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排序方式: 共有143条查询结果,搜索用时 125 毫秒
31.
PETER EMIL KALAND KNUT KRZYWINSKI BJØRG STABELL 《Boreas: An International Journal of Quaternary Research》1984,13(2):243-258
In three different areas in western Norway, large errors are obtained in the radiocarbon dates from lacustrine sediments close to marine/lacustrine sediment boundaries. Differences occur between radiocar-bon and pollenanalytic dates and between dates at isolation/ingression contacts for lakes of the same altitude above the present sea level. Younger radiocarbon dates are also obtained below older ones in undisturbed sediments. When divergent dates occur, the radiocarbon dates always seem to be the youngest. Large differences are also found between NaOH soluble and insoluble fractions of the same sediment samples. Insoluble fractions generally yield younger dates than the soluble. Differences are not found, however, for dates younger than about 8,000 B.P. The dating errors are connected to periods with more oligotrophic conditions with isoetides. Their roots penetrate older sediments. Due to contamination of the organic part of the sediment from partly decomposed roots, some radiocarbon dates will be too young. The isoetide vegetation and the dating errors disappear when the lakes become dystrophic. 相似文献
32.
33.
A ROBUST PLS PROCEDURE 总被引:1,自引:0,他引:1
A robust partial least squares(PLS)regression algorithm is developed.This is achieved by substitutionof the univariate regression steps in the iterative PLS2 algorithm by a robust alternative.The anglebetween loading vectors from both perturbed and unperturbed solutions is used as a measure ofrobustness.By means of a perturbation study on a structure-activity data set,it is demonstrated thatthe stability of the robust method is superior to standard PLS. 相似文献
34.
This paper assesses variations in quantitative reconstructions of late Holocene relative sea‐level (RSL) change arising from using modern diatom datasets from different spatial scales, applied to case studies from Alaska. We investigate the implications of model choice in transfer functions using local‐, sub‐regional‐ and regional‐scale modern training sets, and produce recommendations on the creation and selection of modern datasets for reconstructing RSL change over Holocene timescales in tidal marsh environments comparable with those in Alaska. We show that regional modern training sets perform best in terms of providing fossil samples with good modern analogues, and in producing reconstructions that most closely match observations, where these are available. Local training sets are frequently insufficient to provide fossil samples with good modern analogues and may over‐estimate the precision of RSL reconstructions. This is particularly apparent when reconstructing RSL change for periods beyond the last century. For reconstructing RSL change we recommend using regional modern training sets enhanced by local samples. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
35.
During February 2010, studies of primary production (PP) and physiology were conducted at five selected sites in the KwaZulu-Natal (KZN) Bight of the Agulhas ecosystem as part of a programme to elucidate the influence of major physical driving forces and nutrient inputs on the structure and functioning of biological communities. These sites were located in the vicinity of the Durban lee eddy, in the midshelf region of the central part of the bight, off the Thukela Mouth, and to the north and south of Richards Bay. At four of the sites, chlorophyll a ranged from 0.10 to 1.44?mg m–3 and integrated PP ranged between 0.35 and 2.58?g C m–2 d–1. The highest biomass and PP, which were comparable to those observed in a wind-driven upwelling system, were associated with a diatom community observed at the midshelf site, and varied between 0.26 and 4.27?mg m–3 and 7.22 and 9.89?g C m–2 d–1, respectively. Environmental conditions at each of the sites differed substantially and appeared to be influential in initiating and controlling the development and distribution of phytoplankton biomass and production. Phytoplankton adaptation to variable environmental conditions was characterised by a decreased light-limited slope (αB) and increased rate of photosynthesis (Pm ) and light saturation (Ek) with elevated temperatures. The converse (increased αB and decreased Pm and Ek) was observed as irradiance levels declined. Generalised additive models indicated that irradiance, temperature and biomass were important variables influencing photosynthetic parameters and photosynthetic rates. 相似文献
36.
