全文获取类型
收费全文 | 79375篇 |
免费 | 1491篇 |
国内免费 | 710篇 |
专业分类
测绘学 | 1894篇 |
大气科学 | 5540篇 |
地球物理 | 15810篇 |
地质学 | 28558篇 |
海洋学 | 7014篇 |
天文学 | 18031篇 |
综合类 | 230篇 |
自然地理 | 4499篇 |
出版年
2022年 | 483篇 |
2021年 | 833篇 |
2020年 | 905篇 |
2019年 | 968篇 |
2018年 | 2212篇 |
2017年 | 2035篇 |
2016年 | 2561篇 |
2015年 | 1442篇 |
2014年 | 2457篇 |
2013年 | 4212篇 |
2012年 | 2586篇 |
2011年 | 3482篇 |
2010年 | 2980篇 |
2009年 | 3919篇 |
2008年 | 3636篇 |
2007年 | 3435篇 |
2006年 | 3198篇 |
2005年 | 2571篇 |
2004年 | 2454篇 |
2003年 | 2304篇 |
2002年 | 2118篇 |
2001年 | 1955篇 |
2000年 | 1886篇 |
1999年 | 1502篇 |
1998年 | 1601篇 |
1997年 | 1510篇 |
1996年 | 1186篇 |
1995年 | 1244篇 |
1994年 | 1057篇 |
1993年 | 947篇 |
1992年 | 916篇 |
1991年 | 837篇 |
1990年 | 937篇 |
1989年 | 769篇 |
1988年 | 722篇 |
1987年 | 896篇 |
1986年 | 735篇 |
1985年 | 957篇 |
1984年 | 1012篇 |
1983年 | 950篇 |
1982年 | 916篇 |
1981年 | 782篇 |
1980年 | 746篇 |
1979年 | 694篇 |
1978年 | 689篇 |
1977年 | 626篇 |
1976年 | 598篇 |
1975年 | 565篇 |
1974年 | 576篇 |
1973年 | 530篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
351.
Globorotalia puncticulata and Globorotalia margaritae are critical species that define internationally recognized planktonic foraminiferal biozones in the Pliocene. These biozones are defined from stratotype sections on Sicily and their fauna are commonly considered to have been introduced to the Mediterranean after an influx of Atlantic water that terminated the late Miocene desiccation of the basin. Herein new discoveries of these species in rocks that predate the late Miocene desiccation are described. These data are supported by magneto- and lithostratigraphy that have been integrated at a single continuous section. Not only do these discoveries question the existing foraminiferal biozone stratigraphy, they also suggest new models for the dispersal of planktonic species. It is proposed that Globorotalia puncticulata and perhaps even Globorotalia margaritae evolved in the Mediterranean during earliest Messinian times (during or before chron C3Bn1n) and dispersed into the Atlantic. This suggests that a marine connection remained between the two sea areas until at least chron C3An.1n. Using the existing geomagnetic polarity time scale, these occurrences are some 2 Myr earlier than previously recorded in the Mediterranean. The distribution of G. margaritae and G. puncticulata in Mediterranean sections is likely to reflect palaeoenvironment or the preservation of deposits rather than the absolute age of the sediments. 相似文献
352.
M.J. Bickle 《地学学报》1996,8(3):270-276
The seawater 87 Sr/86 Sr curve implies a 50–100 Myr episodicity in weathering rate which requires a corresponding variation in CO2 degassing from the solid earth to the atmosphere. It is proposed that this is caused by orogenesis, which both produces CO2 as a result of metamorphic decarbonation reactions, and consumes extra CO2 as a consequence of erosion-enhanced weathering. Global climate on the geological time-scale is therefore contTolled by the difference between the relatively large and variable orogenic-moderated degassing and weathering CO2 fluxes. 相似文献
353.
354.
