全文获取类型
收费全文 | 315篇 |
免费 | 24篇 |
国内免费 | 3篇 |
专业分类
测绘学 | 5篇 |
大气科学 | 26篇 |
地球物理 | 64篇 |
地质学 | 102篇 |
海洋学 | 54篇 |
天文学 | 41篇 |
综合类 | 3篇 |
自然地理 | 47篇 |
出版年
2022年 | 2篇 |
2021年 | 9篇 |
2020年 | 4篇 |
2019年 | 5篇 |
2018年 | 11篇 |
2017年 | 9篇 |
2016年 | 14篇 |
2015年 | 9篇 |
2014年 | 9篇 |
2013年 | 35篇 |
2012年 | 18篇 |
2011年 | 14篇 |
2010年 | 22篇 |
2009年 | 17篇 |
2008年 | 12篇 |
2007年 | 11篇 |
2006年 | 9篇 |
2005年 | 9篇 |
2004年 | 8篇 |
2003年 | 10篇 |
2002年 | 7篇 |
2001年 | 4篇 |
2000年 | 4篇 |
1999年 | 3篇 |
1998年 | 5篇 |
1997年 | 3篇 |
1996年 | 3篇 |
1995年 | 5篇 |
1994年 | 2篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1991年 | 5篇 |
1990年 | 4篇 |
1989年 | 3篇 |
1988年 | 8篇 |
1987年 | 3篇 |
1986年 | 6篇 |
1984年 | 2篇 |
1983年 | 5篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1970年 | 1篇 |
1969年 | 2篇 |
1968年 | 1篇 |
1966年 | 1篇 |
排序方式: 共有342条查询结果,搜索用时 15 毫秒
61.
Using the SPICAV-UV spectrometer aboard Venus Express in nadir mode, we were able to derive spectral radiance factors in the middle atmosphere of Venus in the 170-320 nm range at a spectral resolution of R ? 200 during 2006 and 2007 in the northern hemisphere. By comparison with a radiative transfer model of the upper atmosphere of Venus, we could derive column abundance above the visible cloud top for SO2 using its spectral absorption bands near 280 and 220 nm. SO2 column densities show large temporal and spatial variations on a horizontal scale of a few hundred kilometers. Typical SO2 column densities at low latitudes (up to 50°N) were found between 5 and 50 μm-atm, whereas in the northern polar region SO2 content was usually below 5 μm-atm. The observed latitudinal variations follow closely the cloud top altitude derived by SPICAV-IR and are thought to be of dynamical origin. Also, a sudden increase of SO2 column density in the whole northern hemisphere has been observed in early 2007, possibly related to a convective episode advecting some deep SO2 into the upper atmosphere. 相似文献
62.
Tatiana F. Maria Jan Vanaverbeke Ruth Gingold André M. Esteves Ann Vanreusel 《Marine Ecology》2013,34(2):207-217
Lately, across‐shore zonation has been found to be more important in structuring the nematode community of a tropical macrotidal sandy beach than microhabitat heterogeneity. To evaluate whether this zonation pattern applies to a temperate beach, a macrotidal ridge‐and‐runnels sandy beach in the North Sea was studied. We investigated whether a similar zonation occurs in sandbar and runnel microhabitats, and whether the runnels harbour a different community from the subtidal. Our results indicate that nematode communities from runnel and sandbar habitats are significantly different. In addition, horizontal zonation patterns for nematode communities differ between both habitats. Nematode assemblages from sandbars are divided to lower, middle and upper beach while upper and middle runnels cluster together. The subtidal and upper runnels showed dissimilar nematode assemblages, although runnels showed the same dominant species (Daptonema normandicum), which increases its abundance towards the upper runnels. This study illustrates the importance of microhabitat heterogeneity, which resulted in different zonation patterns across the sandy beach examined. The divergent zonation between sandbars and runnels in the macrotidal temperate sandy beach, compared with the pattern observed for a subtropical sandy beach with similar morphodynamics, indicates that generalizations about nematode distribution patterns should be made with caution. 相似文献
63.
