turbidite sequences alternating with pelagic sediments. The sediment, comprised of highly porous sands and silts, averaged 80% calcium carbonate consisting of aragonite, low-Mg calcite and high-Mg calcite.Normal methods for predicting sound velocity from the physical properties of the deposits were found to be inaccurate for these samples. The established relationships of grain size and porosity to sound velocity were invalid because the sands found in the cores consisted of hollow-foram tests, causing high porosity independent of grain size. The rigidity of the deposit was the most significant factor determining sediment sound velocity and was itself controlled by the sediment source, transportation effects and the packing of the deposit. Future work is needed to accurately measure the effect of these factors on the rigidity modulus. 相似文献
76.
A nearly continuous zone of sediment waves is present on the lower continental rise off western Morocco which parallels the regional bathymetric trends. The individual sediment waves within the zone migrate upslope with time and, in general, also trend parallel to the regional bathymetric contours. These observations suggest that geostrophic contour currents are responsible for the formation of sediment waves. Physical oceanographic measurements and sea-floor photographs indicate only a very weak bottom circulation in this region. This suggests either that strong bottom currents are not essential for the formation of sediment waves or that relatively stronger bottom currents flowed along the continental margin of Morocco in the recent past. Turbidity flows may also influence the distribution of these sediment waves. 相似文献
77.
An analytical and experimental study of zoning in plagioclase 总被引:1,自引:0,他引:1
A detailed electron microprobe study has been conducted on natural and experimentally grown zoned plagioclase feldspars. Discontinuous, sector, and oscillatory chemical zoning are observed superimposed on continuous normal or reverse zoning trends. The relative accuracy of 3 element (Na, Ca, K) microprobe traverses was found statistically to be 2 mole percent. Comparison of microprobe data on natural zoned plagioclase with zoned plagioclase grown in controlled experiments has shown that it may be possible to distinguish zonal development resulting from physio-chemical changes to the bulk magma from zoning related to local kinetic control on the growth of individual crystals. 相似文献
78.
Robert Thunell Alan Federman Stephen Sparks Douglas Williams 《Quaternary Research》1979,12(2):241-253
The Y-5 ash is the most widespread layer in deep-sea sediments from the eastern Mediterranean. This ash layer was previously correlated with the Citara-Serrara tuff on Ischia Island and dated at approximately 25,000 yr B.P. New data on the glass chemistry of the Y-5 ash and pyroclastic deposits from the Neopolitan volcanic province suggest that the layer is correlative with the large-volume Campanian ignimbrite and not with the deposit from Ischia Island. The volume of the Y-5 ash is approximately 65 km3 which is comparable in magnitude to the volume of the Campanian ignimbrite. An interpolated age of approximately 38,000 yr B.P. is estimated based on sedimentation rates derived from δ18O stratigraphy. There is a discrepancy between this estimate and previously reported radiocarbon ages which range from 24,000 to 35,000 yr B.P. We propose that the “Campanian tuff ash layer” should be adopted as the full stratigraphic name for the Y-5 ash. The deep-sea ash layer is divisible into two units in proximal localities, probably correlating with two major phases of the eruption: plinian and ignimbrite. 相似文献
79.
Robert W Luth 《Geochimica et cosmochimica acta》2002,66(12):2091-2098
The melting reaction at the solidus of mantle peridotite is commonly peritectic in nature, with liquid and one or more solid phases produced upon melting. In some situations, one of the phases participating on the reactant side of the reaction is present in low abundance. This article explores the possible effects of the low abundance of a reactant phase on the melting behavior of mantle peridotite.For example, spinel lherzolite begins to melt via the peritectic reaction, clinopyroxene + orthopyroxene + spinel = olivine + liquid in the ∼1- to 2-GPa pressure range. In natural spinel lherzolites, spinel is a modally minor mineral and may be infrequently in contact with both clinopyroxene and orthopyroxene. If these mutual contacts are insufficient to generate an interconnected melt, then significant melting may not occur until a combination of minerals that are modally abundant and in contact begin to melt. This scenario could have implications for the physical process of melting and for the timing of formation of an interconnected melt network and separation of the melt from the residue.To begin to investigate this possibility, the spatial relationships between the constituent minerals in two fertile spinel lherzolites were determined by elemental mapping with the electron microprobe. Olivine, orthopyroxene, and clinopyroxene are of similar size, whereas the spinel was smaller and interstitial. Spinel and clinopyroxene are frequently in contact, but mutual contacts of spinel, clinopyroxene, and orthopyroxene are rare. Because of the changes in modal mineralogy anticipated for these lherzolites with increasing temperature, these mutual contacts will be even less common at the solidus. Therefore, an interconnected, potentially extractable, melt may not occur by the solidus spinel + orthopyroxene + clinopyroxene melting reaction. 相似文献
80.
新疆北部古地磁研究 总被引:1,自引:0,他引:1
通过和布克赛尔、克拉玛依、玛纳斯—乌鲁木齐地区泥盆纪到白垩纪古地磁研究,主要取得以下结果:(1)首次建立了准噶尔西北缘及南缘石炭纪—白垩纪古地磁极移曲线,由石炭纪到二叠纪的古地磁极位置基本在同一区间,说明该时期这些地区为一个统一的构造单元,而其古地磁极明显与塔里木、哈萨克斯坦、西伯利亚地块存在着差异。(2)该地区侏罗纪及白垩纪古地磁结果与塔里木地块结果一致,侏罗纪乌鲁木齐与和布克赛尔磁偏角相差30°左右,说明和布克赛尔地区相对乌鲁木齐地区逆时针旋转了30°左右,晚古生代以后曾发生过南向移动,而侏罗、白垩纪以来均向北发生了相当规模的北向运动,并发生了相对旋转,目前东、西准噶尔的构造格局可能就是由于局部相对旋转造成的。(3)中国大陆在早二叠世还不是一个联合的整体,而是以相互分离的独立块体分布于45°N—15°S的古特提斯洋中。(4)该地区二叠纪的磁偏角为165°—168°,而塔里木为218°,哈萨克斯坦为229°,说明存在35°—55°的逆时针旋转,这个旋转可能是由于西部推覆构造造成的。如果将西准噶尔超基性岩带顺时针旋转35°—55°后,东准噶尔超基性岩带、西准噶尔超基性岩带和斋桑泊—鲁布佐夫斯克超基性岩带应在同一构造带上。(5)该地区晚古生代古纬度变化不明显,位于30°—45 相似文献