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排序方式: 共有131条查询结果,搜索用时 312 毫秒
1.
J. van der Plicht B. van Geel S. J. P. Bohncke J. A. A. Bos M. Blaauw A. O. M. Speranza R. Muscheler S. Bjrck 《第四纪科学杂志》2004,19(3):263-269
Accurate chronologies are essential for linking palaeoclimate archives. Carbon‐14 wiggle‐match dating was used to produce an accurate chronology for part of an early Holocene peat sequence from the Borchert (The Netherlands). Following the Younger Dryas–Preboreal transition, two climatic shifts could be inferred. Around 11 400 cal. yr BP the expansion of birch (Betula) forest was interrupted by a dry continental phase with dominantly open grassland vegetation, coeval with the PBO (Preboreal Oscillation), as observed in the GRIP ice core. At 11 250 cal. yr BP a sudden shift to a humid climate occurred. This second change appears to be contemporaneous with: (i) a sharp increase of atmospheric 14C; (ii) a temporary decline of atmospheric CO2; and (iii) an increase in the GRIP 10Be flux. The close correspondence with excursions of cosmogenic nuclides points to a decline in solar activity, which may have forced the changes in climate and vegetation at around 11 250 cal. yr BP. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
2.
The construction of reservoir models is frustrated by the fact that core and well cover only a fraction of the reservoir volume and it is therefore difficult to determine features like facies shape, -size, and -distribution, inter- and intra-facies boundaries and lateral trends from them. These features are, however, critical to fluid flow and they should necessarily be incorporated in the reservoir model and we therefore propose to systematically describe geometry and distribution of facies. To this end we make use of “standard facies models” that a priori contain all elements and boundaries of facies for a number of typical depositional environments. 相似文献
3.
The measurement of the signal to noise ratio in continuous seismic profiling systems is carried out by means of a small general purpose digital computer equipped with an analog to digital converter. The computer is programmed to compute and compare the power spectrum of a sample of noise to the power spectrum of a sample of noise plus an echo. Power spectra of some twenty to fifty shots are averaged to yield reliable estimates of the signal to noise ratio as a function of frequency. The observed spectra agree well with spectra calculated from signals measured near to an air-gun. This measurement of signal to noise ratio as a function of frequency provides unambiguous information for the adjustment of the necessary band-pass filter thus improving the operation of a continuous seismic profiler. 相似文献
4.
Bas van Geel Daniel C. Fisher Adam N. Rountrey Jan van Arkel Joost F. Duivenvoorden Aline M. Nieman Guido B.A. van Reenen Alexei N. Tikhonov Bernard Buigues Barbara Gravendeel 《Quaternary Science Reviews》2011,30(27-28):3935-3946
Intestinal samples from the one-month-old Siberian mammoth calf ‘Lyuba’ were studied using light microscopy and ancient DNA to reconstruct its palaeo-environment and diet. The palynological record indicates a ‘mammoth steppe’. At least some pollen of arboreal taxa was reworked, and thus the presence of trees on the landscape is uncertain. In addition to visual comparison of 11 microfossil spectra, a PCA analysis contributed to diet reconstruction. This yielded two clusters: one of samples from the small intestine and the other of large-intestine samples, indicating compositional differences in food remains along the intestinal tract, possibly reflecting different episodes of ingestion. Based on observed morphological damage we conclude that the cyperaceous plant remains and some remains of dwarf willows were originally eaten by a mature mammoth, most likely Lyuba’s mother. The mammoth calf probably unintentionally swallowed well-preserved mosses and mineral particles while eating fecal material deposited on a soil surface covered with mosses. Coprophagy may have been a common habit for mammoths, and we therefore propose that fecal material should not be used to infer season of death of mammoths. DNA sequences of trnL and rbcL genes amplified from ancient DNA extracted from intestinal samples confirmed and supplemented plant identifications based on microfossils and macro-remains. Results from different extraction methods and barcoding markers complemented each other and show the value of longer protocols in addition to fast and commercially available extraction kits. 相似文献
5.
