全文获取类型
收费全文 | 4539篇 |
免费 | 1323篇 |
国内免费 | 26篇 |
专业分类
测绘学 | 84篇 |
大气科学 | 194篇 |
地球物理 | 2310篇 |
地质学 | 1733篇 |
海洋学 | 320篇 |
天文学 | 840篇 |
综合类 | 3篇 |
自然地理 | 404篇 |
出版年
2021年 | 48篇 |
2020年 | 74篇 |
2019年 | 216篇 |
2018年 | 232篇 |
2017年 | 332篇 |
2016年 | 366篇 |
2015年 | 372篇 |
2014年 | 400篇 |
2013年 | 495篇 |
2012年 | 331篇 |
2011年 | 317篇 |
2010年 | 294篇 |
2009年 | 234篇 |
2008年 | 272篇 |
2007年 | 206篇 |
2006年 | 166篇 |
2005年 | 167篇 |
2004年 | 155篇 |
2003年 | 155篇 |
2002年 | 147篇 |
2001年 | 127篇 |
2000年 | 134篇 |
1999年 | 36篇 |
1998年 | 40篇 |
1997年 | 26篇 |
1996年 | 24篇 |
1995年 | 25篇 |
1994年 | 26篇 |
1993年 | 19篇 |
1992年 | 15篇 |
1991年 | 18篇 |
1990年 | 17篇 |
1989年 | 19篇 |
1988年 | 15篇 |
1987年 | 20篇 |
1986年 | 28篇 |
1985年 | 23篇 |
1984年 | 17篇 |
1983年 | 30篇 |
1982年 | 32篇 |
1981年 | 26篇 |
1980年 | 26篇 |
1979年 | 18篇 |
1978年 | 12篇 |
1977年 | 12篇 |
1976年 | 17篇 |
1975年 | 15篇 |
1974年 | 13篇 |
1973年 | 16篇 |
1971年 | 11篇 |
排序方式: 共有5888条查询结果,搜索用时 31 毫秒
31.
This paper is a continuation of previous research, which obtained a convenient solution for arbitrary surface fluxes before ponding. By means of Fourier Transformation this has been extended to derive analytical solutions of a linearized Richards' equation for arbitrary input fluxes after surface saturation. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
32.
33.
A model of the predawn bulge ionosphere composition and structure is constructed and compared with the ion mass spectrometer measurements from the Pioneer Venus Orbiter during orbits 117 and 120. Particular emphasis is given to the identification of the mass-2 ion which we find unequivocally due to D+ (and not H2+). The atmospheric D/H ratio of 1.4% and 2.5% is obtained at the homopause (~ 130 km) for the two orbits. The H2+ contribution to the mass-2 ion density is less than 10%, and the H2 mixing ratio must be <0.1 ppm at 130 km altitude. The He+ data require a downward He+ flux of ~2 × 107 cm?2 sec?1 in the predawn region which suggest that the light ions also flow across the terminator from day to night along with the observed O+ ion flow. 相似文献
34.
Methane, a potential biosignature, has recently been detected in the martian atmosphere. This Note focuses on field investigations/operational simulations and laboratory studies which resulted in successful detection of methane within arid terrestrial soils, as distinct from the usual methanogen environment, but in at least partial analogy to martian conditions. 相似文献
35.
Abstract— Quantifying the peak temperatures achieved during metamorphism is critical for understanding the thermal histories of ordinary chondrite parent bodies. Various geothermometers have been used to estimate equilibration temperatures for chondrites of the highest metamorphic grade (type 6), but results are inconsistent and span hundreds of degrees. Because different geothermometers and calibration models were used with different meteorites, it is unclear whether variations in peak temperatures represent actual ranges of metamorphic conditions within type 6 chondrites or differences in model calibrations. We addressed this problem by performing twopyroxene geothermometry, using QUILF95, on the same type 6 chondrites for which peak temperatures were estimated using the plagioclase geothermometer (Nakamuta and Motomura 1999). We also calculated temperatures for published pyroxene analyses from other type 6 H, L, and LL chondrites to determine the most representative peak metamorphic temperatures for ordinary chondrites. Pyroxenes record a narrow, overlapping range of temperatures in H6 (865–926 °C), L6 (812–934 °C), and LL6 (874–945 °C) chondrites. Plagioclase temperature estimates are 96–179 °C lower than pyroxenes in the same type 6 meteorites. Plagioclase estimates may not reflect peak metamorphic temperatures because chondrule glass probably recrystallized to plagioclase prior to reaching the metamorphic peak. The average temperature for H, L, and LL chondrites (~900 °C), which agrees with previously published oxygen isotope geothermometry, is at least 50 °C lower than the peak temperatures used in current asteroid thermal evolution models. This difference may require minor adjustments to thermal model calculations. 相似文献
36.
37.
