全文获取类型
收费全文 | 1412篇 |
免费 | 221篇 |
国内免费 | 441篇 |
专业分类
测绘学 | 17篇 |
大气科学 | 214篇 |
地球物理 | 298篇 |
地质学 | 960篇 |
海洋学 | 316篇 |
天文学 | 7篇 |
综合类 | 47篇 |
自然地理 | 215篇 |
出版年
2024年 | 7篇 |
2023年 | 20篇 |
2022年 | 40篇 |
2021年 | 50篇 |
2020年 | 48篇 |
2019年 | 72篇 |
2018年 | 44篇 |
2017年 | 71篇 |
2016年 | 66篇 |
2015年 | 71篇 |
2014年 | 112篇 |
2013年 | 139篇 |
2012年 | 86篇 |
2011年 | 85篇 |
2010年 | 76篇 |
2009年 | 116篇 |
2008年 | 117篇 |
2007年 | 107篇 |
2006年 | 96篇 |
2005年 | 68篇 |
2004年 | 95篇 |
2003年 | 67篇 |
2002年 | 64篇 |
2001年 | 37篇 |
2000年 | 40篇 |
1999年 | 29篇 |
1998年 | 43篇 |
1997年 | 27篇 |
1996年 | 25篇 |
1995年 | 22篇 |
1994年 | 27篇 |
1993年 | 19篇 |
1992年 | 15篇 |
1991年 | 14篇 |
1990年 | 10篇 |
1989年 | 12篇 |
1988年 | 9篇 |
1987年 | 6篇 |
1986年 | 8篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有2074条查询结果,搜索用时 15 毫秒
31.
Lipid distribution in a subtropical southern China stalagmite as a record of soil ecosystem response to paleoclimate change 总被引:3,自引:0,他引:3
Shucheng Xie Yi Yi Junhua Huang Chaoyong Hu Yanjun Cai Matthew Collins Andy Baker 《Quaternary Research》2003,60(3):340-347
Lipid extracts from a 61.7-cm-long subtropical stalagmite in southern China, spanning the period of ca. 10,000–21,000 yr ago as constrained by U–Th dating, were analyzed using gas chromatography–mass spectrometry. The higher plants and microorganisms in the overlying soils contribute a proportion of n-alkanes identified in the stalagmite. The occurrence of LMW (lower molecular weight) n-alkanols and n-alkan-2-ones in the stalagmite was mainly related to the soil microorganisms. We suggest that HMW (higher molecular weight) n-alkanols and n-alkan-2-ones identified in the stalagmite originate from soil organics and reflect input from contemporary vegetation. Shifts in the ratio of LMW to HMW n-alkanols or n-alkan-2-ones indicative of the variation of soil ecosystems (e.g., microbial degradation of organic matter and/or the relative abundance of soil microorganisms to higher plants) are comparable with the subtropical alkenone-SST (sea surface temperature) record of the same period. The similar trends seen in the δ13C data and the lipid parameters in this stalagmite imply that the overlying soil ecosystem response to climate might be responsible for the variation of δ13C values. 相似文献
32.
D. S. Cohan G. A. Sturrock A. P. Biazar P. J. Fraser 《Journal of Atmospheric Chemistry》2003,44(2):131-150
Atmospheric mixing ratios of methyl iodide (CH3I) and other methyl halides have been measured at Cape Grim, Tasmania (41°S, 145°E), since early 1998 as part of the Advanced Global Atmospheric Gases Experiment (AGAGE). This paper analyses about 1700 ambient air CH3I measurements from the 14-month period (March 1998–April 1999). Mixing ratios peaked during the summer, despite faster photolytic loss, suggesting local oceanic emissions were about 2.2–3.6 times stronger in summer than in winter. Back trajectories show that CH3I levels are strongly dependent on air mass origin, with highest mixing ratios in air from the Tasman Sea/Bass Strait region and lowest levels in air originating from the Southern Ocean at higher latitudes. CH3I mixing ratios were not well correlated with other methyl halides in unpolluted marine air. The large variations with season and air mass origin suggest that high frequency, continuous data from key locations will make a significant contribution to the understanding of sources and sinks of this important short-lived atmospheric species. 相似文献
33.
