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991.
Anke Putschew Christine Schaeffer-Reiss Philippe Schaeffer Martin P Koopmans Jan W de Leeuw Michael D Lewan Jaap S Sinninghe Damsté James R Maxwell 《Organic Geochemistry》1998,29(8):1875-1890
An immature sulfur-rich marl from the Gessosso-solfifera Formation of the Vena del Gesso Basin (Messinian, Italy) has been subjected to hydrous pyrolysis (160 to 330°C) to simulate maturation under natural conditions. The kerogen of the unheated and heated samples was isolated and the hydrocarbons released by selective chemical degradation (Li/EtNH2 and HI/LiAlH4) were analysed to allow a study of the fate of sulfur- and oxygen-bound species with increasing temperature. The residues from the chemical treatments were also subjected to pyrolysis–GC to follow structural changes in the kerogens. In general, with increasing hydrous pyrolysis temperature, the amounts of sulfide- and ether-bound components in the kerogen decreased significantly. At the temperature at which the generation of expelled oil began (260°C), almost all of the bound components initially present in the unheated sample were released from the kerogen. Comparison with an earlier study of the extractable organic matter using a similar approach and the same samples provides molecular evidence that, with increasing maturation, solvent-soluble macromolecular material was initially released from the kerogen, notably as a result of thermal cleavage of weak carbon–heteroatom bonds (sulfide, ester, ether) even at temperatures as low as 220°C. This solvent-soluble macromolecular material then underwent thermal cleavage to generate hydrocarbons at higher temperatures. This early generation of bitumen may explain the presence of unusually high amounts of extractable organic matter of macromolecular nature in very immature S-rich sediments. 相似文献
992.
The main driving force behind Al/Si ordering in tetrahedral framework aluminosilicates is nearest-neighbour Al/Al avoidance.
Computer simulation is used to explore the direct consequences of such Al/Al avoidance. The main result is that the order-disorder
transition temperature T
c
falls dramatically as the concentration x of Al in the structure is reduced, and if the only interactions are those associated with nearest-neighbour Al/Al avoidance,
T
c
becomes zero for x less than some critical value x
c
, where x
c
=0.31 for the feldspar framework and x
c
=0.34 for cordierite. Also a large degree of short range order is found above T
c
. Both results differ radically from the standard Bragg-Williams model. Plots of entropy and enthalpy of ordering are given
as functions of x and T, which may be used to interpret experimental data or for extrapolation into ranges of x and T inaccessible to experiment.
Received: 14 May 1997 / Revised, accepted: 2 June 1997 相似文献
993.
Martin Beech 《Monthly notices of the Royal Astronomical Society》1998,294(2):259-264
This study is motivated by the possibility of determining the large-body meteoroid flux at the orbit of Venus. Towards this end, we attempt to estimate the times at which enhanced meteoric activity might be observed in the planet's atmosphere. While a number of meteoroid streams are identified as satisfying common Earth and Venus intercept conditions, it is not clear from the Earth-observed data if these streams contain large-body meteoroids. A subset of the Taurid Complex objects may produce fireball-rich meteor showers on Venus. A total of 11 short-period, periodic comets and 46 near-Earth asteroids approach the orbit of Venus to within 0.1 au, and these objects may have associated meteoroid streams. Comets 27P/Crommelin and 7P/Pons–Winnecke are identified as candidate parents to possible periodic meteor showers at the orbit of Venus. 相似文献
994.
D'Amico Stappers Bailes Martin Bell Lyne & Manchester 《Monthly notices of the Royal Astronomical Society》1998,297(1):28-40
The Parkes survey of the entire southern sky for millisecond and other low-luminosity pulsars has now been completed. The survey system and initial results were described by Manchester et al. and the final results and population studies are described by Lyne et al. A total of 298 pulsars were detected, including 101 new discoveries of which 17 were millisecond pulsars. Here we report on timing observations at Parkes and Jodrell Bank of the 84 long-period (non-millisecond) pulsars discovered in the survey, including timing solutions for 78 of them. Pulse profiles and arrival times were obtained at several different frequencies over intervals of more than a year, yielding a position, period, period derivative and dispersion measure for each pulsar. Pulse profiles at frequencies near 400 MHz and 600 or 1400 MHz are presented for most of the observed pulsars. Significant timing noise was detected for five pulsars and a glitch was observed in the period of PSR J1123–6259. 相似文献
995.
Richard I. Davies Hajime Sugai & Martin J. Ward 《Monthly notices of the Royal Astronomical Society》1998,300(2):388-396
We present near-infrared emission-line images of the circumnuclear ring in NGC 1068. We have measured the Brγ fluxes in a number of star-forming complexes and derived the extinction for each of these by comparison with Hα. We investigate the star-forming histories of these regions and find that a short burst of star formation occurred coevally throughout the ring within the last 30–40 Myr, and perhaps as recently as 4–7 Myr ago. The 1–0 S(1) flux and S(1)/Brγ ratios indicate that as well as fluorescence, shock-excited H2 emission contributes to the total flux. There is excess H2 flux to the north-west where the ionization cone crosses the ring, and we show that it is possible that the non-stellar continuum from the Seyfert nucleus which produces the high-excitation lines could also be causing fluorescence at the edges of molecular clouds in the ring. The nuclear 1–0 S(1) is more extended than previously realized but only along the bar's major axis, and we consider mechanisms for its excitation. 相似文献
996.
