全文获取类型
收费全文 | 1755篇 |
免费 | 569篇 |
国内免费 | 168篇 |
专业分类
测绘学 | 92篇 |
大气科学 | 51篇 |
地球物理 | 512篇 |
地质学 | 1054篇 |
海洋学 | 112篇 |
天文学 | 9篇 |
综合类 | 77篇 |
自然地理 | 585篇 |
出版年
2025年 | 4篇 |
2024年 | 51篇 |
2023年 | 51篇 |
2022年 | 77篇 |
2021年 | 85篇 |
2020年 | 97篇 |
2019年 | 83篇 |
2018年 | 66篇 |
2017年 | 63篇 |
2016年 | 88篇 |
2015年 | 75篇 |
2014年 | 93篇 |
2013年 | 108篇 |
2012年 | 102篇 |
2011年 | 93篇 |
2010年 | 88篇 |
2009年 | 103篇 |
2008年 | 115篇 |
2007年 | 102篇 |
2006年 | 82篇 |
2005年 | 72篇 |
2004年 | 106篇 |
2003年 | 76篇 |
2002年 | 67篇 |
2001年 | 64篇 |
2000年 | 78篇 |
1999年 | 55篇 |
1998年 | 60篇 |
1997年 | 49篇 |
1996年 | 37篇 |
1995年 | 35篇 |
1994年 | 37篇 |
1993年 | 30篇 |
1992年 | 30篇 |
1991年 | 27篇 |
1990年 | 17篇 |
1989年 | 9篇 |
1988年 | 9篇 |
1987年 | 1篇 |
1986年 | 4篇 |
1985年 | 1篇 |
1981年 | 1篇 |
1975年 | 1篇 |
排序方式: 共有2492条查询结果,搜索用时 15 毫秒
1.
Lakes in arid zone are sensitive to climatic changes. The lacustrine sediment sequence in Sogo Nur has well and truly recorded climatic events such as the Sui-Tang Dynasty Warm Period, the Song-Liao Dynasty Cold Period, the Medieval Warm Period, the Little Ice Age and the 20th Century Warm Period. Commonly, the climate in warm Periods was relatively humid,accordingly the lake area extended and water level rose, and vice versa. Apart from climatic change, human activity is also an important factor of influencing lake vicissitude, and they played the dominant role alternatively during different periods; the factor of climatic change predominated in historical period, while since the beginning of the 20th century the utilization of water resources by human has became decisive. 相似文献
2.
The Holocene sedimentary diatom record from Otasan Lake, Alberta, has been analyzed to determine the development of this presently slightly acidic lake. The changes in the lake have been linked to the development of the Sphagnum-dominated catchment. Analysis of the stratigraphic data revealed four distinct zones. The lake record began ca. 8200 yrs BP with a benthic and alkaline diatom assemblage dominated by Ellerbeckia arenaria (Moore) Crawford. At ca. 7300 yrs BP planktonic species began to increase and dominate indicating increased water levels, decreased turbidity, and increased nutrient levels. Throughout the Holocene the peatland in the catchment encroached toward the modern lake margin and by ca. 5000 yrs BP lake acidity had changed sufficiently such that acidic diatom species dominated. Tabellaria flocculosa (Roth) Kütz.v. flocculosa Strain IIIp sensu Koppen dominated the record from ca. 5000 to ca. 3100 yrs BP. The lowest lake water pH was inferred for this zone. From ca. 3100 yrs BP to the present Fragilaria species, primarily F. construens v. venter (Ehr.) Hustedt, dominated the diatom assemblage. Diatom productivity and inferred pH were interpreted as stable. From correspondence analysis of the fossil samples, and from species assemblages, underlying gradients of pH, nutrient level, and water depth were inferred. The change from alkaline to slightly acidic conditions took place between ca. 8200 and ca. 5000 yrs BP. From ca. 3000 yrs BP to the present, lake water pH has remained fairly constant. Nutrient levels and water depth were inferred to have altered together. After ca. 8200 yrs BP, nutrients and water level began to increase until ca. 6000 yrs BP. Then, there was a gradual decline in these variables over the most acidic zone until ca. 3000 yrs BP, after which they, too, have remained fairly constant. Dominant Boreal Upland Vegetation was established by ca. 7200 yrs BP, and it was inferred that dominant climate patterns had been established at that time, but small changes in climate have occurred and the landscape in northeastern Alberta has only been stable for the last 3000 years. 相似文献
3.
