全文获取类型
收费全文 | 355篇 |
免费 | 17篇 |
国内免费 | 8篇 |
专业分类
测绘学 | 3篇 |
大气科学 | 37篇 |
地球物理 | 75篇 |
地质学 | 157篇 |
海洋学 | 21篇 |
天文学 | 50篇 |
自然地理 | 37篇 |
出版年
2023年 | 3篇 |
2021年 | 4篇 |
2020年 | 7篇 |
2019年 | 11篇 |
2018年 | 7篇 |
2017年 | 6篇 |
2016年 | 9篇 |
2015年 | 14篇 |
2014年 | 10篇 |
2013年 | 26篇 |
2012年 | 19篇 |
2011年 | 17篇 |
2010年 | 9篇 |
2009年 | 24篇 |
2008年 | 22篇 |
2007年 | 16篇 |
2006年 | 10篇 |
2005年 | 12篇 |
2004年 | 11篇 |
2003年 | 11篇 |
2002年 | 11篇 |
2001年 | 17篇 |
2000年 | 11篇 |
1999年 | 5篇 |
1998年 | 6篇 |
1997年 | 4篇 |
1996年 | 9篇 |
1995年 | 4篇 |
1994年 | 4篇 |
1993年 | 8篇 |
1992年 | 9篇 |
1990年 | 1篇 |
1989年 | 4篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 1篇 |
1984年 | 8篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1972年 | 1篇 |
1971年 | 2篇 |
1970年 | 1篇 |
排序方式: 共有380条查询结果,搜索用时 448 毫秒
241.
242.
Impacts of rising air temperatures and emissions mitigation on electricity demand and supply in the United States: a multi-model comparison 总被引:2,自引:2,他引:0
243.
244.
The Superior lobe of the Laurentide ice sheet, during its last advance into northwestern Wisconsin, may have moved and transported sediment primarily through deep, widespread shear of its bed to high strains (>100). To study the kinematics of this deformation, we measured the anisotropy of magnetic susceptibility of 3750 intact samples of its basal till—the Douglas member of the Miller Creek formation—collected from eight profiles at 0.2 m depth increments. A benchmark for interpretations was provided by results of ring-shear experiments conducted on the Douglas Till, which provided fabric characteristics for the case of horizontal simple shear. Orientations of maximum susceptibility (k1) were controlled by preferred orientations of silt-sized magnetite grains and were similar to sand-grain orientations measured in thin sections. Strengths of fabrics formed by orientations of k1 indicate that most of the till sampled was deformed but to variable strains. Deformation averaged over the 20 km spanned by the study approximated horizontal simple shear. However, large differences in k1 fabric azimuths (>45°) over lateral distances of meters to tens of meters indicate highly heterogeneous bed deformation, perhaps focused in anastomosing zones with associated divergent and convergent till shear. This interpretation is supported by orientations of principal susceptibilities that imply, in many cases, either that shear planes were steeply dipping, particularly transverse to the shearing direction, or that there were major components of pure shear. Variations in k1 fabric azimuth with depth indicate that most of the till thickness did not shear simultaneously; rather, till accumulated at the bed as shear direction changed in response to temporally shifting zones of shallow deformation (<1 m). This heterogeneous, temporally variable deformation of the bed differs from many applications of the bed-deformation model but is consistent with subglacial measurements at modern glaciers. 相似文献
245.
All groundwater pumped is balanced by removal of water somewhere, initially from storage in the aquifer and later from capture in the form of increase in recharge and decrease in discharge. Capture that results in a loss of water in streams, rivers, and wetlands now is a concern in many parts of the United States. Hydrologists commonly use analytical and numerical approaches to study temporal variations in sources of water to wells for select points of interest. Much can be learned about coupled surface/groundwater systems, however, by looking at the spatial distribution of theoretical capture for select times of interest. Development of maps of capture requires (1) a reasonably well-constructed transient or steady state model of an aquifer with head-dependent flow boundaries representing surface water features or evapotranspiration and (2) an automated procedure to run the model repeatedly and extract results, each time with a well in a different location. This paper presents new methods for simulating and mapping capture using three-dimensional groundwater flow models and presents examples from Arizona, Oregon, and Michigan. 相似文献
246.
