全文获取类型
收费全文 | 108847篇 |
免费 | 1266篇 |
国内免费 | 664篇 |
专业分类
测绘学 | 2519篇 |
大气科学 | 7122篇 |
地球物理 | 20427篇 |
地质学 | 40933篇 |
海洋学 | 9424篇 |
天文学 | 24343篇 |
综合类 | 366篇 |
自然地理 | 5643篇 |
出版年
2022年 | 710篇 |
2021年 | 1182篇 |
2020年 | 1236篇 |
2019年 | 1396篇 |
2018年 | 4442篇 |
2017年 | 4017篇 |
2016年 | 3884篇 |
2015年 | 1592篇 |
2014年 | 2971篇 |
2013年 | 5110篇 |
2012年 | 3811篇 |
2011年 | 5658篇 |
2010年 | 5052篇 |
2009年 | 6224篇 |
2008年 | 5268篇 |
2007年 | 5756篇 |
2006年 | 4038篇 |
2005年 | 3018篇 |
2004年 | 2937篇 |
2003年 | 2836篇 |
2002年 | 2764篇 |
2001年 | 2282篇 |
2000年 | 2202篇 |
1999年 | 1736篇 |
1998年 | 1776篇 |
1997年 | 1659篇 |
1996年 | 1420篇 |
1995年 | 1401篇 |
1994年 | 1214篇 |
1993年 | 1129篇 |
1992年 | 1073篇 |
1991年 | 1080篇 |
1990年 | 1061篇 |
1989年 | 943篇 |
1988年 | 855篇 |
1987年 | 1026篇 |
1986年 | 896篇 |
1985年 | 1092篇 |
1984年 | 1212篇 |
1983年 | 1177篇 |
1982年 | 1083篇 |
1981年 | 1017篇 |
1980年 | 924篇 |
1979年 | 886篇 |
1978年 | 845篇 |
1977年 | 728篇 |
1976年 | 717篇 |
1975年 | 716篇 |
1974年 | 685篇 |
1973年 | 774篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
61.
Surface waters of Alsea Bay, an unpolluted estuary on the Oregon coast, were analysed for nitrous oxide, nitrate and nitrite on a weekly or biweekly basis during the summer of 1979. The estuary was found to be a variable source of N2O to the atmosphere. Large and rapid increases in the concentrations of N2O, NO3?, and NO2? occurred at the beginning of the sampling period and are attributed to the influx of nutrient-rich upwelling water into the estuary with the tide. The subsequent decline in concentrations of nitrate, nitrite and nitrous oxide over the remainder of the summer is attributed to a decrease in upwelling intensity, a decline in nitrification rates and to assimilatory nitrate reduction. Measurements of nitrous oxide at six stations along the Alsea River were also made in September and October before and after the onset of the rainy season. Samples taken after flood conditions were established were systematically 50% higher than pre-flood samples. The data suggest that soil runoff results in elevated concentrations of N2O in rivers. 相似文献
62.
63.
Alan M Bond Julie R Bradbury Henry A Hudson John S Garnham Peter J Hanna Stanley Strother 《Marine Chemistry》1985,16(1):1-9
Evidence was found that uptake of lead from seawater in both model and natural systems by the leaves of the seagrass Zostera muelleri does occur for live, dead and scraped leaves at all the lead concentrations tested. Positive uptake of lead was measured using the three analytical techniques of radio-tracer, differential pulse anodic stripping voltammetry and atomic absorption spectrometry; similar uptake profiles were obtained for each technique. Profiles always showed an initial rapid uptake phase followed by a gradual transition to a plateau phase at which an approximate equilibrium between lead attached to seagrass and free ionic lead in seawater was reached. The presence of active lead uptake processes was indicated in experiments which attempted to remove lead from leaves by a chelating agent. 相似文献
64.
