全文获取类型
收费全文 | 253篇 |
免费 | 15篇 |
专业分类
测绘学 | 5篇 |
大气科学 | 7篇 |
地球物理 | 42篇 |
地质学 | 143篇 |
海洋学 | 27篇 |
天文学 | 16篇 |
自然地理 | 28篇 |
出版年
2023年 | 2篇 |
2020年 | 2篇 |
2019年 | 3篇 |
2018年 | 7篇 |
2016年 | 6篇 |
2015年 | 7篇 |
2014年 | 6篇 |
2013年 | 17篇 |
2012年 | 5篇 |
2011年 | 16篇 |
2010年 | 8篇 |
2009年 | 14篇 |
2008年 | 10篇 |
2007年 | 4篇 |
2006年 | 9篇 |
2005年 | 5篇 |
2004年 | 6篇 |
2003年 | 5篇 |
2002年 | 9篇 |
2001年 | 10篇 |
2000年 | 6篇 |
1998年 | 8篇 |
1997年 | 7篇 |
1996年 | 4篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1993年 | 5篇 |
1992年 | 3篇 |
1991年 | 4篇 |
1990年 | 5篇 |
1989年 | 3篇 |
1988年 | 6篇 |
1987年 | 2篇 |
1986年 | 5篇 |
1985年 | 2篇 |
1984年 | 4篇 |
1983年 | 6篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1975年 | 4篇 |
1974年 | 3篇 |
1972年 | 1篇 |
1961年 | 1篇 |
1955年 | 2篇 |
1937年 | 1篇 |
排序方式: 共有268条查询结果,搜索用时 15 毫秒
121.
Ron Day 《Earth and Planetary Science Letters》1975,27(1):95-100
A study of the thermomagnetic curves of synthetic titanomagnetite powders, heated in air, in non-saturation applied magnetic fields showed that the shape of the curves was related to the magnitude of the applied field, the coercivity of the samples, and the effects of oxidation. The most important result to emerge from this study was the occurrence of reproducible peaks at low temperatures (~100–200°C). The temperature at which the peaks occur was found to increase with increasing coercive force or decreasing applied field. It is suggested that the peaks are the result of the “unblocking” of the magnetization, and should not be confused with thermomagnetic characteristics used to classify ferrites. The “unblocking” temperature is dependent on both the coercive force and the applied magnetic field, as is predicted by most models of thermo-remanent magnetization. 相似文献
122.
Dissolution of eight clay minerals, four zeolites, and quartz in seawater has been monitored for81/2 years. For most of the minerals, dissolution can be described as a first-order reaction in which dissolved silica approaches from undersaturation steady concentration values with time. Characteristic reaction rate constants (k1) are of the order of 10?7 sec?1. One of the zeolites, clinoptilolite, shows a different dissolution behavior: SiO2 concentration in solution reaches a high value within one year, followed by a decline to a lower value, suggestive of precipitation of another silicate phase (possibly sepiolite).A mathematical solution is given for a kinetic equation combining the parabolic-rate and first-order rate processes. It is shown that in a wide range of silicate dissolution reactions taking place over long periods of time, the presence of the parabolic-rate dissolution processes cannot be detected, thereby making its inclusion in the kinetic equations unnecessary. The experimental rates of dissolution are comparable to the SiO2? dissolution rates in oceanic sediments near the sediment/water interface. But deeper in the sediment, the calculated dissolution rates are significantly lower than the near-interface and experimental values. 相似文献
123.
Pleistocene to Recent stratovolcanoes in the Highlands of Papua New Guinea are made up of calc-alkaline to shoshonitic lava, tuff, agglomerate, ash, and lahar deposits. The volcanic rocks are characterized by high and variable Al, high K and total alkalis, and low Fe, Mg, and Ca. There is a continuous variation between high-K calc-alkaline, low-Si high-K calc-alkaline, and shoshonitic rocks. The elements V, Rb, Sr, Zn, Nb, and Ba are high relative to general andesitic abundances, particularly in the shoshonites. The Highlands volcanic rocks originated either in the base of thick sialic crust which is undergoing stabilization after major orogeny and uplift, or more probably, in eclogite sinking through the underlying mantle. Variation in content of K and other incompatible elements was either inherited from the source rocks in the base of the crust, or was produced by zone refining in a thick upper mantle zone containing interstitial fluid rich in these elements. Further variation, mainly in Fe/Mg, Si, and total alkalis, was caused by minor low-pressure crystal fractionation involving olivine, clinopyroxene, and, to a lesser extent, calcic plagioclase. 相似文献
124.
The Flow of Heat through the Floor of the Atlantic Ocean 总被引:2,自引:0,他引:2
125.
In recent decades, humans have become a very important force in the Earth system, demonstrating that emissions (gaseous, liquid, and solid) are the cause of many of our environmental issues. These emissions are responsible for major global reorganizations of the biogeochemical cycles. The oceans are now a net sink of atmospheric CO2, whereas in their preindustrial state they were a source; the trophic state of the coastal oceans is progressively moving toward increased heterotrophy; and the terrestrial realm is now vacillating between trophic states, whereas in preindustrial times it was autotrophic. In this paper, we present model calculations that underscore the role of human-induced perturbations in changing Earth's climate, specifically the role of anthropogenic nitrogen and phosphorus in controlling processes in the global carbon cycle since the year 1850 with projections to the year 2035. Our studies show that since the late 1940's emissions of nitrogen and phosphorus have been sequestered in the terrestrial living phytomass and groundwater. This nutrient-enhanced fertilization of terrestrial biota, coupled with rising atmospheric CO2 and global temperature, has induced a sink of anthropogenic CO2 that roughly balances the emission of CO2 owing to land use change. In the year 2000, for example, the model-calculated terrestrial biotic sink was 1730 Mtons C/year, while the emission of CO2 from changes in land use was 1820 Mtons C/year, a net flux of 90 Mtons C/year emitted to the atmosphere. In the global aquatic environment, enhanced terrestrial inputs of biotically reactive phosphorus (about 8.5 Mtons P/year) and inorganic nitrogen (about 54 Mtons N/year), have induced increased new production and burial of organic carbon in marine sediments, which is a small sink of anthropogenic CO2. It is predicted that the response of the global land reservoirs of C, N, and P to sustained anthropogenic perturbations will be maintained in the same direction of change over the range of projected scenarios of global population increase and temperature change for the next 35 years. The magnitude of change is significantly larger when the global temperature increase is maximum, especially with respect to the processes of remobilization of the biotically important nutrients nitrogen and phosphorus. 相似文献
126.