Juliana Finoto Bueno Elson Paiva Oliveira Neal J. McNaughton Jorge Henrique Laux 《Gondwana Research》2009,15(1):86-97
The Sergipano Belt is the outcome of collision between the Pernambuco–Alagoas Massif and the São Francisco Craton during Neoproterozoic assembly of West Gondwana. Field relationships and U–Pb geochronology of granites intruded in garnet micaschists of the Macururé Domain are used to constrain the main collisional event (D2) in the belt. The granites are divided into two groups, the pre-collisional granites (pre- to early-D2) and the syn-collisional granites (syn- to tardi-D2), the latter were emplaced as sheets along the S2 axial plane foliation or they were collected at the hinge zones of F2 folds. A U–Pb SHRIMP zircon age of 628 ± 12 Ma was obtained for the pre-collisional Camará tonalite. Two U–Pb TIMS titanite ages were obtained for the syn-collisional granites, 584 ± 10 Ma for the Angico granite and 571 ± 9 Ma for the Pedra Furada granite, and these ages are close to the garnet-whole rock Sm–Nd isochron of 570 Ma found for the peak of metamorphism in the Sergipano Belt. The ages of the Camará tonalite (628 Ma) and the Pedra Furada granite (571 Ma) mark respectively the maximum age for beginning of the D2 event and minimum age for the end in the Macururé Domain. Using these ages, the main Neoproterozoic D2 collisional event has been in operation in the Sergipano Belt for at least 57 million years. Correlation with coeval granitoids farther north in the Borborema Province indicate that while in the Sergipano Belt the syn-D2 granites (ca. 590–570 Ma) were emplaced under compression, in the Borborema Province they emplaced under extensional conditions related to regional strike-slip shear zones. These contrasting emplacement settings for contemporaneous Neoproterozoic granitoids are explained by a combination of continent–continent collision and extrusion tectonics. 相似文献
247.
Alan D. Wanamaker Jr Karl J. Kreutz Bernd R. Schöne Neal Pettigrew Harold W. Borns Douglas S. Introne Daniel Belknap Kirk A. Maasch Scott Feindel 《Climate Dynamics》2008,31(2-3):183-194
To investigate ocean variability during the last millennium in the Western Gulf of Maine (GOM), we collected a 142-year-old living bivalve (Arctica islandica L.) in 2004, and three fossil A. islandica shells (calibrated 14CAMS = 1030 ± 78 ad; 1320 ± 45 ad; 1357 ± 40 ad) for stable isotope and growth increment analysis. A statistically significant relationship exists between modern GOM temperature records [shell isotope-derived (30 m) (r = ?0.79; P < 0.007), Prince 5 (50 m) (r = ?0.72; P < 0.019), Boothbay Harbor SST (r = ?0.76; P < 0.011)], and Labrador Current (LC) transport data from the Eastern Newfoundland Slope during 1993–2003. In all cases, as LC transport increased, GOM water temperatures decreased the following year. Decadal trends in the North Atlantic Oscillation (NAO) and the Atlantic Multidecadal Oscillation (AMO) influence GOM water temperatures in the most recent period, with water temperatures decreasing during NAO and AMO negative modes most likely linked to LC transport and Gulf Stream interaction. Mean shell-derived isotopic changes (δ18Oc) during the last 1,000 years were +0.47‰ and likely reflect a 1–2°C cooling from 1000 ad to present. Based on these results, we suggest that observed cooling in the GOM during the last millennium was due to increased transport and/or cooling of the LC, and decreased Gulf Stream influence on the GOM. 相似文献
248.
