全文获取类型
收费全文 | 87585篇 |
免费 | 1412篇 |
国内免费 | 681篇 |
专业分类
测绘学 | 2223篇 |
大气科学 | 6813篇 |
地球物理 | 18006篇 |
地质学 | 29758篇 |
海洋学 | 7528篇 |
天文学 | 19219篇 |
综合类 | 188篇 |
自然地理 | 5943篇 |
出版年
2021年 | 577篇 |
2020年 | 730篇 |
2019年 | 757篇 |
2018年 | 1539篇 |
2017年 | 1537篇 |
2016年 | 2025篇 |
2015年 | 1354篇 |
2014年 | 2034篇 |
2013年 | 4394篇 |
2012年 | 2210篇 |
2011年 | 3170篇 |
2010年 | 2767篇 |
2009年 | 3857篇 |
2008年 | 3477篇 |
2007年 | 3218篇 |
2006年 | 3250篇 |
2005年 | 2688篇 |
2004年 | 2764篇 |
2003年 | 2570篇 |
2002年 | 2460篇 |
2001年 | 2242篇 |
2000年 | 2204篇 |
1999年 | 1848篇 |
1998年 | 1825篇 |
1997年 | 1856篇 |
1996年 | 1587篇 |
1995年 | 1528篇 |
1994年 | 1407篇 |
1993年 | 1262篇 |
1992年 | 1219篇 |
1991年 | 1065篇 |
1990年 | 1256篇 |
1989年 | 1103篇 |
1988年 | 1007篇 |
1987年 | 1203篇 |
1986年 | 1046篇 |
1985年 | 1346篇 |
1984年 | 1480篇 |
1983年 | 1430篇 |
1982年 | 1314篇 |
1981年 | 1230篇 |
1980年 | 1126篇 |
1979年 | 1054篇 |
1978年 | 1084篇 |
1977年 | 1005篇 |
1976年 | 937篇 |
1975年 | 889篇 |
1974年 | 893篇 |
1973年 | 897篇 |
1972年 | 567篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
971.
B. Panteleit K. Hamer R. Kringel W. Kessels H. D. Schulz 《Environmental Earth Sciences》2011,62(1):77-91
Geochemical processes, occurring in a stable transition zone between saltwater and freshwater, were simulated in a 2D, multi-layer
flow chamber experiment. Mixing, calcite dissolution, and oxidative degradation of organic matter were identified as the main
controlling factors. The results of the chamber experiment were compared to field data and verified by thermodynamic modeling.
Similarity in most ion distributions suggests the general applicability of the experimental method. Differences in the redox
conditions between field and experiment were reflected by the oxidants involved in the mineralization of organic carbon; while
field data show evidence of sulfate reduction, the presence of oxygen in the laboratory experiment resulted in the reoxidation
of sulfides. 相似文献
972.
Lamprophyric lavas in the Colima graben,SW Mexico 总被引:2,自引:3,他引:2
The Colima graben, located in SW Mexico, is one of three grabens which intersect about 50 km SSW of Guadalajara, forming a triple junction. The 90 km long, 20–60 km wide Colima graben represents a N-S rift of the E-W trending Mexican Volcanic Belt. Since the Early Pliocene, the Colima graben has served as a locus for the eruption of alkaline lavas, the most recent of which are basanites and minettes erupted from Late Pleistocene cinder cones (Luhr and Carmichael 1981). In this paper, we report on older alkaline lavas which crop out in the graben's walls. These rocks include phlogopite- and hornblende-bearing lamprophyres, a phlogopite-kalsilite-ankaratrite, and high-K andesites. These lavas crop out throughout the Colima graben area, and are intimately associated with calc-alkaline lavas in the field. Compared to these, the alkaline rocks are strikingly enriched in the incompatible elements, particularly Ba, Sr, P, and the LREE. Unlike the younger Late Pleistocene alkaline cinder cone lavas, most of the graben wall lamprophyres and the high-K andesites represent magmas that appear to have undergone significant evolution since their generation, including fractionation, crustal contamination, and possible magma mixing. Least-squares modeling indicates that the cinder cone minettes represent reasonable parental magmas for the graben lamprophyres. The occurrence of these alkaline lavas in an active calc-alkaline volcanic arc is unusual, and we suggest that they are a manifestation of the rifting processes which produced the Colima graben. 相似文献
973.
