全文获取类型
收费全文 | 913篇 |
免费 | 39篇 |
国内免费 | 5篇 |
专业分类
测绘学 | 25篇 |
大气科学 | 84篇 |
地球物理 | 280篇 |
地质学 | 217篇 |
海洋学 | 88篇 |
天文学 | 161篇 |
自然地理 | 102篇 |
出版年
2021年 | 8篇 |
2020年 | 10篇 |
2019年 | 10篇 |
2018年 | 13篇 |
2017年 | 13篇 |
2016年 | 23篇 |
2015年 | 16篇 |
2014年 | 20篇 |
2013年 | 42篇 |
2012年 | 23篇 |
2011年 | 40篇 |
2010年 | 38篇 |
2009年 | 50篇 |
2008年 | 34篇 |
2007年 | 34篇 |
2006年 | 49篇 |
2005年 | 43篇 |
2004年 | 48篇 |
2003年 | 40篇 |
2002年 | 29篇 |
2001年 | 33篇 |
2000年 | 22篇 |
1999年 | 17篇 |
1998年 | 23篇 |
1997年 | 18篇 |
1996年 | 12篇 |
1995年 | 13篇 |
1994年 | 18篇 |
1993年 | 12篇 |
1992年 | 22篇 |
1991年 | 8篇 |
1990年 | 15篇 |
1989年 | 8篇 |
1988年 | 8篇 |
1987年 | 13篇 |
1986年 | 11篇 |
1985年 | 14篇 |
1984年 | 9篇 |
1983年 | 16篇 |
1982年 | 13篇 |
1981年 | 13篇 |
1980年 | 10篇 |
1979年 | 7篇 |
1978年 | 7篇 |
1977年 | 9篇 |
1976年 | 3篇 |
1975年 | 5篇 |
1974年 | 3篇 |
1973年 | 5篇 |
1970年 | 2篇 |
排序方式: 共有957条查询结果,搜索用时 234 毫秒
931.
Thomas J. Whitmore Mark Brenner Keith V. Kolasa William F. Kenney Melanie A. Riedinger-Whitmore Jason H. Curtis Joseph M. Smoak 《Journal of Paleolimnology》2006,36(4):353-370
We evaluate the effects of land-use change since c.1890 on Little Lake Jackson in south-central Florida, USA. The lake currently is alkaline despite the prevalence of acidic lakes in the region. Watershed soils are acidic and poorly drained, but are underlain by limestone bedrock. Limnetic pH inferences, based on weighted-averaging tolerance regression of sedimented diatoms, indicate that Little Lake Jackson became significantly alkalized during the 1900s. Two driving forces that appear to be responsible for water-quality change are increased ionic loading and increased nutrient loading. Golf courses and residential lawns in the watershed receive substantial applications of lime, fertilizer, and irrigation with alkaline waters from deep wells, some of which reaches the lake in channelized runoff. Stormwater runoff and septic leachate also contribute to nutrient and solute loading. Sedimentary total P accumulation increased 5-fold and total N accumulation increased 3-fold since c. 1890. δ15N values suggest agricultural and septic sources for N loading. Sedimented pigments, inferred limnetic chlorophyll a values, and δ13C values of organic matter indicate that increased primary productivity occurred. Surface and subsurface inflow is nutrient-rich but low in hardness. Increased cation deposition in sediments indicates that ionic input might have reduced the lake’s natural resistance to alkalization. Lake waters remain low in ionic content, which suggests that the addition of base from carbonate sources is not responsible for all of the observed alkalization. Acid neutralization might have been facilitated by phosphate loading that led to increased base generation through greater nitrate assimilation. Inadvertent alkalization might occur commonly in regions where poorly buffered lakes are subject to significant ionic and nutrient loading from agriculture, turfgrass, and septic sources in their watersheds. 相似文献
932.
