全文获取类型
收费全文 | 1728篇 |
免费 | 67篇 |
国内免费 | 18篇 |
专业分类
测绘学 | 40篇 |
大气科学 | 123篇 |
地球物理 | 497篇 |
地质学 | 573篇 |
海洋学 | 166篇 |
天文学 | 281篇 |
综合类 | 6篇 |
自然地理 | 127篇 |
出版年
2021年 | 19篇 |
2020年 | 24篇 |
2019年 | 24篇 |
2018年 | 42篇 |
2017年 | 32篇 |
2016年 | 56篇 |
2015年 | 36篇 |
2014年 | 42篇 |
2013年 | 83篇 |
2012年 | 46篇 |
2011年 | 63篇 |
2010年 | 78篇 |
2009年 | 75篇 |
2008年 | 67篇 |
2007年 | 83篇 |
2006年 | 62篇 |
2005年 | 55篇 |
2004年 | 51篇 |
2003年 | 53篇 |
2002年 | 49篇 |
2001年 | 26篇 |
2000年 | 33篇 |
1999年 | 28篇 |
1998年 | 27篇 |
1997年 | 28篇 |
1996年 | 26篇 |
1995年 | 29篇 |
1994年 | 31篇 |
1993年 | 14篇 |
1992年 | 31篇 |
1991年 | 30篇 |
1990年 | 38篇 |
1989年 | 25篇 |
1988年 | 21篇 |
1987年 | 17篇 |
1986年 | 17篇 |
1985年 | 28篇 |
1984年 | 35篇 |
1983年 | 26篇 |
1982年 | 19篇 |
1981年 | 31篇 |
1980年 | 23篇 |
1979年 | 21篇 |
1978年 | 21篇 |
1977年 | 15篇 |
1976年 | 12篇 |
1975年 | 12篇 |
1974年 | 14篇 |
1973年 | 15篇 |
1971年 | 11篇 |
排序方式: 共有1813条查询结果,搜索用时 31 毫秒
951.
Steven A. Murawski Gary R. Clayton Roger J. Reed Charles F. Cole 《Estuaries and Coasts》1980,3(4):308-314
Movements of spawning rainbow smelt, Osmerus mordax, were followed in the Parker River estuary, Massachusetts during 1974 and 1975. Fish marked with vinyl subcutaneous tags (n=1,492) or fin clips (n=577) were recaptured on three separate spawning sites in three different tributaries; the distribution patterns of marked fish indicated a homogeneous spawning stock. Interstream movement may have been facilitated by tidal transport since smelt ascended to the spawning sites on flood tides and moved downstream as tides ebbed. Rates of recapture of fish tagged on the spawning areas were 2.61 and 5.61 times greater for males than females in 1974 and 1975 respectively. Individual tagged males were recovered up to four times during the spawning period; females were recaptured a maximum of once. The proportion of age II and older females sampled from the angling fishery prior to spawning in 1975 (47.38%) was greater than the cumulative proportion sampled on the spawning sites (11.93%) due to longer spawning period of individual males. 相似文献
952.
Hans P. Eugster Charles E. Harvie John H. Weare 《Geochimica et cosmochimica acta》1980,44(9):1335-1347
Phase relations in the 6-component system Na-K-Mg-Ca-SO4-Cl-H2O have been calculated for halite saturation, 25°C and 1 atm pressure. Using a Jänecke projection with the apices Ca-Mg-K2-SO4, 27 stable invariant points have been located which are connected by 69 univariant curves. Polyhalite is the only quaternary solid, but anhydrite occupies the bulk of the interior tetrahedral space. Consequently, 24 of the invariant points lie very close to the Ca-free base, Mg-K2-SO4. The remaining three points involve tachyhydrite and/or antarcticite. All points but two (20,27) represent peritectic conditions. Metastable equilibria have been calculated for the Ca-free system and yield relations corresponding to the solar diagram.Seawater lies in the subspace anhydrite-halite-carnallite-kieserite-bischofite (point 20) and its evaporation has been discussed for conditions of equilibrium and fractional crystallization. After gypsum is converted to anhydrite, halite precipitates. The next phase, under equilibrium conditions, is glauberite, crystallizing at the expense of anhydrite. Continued evaporation leads to glauberite resorption and eventual replacement by polyhalite. Then follow the magnesium sulfates epsomite, hexahydrite and kieserite, which are joined by carnallite. Polyhalite is replaced by anhydrite and bischoflte is added at the final invariant condition. Kainite does not appear as a primary phase under equilibrium conditions, but it is an important phase during fractional crystallization, where Ca-phases are not allowed to back-react with the brine.Up to the appearance of glauberite, thickness ratios of halite: anhydrite couplets (equilibrium or fractionation) can vary from 0 to 7, the relative amount of halite increasing with more intense evaporation. During evaporation, the activity of H2O decreases from 0.98 (seawater) to 0.34 (final invariant brine). The data provided can be used to evaluate the effects of mineral precipitation, evaporation and brine mixing for a wide variety of natural brines. 相似文献
953.