Exploring the impact of diagenesis on (isotope) geochemical and microstructural alteration features in biogenic aragonite 下载免费PDF全文
Ann‐Christine Ritter Vasileios Mavromatis Martin Dietzel Ola Kwiecien Felix Wiethoff Erika Griesshaber Laura A. Casella Wolfgang W. Schmahl Jennifer Koelen Rolf D. Neuser Albrecht Leis Dieter Buhl Andrea Niedermayr Sebastian F. M. Breitenbach Stefano M. Bernasconi Adrian Immenhauser 《Sedimentology》2017,64(5):1354-1380
For the Quaternary and Neogene, aragonitic biogenic and abiogenic carbonates are frequently exploited as archives of their environment. Conversely, pre‐Neogene aragonite is often diagenetically altered and calcite archives are studied instead. Nevertheless, the exact sequence of diagenetic processes and products is difficult to disclose from naturally altered material. Here, experiments were performed to understand biogenic aragonite alteration processes and products. Shell subsamples of the bivalve Arctica islandica were exposed to hydrothermal alteration. Thermal boundary conditions were set at 100°C, 175°C and 200°C. These comparably high temperatures were chosen to shorten experimental durations. Subsamples were exposed to different 18O‐depleted fluids for durations between two and twenty weeks. Alteration was documented using X‐ray diffraction, cathodoluminescence, fluorescence and scanning electron microscopy, as well as conventional and clumped isotope analyses. Experiments performed at 100°C show redistribution and darkening of organic matter, but lack evidence for diagenetic alteration, except in Δ47 which show the effects of annealing processes. At 175°C, valves undergo significant aragonite to calcite transformation and neomorphism. The δ18O signature supports transformation via dissolution and reprecipitation, but isotopic exchange is limited by fluid migration through the subsamples. Individual growth increments in these subsamples exhibit bright orange luminescence. At 200°C, valves are fully transformed to calcite and exhibit purple‐blue luminescence with orange bands. The δ18O and Δ47 signatures reveal exchange with the aqueous fluid, whereas δ13C remains unaltered in all experiments, indicating a carbonate‐buffered system. Clumped isotope temperatures in high‐temperature experiments show compositions in broad agreement with the measured temperature. Experimentally induced alteration patterns are comparable with individual features present in Pleistocene shells. This study represents a significant step towards sequential analysis of diagenetic features in biogenic aragonites and sheds light on reaction times and threshold limits. The limitations of a study restricted to a single test organism are acknowledged and call for refined follow‐up experiments. 相似文献
64.
65.
66.
The past decade has seen what could reasonably be called an explosion in the number of food-related crises in China. Food safety issues represent a national crisis threatening the physical and psychological health of Chinese citizens, despite the repeated adoption of stringent food safety laws and regulations. This project, based on a stratified random sample of 337 households in Nanjing, assesses perceptions related to different food safety concerns and issues among urban residents and explores differences in perceptions across social strata as defined by socioeconomic and demographic variables using principal component analysis and standard statistical tests. 相似文献
67.
Elke Allers Raeid M.M. Abed Laura M. Wehrmann Tao Wang Ann I. Larsson Autun Purser Dirk de Beer 《Marine pollution bulletin》2013
In laboratory experiments, the cold-water coral Lophelia pertusa was exposed to settling particles. The effects of reef sediment, petroleum drill cuttings and a mix of both, on the development of anoxia at the coral surface were studied using O2, pH and H2S microsensors and by assessing coral polyp mortality. Due to the branching morphology of L. pertusa and the release of coral mucus, accumulation rates of settling material on coral branches were low. Microsensors detected H2S production in only a few samples, and sulfate reduction rates of natural reef sediment slurries were low (<0.3 nmol S cm−3 d−1). While the exposure to sediment clearly reduced the coral’s accessibility to oxygen, L. pertusa tolerated both partial low-oxygen and anoxic conditions without any visible detrimental short-term effect, such as tissue damage or death. However, complete burial of coral branches for >24 h in reef sediment resulted in suffocation. 相似文献
68.