From September 1994 through October 1995 aboveground and belowground production ofSarcocornia fruticosa andPhragmites australis was studied at two sites in the Po Delta. In 1995, aboveground production forS. fruticosa in an intertidal site was 678 g dw m−2 yr−1 with a peak live biomass of 1,008 g m−2; belowground production was 1,260 g m−2 with a peak live biomass of 3,735 g m−2. A litter bag decomposition study showed that after 69 wk there were 3.7%, 64.3%, and 66.6% of the original mass of leafy stems, woody stems, and roots, respectively. In a reed bed, which experiences brackish conditions,P. australis aboveground production was 876 g m−2 with a peak live biomass of 780 g m−2; belowground production was 2,263 g m−2 with a peak live biomass of 4,087 g m−2. After 65 wk, there was 45.4%, 50.4%, and 29.3%, respectively, of leaves, stems, and rhizomes remaining of the initial biomass. At both sites, regular submersion by salt water probably leads to lower aboveground biomass and higher belowground biomass than reported for other Mediterranean coastal sites. The high belowground biomass can contribute to accretion to offset rising sea level. 相似文献
127.
Summary Economic 'development' driven by global economic forces produces specific expressions of 'community' in places where large new economic projects are to be located. This paper draws on contemporary geopolitical literature to theorise community identity as partly formulated in response to external 'threats'. A comparative study of community mobilisation in response to proposals to locate coastal superquarries on the Isle of Harris, Outer Hebrides, Scotland, and Cape Breton, Nova Scotia, Canada, suggests the applicability of this theoretical framework for extending geographical analysis of community identity and the politics of place. 相似文献
128.
This investigation used two approaches to examine the flux of bank-derived carbonate particles and determine the potential influence of benthic carbonate particle dissolution on the carbon chemistry of the waters around the Hawaiian Archipelago. First, the particle flux near several representative carbonate banks in the Hawaiian Archipelago was measured and compared with the flux at a distal site (ALOHA) approximately 100 km north of Oahu, Hawaii. The results of four sediment trap deployments on three carbonate banks in the Hawaiian Archipelago demonstrate that the flux of bank-derived carbonate particles are consistently one to two orders of magnitude higher than the fluxes at the distal station. Furthermore, the mineralogy of the carbonate flux near the banks, which includes very soluble bank-derived aragonite and magnesian calcite particles, is distinctly different from that of the distal fluxes. Second, the chemistry of the waters at each bank station along the archipelago was characterized and compared with the chemistry of the distal waters to determine if differences in the particle flux were reflected in the carbon chemistry. Higher alkalinity and carbonate ion concentrations were observed around all of the banks studied. The saturation state of these waters suggests that the dissolution of some magnesian calcite and aragonite phases could explain the higher alkalinity values. Calculations suggest that the dissolution of benthically-derived aragonite and magnesian calcite may be an important component of the North Pacific alkalinity budget and a potential sink for anthropogenic CO2. 相似文献
129.
Large (60 kg, minus 2 mm) sediment samples collected from a 5-km reach of a mature highland stream were used to investigate the effects of bar morphology and channel slope on the downstream dispersion of Au. Results are compared with those for an abundant heavy mineral (magnetite) and are discussed in terms of differential entrapment of light and heavy minerals by gravels and differences in supply of magnetite and Au to the study reach. Gold is selectively trapped in gravels; therefore, highest Au concentrations, a long dispersion train and lowest between-sampling-location variability were obtained with heavy-mineral concentrates of the minus 105+74 μm fraction from sandy gravels. In contrast, sands in bar-tail eddy pools gave the least reliable Au concentrations, due to random (Poisson) sampling errors and local post-depositional winnowing, and show rapid downstream anomaly decay. 相似文献
130.
Michael Day 《地球表面变化过程与地形》1987,12(5):497-505
Detailed slope surveys were made in seven karst depressions in central Belize. Four major slope types were identified—inclined bedrock slopes, staircases, broken cliffs, and talus slopes. Depression bases are dominated by inclined bedrock and talus slopes, midslopes largely by broken cliffs, and near hill summits by staircases. Erosional weight loss tablets deployed in 1975–1981 and 1980–1985 show significant variations between the different slope types, with a maximum near depression bases. Throughfall measurements in 1980 and 1985 show that midslope and near-summit sites receive 10–20 per cent less water than do depression bases. Surface runoff, monitored using coloured dyes, is of limited distance and duration but still contributes to surface corrosion and morphology. Vadose seepage, monitored by tracing water from the surface to a small, shallow cave, is rapid, and subsurface corrosion exceeds that at the surface. Overall, near-surface form and process are consistent with hydrological models of depression development (Williams, 1983, 1985) and surface processes contribute significantly to overall depression form. Moreover, surface activity complements that in the subcutaneous zone by directing recharge into the epikarstic aquifer. 相似文献