全文获取类型
收费全文 | 86481篇 |
免费 | 816篇 |
国内免费 | 1446篇 |
专业分类
测绘学 | 2933篇 |
大气科学 | 5754篇 |
地球物理 | 16730篇 |
地质学 | 34280篇 |
海洋学 | 6517篇 |
天文学 | 16589篇 |
综合类 | 2360篇 |
自然地理 | 3580篇 |
出版年
2022年 | 603篇 |
2021年 | 897篇 |
2020年 | 938篇 |
2019年 | 1034篇 |
2018年 | 6691篇 |
2017年 | 5818篇 |
2016年 | 4838篇 |
2015年 | 1329篇 |
2014年 | 2236篇 |
2013年 | 3423篇 |
2012年 | 3323篇 |
2011年 | 5384篇 |
2010年 | 4501篇 |
2009年 | 5233篇 |
2008年 | 4491篇 |
2007年 | 5093篇 |
2006年 | 2755篇 |
2005年 | 1926篇 |
2004年 | 1959篇 |
2003年 | 1916篇 |
2002年 | 1739篇 |
2001年 | 1448篇 |
2000年 | 1324篇 |
1999年 | 1027篇 |
1998年 | 1010篇 |
1997年 | 1023篇 |
1996年 | 852篇 |
1995年 | 831篇 |
1994年 | 786篇 |
1993年 | 654篇 |
1992年 | 630篇 |
1991年 | 651篇 |
1990年 | 695篇 |
1989年 | 608篇 |
1988年 | 576篇 |
1987年 | 651篇 |
1986年 | 521篇 |
1985年 | 705篇 |
1984年 | 739篇 |
1983年 | 725篇 |
1982年 | 669篇 |
1981年 | 583篇 |
1980年 | 605篇 |
1979年 | 493篇 |
1978年 | 521篇 |
1977年 | 474篇 |
1976年 | 419篇 |
1975年 | 415篇 |
1974年 | 430篇 |
1973年 | 488篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
891.
Since the late Quaternary Period, the rapid rising of the northeastern Qinghai-Tibetan Plateau has greatly changed the East
Asian climate and formed different regional monsoon climates. Various microstructures of the loess in northern China have
formed under a variety of climatic conditions. The scanning electron microscope (SEM) and energy spectrum analysis are used
in this paper to study the relationship between the indexes of loess microstructures and the forming climates, and the indexes
include the diameter of sand grains, Ca/Fe value and the characteristic shapes of the loess from SEM microstructures pictures.
In terms of their respective climatic circumstances, the major microstructures of regional loess may be classified into nine
categories according to their forms and particle sizes, such as loess formed in near-sand source, loess of granule sand dropping,
and loess formed in hot and rainy places, i.e. with the loess SEM pictures, climates of about 15 ka b.p. in different places may be evaluated. In conclusion, the clear and distinguishable loess microstructures and their indexes
in the Loess Plateau of China can also deduce the evolution of climate, such as warm or cold and wet or dry, and even sandstorm
activities when the loess is deposited. 相似文献
892.
M. A. Akhmet’ev N. I. Zaporozhets A. I. Iakovleva G. N. Aleksandrova V. N. Beniamovsky T. V. Oreshkina Z. N. Gnibidenko Zh. A. Dolya 《Stratigraphy and Geological Correlation》2010,18(6):635-659
The analysis of the main biospheric events that took place in West Siberia and the Arctic region during the Early Paleogene
revealed the paleogeographic and paleobiogeographic unity of marine sedimentation basins and close biogeographic relations
between their separate parts. Most biotic and abiotic events of the first half of the Paleogene in the Arctic region and West
Siberia were synchronous, unidirectional, and interrelated. Shelf settings, sedimentation breaks, and microfaunal assemblages
characteristic of these basins during the Paleogene are compared. The comparative analysis primarily concerned events of the
Paleocene-Eocene thermal maximum (PETM) and beds with Azolla (aquatic fern). The formation of the Eocene Azolla Beds in the Arctic region and West Siberia was asynchronous, although it proceeded in line with a common scenario related
to the development of a system of estuarine-type currents in a sea basin partly isolated from the World Ocean. 相似文献
893.
C. Labaune M. Tesson B. Gensous O. Parize P. Imbert V. Delhaye-Prat 《Sedimentary Geology》2010,223(3-4):360-379
Quaternary incised valley systems are usually characterized by the preservation of a single valley-fill attributed to the last post-glacial period. Moreover, there are very few cases of correlation between incised valley system developed on inner shelf and sedimentary units observed on the mid to outer shelf, mainly forced regressive wedges. The Roussillon shelf, in the western part of the Gulf of Lion, is a particular example of preserved Quaternary compound incised valley system also characterized by a direct correlation with the forced regressive lowstand wedges on the mid-outer shelf. High-resolution seismic data and a borehole, 60 m deep, located on the beach barrier permit an accurate study of the geometry and lithology of the system. Six imbricated and more or less preserved incised valleys and valley-fills are observed up to the inner to mid-shelf. The key surfaces associated to the incised valleys are correlated to the boundaries of the forced regressive wedges. They are assumed to be reworked surfaces. At the borehole location, only few thin layers, less than 1 m thick, of coarse grain and/or floating pebbles, are observed and should correspond to preserved fluvial lowstand deposits reworked under marine influence. The valley fills are mainly composed of estuarine muddy silts. From AMS 14C age dating it is inferred that the uppermost incised valley system is younger than 45 ky cal BP. Based on those observations, the six preserved incised valley systems are assumed to be controlled by the last six 4th order sea-level cycles — 100 ky — of the middle to late Quaternary. The paleo-topography of the underlying Plio-Quaternary deposits controls the compound incised valley system location. The deep topography of the Messinian Erosionnal Surface is a controlling factor at a lower degree. The partial preservation of the successive valley fill is attributed not only to the differential subsidence but also to the lateral migration of each incision and to the hydrodynamic regime. 相似文献
894.
