全文获取类型
收费全文 | 429篇 |
免费 | 23篇 |
专业分类
测绘学 | 15篇 |
大气科学 | 16篇 |
地球物理 | 185篇 |
地质学 | 163篇 |
海洋学 | 8篇 |
天文学 | 36篇 |
自然地理 | 29篇 |
出版年
2019年 | 5篇 |
2018年 | 5篇 |
2017年 | 9篇 |
2016年 | 14篇 |
2015年 | 14篇 |
2014年 | 8篇 |
2013年 | 13篇 |
2012年 | 8篇 |
2011年 | 20篇 |
2010年 | 22篇 |
2009年 | 25篇 |
2008年 | 15篇 |
2007年 | 15篇 |
2006年 | 10篇 |
2005年 | 24篇 |
2004年 | 12篇 |
2003年 | 16篇 |
2002年 | 8篇 |
2001年 | 5篇 |
2000年 | 6篇 |
1997年 | 5篇 |
1994年 | 5篇 |
1992年 | 4篇 |
1990年 | 3篇 |
1989年 | 5篇 |
1986年 | 3篇 |
1984年 | 7篇 |
1983年 | 5篇 |
1982年 | 3篇 |
1979年 | 3篇 |
1976年 | 3篇 |
1974年 | 4篇 |
1971年 | 3篇 |
1970年 | 5篇 |
1969年 | 4篇 |
1968年 | 6篇 |
1967年 | 8篇 |
1966年 | 7篇 |
1965年 | 9篇 |
1964年 | 7篇 |
1962年 | 5篇 |
1961年 | 6篇 |
1960年 | 4篇 |
1959年 | 7篇 |
1958年 | 9篇 |
1956年 | 5篇 |
1955年 | 4篇 |
1953年 | 3篇 |
1950年 | 5篇 |
1948年 | 6篇 |
排序方式: 共有452条查询结果,搜索用时 31 毫秒
51.
The morphology of the Alpine and adjacent landscapes is directly related to glacial erosion and associated sediment transport.
Here we report the effects of glacio-hydrologic erosion on bedrock topography in the Swiss Plateau. Specifically, we identify
the presence of subsurface valleys beneath the city of Bern and discuss their genesis. Stratigraphic investigations of more
than 4,000 borehole data within a 430 km2-large area reveal the presence of a network of >200 m-deep and 1,000 m-wide valleys. They are flat floored with steep sided
walls and are filled by Quaternary glacial deposits. The central valley beneath Bern is straight and oriented towards the
NNW, with valley flanks more than 20° steep. The valley bottom has an irregular undulating profile along the thalweg, with
differences between sills and hollows higher than 50–100 m over a reach of 4 km length. Approximately 500 m high bedrock highlands
flank the valley network. The highlands are dissected by up to 80 m-deep and 500 m-broad hanging valleys that currently drain
away from the axis of the main valley. We interpret the valleys beneath the city of Bern to be a tunnel valley network which
originated from subglacial erosion by melt water. The highland valleys served as proglacial meltwater paths and are hanging
with respect to the trunk system, indicating that these incipient highland systems as well as the main gorge beneath Bern
formed by glacial melt water under pressure. 相似文献
52.
Following deglaciation, the long-term pattern of change in diatom communities and the inferred history of the aquatic environment are affected by a hierarchy of environmental controls. These include direct climate impacts on a lake’s thermal and hydrologic budgets, as well as the indirect affects of climate on catchment processes, such as weathering, soil development, microbial activity, fire, and vegetation composition and productivity, which affect the transfer of solutes and particulates from the terrestrial ecosystem into the lake. Some of these catchment influences on lacustrine systems operate as time-dependent patterns of primary succession that are set in motion by glacier retreat. This paper provides a conceptual model of some dominant pathways of catchment influence on long-term lake development in glaciated regions and uses a series of paleolimnological examples from arctic, boreal, and temperate regions to evaluate the relative role of direct climate influences and of catchment processes in affecting the trajectory of aquatic ecosystems during the Holocene in different environmental contexts. 相似文献
53.