A wind-tunnel study was conducted to investigate ventilation of scalars from urban-like geometries at neighbourhood scale
by exploring two different geometries a uniform height roughness and a non-uniform height roughness, both with an equal plan
and frontal density of λ
p
= λ
f
= 25%. In both configurations a sub-unit of the idealized urban surface was coated with a thin layer of naphthalene to represent
area sources. The naphthalene sublimation method was used to measure directly total area-averaged transport of scalars out
of the complex geometries. At the same time, naphthalene vapour concentrations controlled by the turbulent fluxes were detected
using a fast Flame Ionisation Detection (FID) technique. This paper describes the novel use of a naphthalene coated surface
as an area source in dispersion studies. Particular emphasis was also given to testing whether the concentration measurements
were independent of Reynolds number. For low wind speeds, transfer from the naphthalene surface is determined by a combination
of forced and natural convection. Compared with a propane point source release, a 25% higher free stream velocity was needed
for the naphthalene area source to yield Reynolds-number-independent concentration fields. Ventilation transfer coefficients
w
T
/U derived from the naphthalene sublimation method showed that, whilst there was enhanced vertical momentum exchange due to
obstacle height variability, advection was reduced and dispersion from the source area was not enhanced. Thus, the height
variability of a canopy is an important parameter when generalising urban dispersion. Fine resolution concentration measurements
in the canopy showed the effect of height variability on dispersion at street scale. Rapid vertical transport in the wake
of individual high-rise obstacles was found to generate elevated point-like sources. A Gaussian plume model was used to analyse
differences in the downstream plumes. Intensified lateral and vertical plume spread and plume dilution with height was found
for the non-uniform height roughness. 相似文献
37.
Colby Loucks Shannon Barber-Meyer Md. Abdullah Abraham Hossain Adam Barlow Ruhul Mohaiman Chowdhury 《Climatic change》2010,98(1-2):291-298
The Sundarbans mangrove ecosystem, shared by India and Bangladesh, is recognized as a global priority for biodiversity conservation. Sea level rise, due to climate change, threatens the long term persistence of the Sundarbans forests and its biodiversity. Among the forests’ biota is the only tiger (Panthera tigris) population in the world adapted for life in mangrove forests. Prior predictions on the impacts of sea level rise on the Sundarbans have been hampered by coarse elevation data in this low-lying region, where every centimeter counts. Using high resolution elevation data, we estimate that with a 28 cm rise above 2000 sea levels, remaining tiger habitat in Bangladesh’s Sundarbans would decline by 96% and the number of breeding individuals would be reduced to less than 20. Assuming current sea level rise predictions and local conditions do not change, a 28 cm sea level rise is likely to occur in the next 50–90 years. If actions to both limit green house gas emissions and increase resilience of the Sundarbans are not initiated soon, the tigers of the Sundarbans may join the Arctic’s polar bears (Ursus maritimus) as early victims of climate change-induced habitat loss. 相似文献
38.
39.
Transport of pollution and heatout of streets into the boundary layer above is not currently understood and so fluxes cannot be quantified. Scalar concentration within the street is determined by the flux out of it and so quantifying fluxes for turbulent flow over a rough urban surface is essential. We have developed a naphthalene sublimation technique to measure transfer from a two-dimensional street canyon in a wind tunnel for the case of flow perpendicular to the street. The street was coated with naphthalene, which sublimes at room temperature, so that the vapour represented the scalar source. The transfer velocity wT relates the flux out of the canyon to the concentration within it and is shown to be linearly related to windspeed above the street. The dimensionless transfer coefficient wT/U represents the ventilation efficiency of the canyon (here, wT is a transfer velocity,U is the wind speed at the boundary-layer top). Observed values are between 1.5 and 2.7 ×10-3 and, for the case where H/W0 (ratio of buildingheight to street width), values are in the same range as estimates of transfer from a flat plate, giving confidence that the technique yields accurate values for street canyon scalar transfer. wT/U varies with aspect ratio (H/W), reaching a maximum in the wake interference regime (0.3 < H/W < 0.65). However, when upstream roughness is increased, the maximum in wT/U reduces, suggesting that street ventilation is less sensitive to H/W when the flow is in equilibrium with the urban surface. The results suggest that using naphthalene sublimation with wind-tunnel models of urban surfaces can provide a direct measure of area-averaged scalar fluxes. 相似文献
40.
R. J. Sylvester C. J. Skinner & M. J. Barlow 《Monthly notices of the Royal Astronomical Society》1998,301(4):1083-1094
Following our discovery of unidentified infrared (UIR) band emission in a number of M supergiants in h and χ Per, we have obtained 10-μm spectra of a sample of 60 galactic M supergiants. Only three new sources, V1749 Cyg, UW Aql and IRC+40 427, appear to show the UIR bands; the others show the expected silicate emission or a featureless continuum. The occurrence of UIR-band emission in M supergiants is therefore much higher in the h and χ Per cluster than in the Galaxy as a whole. Possible explanations for the origin and distribution of UIR bands in oxygen-rich supergiants are discussed. We use our spectra to derive mass-loss rates ranging from 10−8 to 10−4 M⊙ yr−1 for the new sample, based on the power emitted in the silicate feature. The relationship between mass-loss rate and luminosity for M supergiants is discussed, and correlations are explored between their mid-infrared emission properties. 相似文献