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In order to study the structure of a chemically homogeneous star in equilibrium, a density profile of the form T
N exp(–µm(–)/kT) is suggested. As for polytropes, qualitative aspects of the resulting stellar model can be discussed analytically. In particular it is shown that one reobtains forN=3 Eddington's standard model, whereas forN<3 nearly polytropic models result. WhenN>3, the effective polytropic index does vary appreciably over the star. Numerical results indicate that the proposed density profile is quite reasonable in view of the simplicity of the model. From a comparison of the degree of precision of a polytropic approximation with that of the newly proposed model it follows that the new approximation is definitely better than the polytropic one. It is suggested that the model may be useful to study the structure of stellar clouds, clusters and (spherical) galaxies.Now at Department of Applied Mathematics, Queen Mary College, University of London, England. 相似文献
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N. Van der Putten C. Verbruggen R. Ochyra S. Spassov J.-L. de Beaulieu M. De Dapper J. Hus N. Thouveny 《Quaternary Science Reviews》2009,28(1-2):65-79
Botanical macrofossil analysis of a more than 9000 years old, radiocarbon dated peat sequence of a moss peat bank from South Georgia, shows a clear evolution in the vegetation. Seven ecological phases could be distinguished and they can be interpreted in terms of climate development during the Holocene. Until 2200 years ago, Warnstorfia fontinaliopsis was the dominant moss species pointing to a wet environment. Lower numbers of this species in association with the presence of drier species are assumed to indicate drier periods, such as occurring between ca 6000–5200 and 4400–3400 cal yr BP. The most prominent and definitive vegetation change took place around 2200 cal yr BP. A Polytrichum–Chorisodontium moss peat bank was formed, which is still growing there today. The forcing mechanism for this vegetation change is thought to be a temperature decrease, rather than a precipitation decrease. This conclusion is mainly based on the fact that, today, moss peat banks have their optimal occurrence range in the maritime Antarctic, a region were the mean annual temperature is ca 4 °C lower than on South Georgia. The remarkable change in the moss bank vegetation at 2200 cal yr BP raises the question whether this moment was only a short climatic deterioration, or a definitive change to a cooler and wetter climate after a Holocene climatic optimum period. 相似文献
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M. J. C. Walker S. J. P. Bohncke G. R. Coope M. O'Connell H. Usinger C. Verbruggen 《第四纪科学杂志》1994,9(2):109-118
A synthesis of environmental changes during the Devensian (Weichselian) Lateglacial period (14-9 ka BP) is presented for an area extending from Ireland in the west to northwest Germany in the east. Following a brief reference to the problems of chronology, the principal changes in geomorphology and soils, vegetation history and climate experienced in the region during the Late-glacial are described. Reconstructions of thermal variations during the Late-glacial period are attempted independently for Ireland, England and Wales, Scotland (Highlands and Islanads), north Belgium, The Netherlands and northwest Germany. The collective palaeotemperature data, based mainly uppoln pollen data but also in Britain and The Netherlands on coleopteran data using the ‘mutual climatic range’ approach, provide an overview of regional differences along an east-west transect in northwest Europe. 相似文献
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Macroscopic opal-A concretions were observed in lake marl deposited in a small Flemish lake (Belgium) during the Allerød biozone of the Weichselian Late-glacial (ca. 12–11 ka BP). The silica from these concretions was derived within the profile, by the leaching of siliceous microfossils – mainly diatom frustules. Formation of the concretions probably resulted from pH- and/or evaporation related precipitation of the silica at a lower stratigraphic level, presumably corresponding more or less to a former low position of the groundwater table. The presence of these concretions is probably related to alternatingly wet and dry local conditions during the middle and later part of the Allerød. 相似文献
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F. Verbruggen O. Heiri G.-J. Reichart J. W. De Leeuw K. G. J. Nierop A. F. Lotter 《Journal of Paleolimnology》2010,43(4):857-872
Stable oxygen isotope measurements on fossil chironomid head capsules from lake sediments show that these chitinous remains
can be used to reconstruct past lake water δ18O and, indirectly, past climate change. We examined the impact of chemical pretreatment procedures on the chemical and stable
oxygen isotope composition, and morphology of chironomid cuticles. Use of alkali, acids, and sodium chlorite alters the chemical
composition and the morphological structure of chironomid cuticles by selective removal of chitin or proteins. Gas chromatograms