Nira L. Salant Carl E. Renshaw Francis J. Magilligan James M. Kaste Keith H. Nislow Arjun M. Heimsath 《地球表面变化过程与地形》2007,32(4):509-524
We investigate the use of the short‐lived fallout radionuclide beryllium‐7 (7Be; t1/2 = 53·4 days) as a tracer of medium and coarse sand (0·25–2 mm), which transitions between transport in suspension and as bed load, and evaluate the effects of impoundment on seasonal and spatial variations in bed sedimentation. We measure 7Be activities in approximately monthly samples from point bar and streambed sediments in one unregulated and one regulated stream. In the regulated stream our sampling spanned an array of flow and management conditions during the annual transition from flood control in the winter and early spring to run‐of‐the‐river operation from late spring to autumn. Sediment stored behind the dam during the winter quickly became depleted in 7Be activity. This resulted in a pulse of ‘dead’ sediment released when the dam gates were opened in the spring which could be tracked as it moved downstream. Measured average sediment transport velocities (30–80 metres per day (m d?1)) exceed those typically reported for bulk bed load transport and are remarkably constant across varied flow regimes, possibly due to corresponding changes in bed sand fraction. Results also show that the length scale of the downstream impact of dam management on sediment transport is short (c. 1 km); beyond this distance the sediment trapped by the dam is replaced by new sediment from tributaries and other downstream sources. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
933.
D. Keith Wilson 《Boundary-Layer Meteorology》2001,99(1):151-158
The function ()=(1+|z/L|2/3)1/2,where z is the height, L the Obukhov length, and a constant,is proposed for the nondimensional wind speed and temperaturegradients (flux-profile relationships) in anunstable surface layer. This function agrees quite well withboth wind speed and temperature data,has the theoretically correct behaviour in convective conditions,and leads to simple results when integrated to produce the mean profiles. 相似文献
934.
The July 2007 rock and ice avalanches at Mount Steele, St. Elias Mountains, Yukon, Canada 总被引:1,自引:0,他引:1
Panya S. Lipovsky Stephen G. Evans John J. Clague Chris Hopkinson Réjean Couture Peter Bobrowsky Göran Ekström Michael N. Demuth Keith B. Delaney Nicholas J. Roberts Garry Clarke Andrew Schaeffer 《Landslides》2008,5(4):445-455
A large rock and ice avalanche occurred on the north face of Mount Steele, southwest Yukon Territory, Canada, on July 24,
2007. In the days and weeks preceding the landslide, several smaller avalanches initiated from the same slope. The ice and
rock debris traveled a maximum horizontal distance 5.76 km with a maximum vertical descent of 2,160 m, leaving a deposit 3.66 km2 in area on Steele Glacier. The seismic magnitude estimated from long-period surface waves (M
s) is 5.2. Modeling of the waveforms suggests an estimated duration of approximately 100 s and an average velocity of between
35 and 65 m/s. This landslide is one of 18 large rock avalanches known to have occurred since 1899 on slopes adjacent to glaciers
in western Canada. We describe the setting, reconstruct the event chronology and present a preliminary characterization of
the Mount Steele ice and rock avalanches based on field reconnaissance, analysis of seismic records and an airborne LiDAR
survey. We also present the results of a successful dynamic simulation for the July 24 event. 相似文献
935.
The MS3DJH series of simple models of flow over low hills and other terrain features described in earlier papers (see Taylor et al., 1983) required that the terrain was of uniform surface roughness. In the present paper, we describe an approximate theory of flow above variations in surface roughness using a similar structure to that established by Jackson and Hunt (1975) for flow over hills. This then allows us to include the calculation of flow perturbations due to roughness variations within a modified version of our model which we designate as MS3DJH/3R. Comparisons are made with alternative calculations for simple two-dimensional flows; and sample three-dimensional calculations are presented. The model retains its essential features of high spatial resolution and low computing cost.Summer student, 1981 相似文献
936.
Keith Paterson 《地球表面变化过程与地形》1987,12(5):575-581
This paper reviews the educational merits and application of limestone landform studies in British secondary schools and considers these in the context of recent developments within geomorphology and the geography curriculum. The relevance of both traditional and newer approaches to the field study of karstforms is highlighted together with developments in classroom activities. School-based work is shown to have benefitted considerably from advances in educational technology. 相似文献
937.