H. A. Taylor Jr. P. A. Cloutier M. Dryer S. T. Suess A. Barnes R. S. Wolff 《Earth, Moon, and Planets》1985,32(3):275-290
Corotating solar wind streams emanating from stable coronal structures provide an unique opportunity to compare the response of planetary ionospheres to the energy conveyed in the streams. For recurrent solar conditions the signal propagating outward along spiral paths in interplanetary space can at times exhibit rather similar content at quite different downstream locations in the ecliptic plane. Using solar wind measurements from plasma detectors on ISEE-3, Pioneer Venus Orbiter (PVO) and Helios-A, as well as in-situ ion composition measurements from Bennett Ion Mass Spectrometers on the Atmosphere Explorer-E and PVO spacecraft, corotating stream interactions are examined at Earth and Venus. During May–July 1979 a sequence of distinct, recurrent coronal regions developed at the Sun. Analysis of these regions and the associated solar wind characteristics indicates a corrresponding sequence of corotating streams, identifiable over wide distances. The time series of solar wind velocity variations observed at Earth, Venus, and the Helios-A positions during June–July attests to intervals of corotating stream propagation. The characteristics of the stream which passed Earth on July 3, are observed at Helios-A and at Venus (PVO) about 8 days later, consistent with the spiral path propagation delay times between the locations in the ecliptic plane. On July 3, Earth and Venus have a wide azimuthal separation of about 142 . Although the planetary environments are distinctly different, pronounced and somewhat analagous ionospheric responses to the stream passage are observed at both Earth and Venus. The response to the intercepted stream is consistent with independent investigations which have shown that the variability of the solar wind momentum flux is an important factor in the solar wind-ionosphere interaction at both planets. 相似文献
38.
RAÚL DE LA HORRA Ma ISABEL BENITO JOSE LÓPEZ‐GÓMEZ ALFREDO ARCHE JOSÉ F. BARRENECHEA JAVIER LUQUE 《Sedimentology》2008,55(6):1849-1873
The Late Permian (Wuchiapingian) Alcotas Formation in the SE Iberian Ranges consists of one red alluvial succession where abundant soil profiles developed. Detailed petrographical and sedimentological studies in seven sections of the Alcotas Formation allow six different types of palaeosols, with distinctive characteristics and different palaeogeographical distribution, to be distinguished throughout the South‐eastern Iberian Basin. These characteristics are, in turn, related to topographic, climatic and tectonic controls. The vertical distribution of the palaeosols is used to differentiate the formation in three parts from bottom to top showing both drastic and gradual vertical upwards palaeoenvironmental changes in the sections. Reconstruction of palaeoenvironmental conditions based on palaeosols provides evidence for understanding the events that occurred during the Late Permian, some few millions of years before the well‐known Permian‐Triassic global crisis. 相似文献
39.
GERNOT ARP CHRISTIAN OSTERTAG‐HENNING SELÇÜK YÜCEKENT JOACHIM REITNER VOLKER THIEL 《Sedimentology》2008,55(5):1227-1251
Fossil stromatolites may reveal information about their hydrochemical palaeoenvironment, provided that assignment to a specific microbial community and a corresponding biogeochemical mechanism of formation can be made. Tithonian stromatolites of the Münder Formation at Thüste, north Germany, have traditionally been considered as formed by intertidal cyanobacterial communities. However, thin sections of the stromatolites show elongated angular traces of former gypsum crystals in a dense arrangement, but no algal or cyanobacterial filament traces. Moreover, high Fe2+ and Mn2+ contents, oxygen‐isotope and sulphur‐isotope ratios of carbonate‐bound sulphates, and sulphurized hydrocarbon biomarkers of the stromatolitic carbonate indicate that CaCO3 precipitation occurred near the oxic–anoxic interface as a result of intensive bacterial sulphur cycling rather than photosynthetic activity. Furthermore, anaerobic oxidation of methane by Archaea may have driven CaCO3 precipitation in deeper parts of the biofilm community, as reflected by high concentrations of squalane with a strongly negative δ13C in conjunction with evaporite pseudomorphs showing extremely low δ13CCarb ratios. Consequently, the Thüste stromatolites are now interpreted as having initially formed by gypsum impregnation of biofilms. Subsequently, early Mg‐calcitic calcitization within the biofilms occurred because of combined bacterial iron, manganese and sulphate reduction, with an increasing contribution of anaerobic oxidation of methane with depth. This model plausibly explains the prominent preservation of signals derived from oxygen‐independent metabolic pathways, whereas virtually no geochemical record exists for an aerobic community that may, nevertheless, have prevailed at the stromatolite surface. Photic‐zone stromatolites with a prominent signal of anaerobic oxidation of methane may be common in, and indicative of, oxygen‐depleted sulphate‐bearing environments with high rates of methane production, conditions that possibly were fulfilled at the Archaean to Proterozoic transition. 相似文献
40.
The purpose of this study was to determine whether there was a sufficiently high residual salt load in the dry sediments of
the St Lucia Estuary to cause salinity problems should it later fill up with either freshwater or seawater. The estuary lakes
have suffered the effects of a severe drought since 2002 with the result that many areas were dry, and the salinity of the
residual water varied between 4 psu and up to five times that of seawater. Measurements of the salts content in the sediments
to a depth of 20 cm showed that more than 2 million tonnes of salt was held in this layer of the sediment in 2006. Recent
management of the estuary (since 1970) has ensured that the mouth was not artificially opened. This was to prevent the inflow
of seawater, with its salts, that would otherwise enter while the drought was in place. The results of the sediment salinity
data showed that if the drought had been broken and the lake area filled with rain and river water, the resulting salinity
would be about 6 psu. In March 2007, Cyclone Gamede in the Indian Ocean off the east coast of South Africa produced a wave
climate at sea that resulted in the mouth breaching; thus introducing an estimated 12 million tonnes of salts. The high salinity
in the system resulting from this breach is expected to have an adverse effect on the ecology of the system, whereas the residual
salinity in the sediments would not have caused an environmental problem. If the estuary and lake system were to fill completely
with seawater, the residual salts together with seawater will raise the salinity to an initial value higher than 40 psu, which
will have the effect of suppressing much of the important submerged vegetation that is vital for sustaining juvenile fish
in the system. Many of the large fauna will also suffer from a shortage of freshwater. 相似文献