The number of migrating alewives in the Parker River, Massachusetts, can be counted visually by standing at the upper end of each of the 6 fish ladders in the system. Estimating the entire run of up to a month’s duration by this procedure is not economically feasible, but we suggest a subsampling procedure that can provide sufficient accuracy and be economical. The reliability of periodic sample counts when used to estimate population numbers of anadromous alewives (Alosa pseudoharengus) was indicated by the low variance of paired counts, small random errors of individual counts, and by the low variability between counts on days of large runs in the Parker River. Analyses of computer-simulated runs using actual data suggested that short counts taken frequently are superior to longer counts taken less often. Both field and computer-simulated data suggested that ten-minute counts taken hourly will estimate the true population of alewife runs within 10% error at the .05 level of probability. 相似文献
954.
Temperature preference tests were conducted on fresh water white perch (Morone americana), collected from the Wicomico River, Maryland. Collection temperature was 27 C and acclimation temperatures used in temperature preference tests were 6, 12, 18, 24, 30, and 33 C. The following methods were used to determine the final temperature preference: linear regression, quadratic equation, and “eyeball” plots. Recorded final temperature preference values were 28.9, 29.3 and 30.6 C using each method respectively. 相似文献
955.
The Use of Microorganisms as Ground-Water Tracers: A Review 总被引:3,自引:0,他引:3
956.
B.H.W.S de Jong Keith D Keefer Gordon E Brown Charles M Taylor 《Geochimica et cosmochimica acta》1981,45(8):1291-1308
The local, up to second nearest neighbor, around Si atoms in alkali and alkaline earth-silicate glasses has been characterized by SiKβ X-ray emission spectroscopy. Principally two types of Si atoms can be distinguished. These are Si atoms with only other Si atoms as second nearest neighbours, and those with one or more alkali or alkaline earth atoms in their second coordination sphere. The spectroscopic results indicate that the lower molecular weight alkali and alkaline earth-silicate glasses tend towards a bimodal distribution of local Si environments, which is designated Q4-Q0 following the assignment by Engelhardtet al. (1975) for silica species in aqueous solutions. From a different perspective the outcome of these experiments suggests that, though the concentration of bridging oxygens (O(br)) and non-bridging oxygens (O(nbr)) is fixed by the stoichiometry of the glass, the distribution of O(nbr) in the glass varies according to the kind of alkali or alkaline earth atom present. From observed nucleation data on R2O-SiO2 (R = Li, Na, K) glasses it is inferred that a bimodal Q distribution and in particular the presence of Q0 species dominates the internal nucleation process in the alkali and alkaline earth-silicate glasses studied. Using this inference rationalizations can readily be found to explain the observed resistance to thermal shock and devitrification rates of these glasses. 相似文献
957.
