全文获取类型
收费全文 | 79456篇 |
免费 | 1258篇 |
国内免费 | 603篇 |
专业分类
测绘学 | 1951篇 |
大气科学 | 5431篇 |
地球物理 | 15879篇 |
地质学 | 27994篇 |
海洋学 | 6947篇 |
天文学 | 18486篇 |
综合类 | 185篇 |
自然地理 | 4444篇 |
出版年
2021年 | 712篇 |
2020年 | 780篇 |
2019年 | 821篇 |
2018年 | 2052篇 |
2017年 | 1889篇 |
2016年 | 2163篇 |
2015年 | 1209篇 |
2014年 | 2005篇 |
2013年 | 3861篇 |
2012年 | 2287篇 |
2011年 | 3054篇 |
2010年 | 2723篇 |
2009年 | 3711篇 |
2008年 | 3105篇 |
2007年 | 3199篇 |
2006年 | 2937篇 |
2005年 | 2692篇 |
2004年 | 2830篇 |
2003年 | 2529篇 |
2002年 | 2140篇 |
2001年 | 1888篇 |
2000年 | 1727篇 |
1999年 | 1489篇 |
1998年 | 1495篇 |
1997年 | 1482篇 |
1996年 | 1248篇 |
1995年 | 1220篇 |
1994年 | 1110篇 |
1993年 | 992篇 |
1992年 | 930篇 |
1991年 | 914篇 |
1990年 | 996篇 |
1989年 | 921篇 |
1988年 | 855篇 |
1987年 | 1050篇 |
1986年 | 895篇 |
1985年 | 1112篇 |
1984年 | 1233篇 |
1983年 | 1145篇 |
1982年 | 1037篇 |
1981年 | 1082篇 |
1980年 | 893篇 |
1979年 | 861篇 |
1978年 | 834篇 |
1977年 | 809篇 |
1976年 | 740篇 |
1975年 | 719篇 |
1974年 | 723篇 |
1973年 | 751篇 |
1971年 | 453篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
A. Winter K.O. Coyle G. Swartzman 《Deep Sea Research Part II: Topical Studies in Oceanography》2007,54(23-26):2869
Cohort abundance of walleye pollock (Theragra chalcogramma) is subject to strong interannual variation in the eastern Bering Sea, and this variation is known to be determined largely at the age-0 stage. We estimated the spatial distributions and densities of age-0 walleye pollock in five nursery areas around the eastern Bering shelf in three successive years (1997–1999) from acoustic survey data. Concurrently, we calculated estimates of the spatial distribution of euphausiids, a major prey of age-0 walleye pollock, and estimates of spatial overlap of groundfish predators with the age-0 walleye pollock. The analyses showed that all nursery areas had low densities of age-0 walleye pollock in 1997, which ultimately produced the weakest adult year-class. In the intermediate year of 1998, age-0 densities were low to medium, and in 1999, which produced the strongest of the three adult year-classes, all nursery areas had medium to high age-0 walleye pollock densities. Euphausiid distributions had a consistently positive spatial relationship with age-0 walleye pollock. Groundfish predator density ratios were positively related to age-0 walleye pollock density when age-0 walleye pollock were displaced relatively northward. Our results suggest that abundance of age-0 walleye pollock, and hence of adult cohorts in the eastern Bering Sea, can be predictable from a concise set of indicators: the densities of age-0 walleye pollock at nursery areas in mid- to late-summer, their spatial relationship to euphausiids and groundfish predators, and the latitudinal trend of their distributions. The 3 years 1997–1999 had significant differences of physical conditions in the eastern Bering Sea, and represent an advantageous framework for testing these hypotheses. 相似文献
993.
994.
Spatio-temporal variability of small copepods (especially Oithona plumifera) in shallow nearshore waters off the south coast of South Africa 总被引:1,自引:0,他引:1
Francesca Porri Christopher D. McQuaid William P. Froneman 《Estuarine, Coastal and Shelf Science》2007,72(4):711-720
Although small copepods are one of the main dietary sources for many commercially important fish, their role in the pelagic trophic dynamics has traditionally been underestimated due to the methodology commonly used in plankton sampling. Temporal variation in abundance of adults and nauplii of small copepods (particularly Oithona plumifera) in nearshore waters on the south coast of South Africa was investigated fortnightly over 14 months at site (km) and location (100 m) scales. Sampling was within <500 m of the shore, where depth was ca. 10 m, using vertical hauls of an 80-μm mesh plankton net from 1 m above the seabed to the surface. Twenty-seven adult copepod taxa were recorded, but Oithona spp. was consistently the most abundant. Taxon richness was 7–19 on each sampling occasion. There was strong temporal variation (Oithona varied between 0 and 2300 m−3), but much of this was short-term variability (e.g. between consecutive sampling sessions), with no seasonality or other long-term discernable patterns. There were periods of consistently low numbers, but very high numbers often followed samples with low abundances. Nor was there spatial structure at the location scale, though numbers differed between sites. Despite considerable variability at the location scale within sites, Kenton consistently showed higher densities than High Rocks. Separate analyses, with Bonferroni adjustment, showed that this difference was significant on eight out of 21 occasions for Oithona, six for other pelagic copepods and three for nauplii. This suggests that hydrodynamics favour aggregation of plankton at Kenton. A high degree of short-term variability, with a tendency for aggregation of small zooplankton at certain sites has implications for both pelagic processes and food-web links between the benthic and pelagic environments. 相似文献
995.