We present results of investigations into chromospheric velocity oscillations in sunspots, carried out at the Sayan Solar
Observatory. It is shown that the “chevron” structures in the space-time diagrams demonstrate wavetrain properties. Such structures
are indicators of a propagating wave process and they are typical of many sunspots. In the authors’ opinion, three-minute
umbral oscillations are not the source of running penumbral waves (RPW). It is very likely that umbral oscillations and RPW
initially propagate along different magnetic field lines. We explain the decrease in RPW propagation velocity and frequency
in the outer penumbra, as compared with the inner, by the combined action of different frequency modes. To better reveal the
properties of these modes, frequency filtering was used. Our measurements of the RPW (five-minute mode) wavelength and RPW
propagation velocity in different sunspots vary from 12″ to 30″ and from 28 to 60 – 70 km s−1 correspondingly. 相似文献
65.
66.
Recently, exact spatially-homogeneous solutions have been found for a conformally invariant massless scalar fieldS(t) coupled to a Robertson-Walker geometry. We investigate extending these solutions to inhomogeneous scalar fieldsS(t, x
l
). For an isotropic scalar fieldS(t, r) we find a solution satisfying three of the five field equations. If we use REDUCE, we show that the remaining equations forceS=S(t). 相似文献
67.
68.
Summary We present compositions of reheated melt inclusions in clinopyroxene phenocrysts from three mafic xenoliths in Breccia Museo,
Campi Flegrei, Italy. Melt inclusion compositions are remarkably different from the compositions of known contemporary Campi
Flegrei lavas, being significantly enriched in K2O and depleted in Na2O. Some differences are also evident in FeO* (total Fe as FeO) and TiO2 contents. The clinopyroxene phenocrysts could not have crystallised from Campi Flegrei magmas. We suggest that they originated
from a volcanic system genetically very similar to, and possibly linked with, the >14 ka volcanic system of Mt. Somma, another
Campanian volcano ∼ 30 km east from Campi Flegrei, from which Vesuvius subsequently developed. This result indicates a close
relationship (or link) between the two volcanic systems which have until now been considered separate. We speculate that the
link was established prior to eruption of the Neapolitan Yellow Tuff (NYT) (∼ 12 ka). The xenoliths were derived from a volcanic
system older than the host breccias themselves. We suggest that this older volcanism had close similarities with the volcanism
of the older products of Mt. Somma (∼25 ka).
Received March 20, 2000; accepted November 2, 2000 相似文献
69.
L. A. L. Da Silva 《Astrophysics and Space Science》1990,165(2):255-260
A comparison is made of the light and colour curves of the SN 1987A in the first 280 days with the mean properties of type II supernovae. The conclusion is that even though it has H lines, and was not a typical type II supernova, but a peculiar object from a photometric point of view. The implications for inclusion of SN 1987A in the context of the usual Minkowski-Zwicky classification are explicitly discussed. 相似文献
70.
Geophysical data from Gerlache Strait, Croker Passage, Bismarck Strait and the adjacent continental shelf reveal streamlined subglacial bedforms that were produced at the bed of the Antarctic Peninsula Ice Sheet (APIS) during the last glaciation. The spatial arrangement and orientation of these bedforms record the former drainage pattern and flow dynamics of an APIS outlet up‐flow, and feeding into, a palaeo‐ice stream in the Western Bransfield Basin. Evidence suggests that together, they represent a single ice‐flow system that drained the APIS during the last glaciation. The ice‐sheet outlet flowed north/northeastwards through Gerlache Strait and Croker Passage and converged with a second, more easterly ice‐flow tributary on the middle shelf to form the main palaeo‐ice stream. The dominance of drumlins with low elongation ratios suggests that ice‐sheet outlet draining through Gerlache Strait was comparatively slower than the main palaeo‐ice stream in the Western Bransfield Basin, although the low elongation ratios may also partly reflect the lack of sediment. Progressive elongation of drumlins further down‐flow indicates that the ice sheet accelerated through Croker Passage and the western tributary trough, and fed into the main zone of streaming flow in the Western Bransfield Basin. Topography would have exerted a strong control on the development of the palaeo‐ice stream system but subglacial geology may also have been significant given the transition from crystalline bedrock to sedimentary strata on the inner–mid‐shelf. In the broader context, the APIS was drained by a number of major fast‐flowing outlets through cross‐shelf troughs to the outer continental shelf during the last glaciation. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献