Experimental material was obtained supporting a hypothesis on the interaction between p- and o-diphenol toxicity and their oxidation. It is shown that quinones – the products of pyrocatechol and hydroquinone oxidation – possess increased toxicity towards luminous bacteria cells. 相似文献
6.
7.
Kees Wapenaar 《Geophysical Journal International》1998,135(2):585-594
The derivation of seismic reflection and transmission coefficients is generally based on the assumption that the medium parameters behave as step functions of depth, at least in a finite region around the interface. However, outliers observed in well logs generally behave quite differently from step functions. In this paper we represent an interface by a self-similar singularity, embedded between two homogeneous half-spaces, and we derive its frequency-dependent normal-incidence reflection and transmission coefficients. For ω → 0 the expressions for the coefficients reduce to those for a discrete boundary between two homogeneous half-spaces; for ω → ∞ they become frequency-independent. These asymptotic expressions have a relatively simple form and depend on the singularity exponent α .
The exact as well as the asymptotic expressions are used to evaluate the time-domain reflection and transmission responses of a self-similar interface. Finally, we use a numerical method to model the response of a smoothed version of a self-similar interface (note that the velocity of a smoothed singularity remains finite). It turns out that smoothing has hardly any effect on the response, provided that the smoothing does not affect the scales corresponding to the seismic frequency range. 相似文献
The exact as well as the asymptotic expressions are used to evaluate the time-domain reflection and transmission responses of a self-similar interface. Finally, we use a numerical method to model the response of a smoothed version of a self-similar interface (note that the velocity of a smoothed singularity remains finite). It turns out that smoothing has hardly any effect on the response, provided that the smoothing does not affect the scales corresponding to the seismic frequency range. 相似文献
8.
Surf Scoters and Greater Scaup and samples of species which comprise the major part of their diets, from Iona Island and Roberts Bank tidal flats, near Vancouver, British Columbia, were analysed for metal residues. Significantly higher silver, copper, lead and zinc levels were found in Greater Scaup than Surf Scoters. The latter species had higher mercury levels. Differences in residue levels between the species reflect interspecific variation in prey as well as feeding habitat. Significantly higher silver, mercury, lead and zinc in Surf Scoters and silver and lead in Greater Scaup observed at Iona Island than at Roberts Bank can be explained by higher levels of those metals occurring in a more metal-contaminated prey and sediment near the Iona Island sewer outfall. Ducks feeding on organisms in different intertidal zones show different accumulation of metal residues and constitute valuable indicators of pathways of toxic chemicals in specific estuarine regions. 相似文献
9.
Cross-country comparison of DDE and PCB residue levels in cormorant, gull and tern eggs in Canada reveal that bird populations at the Great Lakes are most contaminated with those pollutants. DDE levels have been correlated with reproductive failure in Double-crested Cormorants in the Great Lakes with eggshell thinning as a major factor. Low reproductive success in Herring Gull colonies at Lake Ontario is associated with high chlorinated hydrocarbon levels in eggs. Fish-eating birds in the Wabigoon River system, northwestern Ontario, are among the most known mercury contaminated birds. It is suggested that the effects of mercury on the reproduction of fish-eating birds should be further examined there.Fish-eating birds occupy the highest levels of the food web and magnification of toxic chemicals through prey organisms in this web makes those birds vulnerable to the effects of environmental contaminants. Since fish-eating birds are present everywhere in Canada's freshwater and marine habitats and occupy various niches there, they may serve as pollution indicators in various food chains of our aquatic environment. Colonial birds are especially valuable indicators as pollution effects on total bird populations can be studied. Baseline information on fish-eating bird populations should now be collected everywhere in Canada for measuring present and future effects of environmental pollutants, as well as other man-made disturbances on their populations. 相似文献
10.