Sloshing in a two-dimensional rectangular tank in horizontal motion is analysed based on the velocity potential theory. It is found that even when the first-order excitation is away from all the natural frequencies of the tank, second-order resonance can still occur when the sum-frequency or the difference-frequency is equal to one of the natural frequencies corresponding to the even mode. However, such resonance is not excited when the sum or difference frequency is equal to the natural frequency of an odd mode. 相似文献
996.
S. D. Nikolaev 《Oceanology》2006,46(4):513-525
The objective of this study was to present details of the evolution of the thermal field of the ocean during the last Cenozoic glacial megacycle. The main causes of the phenomena observed are discussed based on the generalization of original data accumulated during many years and numerous publications. It is shown that, in the Eocene and Early Oligocene, the climatic conditions in the World Ocean were uniform and featured no evident zonal differences, the meridional temperature gradients were low, and the mean global temperature was close to the present-day value. Starting from the Late Oligocene and throughout the entire Neogene, the meridional contrasts in the temperature and hydrological conditions have increased, and, in the Late Pliocene, the climatic pattern resembled the present-day situation. The general tendency in the climate changes since the Eocene until the Pliocene consisted of its latitudinal differentiation and growing contrasts rather than of general climate cooling. In the Pleistocene, with the onset of the glaciation in the Northern Hemisphere, the character of the temperature field changed and variations in the mean global temperature values began to dominate. The factors that result from the climate changes observed are discussed and the causes are shown that lead to the differences between the author’s data and the existing concepts. 相似文献
997.
Erdal Dolu Erkan Gökaşan Engin Meriç Mustafa Ergin Tolga Görüm Hüseyin Tur Berkan Ecevitoğlu Niyazi Avşar Muhittin Görmüş Fatmagül Batuk Buğser Tok Oktay Çetin 《Geo-Marine Letters》2007,27(6):355-381
The Quaternary evolution of the Gulf of İzmit, situated on the tectonically active North Anatolian Fault Zone (NAFZ), was
investigated using seismic reflection, paleontologic, and sediment textural data. On the basis of seismic stratigraphic and
sedimentologic-paleontologic interpretations, four depositional units were distinguished within the Plio-Quaternary sequence
of the Gulf of İzmit. According to these data, Plio-Quaternary deposits supplied from the northern terrestrial area started
to accumulate during a progradational phase, in a south-facing half-graben. A coarse-grained sedimentary unit prograding into
the gulf from the south since 200 ka b.p. indicates a dramatic variation in the evolution of the gulf, with the initiation of a new strike-slip fault of the NAFZ
and a corresponding uplift of the Armutlu Peninsula in the south of the gulf. During the evolution of this fault from a wide
shear zone consisting of right-stepped strike-slip faults and pull-apart basins to a localized principal fault zone, sediments
were deposited under the influence of northerly prograding terrestrial and shallow-marine conditions due to relative sea-level
fluctuations in the Marmara Sea. During this period, the Gulf of İzmit was invaded mainly by Mediterranean and partly by Black
Sea waters. In the latest glacial period, shallow areas in the gulf became subaerially exposed, whereas the central and western
sub-basins of the gulf turned into lakes. The present evolution of the Gulf of İzmit is controlled by the after effects of
the new rupture of the NAFZ and the estuarine nature of the gulf environment. 相似文献
998.
Interannual variability in Nordic seas primary production 总被引:1,自引:0,他引:1
999.
1000.
This paper presents the findings from an in-depth analysis of the (axial) stiffness data recorded during tension–tension fatigue tests on wire ropes, particularly in relation to how changes in stiffness during testing relate to changes in rope strength. A linear relationship between stiffness and strength is shown to exist and a methodology presented for quantifying residual strength with applied cycles. New lower bound fatigue lines for six-strand rope and spiral strand are presented which are based on a 10% loss of strength. These new lines have the advantage of having been established using a common discard criterion for wire ropes. 相似文献