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51.
Zofia Ecaterina Taranu Dörte Köster Roland I. Hall Theo Charette Francine Forrest Les C. Cwynar Irene Gregory-Eaves 《Aquatic Sciences - Research Across Boundaries》2010,72(1):97-115
Although comparative analyses between dimictic and polymictic lakes have noted the key role of mixing regime in governing
water quality, limnologists have historically focused on dimictic lakes, leaving polymictic lakes relatively understudied.
In this study, we investigated whether the effects of agricultural development on water quality differed between dimictic
and polymictic lakes in a naturally nutrient-rich region of Alberta, Canada. Through a spatial limnological analysis of 36
sites, we found that the relationship between surface water total phosphorus concentration and the percent of agriculture
(% Agr) in the catchments differed between dimictic and polymictic lakes, where the proportion of variance explained was much
more pronounced in the dimictic (79% explained) than in the polymictic systems (7% explained). Paleolimnological analyses
of subfossil chironomids in surface sediment samples (0–1 cm) from 18 of the 36 study lakes, and in sediment core profiles
from the dimictic and polymictic basins of a eutrophic lake, showed that water quality differed between mixis groups. We found
that the surface sediment chironomid assemblages differed significantly between polymictic and dimictic lakes. Detailed analyses
of the sediment cores demonstrated that the two basin types differed in their responses to land-use change through time, as
only the dimictic basin showed a parallel increase in anoxia with increasing agricultural development. We suggest that in
naturally-fertile landscapes, external nutrient loading exerts a more notable effect on dimictic lakes, whereas internal nutrient
loading is more important in polymictic systems. 相似文献
52.
Glen M. MacDonald Andrei A. Velichko Constantine V. Kremenetski Olga K. Borisova Aleksandra A. Goleva Andrei A. Andreev Les C. Cwynar Richard T. Riding Steven L. Forman Tom W. D. Edwards Ramon Aravena Dan Hammarlund Julian M. Szeicz Valery N. Gattaulin 《Quaternary Research》2000,53(3):302
Radiocarbon-dated macrofossils are used to document Holocene treeline history across northern Russia (including Siberia). Boreal forest development in this region commenced by 10,000 yr B.P. Over most of Russia, forest advanced to or near the current arctic coastline between 9000 and 7000 yr B.P. and retreated to its present position by between 4000 and 3000 yr B.P. Forest establishment and retreat was roughly synchronous across most of northern Russia. Treeline advance on the Kola Peninsula, however, appears to have occurred later than in other regions. During the period of maximum forest extension, the mean July temperatures along the northern coastline of Russia may have been 2.5° to 7.0°C warmer than modern. The development of forest and expansion of treeline likely reflects a number of complimentary environmental conditions, including heightened summer insolation, the demise of Eurasian ice sheets, reduced sea-ice cover, greater continentality with eustatically lower sea level, and extreme Arctic penetration of warm North Atlantic waters. The late Holocene retreat of Eurasian treeline coincides with declining summer insolation, cooling arctic waters, and neoglaciation. 相似文献
53.
Assessment of slope stability, soil management or contaminant transport problems usually requires numerous, yet accurate point measurements of permeability. This technical note describes a new method for the rapid field assessment of permeability in near-surface soils or unconsolidated sediments. The procedure is known as ‘ring permeametry’ and is an ex situ core-based method giving measurements which can be guaranteed to be stratum-specific, unlike measurements from some in situ techniques. The potential sources of precision and bias error within the method are quantified and their effect on the uncertainty of permeability estimates is illustrated. © 1997 John Wiley & Sons, Ltd. 相似文献
54.
Svante Bjrck Michael J. C. Walker Les C. Cwynar Sigfus Johnsen Karen-Luise Knudsen J. John Lowe Barbara Wohlfarth 《第四纪科学杂志》1998,13(4):283-292
It is suggested that the GRIP Greenland ice-core should constitute the stratotype for the Last Termination. Based on the oxygen isotope signal in that core, a new event stratigraphy spanning the time interval from ca. 22.0 to 11.5 k GRIP yr BP (ca. 19.0–10.0 k 14C yr BP) is proposed for the North Atlantic region. This covers the period from the Last Glacial Maximum, through Termination 1 of the deep-ocean record, to the Pleistocene–Holocene boundary, and encompasses the Last Glacial Late-glacial of the traditional northwest European stratigraphy. The isotopic record for this period is divided into two stadial episodes, Greenland Stadials 1 (GS-1) and 2 (GS-2), and two interstadial events, Greenland Interstadials 1 (GI-1) and2 (GI-2). In addition, GI-1 and GS-2 are further subdivided into shorter episodes. The event stratigraphy is equally applicable to ice-core, marine and terrestrial records and is considered to be a more appropriate classificatory scheme than the terrestrially based radiocarbon-dated chronostratigraphy that has been used hitherto. © 1998 John Wiley & Sons, Ltd. 相似文献
55.
