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51.
Gerald Manners 《Geoforum》1979,10(3):323-332
This paper suggests that the major uncertainties overhanging the future geography of mineral supply in the non-Communist world derive from intellectual rather than resource inadequacies. In particular, the paucity of medium-term, macro-economic forecasts of growth in the economies of the OECD seriously undermine the value of mineral-demand forecasts. At the same time, the altered investment climate for mineral exploration and development will have a substantial impact upon mineral supplies, but its magnitude and geography remain a matter for dispute. And, third, the future pattern of mineral trade, shaped by the shifting geography of mineral-processing activities and metal manufacture, continues to elude convincing analysis. 相似文献
52.
53.
Jay E. Leonard Barry Cameron Orrin H. Pilkey Gerald M. Friedman 《Sedimentary Geology》1981,28(1):1-28
The common belief currently shared by many geoscientists concerning the climatic interpretation of limestones is that a warm-water environment is essential. This concept is not necessarily true because the rate and extent of terrigenous sediment dilution, rather than water temperature, is the primary factor determining whether or not a limestone forms at nearshore or continental shelf depths. Because carbonate productivity is lowest in cold climates, however, CaCO3 abundance and the thickness of carbonate accumulations tend to be least at high latitudes. In this regard present-day continental shelves and beaches offer a poor model for comparing cold-water and warm-water carbonates because of the generally emergent continental tectonic framework, recent eustatic sea-level changes, and the presence of ice caps at the modern poles.Typically, the influence of climate on non-reef continental shelf and beach environments cannot be clearly distinguished by the presence or absence of major taxonomic groups. Faunal diversity and equitability are more sensitive in this regard. The absence of shelf-depth inorganic carbonate precipitates, micrite envelopes, and peloids may also point to the cold-water origin of a rock. Skeletal mineralogy and oxygen isotopes of certain unrecrystallized carbonates may be good paleoclimatic indicators; however, trace elements and physical-textural attributes of the carbonate fraction are probably temperature insensitive.Previous studies of high-latitude continental shelves have concentrated merely on the abundance of calcareous material and there is seemingly a disproportionate amount of information with respect to low-latitude carbonate studies. Further research on cold-water carbonates may open up new avenues for alternative paleoenvironmental and paleoclimatic interpretations. 相似文献
54.
Enceladus: Present internal structure and differentiation by early and long-term radiogenic heating 总被引:1,自引:0,他引:1
Pre-Cassini images of Saturn's small icy moon Enceladus provided the first indication that this satellite has undergone extensive resurfacing and tectonism. Data returned by the Cassini spacecraft have proven Enceladus to be one of the most geologically dynamic bodies in the Solar System. Given that the diameter of Enceladus is only about 500 km, this is a surprising discovery and has made Enceladus an object of much interest. Determining Enceladus' interior structure is key to understanding its current activity. Here we use the mean density of Enceladus (as determined by the Cassini mission to Saturn), Cassini observations of endogenic activity on Enceladus, and numerical simulations of Enceladus' thermal evolution to infer that this satellite is most likely a differentiated body with a large rock-metal core of radius about 150 to 170 km surrounded by a liquid water-ice shell. With a silicate mass fraction of 50% or more, long-term radiogenic heating alone might melt most of the ice in a homogeneous Enceladus after about 500 Myr assuming an initial accretion temperature of about 200 K, no subsolidus convection of the ice, and either a surface temperature higher than at present or a porous, insulating surface. Short-lived radioactivity, e.g., the decay of 26Al, would melt all of the ice and differentiate Enceladus within a few million years of accretion assuming formation of Enceladus at a propitious time prior to the decay of 26Al. Long-lived radioactivity facilitates tidal heating as a source of energy for differentiation by warming the ice in Enceladus so that tidal deformation can become effective. This could explain the difference between Enceladus and Mimas. Mimas, with only a small rock fraction, has experienced relatively little long-term radiogenic heating; it has remained cold and stiff and less susceptible to tidal heating despite its proximity to Saturn and larger eccentricity than Enceladus. It is shown that the shape of Enceladus is not that of a body in hydrostatic equilibrium at its present orbital location and rotation rate. The present shape could be an equilibrium shape corresponding to a time when Enceladus was closer to Saturn and spinning more rapidly, or more likely, to a time when Enceladus was spinning more rapidly at its present orbital location. A liquid water layer on Enceladus is a possible source for the plume in the south polar region assuming the survivability of such a layer to the present. These results could place Enceladus in a category similar to the large satellites of Jupiter, with the core having a rock-metal composition similar to Io, and with a deep overlying ice shell similar to Europa and Ganymede. Indeed, the moment of inertia factor of a differentiated Enceladus, C/MR2, could be as small as that of Ganymede, about 0.31. 相似文献
55.
