全文获取类型
收费全文 | 966篇 |
免费 | 21篇 |
国内免费 | 2篇 |
专业分类
测绘学 | 17篇 |
大气科学 | 79篇 |
地球物理 | 249篇 |
地质学 | 275篇 |
海洋学 | 94篇 |
天文学 | 134篇 |
综合类 | 3篇 |
自然地理 | 138篇 |
出版年
2020年 | 8篇 |
2019年 | 9篇 |
2018年 | 27篇 |
2017年 | 15篇 |
2016年 | 18篇 |
2015年 | 16篇 |
2014年 | 21篇 |
2013年 | 41篇 |
2012年 | 38篇 |
2011年 | 46篇 |
2010年 | 34篇 |
2009年 | 32篇 |
2008年 | 41篇 |
2007年 | 35篇 |
2006年 | 34篇 |
2005年 | 34篇 |
2004年 | 24篇 |
2003年 | 25篇 |
2002年 | 34篇 |
2001年 | 19篇 |
2000年 | 18篇 |
1999年 | 15篇 |
1998年 | 20篇 |
1997年 | 16篇 |
1996年 | 9篇 |
1995年 | 16篇 |
1994年 | 12篇 |
1993年 | 19篇 |
1992年 | 11篇 |
1991年 | 5篇 |
1990年 | 13篇 |
1989年 | 10篇 |
1988年 | 10篇 |
1987年 | 12篇 |
1986年 | 18篇 |
1985年 | 19篇 |
1984年 | 17篇 |
1983年 | 23篇 |
1982年 | 18篇 |
1981年 | 8篇 |
1980年 | 19篇 |
1979年 | 15篇 |
1978年 | 20篇 |
1977年 | 12篇 |
1976年 | 16篇 |
1975年 | 7篇 |
1974年 | 11篇 |
1973年 | 12篇 |
1972年 | 5篇 |
1971年 | 5篇 |
排序方式: 共有989条查询结果,搜索用时 15 毫秒
1.
Lawrence M. Mayer Peter T. Rahaim William Guerin Stephen A. Macko Les Watling Franz E. Anderson 《Estuarine, Coastal and Shelf Science》1985,20(4):491-503
A variety of measures of organic matter concentration and quality were made on samples collected from the top few mm of intertidal mudflat sediment over the course of a year, in order to assess the relative importance of biological and sedimentological influences on sedimentary organic matter. Winter and summer were times of relatively fine-grained sediment accumulation, caused by biological deposition or stabilization processes and resulting in higher organic matter concentrations. Stable carbon isotope and Br:C ratios indicated a planktonic source of bulk organic matter. Ratios of organic carbon to specific surface area of the sediments were consistent with an organic monolayer coverage of sediment grains. Correction for changing grain size during the year showed no change in the organic concentration per unit surface area, in spite of organic matter inputs by in situ primary production, buildup of heterotroph biomass and mucus coatings, and biodeposition of organic-rich seston. There were also no indications of changes in bulk organic quality, measured as hydrolyzable carbohydrates and amino acids, in response to these biological processes. It is concluded that biological processes on a seasonal time scale affect the bulk organic matter of these sediments via a modulation of grain size rather than creation or decay of organic matter. 相似文献
2.
3.
The prevalence of dermal neoplasia among neotenic tiger salamanders (Ambystoma tigrinum) collected from the Reese Air Force Base sewage lagoon far exceeded that among salamanders from nearby uncontaminated lagoons. Perylene was the predominant pglycyclic aromatic hydrocarbon contaminant in this lagoon. Perylene was shown to be a substrate for monooxygenases in skin and liver homogenates and formed DNA-binding metabolites. Perylene exposure appeared to augment subsequent perylene metabolism by dermal enzymes but it was not a strong inducer of hepatic monooxygenases. Aroclor 1254 induced benzo(a)pyrene and perylene metabolism by both skin and liver enzymes. However, perylene apparently was not activated by salamander enzymes, as suggested by its lack of in vitro mutagenicity and failure to induce tumors. 相似文献
4.