This paper presents one of the richest and most complete vertebrate faunas of the late Middle Miocene (~12 Ma) of Central Europe. Up to now, sixty-two vertebrate taxa, comprising all major groups (fishes, amphibians, reptiles, birds, mammals), have been recorded. Based on sedimentological and palaeobiological evidences, this Fossillagerstätte is assumed to originate from a floodplain paleosol formed on top of a braided river sequence. The fauna points to a highly structured, somewhat vegetated landscape with a wide array of habitats (e.g., fluvial channels, sporadically moist floodplains, short-lived ponds, savannah-like open areas and screes). It was preserved due to a rapid drowning and the switch to a freshwater lake environment. Palaeoclimatological data, derived from pedogenic features as well as from biota, indicate an overall semi-arid, subtropical climate with distinct seasonality (mean annual precipitation 486 ± 252 mm, mean annual temperature ~15°C). This underlines the late Middle/early Late Miocene dry-spell in Central Europe. From taphonomical point of view, the irregularly distributed but roughly associated larger vertebrate remains refer to an in situ accumulation of the bone bed. Splintered bones, gnawing marks as well as rhizoconcretions and root corrosion structures record some pre- and post-burial modification of the taphocoenose. However, the findings of pellet remains argue for a very fast burial and thus to a low degree of time-averaging. For this reason, the fossil fauna reflects the original vertebrate community rather well and is a cornerstone for the understanding of late Middle Miocene terrestrial ecosystems in this region. Certainly, Gratkorn will be one of the key faunas for a high-resolution continental biostratigraphy and the comprehension of Europe’s faunal interchanges near the Middle/Late Miocene transition. 相似文献
974.
Samantha B. Joye 《Geochimica et cosmochimica acta》2005,69(23):5633
975.
Autoclaves are closed vessels in which liquids can be heated above their ‘normal' boiling points. As a consequence the pressure will rise. This equipment has permitted a variety of processes like the syntheses of organics and the leaching of metal ores. Pressure technologies can be used for accelerating reactions, for specific syntheses or they offer an environmental or economical alternative for existing processes. Wet pressure oxidation of sewage sludge, an organic waste product, has become a proven technology. If less oxygen is added than stoichiometrically required for a complete oxidation, cellulose will break down by a different reaction mechanism, releasing electrons. Under these circumstances reduction of inorganic compounds is possible. 相似文献
976.
We investigated the relationship between distance from the ocean and the recruitment of ocean-spawned juvenile fish to seagrass shallows within marine dominated Lake Macquarie, a coastal barrier lagoon in Southeast Australia. Samples were taken by seine net every 6 wk between June 1986 and June 1987, at 20 sites established at various distances from the entrance channel. The fish fauna was diverse: 80 species from 39 families were caught, with the Gobiidae, Monacanthidae, Syngnathidae, Tetraodontidae, Mugilidae, Atherinidae, Clupeidae, Mullidae, Sparidae, and Blenniidae being well represented. Ambassis jacksoniensis, Atherinomorus ogilbyi, and Gerres subfasciatus accounted for 46% of all individuals. Thirty-two species were classified as ocean spawners, 38 as lagoon spawners, and 10 as unknown in terms of spawning area. Newly settled juveniles of ocean spawners were concentrated near the lagoon's entrance, where most recruited in spring. This pattern occurred in the absence of a salinity gradient: distance from the ocean coupled with weak internal water circulation appears to limit larval distribution and hence juvenile recruitment. Small juveniles of Rhabdosargus sarba were sufficiently abundant for their subsequent dispersal to be directly traced. On the basis of results for this species, and indirect evidence of dispersal for several others, it is suggested that ocean-spawned juveniles, after settling near the entrance, gradually disperse as they grow and change their habitat. Thus, further from the entrance, their recruitment to seagrass shallows is later, at larger sizes, and in smaller numbers. Newly settled juveniles of lagoon spawners, however, were widely distributed within the lagoon. The optimal recruitment of ocean-spawned juveniles to similar coastal lagoons may depend on suitable habitat being available near the entrance. *** DIRECT SUPPORT *** A01BY081 00003 相似文献
977.
978.
The Shamrocke ore body is a stratiform deposit of disseminated copper-iron sulphides found within lenses of calcareous meta-arkose occurring in Lomagundi Group graphitic schist (Late Precambrian) at Karoi, Rhodesia. Both sulphides and sediments were subjected to high grades of regional metamorphism. Argon/argon isochron ages indicate a major metamorphic event at 550 m. y. (Damaran orogeny) with later minor argon losses. Ore sulphide 34S values range from +3.0 to +14.8 CDT and a general decrease from footwall to hanging-wall reflects an original sedimentary environment where sulphides formed about the sea bed from hydrothermal fluids progressively mixing with sea water. Isotopically lighter sulphides formed syngenetically in the host rocks from bacterial reduction of sulphate. The pyrrhotite was probably formed from pyrite during metamorphism, and owing to reducing conditions maintained similar 34S values to the original pyrite. Oxygen and carbon isotopic analyses of mineralised and unmineralised carbonate lenses are consistent with deposition as marine limestones in an evaporitic environment and/or near hot spring vents.I. N. S. Contribution No 734 相似文献
979.
980.