Florence R. Weber Thomas D. Hamilton David M. Hopkins Charles A. Repenning Herbert Haas 《Quaternary Research》1981,16(2):167-180
The Canyon Creek vertebrate-fossil locality is an extensive road cut near Fairbanks that exposes sediments that range in age from early Wisconsin to late Holocene. Tanana River gravel at the base of the section evidently formed during the Delta Glaciation of the north-central Alaska Range. Younger layers and lenses of fluvial sand are interbedded with arkosic gravel from Canyon Creek that contains tephra as well as fossil bones of an interstadial fauna about 40,000 years old. Solifluction deposits containing ventifacts, wedge casts, and rodent burrows formed during a subsequent period of periglacial activity that took place during the maximum phase of Donnelly Glaciation about 25,000–17,000 years ago. Overlying sheets of eolian sand are separated by a 9500-year-old paleosol that may correlate with a phase of early Holocene spruce expansion through central Alaska. The Pleistocene fauna from Canyon Creek consists of rodents (indicated by burrows), Mammuthus primigenius (woolly mammoth), Equus lambei (Yukon wild ass), Camelops hesternus (western camel), Bison sp. cf. B. crassicornis (large-horned bison), Ovis sp. cf.O. dalli (mountain sheep), Canis sp. cf. C. lupus (wolf), Lepus sp. cf. L. othus or L. arcticus (tundra hare), and Rangifer sp. (caribou). This assemblage suggests an open landscape in which trees and tall shrubs were either absent or confined to sheltered and moist sites. Camelops evidently was present in eastern Beringia during the middle Wisconsin interstadial interval but may have disappeared during the following glacial episode. The stratigraphic section at Canyon Creek appears to demonstrate that the Delta Glaciation of the north-central Alaska Range is at least in part of early Wisconsin age and was separated from the succeeding Donnelly Glaciation by an interstadial rather than interglacial episode. 相似文献
958.
Jonathan M. Horlick W. Charles Cooper Alan H. Clark 《International Journal of Mineral Processing》1981,8(1):49-59
Chrysocolla in the Cuajone ores appears to be microcrystalline and to exhibit a fibrous structure.Sulphuric acid leaching of the chrysocolla is facilitated by a microcracked surface structure. A marked surface structural change is evidenced as hydrogen ions replace the copper. The end result is a noncontinuous surface layer of silica from which the copper has been essentially completely removed.The results of this study provide confirmatory evidence for the structure and hydrometallurgy of chrysocolla. 相似文献
959.
ABSTRACT This paper addresses warm season hydroclimatic variability in the southern Appalachian region of the southeastern U.S., where precipitation can vary as much as 127?mm or more, with maximum seasonal totals exceeding 736?mm in extreme cases. Despite the occurrence of droughts, floods, and their socioecological impacts, hydroclimate variability is still poorly understood. This study characterizes the regional scale variations in the hydroclimate by examining the daily distribution of precipitation patterns in different topographic environments. Parameter-elevation relationships on independent slopes model (PRISM) gridded precipitation estimates are used to identify the location and frequency of different types of rainfall events. Several types of clustering algorithms are used as a regionalization approach to define areas where the precipitation regime exhibits similarities in its frequency of occurrence. The results are compared with internal validation statistics and a visualization is used to assess how well the resulting hydroclimatic regions align with different topographic environments. This study reveals the intricate spatial footprint of dry and wet regimes and demonstrates how clustering applications can be used with gridded climate data to determine where extremes are most likely to develop across mountain catchments. 相似文献
960.
Charles E. Stearns 《Quaternary Research》1976,6(3):445-449
The absolute elevations of sea level 103,000 and 82,000 years ago have been estimated as ?15 and ?13 m, respectively, from the present elevations of emergent reefs on Barbados (Broecker et al., 1968; Matthews, 1973; Bloom et al., 1974). The “Barbados model” requires two assumptions: (1) that sea level was +6 m 124,000 years ago, and (2) that rates of uplift on short individual traverses have been uniform during the last 125,000 years.A test of the derived values on Barbados itself does not yield uniform rates of uplift between 124,000 and 82,000 years ago. Less reliably dated strand line features on less uplifted coasts suggest that sea level 124,000 years ago differed from sea levels 103,000 and 82,000 years ago by smaller amounts than those suggested by the “Barbados model.” Such smaller differences yield more uniform rates of uplift between 124,000 and 82,000 years ago, on New Guinea as well as on Barbados, than do the larger. The “Barbados model” is not sufficiently precise to yield close estimates of past elevations of sea level. Better values will eventually be derived from low uplift coasts, when stratigraphic and radiometric data from them have achieved the credibility of data from moderate to high uplift coasts. 相似文献