全文获取类型
收费全文 | 108218篇 |
免费 | 1530篇 |
国内免费 | 1437篇 |
专业分类
测绘学 | 3089篇 |
大气科学 | 7468篇 |
地球物理 | 20683篇 |
地质学 | 41284篇 |
海洋学 | 8873篇 |
天文学 | 21426篇 |
综合类 | 2353篇 |
自然地理 | 6009篇 |
出版年
2022年 | 619篇 |
2021年 | 1051篇 |
2020年 | 1125篇 |
2019年 | 1257篇 |
2018年 | 6966篇 |
2017年 | 6083篇 |
2016年 | 5124篇 |
2015年 | 1579篇 |
2014年 | 2696篇 |
2013年 | 4534篇 |
2012年 | 3753篇 |
2011年 | 6167篇 |
2010年 | 5250篇 |
2009年 | 6404篇 |
2008年 | 5496篇 |
2007年 | 6020篇 |
2006年 | 3629篇 |
2005年 | 2725篇 |
2004年 | 2859篇 |
2003年 | 2688篇 |
2002年 | 2526篇 |
2001年 | 2032篇 |
2000年 | 2019篇 |
1999年 | 1560篇 |
1998年 | 1632篇 |
1997年 | 1485篇 |
1996年 | 1255篇 |
1995年 | 1249篇 |
1994年 | 1053篇 |
1993年 | 1018篇 |
1992年 | 940篇 |
1991年 | 988篇 |
1990年 | 938篇 |
1989年 | 823篇 |
1988年 | 758篇 |
1987年 | 893篇 |
1986年 | 775篇 |
1985年 | 960篇 |
1984年 | 1082篇 |
1983年 | 1036篇 |
1982年 | 957篇 |
1981年 | 913篇 |
1980年 | 810篇 |
1979年 | 750篇 |
1978年 | 729篇 |
1977年 | 627篇 |
1976年 | 633篇 |
1975年 | 618篇 |
1974年 | 592篇 |
1973年 | 660篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
731.
732.
Daniel Jean Stanley Harrison Sheng Douglas N. Lambert Peter A. Rona David W. McGrail J.Stacy Jenkyns 《Marine Geology》1981,40(3-4):215-235
Analyses of DSRV “Alvin” core samples on the Cape Hatteras margin indicate major textural and compositional changes at depths of about 1000 and well below 2500 m. The distribution patterns of petrologic parameters correlate well with water mass flow and suspended-sediment plumes measured on this margin by other workers. Our study also shows: (a) vigorous erosion and sediment transport at depths of less than 400 m resulting from the NE-trending Gulf Stream flow; (b) deposition, largely planktonic-rich sediment released from the Gulf Stream, on the upper- to mid-slope, to depths of about 800–1200 m; (c) winnowing, resuspension and deposition induced by periodically intensified slope currents on the mid-slope to uppermost rise, between about 1000 and 2500 m; and (d) prevailing deposition on the upper rise proper (below 2500 m), from transport by the SW-trending Western Boundary Undercurrent. Sediments moved by bottom currents have altered the composition and distribution patterns of material transported downslope by offshelf spillover; this mixing of gravity-emplaced and bottom-current-transported sediment obscures depositional boundaries. Moreover, reworking of the seafloor by benthic organisms alters physical properties and changes erodability of surficial sediments by bottom currents. Measurement of current flow above the seafloor and direct observation of the bottom are insufficient to delineate surficial sediment boundaries. Detailed petrologic analyses are needed to recognize the long-term signature of processes and define depositional provinces. 相似文献
733.
734.
A novel technique in analyzing non-linear wave-wave interaction 总被引:1,自引:0,他引:1
During wave growth non-linear wave–wave interactions cause transfer of some wave energy from lower to higher wave periods as the spectrum grows. Wavelet bicoherence, which is a new technique in the analysis of wind–wave and wave–wave interactions, is used to analyze non-linear wave–wave interactions. A selected record of wind wave that contains the maximum wave height observed during 6 h of wave generation is divided into five segments and wavelet bicoherence is computed for the whole record, and for all divided segments. The study shows that the non-linear wave–wave interaction occurs at different bicoherence levels and these levels are different from one segment to another due to the non-stationarity feature of the examined data set. 相似文献
735.
736.
737.
The paper describes experimental tests carried out on three ring-stiffened cones that were tested to destruction under external hydrostatic pressure. The cones were carefully machined from EN1A Steel. All three cones failed by plastic non-symmetric bifurcation buckling in a mode commonly known as general instability. In this mode the entire ring-shell combination buckles bodily.The paper also provides a design chart using the results obtained from these three vessels, together with the results of six other vessels obtained from other tests. The design chart allows the possibility of obtaining a plastic knock down factor, so that the theoretical buckling pressures, based on elastic theory, can be divided by the plastic knockdown factor, to give the predicted buckling pressure. This method can also be used for the design of full-scale vessels. 相似文献
738.