4.
我国西部地区湖泊水资源利用与湖水咸化状况分析 总被引:13,自引:6,他引:13
湖泊咸化是指湖水含盐量逐步增加.湖泊由淡水湖向咸水湖、再向盐湖和干盐湖的演化过程,这是我国西部干旱、半干旱地区湖泊普遍存在的一种现象。在大量调查的基础上,分析了我国西部干旱、半干旱地区近几十年来湖泊萎缩咸化的状况,湖泊变化的所产生的原因,并初步提出改善我国西部地区湖泊水资源利用的对策。 相似文献
5.
Melanie A. Riedinger Miriam Steinitz-Kannan William M. Last Mark Brenner 《Journal of Paleolimnology》2002,27(1):1-7
Lithostratigrahic and mineralogic analyses of sediments from hypersaline Bainbridge Crater Lake, Galápagos Islands, provide evidence of past El Niño frequency and intensity. Laminated sediments indicate that at least 435 moderate to very strong El Niño events have occurred since 6100 14C yr BP (7130 cal yr BP), and that frequency and intensity of events increased at about 3000 14C yr BP (3100 cal yr BP). El Niño activity was present between 6100 and 4000 14C yr BP (4600 cal yr BP) but infrequent. The Bainbridge record indicates that there has been considerable millennial-scale variability in El Niño since the mid-Holocene. 相似文献
6.
Geochemical samples from part of Lake Geneva were analyzed for 29oxides and trace elements. The variables and samples were subjected to R- and Q-mode analyses. The following techniques were
applied in sequence: data transformation (normalization and standardization), data reduction (principal component and factor
analysis), and automatic classification (dendrograph). The data were treated using various combinations of these techniques,
and the resulting classifications evaluated by means of several criteria. The best classification of the samples is given
by a cluster analysis performed on four principal components computed from standardized variables. The discriminatory power
of the variables also was measured and determined to depend on their degree of intercorrelation. As a final result, the 29original variables were reduced to four components and the sediment samples classified into four facies, leading to easily
interpretable geochemical maps. 相似文献
7.
Because of differential isostatic rebound, many lakes in Canada have continued to change their extent and depth since retreat of the Laurentide Ice Sheet. Using GIS techniques, the changing configuration and bathymetry of Lake of the Woods in Ontario, Manitoba, and Minnesota were reconstructed for 12 points in time, beginning at 11,000 cal yr B.P. (9.6 14C ka B.P.), and were also projected 500 years into the future, based on the assumption that Lake of the Woods continued to have a positive hydrological budget throughout the Holocene. This modeling was done by first compiling a bathymetric database and merging that with subaerial data from the Shuttle Radar Topography Mission (SRTM). This DEM file was then adjusted by: (1) isobase data derived from Lake Agassiz beaches prior to 9000 cal yr B.P. (8.1 14C ka B.P.) and (2) modeled isostatic rebound trend analysis after 9000 cal yr B.P. Just after the end of the Lake Agassiz phase of Lake of the Woods, only the northernmost part of the basin contained water. Differential rebound has resulted in increasing water depth. In the first 3000 years of independence from Lake Agassiz, the lake transgressed >50 km to the south, expanding its area from 858 to 2857 km2, and more than doubling in volume. Continued differential rebound after 6000 cal yr B.P. (5.2 14C ka B.P.) has further expanded the lake, although today it is deepening by only a few cm per century at the southern end. In addition, climate change in the Holocene probably played a role in lake level fluctuations. Based on our calculation of a modern hydrological budget for Lake of the Woods, reducing runoff and precipitation by 65% and increasing evaporation from the lake by 40% would end overflow and cause the level of the lake to fall below the outlets at Kenora. Because this climate change is comparable to that recorded during the mid-Holocene warming across the region, it is likely that the area covered by the lake at this time would have been less than that determined from differential isostatic rebound alone. 相似文献
8.
9.
B. Takano K. Suzuki K. Sugimori T. Ohba S. M. Fazlullin A. Bernard S. Sumarti R. Sukhyar M. Hirabayashi 《Journal of Volcanology and Geothermal Research》2004,135(4):299-329
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
10.