This environmental overview of Eckernförde Bay (northern Germany) summarizes the results of previous studies relevant to the Office of Naval Research's Coastal Benthic Boundary Layer (CBBL) Baltic field exercise conducted during 1993–1994. Significant environmental characteristics include the following: (1) surface sediment distribution is related to water depth, dictated primarily by hydrodynamic reworking of older glacial deposits; (2) the origin and characteristics of small-scale sedimentary structures depend on storm-generated waves and currents; (3) the proximity of the sea surface and sediment —water interface results in a pelagic—benthic coupling that drives biogeochemical processes and produces organicrich, acoustically turbid sediments; and (4) the bay floor is complicated topographically by pockmarks and manmade sedimentary structures. 相似文献
5.
Historical data of total dissolved inorganic carbon (CT), together with nitrate and phosphate, have been used to model the evolution of these constituents over the year in the Atlantic water of the Norwegian Sea. Changes in nutrient concentration in the upper layer of the ocean are largely related to biological activity, but vertical mixing with the underlying water will also have an impact. A mixing factor is estimated and used to compute the entrainment of these constituents into the surface water from below. After taking the mixing contribution into account, the resulting nutrient concentration changes are attributed to biological production or decay. The results of the model show that the change in CT by vertical mixing and by biological activity based on nutrient equivalents needs another sink to balance the carbon budget. It cannot be the atmosphere as the surface water is undersaturated with respect to carbon dioxide and is, thus, a source of CT in this region. Inasmuch as the peak deficit of carbon is more than a month later than for the nutrients, the most plausible explanation is that other nitrogen and phosphate sources than the inorganic salts are used together with dissolved inorganic carbon during this period. As nitrate and phosphate show a similar trend, it is unlikely that the explanation is the use of ammonia or nitrogen fixation but rather dissolved organic nitrogen and phosphate, while dissolved organic carbon is accumulating in the water. 相似文献
6.
Framvaren, a super-anoxic fjord in southern Norway, contains 7–8 mmoll−1 of sulphide and a total carbonate concentration of 18.5 mmol kg−1 in the bottom water. The chemistry of calcium has been studied, considering sources, biogenic and chemical processes and sedimentary sinks. Calcium associated with the bacteria biomass at the redox interface (18m depth) appears to be the primary source of dissolved calcium in the deep, anoxic water. Excess calcium and high total carbonate cause supersaturation of calcite, which is precipitated chemogenically. Calcite (and presumably some aragonite) is identified both in sediment trap material and the bottom sediments below the depth of supersaturation. 相似文献
7.
Overwash is a major controlling factor in the morphology of the mixing zone of coastal aquifers. Conceptual models of the mixing zone describe an interface controlled by tidal oscillations, wave run-up, and other factors; however, few describe the influence of large storm events. In August 1993, Hatteras Island, North Carolina, USA, experienced a 3-m storm surge due to Hurricane Emily. Sound-side flooding infiltrated a wellfield, causing a dramatic increase in TDS levels that persisted for more than 3 years. Two-dimensional simulations with SUTRA, the USGS finite-element model, are calibrated to the TDS breakthrough data of this storm to infer model dispersivity values. Simulations using the calibrated dispersivity values, predicted flooding levels, and 54 years of hurricane records to determine the influence of the overwash events suggest that it is rare for the mixing zone to approximate the conceptual morphology. Even during quiescent periods such as between 1965 and 1975, TDS levels do not return to theoretical levels before being elevated by a subsequent storm event. Thus, while tidal oscillations and other factors are important to mixing zone development, basic wind events and more severe storm events may have more influence and lasting effect on the morphology of the mixing zone. 相似文献
8.
A large amount of interest has recently been expressed pertaining to the quantity of physically adsorbed water by the Martian regolith. Thermodynamic calculations based on experimentally determined adsorption and desorption isotherms and extrapolated to subzero temperatures indicate that physical adsorption of more than one or two monomolecular layers is highly unlikely under Martian conditions. Any additional water would find ice to be the state of lowest energy and therefore the most stable form. To test the validity of the thermodynamic calculations we have measured adsorption and desorption isotherms of sodium montmorillonite at ?5°C. To a first approximation it was found to be valid. 相似文献
9.
10.
The heterogeneous mantle 总被引:1,自引:0,他引:1