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421.
Previous studies confirm that sand can be entrained at the wind velocities recorded on Venus. Present results describe bedforms produced in the Venus Wind Tunnel (VWT) simulating the average Venusian environment. Even at the low wind speeds measured on Venus, dunelike structures form in fine-grained quartz sands (particles 50–200 μm in diameter). The dunelike structures, referred to as microdunes, are considered to be true dunes analogous to those on Earth because they have (1) slip faces, (2) a lack of particle-size sorting, (3) a low ratio of saltation path length to dune length, and (4) internal cross-bedding. The microdunes typically produced in the VWT are 9 cm long and 0.75 cm high. It is proposed that there may be fields of microdunes on Venus that are capable of very fast rates of migration and that they may grow into features much larger than those observed in the VWT. However, neither dunes nor other types of features develop above a wind speed of ~ 1.5 m/sec; at this wind speed, the bed is flat and featureless. Thus, it is predicted that relatively short periods of higher winds may destroy microdunes and other small bedforms which could account for the sparsity of definitive aeolian features observed in Venera images. Some apparent cross-bedding observed in Venera images, however, could represent preserved aeolian structures. 相似文献
422.
The source of metasomatic fluids in iron-oxide–copper–gold districts is contentious with models for magmatic and other fluid sources having been proposed. For this study, δ
18O and δ
13C ratios were measured from carbonate mineral separates in the Proterozoic eastern Mt Isa Block of Northwest Queensland, Australia. Isotopic analyses are supported by petrography, mineral chemistry and cathodoluminescence imagery. Marine meta-carbonate rocks (ca. 20.5‰ δ
18O and 0.5‰ δ
13C calcite) and graphitic meta-sedimentary rocks (ca. 14‰ δ
18O and −18‰ δ
13C calcite) are the main supracrustal reservoirs of carbon and oxygen in the district. The isotopic ratios for calcite from the cores of Na–(Ca) alteration systems strongly cluster around 11‰ δ
18O and −7‰ δ
13C, with shifts towards higher δ
18O values and higher and lower δ
13C values, reflecting interaction with different hostrocks. Na–(Ca)-rich assemblages are out of isotopic equilibrium with their metamorphic hostrocks, and isotopic values are consistent with fluids derived from or equilibrated with igneous rocks. However, igneous rocks in the eastern Mt Isa Block contain negligible carbon and are incapable of buffering the δ
13C signatures of CO2-rich metasomatic fluids associated with Na–(Ca) alteration. In contrast, plutons in the eastern Mt Isa Block have been documented as having exsolved saline CO2-rich fluids and represent the most probable fluid source for Na–(Ca) alteration. Intrusion-proximal, skarn-like Cu–Au orebodies that lack significant K and Fe enrichment (e.g. Mt Elliott) display isotopic ratios that cluster around values of 11‰ δ
18O and −7‰ δ
13C (calcite), indicating an isotopically similar fluid source as for Na–(Ca) alteration and that significant fluid–wallrock interaction was not required in the genesis of these deposits. In contrast, K- and Fe-rich, intrusion-distal deposits (e.g. Ernest Henry) record significant shifts in δ
18O and δ
13C towards values characteristic of the broader hostrocks to the deposits, reflecting fluid–wallrock equilibration before mineralisation. Low temperature, low salinity, low δ
18O (<10‰ calcite) and CO2-poor fluids are documented in retrograde metasomatic assemblages, but these fluids are paragenetically late and have not contributed significantly to the mass budgets of Cu–Au mineralisation. 相似文献
423.
Information technology changes corporate office activity 总被引:1,自引:0,他引:1
Corporate office activity is concentrated close to head offices in South Eastern England. It is often argued that information technology could change the location of offices by reducing the cost of provincial workplaces. The reorganisation of office functions associated with the introduction of information technology could also provide an opportunity to decentralise administrative structures.Two case studies in offices in manufacturing industry suggest that information technology can in fact be used to support either the centralisation or decentralisation of office work. The impact of the technology varies between corporate structures. In Unilever, a decentralised organisation, information technology has encouraged the decentralisation of routine administrative functions. In the more tightly-controlled Ford accounting function the mechanisation of accounts has been associated with the centralisation of administration at key sites. In both corporate structures it seems that information technology is being used to centralise important management functions. 相似文献
424.