We have developed a finite-difference (FD) method to model the response of a multiple-spacing triaxial array induction tool with a multiple-electrode type sleeve. The FD software was verified versus two other independent modeling methods. The model response was checked for the FD grid refinement, while attempting to minimize run time. Many details of the electrode-sleeve geometry were studied using this method that led to the final electrode-sleeve design. The electrode sleeve reduces the response of the transverse couplings to eccentering in a conductive borehole. Quantitative agreement was found between the measurements and calculations. Using the FD software we study the tool response to various effects of 3D geometry. 相似文献
56.
First measurements of SO2 and SO in the Venus mesosphere (70-100 km) are reported. This altitude range is distinctly above the ∼60-70 km range to which nadir-sounding IR and UV investigations are sensitive. Since July 2004, use of ground-based sub-mm spectroscopy has yielded multiple discoveries. Abundance of each molecule varies strongly on many timescales over the entire sub-Earth Venus hemisphere. Diurnal behavior is evident, with more SO2, and less SO, at night than during the day. Non-diurnal variability is also present, with measured SO2 and SO abundances each changing by up to 2× or more between observations conducted on different dates, but at fixed phase, hence identical sub-Earth Venus local times. Change as large and rapid as a 5σ doubling of SO on a one-week timescale is seen. The sum of SO2 and SO abundances varies by an order of magnitude or more, indicating at least one additional sulfur reservoir must be present, and that it must function as both a sink and source for these molecules. The ratio SO2/SO varies by nearly two orders of magnitude, with both diurnal and non-diurnal components. In contrast to the strong time dependence of molecular abundances, their altitude distributions are temporally invariant, with far more SO2 and SO at 85-100 km than at 70-85 km. The observed increase of SO2 mixing ratio with altitude requires that the primary SO2 source be upper mesospheric photochemistry, contrary to atmospheric models which assert upward transport as the only source of above-cloud SO2. Abundance of upper mesospheric aerosol, with assumption that it is composed primarily of sulfuric acid, is at least sufficient to provide the maximum gas phase (SO + SO2) sulfur reported in this study. Sulfate aerosol is thus a plausible source of upper mesospheric SO2. 相似文献
57.
Stephen Edward Lewis Robert A. Henderson Gerald R. Dickens Graham A. Shields Simon Coxhell 《Mineralium Deposita》2010,45(6):599-620
A significant resource of vanadium and molybdenum exists near Julia Creek, Australia, where the middle Cretaceous organic-rich
Toolebuc Formation lies between 0 and 25 m of the surface. We present and discuss a comprehensive geochemical study of the
Toolebuc Formation and its enclosing stratigraphy near Julia Creek to understand this ore deposit. V and Mo contents in fresh
facies are strongly associated with total organic carbon (TOC) contents, but not with Al or CaCO3; this suggests that V and Mo were originally concentrated in the organic fraction. However, chemical extractions using H2O2 indicate that Mo was originally concentrated in pyrite. The data also suggest that V was mobilised from organic matter during
early diagenesis and became associated with clays as little V was extracted by H2O2 in the fresh samples. TOC contents in the Toolebuc Formation were removed during weathering, residually enriching trace metals
including V and Mo, and as a result, the TOC relationship with V and Mo disintegrates. With weathering, both V and Mo predominantly
became associated with iron oxide/hydroxide phases (and possibly other unidentified phases) as these elements in the weathered
facies were highly soluble in the sodium citrate–sodium dithionite digestion. Large shale-hosted V and Mo deposits such as
Julia Creek offer a potentially viable alternative to the currently mined magnetite-hosted deposits. A thorough understanding
of the formation and host mineral phases for V and Mo of these shale deposits, however, is critical to ensure that these valuable
metals can be feasibly extracted. 相似文献
58.