D. R. Engstrom C. Whitlock S. C. Fritz H. E. Wright Jr. 《Journal of Paleolimnology》1991,5(2):139-174
Recent sediments of eight small lakes in the northern winter range of Yellowstone National Park were cored to examine stratigraphic records of past changes in limnology and local environment that might be attributed to grazing and other activities of elk, bison, and other large ungulates. Cores of undisturbed sediment were analyzed at close intervals to depths covering the last 100–150 years according to chronologies established by lead-210 dating. Pollen analyses were made to show change in regional vegetation, and diatom and geochemical analyses were made to reveal possible limnological changes resulting from soil erosion and nutrient input from the lake catchments.Variations in sedimentary components prior to establishment of the Park in 1872 indicate some natural variability in environmental factors e.g., erosional inputs in landslide areas west of Gardiner. All lakes had abundant nutrient inputs.After the Park was founded, fire suppression may have been responsible for small increases in pollen percentages of various conifers and Artemisia tridentata (big sagebrush) at different times in different lakes. Perceptible decreases in pollen of willow, aspen, alder, and birch at different times may reflect local ungulate browsing, although drier climatic conditions may have been a factor as well.The most striking manifestation of accelerated erosion in a catchment was found at a lake located beside a road constructed in the 1930s. In contrast to changes at this site, the record of erosion at other lakes is hardly perceptible. Changes in sediment-accumulation rates seen at most sites result from redistribution of sediment within the lake after initial deposition.In the century following Park establishment, the abundance of planktonic diatoms relative to benthic taxa varies among lakes and may reflect differential nutrient inputs or changes in lake level. Four of the five lakes analyzed for diatoms show in the last few decades an increase in planktonic relative to benthic species, implying elevated nutrient inputs. The recent flora, however, is similar to that in pre-Park levels which suggests that these lakes have not been perturbed outside their normal range. Increased nutrient supply in recent decades for at least two of the lakes is supported by the geochemical data, which show an increase in biogenic silica and in organic matter.As a whole, our investigation of the sedimentary record does not support the hypothesis that ungulate grazing has had a strong direct or indirect effect on the vegetation and soil stability in the lake catchments or on the water quality of the lakes. 相似文献
54.
55.
56.
A prominent structure in the Western Escarpment of the Andes of northern Chile is the Oxaya anticline, dissected at the culmination by >1500 m deep valleys. The distribution of fault and fold structures indicates that the anticline could represent a simple buckle. Buckling thus appears to have accommodated crustal shortening in the plate overriding the subducting plate, between the trench and the Western Cordillera. Cross-cutting relationships between structures and dated strata indicated that the time interval of enhanced buckling coincides with the period when rates of valley formation were enhanced. We thus propose that the formation of this anticline was enhanced by fluvial incision, which is consistent with the results of published theoretical models. 相似文献
57.
We report observations of MeV heavy ions made with Explorer 45 in the earth's radiation belts during the 7-month period June–December 1972 when four major magnetic storms occurred. Significant fluxes of ions heavier than fluorine (i.e. with nuclear charge Z ? 9) were observed stably trapped in the interior of the radiation belts at L ~ 2–4. These energetic very heavy ions, were found to appear suddenly during the August 1972 magnetic storm period and their fluxes decayed during the following months on time scales typically several tens of days. Simultaneously, strong increases in the geomagnetically trapped MeV helium and CNO ion fluxes were observed, and the post-injection flux decay of these ions was found to be slower than that of the Z ? 9 ions. The relative enhancements in trapped fluxes during the storm increased with increasing ion mass and/or increasing ion energy. 相似文献
58.
59.
60.
Fr. Model 《Ocean Dynamics》1958,11(5):217-218
Ohne Zusammenfassung 相似文献