Solubilities of amorphous silica in several aqueous electrolyte solutions up to 300°C (Marshall, 1980a; Chen and Marshall, 1982) fitted the Setchénow equation, as described earlier (Marshall, 1980b) where s0 and s are molal solubilities of silica in pure water and salt solution, respectively, m is the molality of salt, and D is a proportionality constant related to the particular salt and temperature. It is now shown that, to a first approximation, the D parameters for various salts at the same temperature are additive. For instance, D(NaCl) ? D(KCl) = D(NaNO3) ? D(KNO3) or D(MgSO4) = D(MgCl2) + D(Na2S04) ? 2D(NaCl). It also follows that .This additivity principle was used to estimate amorphous silica solubilities in mixed NaCl-Na2SO4, NaCl-MgCl2, NaCl-MgSO4, Na2SO4-MgCl2, Na2SO4-MgSO4, and MgCl2-MgSO4 aqueous solutions up to 300°C. The method produces results that agree reasonably well with experimental values and would be useful for predicting silica solubilities, for example, in seawater and its hydrothermal concentrates and in geothermal energy applications. 相似文献
425.
Residual hydrocarbons were extracted from a number of vertebrate fossils and their surrounding sediments collected at six sites in the Plio-Pleistocene sediments of paleo-Lake Turkana in northern Kenya. Gas chromatographic separation of these alkane fractions showed three recognizable suites of hydrocarbons, a C-15 to C-19 group derived from algae, a C-20 to C-30 group with no odd/ even preference derived from microbial action on soil organics, and a C-21 to C-35 group with high odd/even preference derived from plant waxes. No alkanes identifiable as being derived from the original animal fats were observed. The plant wax alkanes and microbial hydrocarbons were nearly ubiquitous, in sands, silts, clays and the fossils. The algal hydrocarbons, on the other hand, were found only in the sands and silts. There were no consistent differences between the hydrocarbon content of the fossils and the surrounding sediments. There were great local variations in alkane distribution, but no patterns were discernible. The total hydrocarbon content of these arid region sediments is almost vanishingly small compared to sediments in more temperate climates. 相似文献
426.
Tropical cyclone track and intensity prediction: The generation and assimilation of high-density, satellite-derived data 总被引:1,自引:0,他引:1
J. F. Le Marshall L. M. Leslie R. F. Abbey Jr. L. Qi 《Meteorology and Atmospheric Physics》2002,80(1-4):43-57
Summary The impact of recent scientific and technological advances in tropical cyclone track, intensity and structure modeling is
discussed. Since the early 1990s, developments have occurred in remote sensing, data assimilation procedures, numerical models
and high performance computing. In particular, there is now quasi-continuous high spatial and temporal resolution data coverage
over the previously data-sparse oceans where tropical cyclones spend most of their life cycles. There has been a rapid development
of data assimilation methodologies capable of using these data to initialize high-resolution prediction models. Model developments
have reached a stage of maturity where the representation of many of the physical processes necessary for improved tropical
cyclone track and intensity prediction are now included. Finally, available computer power has reached the teraflop range.
Most operational centers have high performance computers capable of tropical cyclone modeling at resolutions necessary for
skillful track and intensity simulations.
This article focuses on combining all of the above developments in a tropical cyclone data analysis and prediction system.
The system has produced statistically significant reductions in the mean forecast error statistics for tropical cyclone track
predictions and resulted in far more realistic simulations of tropical cyclone intensity and structure. A large number of
tropical cyclones have been modeled, with emphasis on those classified as being “difficult” storms to predict accurately.
These difficult storms are most responsible for rapidly growing forecast errors. Our results are illustrated by case studies
of such tropical cyclones.
Received October 9, 2001 Revised December 28, 2001 相似文献
427.