Roberto Cossu Monique Petitdidier Julian Linford Vincent Badoux Luigi Fusco Benoit Gotab Ladislav Hluchy Giuditta Lecca Fabrizio Murgia Camiel Plevier Philippe Renard Horst Schwichtenberg Wim Som de Cerff Viet Tran Gerald Vetois 《Earth Science Informatics》2010,3(3):135-148
Due to its intensive data processing and highly distributed organization, the multidisciplinary Earth Science applications community is uniquely positioned for the uptake and exploitation of Grid technologies. Currently Enabling Grids for E-sciencE, and other large Grid infrastructures are already deployed and capable of operational services. So far however, the adoption and exploitation of Grid technology throughout the Earth Science community has been slower than expected. The Dissemination and Exploitation of GRids in Earth sciencE project, proposed by the European Commission to assist and accelerate this process in a number of different ways, had between its main goals the creation of a roadmap towards Earth Science Grid platform. This paper presents the resulting roadmap. 相似文献
59.
Wassim E. Tabet Amy B. Cerato Gerald A. Miller 《Geotechnical and Geological Engineering》2014,32(5):1253-1260
This paper presents results of the influence of clod size and initial moisture condition on the shearing behavior of a clayey soil with a plasticity index of 22. The clods were divided into different size categories, and then two groups of samples were prepared; samples with large clods (LC = clod size less than 38 mm) and samples with small clods (SC = clod size less than 4.75 mm). Two initial moisture conditions were achieved using two different moisture tempering times of 0 and 14 days. All samples were compacted to the same density and moisture content (95 % of maximum dry density and 2 % dry of optimum moisture content). Triaxial compression tests, including constant water content unsaturated tests and backpressure saturated, isotropically consolidated, undrained compression tests were performed at different confining pressures. At 0-day tempering time, samples prepared using large clods were found to be stiffer and stronger than those prepared using small clods. However, at 14-day tempering time, the strength of both LC and SC samples was similar. 相似文献
60.
Relative to those at sub‐bankfull flow, hydraulic conditions at overbank flow, whether in the channel or on the floodplain, are poorly understood. Here, velocity conditions are analysed over an unusually wide range of flows in the arid zone river of Cooper Creek with its complex system of anastomosing channels and large fluctuations in floodplain width. At‐a‐station hydraulic geometry relationships reveal sharp discontinuities in velocity at the inbank–overbank transition, the nature of the discontinuity varying with the degree of flow confinement and the level of channel–floodplain interaction. However, despite inter‐sectional differences, velocities remain modest throughout the flow range in this low‐gradient river, and the large increases in at‐a‐station discharge are principally accommodated by changes in cross‐sectional area. Velocity distribution plots suggest that within‐channel conditions during overbank flow are characterized by a central band of high velocity which penetrates far toward the bed, helping to maintain already deep cross‐sections. Floodplain resistance along Cooper Creek is concentrated at channel bank tops where vegetation density is highest, and the subsequent flow retardation is transmitted across the surface of the channels over distances as large as 50–70 m. The rough floodplain surface affects flood wave transmission, producing significant decreases in wave speeds downstream. The character of the wave‐speed–discharge relationship also changes longitudinally, from log–linear in the upper reaches to nonlinear where the floodplain broadens appreciably. The nonlinear form is similar in several respects to relationships proposed for more humid rivers, with flood wave speed reaching an intermediate maximum at about four‐fifths bankfull discharge before decreasing to a minimum at approximately Q2·33. It does not regain the value at the intermediate maximum until the 10 year flood, by which time floodplain depths have become relatively large and broad floodways more active. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献