M. Malakootian K. Yaghmaeian 《International Journal of Environmental Science and Technology》2004,1(3):227-231
In this research, an investigation of carbon monoxide in heavy traffic intersections of Kerman municipal districts in 2003–2004 has been performed. Kerman City is located at the south-eastern part of Iran. Carbon monoxide in highly toxic for human health and natural ecosystems in metropolition districts especially due to high traffic and industrial activities. For this reason, it was decided to investigate and find out the high concentration of CO pollutants in heavy traffic intersections of Kerman City. In this regards 21 heavy traffic squares and intersections were selected and the concentration of CO were measured. In the middle of each month during the year, sampling and measurement were scheduled three times at daily times according 7–12 a.m.; 12–16 and 16–20 p.m. In each period, 12 samples of 5 minutes were collected and CO concentration for 1-hr was calculated. The results obtained indicate that the hourly mean concentrations were lower than WHO guidelines and also lower than measurements of 1989, which has already done. In spite of more cases and higher populations, natural gas distribution was the cause of decline in CO concentrations. 相似文献
895.
We analyze observations of the compact GHZ-peaked-spectrum radio source in the nucleus of the weakly active galaxy NGC 1052,
assuming that the low-frequency turnover in its spectrum is due to synchrotron self-absorption. The analysis is based on a
model for an inhomogeneous source of synchrotron radiation. It is shown that the magnetic field is not uniform, but the change
in the field strength from the center to the edge of the compact radio source does not exceed an order of magnitude. The maximum
magnetic-field strength in the nucleus of NGC 1052 is 20 G < H
⊥ < 200 G, and the density of relativistic electrons is 0.018 cm−3 < n
e
< 0.18 cm−3 on scales of 0.1 pc; everywhere in the radio source, the energy density of the magnetic field exceeds the energy density
of the relativistic electrons. The physical conditions are similar to those in the nuclei of the nearby radio galaxies 3C
111 and 3C 465, and differ strongly from those in the nucleus of the radio galaxy 0108+388, which is a compact GHz-peaked-spectrum
source (these three galaxies were studied by the authors earlier using the same method). 相似文献
896.
A.E. Kontorovich A.N. Fomin V.O. Krasavchikov A.V. Istomin 《Russian Geology and Geophysics》2009,50(11):917-929
Thermal maturation for the top and base of the Jurassic in the West Siberian megabasin was modeled on the basis of the vitrinite reflectance (Rvt0) data using mathematical modeling and computer simulations. The values of thermal maturation are found to vary within substages PC3-MC32 for the top (or being equivalent to PC3-MC12 on the periphery and southern part of the basin, or to MC2-MC32 in the north) and PC3-AC3 for the base of the Jurassic (or being equivalent to PC3-MC2 on the periphery and southern part of the basin or MC31-AC3 in the north). Thermal maturity levels of the Jurassic in West Siberia are controlled by depths of burial and peak temperatures which the source rocks were subjected to during this period. The situation is further complicated by high heat flows superimposed on the regional background, which are observed in deep fault zones and in the proximity of numerous igneous bodies. 相似文献
897.
Ajith G. Nair D.S. Suresh Babu K.T. Damodaran R. Shankar C.N. Prabhu 《Journal of Asian Earth Sciences》2009,34(2):115-122
The magnetic fractions of ilmenite from the beach placer deposit of Chavara, southwest India have been studied for mineralogical and chemical composition to assess the range of their physical and chemical variations with weathering. Chavara deposit represents a highly weathered and relatively homogenous concentration. Significant variation in composition has been documented with alteration. The most magnetic of the fractions of ilmenite, separated at 0.15 Å, and with a susceptibility of 3.2 × 10?6 m3 kg?1, indicates the presence of haematite–ilmenite intergrowth. An iron-poor, titanium-rich component of the ilmenite ore has been identified from among the magnetic fractions of the Chavara ilmenite albeit with an undesirably high Nb2O5 (0.28%), Cr2O3 (0.23%) and Th (149 ppm) contents. The ilmenite from Chavara is compared with that from the nearby Manavalakurichi deposit of similar geological setting and provenance. The lower ferrous iron oxide (2.32–14.22%) and higher TiO2 (56.31–66.45%) contents highlight the advanced state of alteration of Chavara. This is also evidenced by the relatively higher Fe3+/Fe2+ ratio compared to Manavalakurichi ilmenite. In fact, the ilmenite has significantly been converted to pseudorutile/leucoxene. 相似文献
898.
899.
900.
E. V. Ruban G. A. Alekseeva A. A. Arkharov E. I. Hagen-Thorn V. D. Galkin I. N. Nikanorova V. V. Novikov V. P. Pakhomov T. Yu. Puzakova 《Astronomy Letters》2006,32(9):604-607
We present the results of our visual and near-infrared spectrophotometric observations for 77 variable stars obtained during 1971–1991 in Chile, Armenia, and Bolivia. The quasi-monochromatic, extraatmospheric fluxes from the stars are given in absolute energy units (W m?2 m?1) at all wavelengths of the spectral range at 2.5-nm intervals. 相似文献