Tatiana F. Maria André M. EstevesJan Vanaverbeke Ann Vanreusel 《Estuarine, Coastal and Shelf Science》2011,94(3):272-280
The effect of an abundant sandy beach polychaete, Scolelepis squamata, on the colonisation of defaunated sediments by marine nematodes indicates that sandy beach fauna can be partially controlled by biological interactions within and across size groups. Experimental cores, equipped with windows allowing infaunal colonisation, were filled with defaunated sandy beach sediment containing two different treatments with and without S. squamata. These cores were inserted into microcosms filled with sediment with indigenous meiofauna collected from the field. The treatments were incubated in the laboratory at ambient temperature and salinity for 2, 7, 14 and 21 days, in order to follow the colonisation process of the defaunated sediments by the indigenous nematode fauna over time. Nematodes initially colonised both treatments, with abundances of up to 10% of the densities in the control; after 2 weeks, nematode densities in the cores without S. squamata surpassed the control densities. Nematode assemblages in both treatments were not species rich, and also differed in composition from the natural assemblages. The most successful colonising species, Enoplolaimus litoralis, was rare in the surrounding sediment, suggesting that colonisation was determined by species-specific characteristics such as body size, motility and feeding strategy. Initially the presence of macrofauna did not affect the nematode community composition, but after 2 weeks of the experiment, the presence of the polychaete seemed to facilitate the earlier establishment of non-opportunistic species. 相似文献
69.
Varved sediments of Lake Yoa (Ounianga Kebir,Chad) reveal progressive drying of the Sahara during the last 6100 years 总被引:1,自引:0,他引:1
Pierre Francus Hans von Suchodoletz Michael Dietze Reik V. Donner Frédéric Bouchard Ann‐Julie Roy Maureen Fagot Stefan Kröpelin 《Sedimentology》2013,60(4):911-934
The sedimentological and geochemical properties of a 7·47 m long laminated sequence from hypersaline Lake Yoa in northern Chad have been investigated, representing a unique, continuous 6100 year long continental record of climate and environmental change in the eastern Central Sahara. These data were used to reconstruct the Mid to Late Holocene history of this currently hyper‐arid region, in order to address the question of whether the Mid Holocene environmental transition from a humid to a dry Sahara was progressive or abrupt. This study involved a suite of analyses, including petrographic and scanning electron microscope examination of thin sections, X‐ray diffraction, X‐radiography, granulometry, loss on ignition and magnetic susceptibility. The potential of micro‐X‐ray fluorescence core scanning was tested at very high resolution. Detailed microscopic investigation revealed the sedimentary processes responsible for the formation of the fine laminations, identified the season during which they were formed, and confirmed their annually rhythmic nature. High‐resolution X‐ray fluorescence core scanning allowed the distinction of each individual lamination over the entire record, opening new perspectives for the study of finely laminated sediment sequences. Geochemical and mineralogical data reveal that, due to decreasing monsoon rainfall combined with continuous and strong evaporation, the hydrologically open and fresh Mid Holocene Lake Yoa slowly evolved into the present‐day hypersaline brine depleted in calcium, which has existed for about the past 1050 years. During the oldest part of the investigated period, Lake Yoa probably contained a permanently stratified lower water column that was nevertheless disrupted relatively frequently by mixing events. Deep‐water anoxia became more stable because of increased salinity‐driven density stratification. In parallel, the sediment grain‐size proxies record a progressive increase of aeolian input in the course of the last 6100 years. Altogether, all geochemical and sedimentological indicators point to a progressive drying of the eastern Central Sahara, strengthening previous conclusions based on palaeoecological indicators. 相似文献
70.
We present absorption cross sections of propane (C3H8) at temperatures from 145 K to 297 K in the 690–1550 cm−1 region. Pure and N2-broadened spectra were measured at pressures from 3 Torr to 742 Torr using a Bruker IFS125 FT-IR spectrometer at JPL. The gas absorption cell, developed at Connecticut College, was cooled by a closed-cycle helium refrigerator. The cross sections were measured and compiled for individual spectra recorded at various experimental conditions covering the planetary atmosphere and Titan. In addition to the cross sections, a propane pseudoline list with a frequency grid of 0.005 cm−1, was fitted to the 34 laboratory spectra. Line intensities and lower state energies were retrieved for each line, assuming a constant width. Validation tests showed that the pseudoline list reproduces discrete absorption features and continuum, the latter contributed by numerous weak and hot band features, in most of the observed spectra within 3%. Based on the pseudoline list, the total intensity in the 690–1550 cm−1 region was determined to be 52.93 (±3%) × 10−19 cm−1/(molecule cm−2) at 296 K; this value is within 3% of the average from four earlier studies. Finally, the merit of the pseudoline approach is addressed for heavy polyatomic molecules in support of spectroscopic observation of atmospheres of Titan and other planets. The cold cross sections will be submitted to the HITRAN database (hitran.harvard.edu), and the list of C3H8 pseudolines will be available from a MK-IV website of JPL (http://mark4sun.jpl.nasa.gov/data/spec/Pseudo). 相似文献