Hayes RA Regondi S Winter MJ Butler PJ Agradi E Taylor EW Kevin Chipman J 《Marine environmental research》2004,58(2-5):665-669
Metallothionein has been assayed in a range of aquatic animal tissues as an indicator of metal exposure. We sequenced chub (Leuciscus cephalus) metallothionein cDNA which showed over 90% homology to common carp, goldfish and stone loach and 77% homology to rainbow trout sequences for metallothionein. We then used the extended primer method to develop an accurate quantitative competitive RT-PCR assay for metallothionein mRNA. RT-PCR was used to measure metallothionein mRNA in feral chub from a range of field sites, with different levels of heavy metal pollution, in the West Midlands, UK. Measurements were complemented by analysis of liver and gill metallothionein protein by capillary electrophoresis. There was no significant difference in the metallothionein protein levels between fish of different rivers and there was no evidence of elevation of mRNA at the sites of highest metal exposure. The level of metal exposure (e.g. zinc, nickel and cadmium each ranging between 15 and 28 microg/l ) at the pH (7.5-8.5) of these rivers appears insufficient to elevate hepatic or gill metallothionein in chub. A lack of elevation of hepatic metallothionein mRNA in chub exposed to zinc, copper and manganese for 24 h and 10 days in the laboratory also suggests a non-responsiveness of this species. 相似文献
739.
The recent sea-ice reduction in the Arctic Ocean is not spatially uniform, but is disproportionally large around the Northwind
Ridge and Chukchi Plateau compared to elsewhere in the Canada Basin. In the Northwind Ridge region, Pacific Summer Water (PSW)
delivered from the Bering Sea occupies the subsurface layer. The spatial distribution of warm PSW shows a quite similar pattern
to the recent ice retreat, suggesting the influence of PSW on the sea-ice reduction. To understand the regionality of the
recent ice retreat, we examine the dynamics and timing of the delivery of the PSW into this region. Here, we adopt a two-layer
linearized potential vorticity equation to investigate the behavior of Rossby waves in the presence of a topographic discontinuity
in the high latitude ocean. The analytical results show a quite different structure from those of mid-latitude basins due
to the small value of β. Incident barotropic waves excited by the sea-ice motion with large annual variation can be scattered
into both barotropic and baroclinic modes at the discontinuity. Since the scattered baroclinic Rossby wave with annual frequency
cannot propagate freely, a strong baroclinic current near the topographic discontinuity is established. The seasonal variation
of current near the topographic discontinuity would cause a kind of selective switching system for shelf water transport into
the basin. In our simple analytical model, the enhanced northward transport of summer water and reduced northward transport
of winter water are well demonstrated. The present study indicates that these basic dynamics imply that a strengthening of
the surface forcing during winter in the Canada Basin could cause sea-ice reduction in the Western Arctic through the changes
of underlying Pacific Summer Water. 相似文献
740.
M. Tiwari R. Ramesh B. L. K. Somayajulu A. J. T. Jull G. S. Burr 《Geo-Marine Letters》2006,26(1):23-30
Rapid shifts in past climate recorded in polar ice sheets have elicited various explanations relating to either thermohaline
circulation changes by ice-rafting or natural greenhouse gas concentrations modulated by climatic conditions in the tropics.
To compare the tropical paleoclimate record with the polar record, one must choose sediment cores from highly productive ocean
regions. Necessarily, such regions reflect the wind records in the tropics, because high productivity is associated with upwelling
driven by winds. Comparing tropical precipitation records with high-latitude records is, however, a more difficult task because
sediments recording paleoprecipitation usually have low sedimentation rates, and offer coarser resolution relative to polar
ice cores. Here, we present δ
18O data of three planktonic species of Foraminifera (a proxy for precipitation) from such a sediment core, spanning the past
35 ka for the equatorial Indian Ocean, which falls under the southwest monsoon (SWM) realm. Results show that minimum SWM
precipitation occurred at the Last Glacial Maximum, with a subsequent increase at Termination IA. During the Holocene, SWM
precipitation intensified uniformly up to the core top (∼2.2 ka b.p.), as revealed by generally decreasing δ
18O values. Variations in precipitation are consistent with climate changes recorded in polar ice sheets. Although the different resolutions of the two records preclude
a rigorous comparison, abrupt cooling/warming events appear to be accompanied by sudden reduction/enhancement in (SWM) rainfall.
Thus, mechanisms with time scales much shorter than a millennium, such as natural greenhouse warming (e.g., CH4 concentration), controlled by emissions from the tropics, could have played a major role in high-latitude climate change. 相似文献