The late Quaternary paleoclimate of eastern Beringia has primarily been studied by drawing qualitative inferences from vegetation shifts. To quantitatively reconstruct summer temperatures, we analyzed lake sediments for fossil chironomids, and additionally we analyzed the sediments for fossil pollen and organic carbon content. A comparison with the δ18O record from Greenland indicates that the general climatic development of the region throughout the last glaciation–Holocene transition differed from that of the North Atlantic region. Between 17 and 15 ka, mean July air temperature was on average 5°C colder than modern, albeit a period of near-modern temperature at 16.5 ka. Total pollen accumulation rates ranged between 180 and 1200 grains cm− 2 yr− 1. At 15 ka, approximately coeval with the Bølling interstadial, temperatures again reached modern values. At 14 ka, nearly 1000 yr after warming began, Betula pollen percentages increased substantially and mark the transition to shrub-dominated pollen contributors. Chironomid-based inferences suggest no evidence of the Younger Dryas stade and only subtle evidence of an early Holocene thermal maximum, as temperatures from 15 ka to the late Holocene were relatively stable. The most recognizable climatic oscillation of the Holocene occurred from 4.5 to 2 ka. 相似文献
249.
The extent of glaciation in northwestern Alaska, the source of sediment supply to the Chukchi shelf and slope, and the movement of sea ice and icebergs across the shelf during the last glacial maximum (LGM) remain poorly constrained. Here we present geophysical and geological data from the outer Chukchi margin that reveal a regionally extensive, heavily ice-scoured surface ∼ 5-8 m below the modern seafloor. Radiocarbon dating of this discrete event yields age estimates between 10,600 and 11,900 14C yr BP, indicating the discharge event occurred during the Younger Dryas. Based on mineralogy of the ice-rafted debris, the icebergs appear to be sourced from the northwestern Alaskan margin, which places important constraints on the ice extent in northern Alaska during the LGM as well as existing circulation models for the region. 相似文献
250.
Temperature variance and temperature power spectra in the unstable surface layer have always presented a problem to the standard
Monin-Obukhov similarity model. Recently that problem has intensified with the demonstration by Smedman et al. (2007, Q J
Roy Meteorol Soc 133: 37–51) that temperature spectra and heat-flux cospectra can have two distinct peaks in slightly unstable
conditions, and by McNaughton et al. (2007, Nonlinear Process Geophys 14: 257–271) who showed that the wavenumber of the peak
of temperature spectra in a convective boundary layer (CBL), closely above the surface friction layer (SFL), can be sensitive
to the CBL depth, z
i. Neither the two-peak form at slight instability nor the dependence of peak position on z
i at large instability is compatible with the Monin-Obukhov model. Here we examine the properties of temperature spectra and
heat-flux cospectra from between these extremes, i.e. from within the unstable SFL, in two experiments. The analysis is based
on McNaughton’s model of the turbulence structure in the SFL. According to this model, heat is transported through most of
the SFL by sheet plumes, created by the action of impinging outer eddies. The smallest and most effective of these outer eddies
have sizes that scale on SFL depth, z
s. The z
s-scale eddies and plumes are organised within the overall convection pattern in the CBL, and in turn they organise the motion
of smaller eddies within the SFL, whose sizes scale on height, z. The main experimental results are: (1) the peak amplitudes of the temperature spectra in the SFL are collapsed with a scaling
factor (zsz)1/3eo2/3{(z_{\rm s}z)^{1/3}\varepsilon_{\rm o}^{2/3}} divided by the square of the surface temperature flux, where eo{\varepsilon_{\rm o}} is the dissipation rate of turbulent energy in the outer CBL (above the SFL); (2) the peak wavenumbers of the temperature
spectra are collapsed with the mixed length scale (z
i
z
s)1/2; (3) the peak wavenumbers of the heat-flux cospectra are collapsed with the doubly-mixed length scale (z
i
z
s)1/4
z
1/2; (4) for z/z
s < 0.03, the peak in the cospectrum is replaced by another peak at a wavenumber about a magnitude larger. This peak’s position
scales on z; (5) all these findings are consistent with the observations of Smedman et al. 相似文献