Matthew J. Wooller Joshua Kurek Benjamin V. Gaglioti Les C. Cwynar Nancy Bigelow Joshua D. Reuther Carol Gelvin-Reymiller John P. Smol 《Journal of Paleolimnology》2012,48(1):83-99
Wetlands and lakes in the Tanana Valley, Alaska, have provided important resources for prehistoric humans who inhabited this region. We examine an ~11,200?cal?yr BP record of environmental and paleolimnological changes from Quartz Lake in the middle Tanana Valley. Our data are also presented in the context of recent archaeological findings in the lake??s general vicinity that have 18 associated AMS 14C dates. We analyzed the stable-carbon and nitrogen isotope composition of total organic matter from the core, coupled with oxygen and carbon isotope analyses of Pisidiidae shells (fingernail clams), in addition to chironomid assemblage changes. Lacustrine sediments began to accumulate at ~11,200?cal?yr BP. Initially, autochthonous production was low and allochthonous organic input was negligible between 11,000 and 10,500?cal?yr BP, and were associated with relatively cool conditions at Quartz Lake at ~10,700?cal?yr BP. After 10,500?cal?yr BP, autochthonous production was higher coincident with a shift to chironomid assemblages dominated by taxa associated with warmer summer climates. A decrease in ??13C values of total organic carbon (TOC) and organic content of the sediment between 9,000 and 4,000?cal?yr BP may indicate declining autochthonous primary production. This period ended with an abrupt (~7???) decrease in the ??18O values from Pisidiidae shells at ~3,000?cal?yr BP, which we hypothesize represented an episodic connection (flood) of the lake with flow from the nearby (~6?km) Tanana River. Our findings coincide with evidence for major flooding at other locations connected to the Tanana River and further afield in Alaska. From ~3,000?cal?yr BP Quartz Lake subsequently appeared to become a relatively closed system, as indicated by the ??18OPisidiidae and ??13CPisidiidae data that are positively correlated and generally higher, which also correlates with a shift to moderately higher abundances of littoral chironomids. The cause of the transition to closed-basin conditions may have been geomorphic rather than climatic. This evidence of a progressively stronger evaporative influence on the lake??s closed hydrology after ~3,000?cal?yr BP is consistent with our modern ??18O and ??D water data from Quartz Lake that plot along a regional evaporative line we base on isotopic measurements from other local lakes and rivers. 相似文献
56.
Les Worrall 《International journal of geographical information science》2013,27(6):545-565
Abstract Justifying GIS in local government is a complex task for two main reasons: first, a GIS implementation can range from a single, well-defined application in one department to a corporate multi-purpose GIS; and, second, local authorities are complex organizations currently facing major pressures for change, and considerable uncertainty about their future structure and role. The purpose of this paper is to review the potential use of cost-benefit analysis, to explore tangible and intangible benefits, to identify the costs involved, to explore the effect that a robust GIS strategy can have on the relationship between costs and benefits, and to review some of the published material on the cost-justification of GIS in local government. 相似文献
57.
58.
Modification of the land use of a small catchment through coniferous afforestation is shown to have influenced stream bank erosion and channel form. Field mapping and erosion pin measurements over a 19-month period provides evidence of more active bank erosion along forested channel reaches than along non-forested. Extrapolation of downstream increases in bankfull width, bankfull depth, and channel capacity with increasing basin area for the non-forested catchment has demonstrated that afforestation of the lower part of the catchment has had a marked effect on channel form. Channel widths within the forest are up to three times greater than that predicted from the regression. These changes in bankfull width have led to stream bed aggradation and the development of wide shallow channels within the forest, and channel capacities within the forest are over two times that predicted from the basin area. The relationship between channel sinuosity and valley gradient for non-forested reaches of the river also indicated decreased sinuosity resulting from afforestation. These changes in channel form result from active bank erosion within the forest with coarse material being deposited within the channel as point-bars and mid-channel bars. Active bank erosion is largely attributed to the suppression by the forest of a thick grass turf and its associated dense network of fine roots, and secondly to the river attempting to bypass log jams and debris dams in the stream channel. 相似文献
59.
60.
Contemporary fiddling in human geography while Rome burns: has quantitative analysis been largely abandoned--and should it be? 总被引:2,自引:0,他引:2
Chris Hamnett [Geoforum 34 (2003) 1] has certainly been provocative in his claim that the nature of much contemporary human geography means that the discipline ‘will cease to be taken seriously in the world beyond the narrow confines of academe’ (p. 1). Much of his critique focuses on what he terms ‘post-modernism, new cultural geography and the interpretative turn’ (p. 2) practiced by those he identifies as ‘the new intellectual dilettanti’ (p. 3). In responding, however, our concern is not with that aspect of his critique, but rather with Hamnett’s misrepresentation of one section of contemporary human geography. 相似文献