The circulation of cold, deep water is one of the controlling factors of the Earth's climate. Forty percent of this water
enters the world ocean through the Southwest Pacific as a deep western boundary current (DWBC) flowing northwards at bathyal
to abyssal depths, east of the New Zealand microcontinent. South of latitude 50°S, the DWBC is intimately linked with the
Antarctic circumpolar current (ACC), which is the prominent force for the shallow-water circulation. The Pacific DWBC is presently
the largest single contributor of deep ocean water, and deciphering its evolution is of fundamental importance to understanding
ocean and climate history, and global ocean hydrography. The evolution of the DWBC system, and of related circum-Antarctic
currents, has taken place since 30–25 Ma when plate movements created the first oceanic gaps south of Australia and South
America. The stratigraphic record preserved in sediment drifts of the Southwest Pacific, in eastern New Zealand, is the best
available for deciphering the Neogene history of Southern Ocean water masses, and of the circulation of the ACC, DWBC and
their precursor systems. Major current activity commenced on the New Zealand margin in the late Eocene or early Oligocene
(Hoiho Drift; early ACC) and was widespread by the mid-late Oligocene (Marshall Paraconformity and Weka Pass Limestone drift;
ACC). During the Neogene the eastern South Island continental shelf built seawards by accretion at its outer edge of large
Miocene current drifts up to tens of kilometres long and hundreds of metres thick (Canterbury drifts). Also commencing in
the mid-Cenozoic, but in depths >2000 m, the DWBC emplaced large deep-water sediment drifts. Rates of drift deposition accelerated
considerably in the late Neogene, when climatic change (and particularly glacial sea-level falls) caused the delivery of large
volumes of turbiditic sediment into the path of the DWBC via the Bounty and Hikurangi channels.
Received: 9 August 1995 / Accepted: 15 January 1996 相似文献
34.
华南西部晚二叠世碳同位素地层学研究 总被引:1,自引:1,他引:1
对黔桂地区7条晚二叠世碳酸盐岩型煤系剖面进行了碳酸盐岩碳氧同位素测定,发现晚二叠世吴家坪阶与长兴阶碳酸盐岩的δ13C值有明显差异,前者δ13C多大于3.0‰,后者多小于3.0‰,进一步对晚二叠世δ13C的变化原因的分析表明,δ13C的变化可能与晚古生代末期聚煤作用的减弱密切相关,后者造成自然界碳库中的12C的富集,从而使得海水以及在其中沉淀出的碳酸盐岩中的δ13C值降低。此外,植物光合作用从大气中吸收CO2的量也随陆地植被减少而减少,结果造成大气中的CO2的大量富集,从而进一步产生温室效应,制约生物界的发展。联系到这一阶段生物界的衰亡灭绝,可以认为温室效应是晚二叠世生物逐渐衰亡及晚二叠世末群体灭绝的主要原因之一。 相似文献
35.
R.M.W. Musson 《Bulletin of Earthquake Engineering》2004,2(1):101-112
Three sites in the UK are taken, representative of low, medium and high hazard levels (by UK standards). For each site, the
hazard value at 10−4 annual probability is computed using a generic seismic source model, and a variety of ground motion parameters: peak ground
acceleration (PGA), spectral acceleration at 10 Hz and 1 Hz, and intensity. Disaggregation is used to determine the nature
of the earthquakes most likely to generate these hazard values. It is found (as might be expected) that the populations are
quite different according to which ground motion parameter is used. When PGA is used, the result is a rather flat magnitude
distribution with a tendency to low magnitude events (\le 4.5 ML) which are probably not really hazardous. Hazard-consistent scenario earthquakes computed using intensity are found
to be in the range 5.8–5.9 ML, which is more in accord with the type of earthquake that one expects to be a worst-case event
in the UK.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
36.
本文分析了天山山区近40 a来年降水变化的基本特征,并与南疆、北疆进行了比较,所得的主要结果如下:(1)天山山区在年降水量干湿变化阶段上与北疆的相似性强于南疆。(2)年降水量的空间分布的同步变化性以北疆为最好,南疆最差,天山山区居中,而年降水量的空间分布的反向变化性,以天山山区为最大,北疆最小,南疆居中。(3)天山山区与南疆从60年代到90年代,年降水均表现出了持续的增加的趋势,北疆年降水从60年代到90年代,除70年代外,不断增多。 相似文献
37.
After a period of considerable and sustained hegemony, many commentators have argued that contemporary processes of globalisation are acting as a challenge to nation state sovereignty. The paper argues that geographers need to focus on the ways in which the nation state continues to act, albeit in a modified manner, within the era of globalisation. This might help to position geography within globalisation debates, which--according to Dicken [Geographers and `globalization': (yet) another missed boat? forthcoming, Transactions of the Institute of British Geographers, 2004]--we have not been centrally involved in. Drawing on the work of Michael Mann, we focus on a neglected dimension of state power--namely, its ideological form--as a means of exploring how the nation state is being differentially re-engineered under globalisation. Using Mann's classification of forms of ideological organisation, we deploy three vignettes in order to demonstrate the evolving nature of ideological power within the contemporary UK State. 相似文献
38.
