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441.
R. M. Schmid 《Hydrobiologia》1990,197(1):305-308
The travertine structure containing Mountford Spring is in the only surface accumulation of CaCO3 on Lake Torrens, a playa in South Australia. Here the first 14C data on Lake Torrens is presented. 14C ages for travertine samples surrounding Mountford Spring, range from 13 770 ± 130 years BP to 22 700 ± 290 years BP, giving a time frame for lacustrine sediments wedged in between. In the absence of recent tectonic movements, the old travertine (22 700 ± 130 years) on the clifftop, surrounded by younger spring deposits (13770 ± 130 years) at a lower level, prove a deflation event at this playa. 相似文献
442.
Gyrfalcons Falco rusticolus use the same nest-sites over long periods of time, and in the cold dry climate of Greenland, guano and other nest debris decay slowly. Nineteen guano samples and three feathers were collected from 13 Gyrfalcon nests with stratified faecal accumulation in central-west and northwest Greenland. Samples were 14 C dated, with the oldest guano sample dating to c . 2740–2360 calendar years (cal yr) before present (BP) and three others were probably > 1000 cal yr BP. Feather samples ranged from 670 to 60 cal yr BP. Although the estimated age of material was correlated with sample depth, both sample depth and guano thickness gave a much less reliable prediction of sample age than use of radiocarbon dating on which the margin of error was less. Older samples were obtained from sites farther from the current Greenland Ice Sheet and at higher elevations, while younger samples were closer to the current ice sheet and at lower elevations. Values for δ13 C showed that Gyrfalcons nesting farther from the Greenland Ice Sheet had a more marine diet, whereas those nesting closer to the ice sheet (= further inland) fed on a more terrestrial diet. The duration of nest-site use by Gyrfalcons is a probable indicator of both the time at which colonization occurred and the palaeoenvironmental conditions and patterns of glacial retreat. Nowhere before has such extreme long-term to present use of raptor nest-sites been documented. 相似文献
443.
Delphine A. Aigoin Nicolas Devos Sanna Huttunen Michael S. Ignatov Juana M. Gonzalez‐Mancebo Alain Vanderpoorten 《Evolution; international journal of organic evolution》2009,63(12):3248-3257
The Macaronesian endemic flora has traditionally been interpreted as a relict of a subtropical element that spanned across Europe in the Tertiary. This hypothesis is revisited in the moss subfamily Helicodontioideae based on molecular divergence estimates derived from two independent calibration techniques either employing fossil evidence or using an Monte Carlo Markov Chain (MCMC) to sample absolute rates of nucleotide substitution from a prior distribution encompassing a wide range of rates documented across land plants. Both analyses suggest that the monotypic Madeiran endemic genus Hedenasiastrum diverged of other Helicodontioideae about 40 million years, that is, well before Macaronesian archipelagos actually emerged, in agreement with the relict hypothesis. Hedenasiastrum is characterized by a plesiomorphic morphology, which is suggestive of a complete morphological stasis over 40 million years. Macaronesian endemic Rhynchostegiella species, whose polyphyletic origin involves multiple colonization events, evolved much more recently, and yet accumulated many more morphological novelties than H. percurrens. The Macaronesian moss flora thus appears as a complex mix of ancient relicts and more recently dispersed, fast‐evolving taxa. 相似文献
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446.
Timothy J. Gallaher Paul M. Peterson Robert J. Soreng Fernando O. Zuloaga De‐Zhu Li Lynn G. Clark Christopher D. Tyrrell Cassiano A.D. Welker Elizabeth A. Kellogg Jordan K. Teisher 《植物分类学报:英文版》2022,60(3):522-569
Grasses are widespread on every continent and are found in all terrestrial biomes. The dominance and spread of grasses and grassland ecosystems have led to significant changes in Earth′s climate, geochemistry, and biodiversity. The abundance of DNA sequence data, particularly chloroplast sequences, and advances in placing grass fossils within the family allows for a reappraisal of the family′s origins, timing, and geographic spread and the factors that have promoted diversification. We reconstructed a time-calibrated grass phylogeny and inferred ancestral areas using chloroplast DNA sequences from nearly 90% of extant grass genera. With a few notable exceptions, the phylogeny is well resolved to the subtribal level. The family began to diversify in the Early–Late Cretaceous (crown age of 98.54 Ma) on West Gondwana before the complete split between Africa and South America. Vicariance from the splitting of Gondwana may be responsible for the initial divergence in the family. However, Africa clearly served as the center of origin for much of the early diversification of the family. With this phylogenetic, temporal, and spatial framework, we review the evolution and biogeography of the family with the aim to facilitate the testing of biogeographical hypotheses about its origins, evolutionary tempo, and diversification. The current classification of the family is discussed with an extensive review of the extant diversity and distribution of species, molecular and morphological evidence supporting the current classification scheme, and the evidence informing our understanding of the biogeographical history of the family. 相似文献
447.