Mineralogical, petrological and geochemical analyses of corals and associated skeletal limestones taken from three transects across the Late Quaternary raised coral reefs of the Huon Peninsula, Papua New Guinea, show that tectonic uplift can be related to the degree of subaerial diagenesis of the reefs. Where the uplift rate is high, Pleistocene corals frequently retain their aragonite mineralogy, even though the annual rainfall is relatively high. In contrast, similar age corals from low‐uplift areas are consistently altered neomorphically to calcite. The transformation of reef skeletal limestones shows a similar, but less pronounced, trend to the corals. Chemical analysis shows that the neomorphic calcite crystals of coral skeletons from the low‐uplift areas have relatively higher Sr and Mg concentrations, compared with those in high‐uplift areas. This may indicate that neomorphism of corals in the low‐uplift terraces takes place at a relatively higher rate and an earlier stage than that in the high‐uplift areas. The pattern of diagenesis of the Huon reefs can be explained by the effects of tectonic uplift on the regional hydrological regime. First, lower uplift rates allow a raised reef or any part of it to remain in the meteoric phreatic zone for a relatively long time. Second, river gorge slopes from low‐uplift regions have lower gradients and reef terraces in these areas have more extensive raised lagoon depressions than in high‐uplift areas. Thus, there is less runoff and consequently more extensive vadose percolation in the former areas. Third, the resulting low‐relief topography in low‐uplift areas prompts formation of soils on the terraces, and further increases the ability of interaction between coral reefs and formation water. 相似文献
428.
Patrick J. Brenchley Jim D. Marshall David A. T. Harper Caroline J. Buttler Charlie J. Underwood 《Geological Journal》2006,41(1):1-22
The growth and decay of the end‐Ordovician Gondwanan glaciation is globally reflected by facies changes in sedimentary sequences, which record a major eustatic fall and subsequent rise in the Hirnantian Stage at the end of the Ordovician. However, there are different reported estimates of the magnitude and pattern of sea‐level change. Particularly good evidence for end‐Ordovician sea‐level change comes from a sequence at Meifod in central Wales, which has a karstified limestone unit within a channel incised into marine shelf sediments. Pre‐glacial (Rawtheyan) mudstones have a diverse fauna suggesting a mid‐to‐deep‐shelf water depth of c. 60 m. The channel, 20 m deep, was incised into these mudstones and partially filled with a mixture of fine sand and detrital carbonate. The taphonomy of bioclasts and intraclasts indicates that many had a long residence time on the sea floor or suffered diagenesis after shallow burial before being resedimented into the channel. The presence of carbonates on the Welsh shelf is atypical and they are interpreted as having accumulated as patches during a minor regression prior to the main glacio‐eustatic fall. Comparison of the carbon stable‐isotopic values of the bioclast material with the global isotopic record confirms that most of the material is of Rawtheyan age, but that some is Hirnantian. The resedimented carbonates lithified rapidly and formed a limestone, several metres thick, in the deepest parts of the channel. As sea‐level fell, this limestone was exposed and eroded into karstic domes and pillars with a relief of over 2 m. The overall, glacio‐eustatic, sea‐level fall is estimated to be in excess of 80 m. A succeeding sea‐level rise estimated to be 40–50 m is recorded in the laminated crust that mantles the karstic domes and pillars. The crust is formed of encrusting bryozoans, associated cystoids, crinoid holdfasts and clusters of the brachiopod Paromalomena, which is normally associated with mid‐shelf environments. Fine sands buried the karst topography and accumulated to fill the channel. In the sandstones at the base of the channel there is a Hirnantia fauna, while in the sandstones high in the channel‐sequence there is cross‐stratification characteristic of mid‐shoreface environments. This would indicate a fall of sea‐level of c. 30 m. The subsequent major transgression marking the end of the glaciation is not recorded at the Meifod locality, but nearby exposures of mudstones suggest a return to mid‐to‐deep‐shelf environments, similar to those that prevailed before the Hirnantian regression. The Meifod sequence provides strong evidence for the magnitude of the Hirnantian sea‐level changes and by implication confirm larger estimates for the size of the ice sheets. Smaller oscillations in relative sea‐level seen at Meifod may be local phenomena or may reflect eustatic changes that have not been widely reported elsewhere. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献