of pyrolyzates show that NaClO2 causes deproteination, whereas the combined use of HCl and HF results in partial chitin removal. Head capsules pretreated
with KOH contained both chitin- and protein-derived moieties, although the concentration of protein was reduced, especially
after KOH treatment at high concentration (28%) and temperature (100°C). Scanning electron microscopy confirmed that a proteinaceous
matrix is still present in modern and fossil head capsules after KOH treatment. This matrix, however, is largely absent in
head capsules pretreated with NaClO2. A change in the proportion of chitin and proteins in our samples was associated with differences in chironomid δ18O values. Our results suggest that deproteination results in a relative increase of chironomid δ18O, whereas removal of chitin leads to decreased δ18O values. We therefore discourage the use of acids or prolonged (≥1 h) exposure to hot alkali (70°C) prior to chironomid δ18O analysis. Chitin purification by sodium chlorite causes significant weight loss, which may preclude down-core chironomid
δ18O measurements. Caution and standardization are required when pretreating samples for chironomid δ18O analysis to ensure reliable, comparable, and reproducible results. 相似文献
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Geochemical Fingerprint and Iron Ore Potential of the Siliceous Itabirite from Palaeoproterozoic Nyong Series,Zambi Area,Southwestern Cameroon 
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下载免费PDF全文 Sylvestre Ganno Cyriel Moudioh Adjija Nzina Nchare Gus Djibril Kouankap Nono Jean Paul Nzenti 《Resource Geology》2016,66(1):71-80
The study area forms part of an emerging iron ore province of southern Cameroon. Geochemistry analyses reveal that the siliceous itabirite has a very simple chemical composition, with Fe2O3 and SiO2 representing more than 96 wt.% of the average composition; suggesting chemical precipitates of silica and iron. Low Al2O3 and TiO2 concentrations and a weak positive correlation between them point to a minor detrital component in the precipitated marine sediments. The Si/Al ratio (average 52.7) indicates the hydrothermal origin of the studied itabirite. The Al–Si discrimination diagram supports this interpretation through the plot of all data in the hydrothermal field. The studied samples have low iron content (about 39.32% Fe), high gangue content (40.97% SiO2 and 1.3 % Al2O3) and low concentration of the deleterious elements (0.16 % P and < 0.01% S). The main gangue mineral is silica which can be efficiently removed from iron ores during preparation of raw materials for the blast furnace process. According to commercial standards for crude iron ores, it may be concluded that the Zambi iron ores are a low‐grade magnetic ore that can be profitably exploited for the production of iron for steel production. 相似文献
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Maarten van Hardenbroek Oliver Heiri Jonathan Grey Paul L. E. Bodelier Frederike Verbruggen André F. Lotter 《Journal of Paleolimnology》2010,43(2):235-245
We used a series of experiments to determine whether stable carbon isotope analysis of modern and fossil larval head capsules
of chironomids allowed identification of their dietary carbon source. Our main focus was to assess whether carbon from naturally
13C-depleted methane-oxidizing bacteria (MOB) can be traced in chironomid cuticles using stable carbon isotope analysis. We
first showed that a minimum sample weight of ~20 μg was required for our equipment to determine head capsule δ13C with a standard deviation of 0.5‰. Such a small minimum sample weight allows taxon-specific δ13C analyses at a precision sufficient to differentiate whether head capsules consist mainly of carbon derived from MOB or from
other food sources commonly encountered in lake ecosystems. We then tested the effect of different chemical pre-treatments
that are commonly used for sediment processing on δ13C measurements on head capsules. Processing with 10% KOH (2 h), 10% HCl (2 h), or 40% HF (18 h) showed no detectable effect
on δ13C, whereas a combination of boiling, accelerated solvent extraction and heavy chemical oxidation resulted in a small (0.2‰)
but statistically significant decrease in δ13C values. Using culturing experiments with MOB grown on 13C-labelled methane, we demonstrated that methanogenic carbon is transferred not only into the larval tissue, but also into
chironomid head capsules. Taxon-specific δ13C of fossil chironomid head capsules from different lake sediments was analyzed. δ13C of head capsules generally ranged from −28 to −25.8‰, but in some instances we observed δ13C values as low as −36.9 to −31.5‰, suggesting that carbon from MOB is traceable in fossil and subfossil chironomid remains.
We demonstrate that stable carbon isotope analyses of fossil chironomid head capsules can give insights into dietary links
and carbon cycling in benthic food webs in the past and that the method has the potential to reconstruct the importance of
MOB in the palaeo-diet of chironomid larvae and, indirectly, to infer past changes in methane flux at the sediment water interface
in lakes. 相似文献
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