In southern Turkey ongoing differential impingement of Arabia into the weak Anatolian collisional collage resulting from subduction of the Neotethyan Ocean has produced one of the most complex crustal interactions along the Alpine–Himalayan Orogen. Several major transforms with disputed motions, including the northward extension of the Dead Sea Fault Zone (DSFZ), meet in this region. To evaluate neotectonic motion on the Amanos and East Hatay fault zones considered to be northward extensions of the DSFZ, the palaeomagnetism of volcanic fields in the Karasu Rift between these faults has been studied. Remanence carriers are low-Ti magnetites and all except 5 of 51 basalt lavas have normal polarity. Morphological, polarity and K–Ar evidence show that rift formation occurred largely during the Brunhes chron with volcanism concentrated at 0.66–0.35 Ma and a subsidiary episode at 0.25–0.05. Forty-four units of normal polarity yield a mean of D/I=8.8°/54.7° with inclination identical to the present-day field and declination rotated clockwise by 8.8±4.0°. Within the 15-km-wide Hassa sector of the Karasu Rift, the volcanic activity is concentrated between the Amanos and East Hatay faults, both with left lateral motions, which have rotated blocks bounded by NW–SE cross faults in a clockwise sense as the Arabian Block has moved northwestwards. An average lava age of 0.5 Ma yields a minimum cumulative slip rate on the system bounding faults of 0.46 cm/year according with the rate deduced from the Africa–Arabia Euler vector and reduced rates of slip on the southern extension of the DSFZ during Plio-Quaternary times. Estimates deduced from offsets of dated lavas flows and morphological features on the Amanos Fault Zone [Tectonophysics 344 (2002) 207] are lower (0.09–0.18 cm/year) probably because they are limited to surface fault breaks and do not embrace the seismogenic crust.Results of this study suggest that most strike slip on the DSFZ is taken up by the Amanos–East Hatay–Afrin fault array in southern Turkey. Comparable estimates of Quaternary slip rate are identified on other faults meeting at an unstable FFF junction (DSFZ, East Anatolian Fault Zone, Karatas Fault Zone). A deceleration in slip rate across the DSFZ and its northward continuation during Plio-Quaternary times correlates with reorganization of the tectonic regime during the last 1–3 Ma including tectonic escape within Anatolia, establishment of the North and East Anatolian Fault Zones bounding the Anatolian collage in mid–late Pliocene times, a contemporaneous transition from transpression to transtension and concentration of all basaltic magmatism in this region within the last 1 Ma. 相似文献
39.
The Calafate Formation crops out in south-western Santa Cruz Province, Argentina, and displays a stacking of asymmetrical coarsening–fining-upward cycles. These cycles are interpreted as the product of short-lived transgressive-regressive events in which the coarsening upward part represents sedimentary aggradation with a stable or decreasing sea level. Sedimentological and palynological analyses indicate nearshore marine conditions. Even though the existence of an estuary or incised valley cannot be determined, this is the most probable palaeogeographic model. Based on dinoflagellate cysts, the base of the section is considered to be not older than Maastrichtian. The presence of the oyster Ambigostrea clarae (Ihering) occurring together with the dinoflagellate cyst species Manumiella druggii (Stover) Bujak and Davies and Eisenackia circumtabulata Drugg in the middle part of the section indicates an age no older than late Maastrichtian. According to sedimentological data, deposits representing the Cretaceous–Palaeogene transition would have been eroded, which is confirmed by the presence of Grapnelispora loncochensis Papú. This megaspore is a consistent component of the Maastrichtian assemblages from Patagonia. 相似文献
40.
Seraphim Alvanides 《Area》2004,36(1):41-49
Local taxation reform is back on the agenda in the light of the recent Local Government Bill. In reviewing the literature on the council tax, it is noted that researchers tend to view this from one of two perspectives: either as a fiscal policy, focusing on the tax burden, or as a housing policy, focusing on the property price valuations. Recent debate on council tax reform has gone as far as suggesting that there are regional inequities in the council tax, but what tends to be neglected remains the council tax incidence at the local scale. A research agenda is proposed here as a means of promoting geographically conscious inter-disciplinary research on local taxation. 相似文献