Located in northeast Scotland, the Lecht manganiferous ironstone occurs as several minor and one principal outcrop within deeply weathered Dalradian meta-sediments. The distribution of these shows is controlled primarily by an underlying porous breccia pipe and not by Dalradian stratigraphy or faulting, as previously suggested. The deposit is composed principally of goethite and cryptomelane, with minor hematite, ramsdellite, pyrolusite, lithiophorite, chalcophanite and woodruffite. The ironstone is enriched in several target and pathfinder elements, particularly Zn and Ba which are primarily concentrated in the manganese oxides. Detailed examination of the geochemistry demonstrates that the enrichments are actually more typical of non-economic ironstones (particularly bog-ore) than gossans (a conclusion supported by field, textural and mineralogical evidence), illustrating the danger of relying upon simple geochemical surveys alone for ironstone-gossan discrimination. No relict sulphides, secondary ore minerals, native metals, gangue minerals or “boxwork” textures were observed in either hand specimen or polished section. The morphology and textures of the Lecht ironstone are typical of those observed in bog-iron ores and in weathered profiles.The Lecht ironstone is considered to have been derived from prolonged weathering of the local Dalradian meta-sediments. These are enriched in target and pathfinder elements and are regarded as a prospective sequence. Cementation of the subsequent regolith by solutions rich in iron, manganese and other elements, combined with bog-ore formation and penetration of the breccia pipe by these solutions, produced the complex and varied morphology and geochemistry seen in the deposit today. The Lecht deposit may represent the distal manganiferous expression of a goldrich zinc-lead exhalative deposit hosted by the Dalradian meta-sediments of the region. 相似文献
938.
Long wavelength magnetic anomalies of crustal origin derived from the POGO and MAGSAT satellite data often display a strong continuity across the now-rifted continental margins when the continents are reassembled into Pangaea. These anomalies predate the breakup of the supercontinent and represent major blocks whose crustal properties are broadly similar even though those blocks are no longer contiguous. 相似文献
939.
Keith A. Hunter 《Geochimica et cosmochimica acta》1983,47(3):467-473
The estuarine mixing of dissolved Fe, Cu, Ni, Si and surface-active organic matter has been investigated in the Taieri Estuary, New Zealand, simultaneously with measurements of the electrokinetic charge on colloidal particles. Dissolved Fe showed almost quantitative removal from solution characteristic of the coagulation of iron-containing colloids by seawater electrolytes. Surface active organic matter behaved conservatively, indicating that a relatively constant fraction of estuarine organic matter is surface active, but that organic species associated with iron during removal are a minor fraction. Results for Cu, Ni and Si were scattered but offered no evidence for gross removal during estuarine mixing. The negative charge on suspended colloids was not reversed by adsorption of seawater cations, but remained uniformly negative throughout the salinity range, decreasing sharply in magnitude during the first few %. salinity. 相似文献
940.
Keith A. Kvenvolden Etta Peterson Jon Wehmiller P.E. Hare 《Geochimica et cosmochimica acta》1973,37(10):2215-2225
Ratios of d- to l-amino acids in acid hydrolysates from foraminifera of two deep-sea cores from the Caribbean Sea and Atlantic Ocean increase with depth and consequently with age over a span from 40,000 to 2,000,000 yr. The changing ratios do not seem to follow first-order reversible rate laws. Valine, leucine and glutamic acid apparently racemize (isoleucine epimerizes) at slower rates than do phenylalanine, alanine, aspartic acid and proline. The general relative order for rates of racemization of total (free and bound) amino acids may depend on the electron-withdrawing capacity of the R substituents of the amino acids and on the rates with which the amino acids are naturally hydrolyzed. In contrast to the total amino acids, the free amino acids in these samples are more extensively racemized, probably as a result of various catalytic and hydrolytic reactions.Previous related work based on ion-exchange chromatography has considered only ratios of alloisoleucine to isoleucine. With the gas chromatographic method used here, d/l ratios of all common asymmetric amino acids can be estimated. Measurement of the extent of racemization of amino acids in marine sediments seems to provide the basis for a geochronological tool covering the last few million years. 相似文献