997.
Summary This paper addresses the digital dissemination of geographically referenced census information in the UK. A number of important weaknesses in the 1991 model of data access are identified, and the possibility of future access to census information via the World Wide Web is then addressed in detail. Two case studies demonstrate the potential to overcome some fundamental weaknesses in earlier access models, including the provision of integrated data and metadata, graphical interfaces to geographical datasets, and an integrated interface and analysis environment. 相似文献
998.
Climate change, tree species distributions and forest dynamics: A case study in the mixed conifer/northern hardwoods zone of northern Europe 总被引:10,自引:0,他引:10
Three bioclimate variables (growing degree days after budburst, temperature of the coldest month and a moisture index) are used in a model (STASH) to predict potential range limits of north European tree species. CO2-induced climate warming scenarios cause major changes in these limits. The dominant conifers of the mixed conifer/northern hardwoods zone, Picea abies and Pinus sylvestris, retreat from the south and west while Fagus sylvatica and other temperate hardwoods spread to the north. A gap model (FORSKA 2) is used to simulate the associated transient responses of a forest reserve within this zone. In both dry Pinus- and moist Picea- dominated forest communities, Picea continues to increase while Pinus sooner or later declines. These changes are continuing successional (non-climatic) responses to the cessation of disturbance 150 years ago. Climate warming speeds up the succession, and allows Fagus to establish and increase. The eventual decline of Picea however is delayed due to the persistence of old-growth stands. Picea forests subject to continuing disturbance show a more rapid shift to dominance by Fagus and other temperate hardwoods. Delayed immigration of new species, including Fagus, would favour earlysuccessional species such as Betula pendula and Quercus spp. in a forest with reduced biomass and diversity. The results emphasise the complex and sometimes counter-intuitive nature of transient responses, and the importance of considering disturbance history and potential migration rates when predicting the impacts of rapid climate change on forests. 相似文献
999.
Environmental variables and their association with faecal coliform and streptococci densities were investigated at a number of Sydney's metropolitan beaches. High variability in faecal coliform and faecal streptococci densities was observed at most beaches. The ability of the various models to explain the observed variation in faecal coliform or streptococci levels was relatively low, although they did provide an insight into the more influential environmental factors involved. Rainfall has the single most important effect on faecal coliform and streptococci densities at Sydney beaches. Whether this is due to increased stormwater runoff at the beaches, increased discharge through the outfalls (both the deepwater ocean outfalls and the few remaining shoreline outfalls), or a combination of both is unclear and is likely to depend on the individual beach being considered. Elevated levels of faecal coliforms and streptococci are still occasionally observed at some beaches, but these levels are usually not as high nor as frequent as they were prior to commissioning of the deepwater outfalls. The significant associations of current and plume entrapment with faecal coliform and streptococci densities at some Sydney beaches suggest that under certain conditions, effluent from the deepwater ocean outfalls can still influence faecal coliform and streptococci densities at these beaches. 相似文献
1000.
Erwan Hallot Philippe Davy Jean de Bremond d'Ars Bernard Auvray Hervé Martin Henri Van Damme 《Journal of Volcanology and Geothermal Research》1996,71(1):31-44
Injection of Newtonian crystal-free magmas into a partially crystallised host which may exhibit non-Newtonian properties produces magmatic structures such as pipes, syn-plutonic dikes or dendritic structures. Field relationships between the structure and the host rock commonly indicate what the rheological contrasts during the injection were. The manner in which a magma deforms in response to injection is mainly linked to crystal content and strain rate (i.e., injection rate). Three kinds of behaviour can be distinguished: (1) Newtonian at low crystal contents; (2) Non-Newtonian at intermediate (40–60%) crystal contents, or at high crystal contents if the strain rate is small; and (3) brittle failure at high crystal content or strain rates.Petrologic observations indicate that injection can take place when the host magma still behaves as a fluid. To investigate the physics of the injection process we review the results of injection experiments in non-Newtonian fluids. These experiments were performed to study viscous fingering in 2-D Hele Shaw cells. They provide the first step to establishing the main non-Newtonian effects during the formation of interfacial instabilities arising when a Newtonian fluid is injected into a more viscous fluid or paste. The qualitative comparison of the morphological features of the interfaces between the fluids in the experiments with those in nature suggests that, in magmas, irregularities of the interfaces (dikes and dendrites) result from non-Newtonian properties of the host. We conclude that fluid-like deformation, rather than brittle behaviour of the host, during injection is likely to produce the general features observed on the field. Cooling effects might be responsible for the widespread phenomenon of fragmentation. We emphasise that the main effect of non-Newtonian properties in partially crystallised magmas is to generate strongly heterogeneous media producing discontinuities which could explain the main morphological features of syn-plutonic injection structures. 相似文献