Reconstruction of human subsistence and husbandry strategies from the Iberian Early Neolithic: A stable isotope approach 下载免费PDF全文
448.
A branched mid-Holocene bamboo coral skeleton of the isidid gorgonian genus Keratoisis (Octocorallia) recovered at southwestern Chatham Rise (New Zealand) from an average water depth of 680 m is described with
respect to sclerochronology and age determination. Growth rates of the Mg-calcitic internodal increments were investigated
by the counting of colour bands and radiocarbon dating. Growth banding is produced by varying orientations of crystal fan
bundles towards the image plane. The skeleton shows three growth interruptions, which are documented in all branches. AMS
14C ages decrease from base to top of the trunk and from the central axes to the margins of the branches, documenting a simultaneous
vertical and lateral growth. The data provide a maximum age of 3,975 ± 35 years BP, and a record spanning 240 ± 35 years. While calculated longitudinal growth rates amount to an average of 5 mm year−1 during a 55-year record, average lateral linear extension rates of 0.4 mm year−1 are an order of magnitude lower, still allowing for a seasonal to annual resolution of colour bands on a macroscopic scale
and for a daily to monthly resolution on microscales of individual crystal generations to fascicle bundles. Hence, the isidid
skeleton provides a high-resolution archive of paleoceanographic dynamics in deeper water masses. Concentric incremental accretion
around the central axis in the early growth stages changed into a unilaterally asymmetric growth during late-stage evolution,
probably triggered by the establishment of a stable system of unidirectional currents and nutrient flux. While colour band
counts, related to the AMS 14C ages, support a seasonal to annual accretion of macroscopic growth bands in the inner concentric and complete outer parts
of the skeleton, incremental growth rates at the condensed side are highly variable, as documented by hiatuses and unconformities.
Thus the specimen proves that growth rates of bamboo corals may vary within individual skeletons and strongly deviate from
the annual mode, hence showing implications on paleoceanographic proxy analyses. 相似文献
449.
450.
Sterkfontein Caves is the single richest early hominin site in the world with deposits yielding one or more species of Australopithecus and possible early Homo, as well as an extensive faunal collection. The inability to date the southern African cave sites accurately or precisely has hindered attempts to integrate the hominin fossil evidence into pan-African scenarios about human evolutionary history, and especially hominin biogeography. We have used U-Pb and U-Th techniques to date sheets of calcium carbonate flowstone inter-bedded between the fossiliferous sediments. For the first time, absolute age ranges can be assigned to the fossil-bearing deposits: Member 2 is between 2.8 ± 0.28 and 2.6 ± 0.30 Ma and Member 4 between 2.65 ± 0.30 and 2.01 ± 0.05 Ma. The age of 2.01 ± 0.05 Ma for the top of Member 4 constrains the last appearance of Australopithecus africanus to 2 Ma. In the Silberberg Grotto we have reproduced the U-Pb age of ∼2.2 Ma of for the flowstones associated with StW573. We believe that these deposits, including the fossil and the flowstones, accumulated rapidly around 2.2 Ma. The stratigraphy of the site is complex as sediments are exposed both in the underground chambers and at surface. We present a new interpretation of the stratigraphy based on surface mapping, boreholes logs and U-Pb ages. Every effort was made to retain the Member system, however, only Members 2 and 4 are recognized in the boreholes. We propose that the deposits formally known as Member 3 are in fact the distal equivalents of Member 4. The sediments of Members 2 and 4 consisted of cone-like deposits and probably never filled up the cave. The U-Th ages show that there are substantial deposits younger than 400 ka in the underground cave, underlying the older deposits, highlighting again that these cave fills are not simple layer-cakes. 相似文献