C. Christiansen F. Gertz M. J. C. Laima L. C. Lund-Hansen T. Vang C. Jürgensen 《Environmental Geology》1997,29(1-2):66-77
The yearly nutrient supply from land and atmosphere to the study area in SW Kattegat is 10 900 tons of N and 365 tons of
P. This is only few percent of the supply from adjacent marine areas, as the yearly transport through the study area is 218 000
tons of N and 18 250 tons of P. Yearly net deposition makes up 1340 tons of N (on average 2.5 g m–2 yr–1) and 477 ton of P (on average 0.9 g m–2 yr–1). Shallow-water parts of the study area have no net deposition because of frequent (>35% of the year) resuspension. Resuspension
frequency in deep water is <1% of the year. Resuspension rates, as averages for the study area, are 10–17 times higher than
net deposition rates. Because of resuspension, shallow-water sediments are coarse lag deposits with small amounts of organic
matter (1.1%) and nutrients (0.04% N and 0.02% P). Deep-water sediments, in contrast, are fine grained with high levels of
organic matter (11.7%) and nutrients (0.43% N and 0.15% P). Laboratory studies showed that resuspension changes the diffusive
sediment water fluxes of nutrients, oxygen consumption, and penetration into the sediment. Fluxes of dissolved reactive phosphate
from sediment to water after resuspension were negative in organic-rich sediments (13.2% organic matter) with low porosity
(56) and close to zero in coarse sediments with a low organic matter content (2.3%) and high porosity (73). Fluxes of inorganic
N after resuspension were reduced to 70% and 0–20% in relation to the rates before resuspension, respectively.
Received: 10 July 1995 · Accepted: 19 January 1996 相似文献
355.
In this paper we discuss the initial phase of chromospheric evaporation during a solar flare observed with instruments on the Solar Maximum Mission on May 21, 1980 at 20:53 UT. Images of the flaring region taken with the Hard X-Ray Imaging Spectrometer in the energy bands from 3.5 to 8 keV and from 16 to 30 keV show that early in the event both the soft and hard X-ray emissions are localized near the footpoints, while they are weaker from the rest of the flaring loop system. This implies that there is no evidence for heating taking place at the top of the loops, but energy is deposited mainly at their base. The spectral analysis of the soft X-ray emission detected with the Bent Crystal Spectrometer evidences an initial phase of the flare, before the impulsive increase in hard X-ray emission, during which most of the thermal plasma at 107 K was moving toward the observer with a mean velocity of about 80 km s-1. At this time the plasma was highly turbulent. In a second phase, in coincidence with the impulsive rise in hard X-ray emission during the major burst, high-velocity (370 km s-1) upward motions were observed. At this time, soft X-rays were still predominantly emitted near the loop footpoints. The energy deposition in the chromosphere by electrons accelerated in the flare region to energies above 25 keV, at the onset of the high-velocity upflows, was of the order of 4 × 1010 erg s-1 cm-2. These observations provide further support for interpreting the plasma upflows as the mechanism responsible for the formation of the soft X-ray flare, identified with chromospheric evaporation. Early in the flare soft X-rays are mainly from evaporating material close to the footpoints, while the magnetically confined coronal region is at lower density. The site where upflows originate is identified with the base of the loop system. Moreover, we can conclude that evaporation occurred in two regimes: an initial slow evaporation, observed as a motion of most of the thermal plasma, followed by a high-speed evaporation lasting as long as the soft X-ray emission of the flare was increasing, that is as long as plasma accumulation was observed in corona. 相似文献
356.
357.
358.
Radiocaesium isotopes, discharged into the North-east Irish Sea from the Sellafield (formerly Windscale) nuclear fuel reprocessing plant in Cumbria, have been employed as flow monitors to update and extend the record of coastal water movement from the Irish Sea to the Clyde Sea area and, further north, to Loch Etive. The temporal trends in radiocaesium levels have been used to determine the extent of water mixing en route and to define mean advection rates. Flow conditions from the Irish Sea have changed considerably since the mid-1970s, the residence time of northern Irish Sea waters being ~12 months during 1978–1980 inclusive. Average transport times of four and six months are estimated for the Sellafield to Clyde and Sellafield to Etive transects respectively. Sellafield 137Cs levels in seawater were diluted by factors of 27 and 50 respectively during current movement to the Clyde and Etive areas. The decrease in salinity-corrected 137Cs concentrations between the Clyde and Etive suggests that dilution by Atlantic water occurs, the latter mainly entering the Firth of Lorne from the west. The majority (~94%) of the radiocaesium supply to Loch Etive enters the Firth of Lorne via the portion of the coastal current circulating west of Islay, only ~6% arriving via the Sound of Jura. 相似文献
359.
360.