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181.
Silke Severmann Rachel A. Mills Anthony E. Fallick 《Geochimica et cosmochimica acta》2004,68(1):73-88
Samples of Fe-oxide-rich hydrothermal sediments were collected from active and inactive portions of the TransAtlantic Geotraverse (TAG) hydrothermal field on the Mid-Atlantic Ridge. Clays separated from TAG metalliferous sediments in this study all consist of Al-poor nontronite. Oxygen isotope thermometry of the clays yields formation temperatures of 54-67°C for samples from the inactive Alvin mound compared with 81-96°C for samples from the active TAG site. The latter are the highest recorded temperatures for authigenic hydrothermal clays. Sr isotope analysis of the clays from the active mound suggests that they precipitated from seawater-dominated fluids, containing less than 15% hydrothermal end-member fluid. In contrast, nontronite from the inactive Alvin mound has 87Sr/86Sr values that closely resemble that of detrital North Atlantic clays, suggesting a dominantly continental source for the Sr. Rare earth element data are consistent with a significant detrital input to the inactive site but also demonstrate the extent of hydrothermal input to the low temperature fluid. Crystallographic fractionation of the trivalent REE is apparent in the heavy REE enrichments for all nontronite samples. The inferred formation-mechanism for nontronite-rich Fe-oxyhydroxide deposits at the surface of the active mound is by direct precipitation from low temperature fluids. At the inactive Alvin site, in contrast, the deposits form during alteration of pelagic sediments by diffuse fluids and replacement of biogenic carbonate with nontronite and Fe-oxyhydroxide. These two modes of formation are both important in seafloor hydrothermal settings where clay minerals are a significant component of the hydrothermal deposit. 相似文献
182.
Edgar L Andreas Rachel E. Jordan Aleksandar P. Makshtas 《Boundary-Layer Meteorology》2005,114(2):439-460
A 4-month deployment on Ice Station Weddell (ISW) in the western Weddell Sea yielded over 2000 h of nearly continuous surface-level meteorological data, including eddy-covariance measurements of the turbulent surface fluxes of momentum, and sensible and latent heat. Those data lead to a new parameterization for the roughness length for wind speed, z0, for snow-covered sea ice that combines three regimes: an aerodynamically smooth regime, a high-wind saltation regime, and an intermediate regime between these two extremes where the macroscale or `permanent' roughness of the snow and ice determines z0. Roughness lengths for temperature, zT, computed from this data set corroborate the theoretical model that Andreas published in 1987. Roughness lengths for humidity,zQ, do not support this model as conclusively but are all, on average, within an order of magnitude of its predictions. Only rarely arezTand zQ equal to z0. These parameterizations have implications for models that treat the atmosphere-ice-ocean system. 相似文献
183.
Can observations of earthquake scaling constrain slip weakening? 总被引:4,自引:0,他引:4
184.
Banking is going through a revolution that is radically altering the way banks work and the way customers use them. It is a revolution driven by global competition and enabled by technology. Unlike the back-room developments of previous decades, contemporary restructuring is directly affecting the network of bank branches and opportunities customers have to interact with them. Today virtually all retailing banks are seeking ways of reducing their extensive and costly network of branches and experimenting with alternative means of interacting with customers. Many of the difficulties of deciding what branches to close relate to predicting the impact of such closures on remaining parts of the bank’s network. The aim of this paper is to illustrate the application of a GIS-based spatial interaction model designed to help banks make that decision. Applied in the New Zealand context, it draws attention to some of the opportunities, as well as difficulties, a spatial interaction model creates in assessing the consequences of closing branches, both for the bank itself and for the wider community. 相似文献
185.
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187.
Journal of Geographical Systems - 相似文献
188.
Journal of Geographical Systems - Although the effects of urban shrinkage on quality of life and the built environment have received a great deal of attention, the characteristics of those... 相似文献
189.
Daniel J. Mortlock Rachel L. Webster 《Monthly notices of the Royal Astronomical Society》2000,319(3):879-892
Gravitationally lensed quasars can be discovered as a by-product of galaxy redshift surveys. Lenses discovered spectroscopically in this way should require less observational effort per event than those found in dedicated lens surveys. Further, the lens galaxies should be relatively nearby, facilitating a number of detailed observations that are impossible for the more common high-redshift lenses. This is epitomized by the wide range of results that have been obtained from Q 2237+0305, which was discovered as part of the Center for Astrophysics redshift survey, and remains the only quasar lens discovered in this way. The likelihood of this survey yielding a lens is calculated to be ∼0.03, which is an order of magnitude larger than previous estimates due to two effects. First, the quasar images themselves increase the observed flux of the lens, so that lens galaxies up to a magnitude fainter than the nominal survey limit must be included in the calculation. Secondly, it is possible for lensed quasars with extremely faint deflectors to enter the survey due to the extended morphology of the multiple images. Extrapolating these results to future surveys, the 2 degree Field galaxy redshift survey should contain between 10 and 50 lenses and the Sloan Digital Sky Survey should yield between 50 and 300 lenses, depending on the cosmological model and the observing conditions. 相似文献
190.
Daniel J. Mortlock Rachel L. Webster 《Monthly notices of the Royal Astronomical Society》2000,319(3):872-878
The absence of any wide-separation gravitational lenses in the Large Bright Quasar Survey is used to place limits on the population of cluster-sized haloes in the universe, and hence constrain a number of cosmological parameters. The results agree with previous investigations in strongly ruling out the standard cold dark matter model but they are consistent with low-density universes in which the primordial fluctuation spectrum matches both cluster abundances and cosmic microwave background measurements. These conclusions are essentially independent of the cosmological constant, which is in stark contrast to the statistics of galaxy lenses. The constraints presented here are nullified if clusters have core radii of ≳10 kpc, but are free of a number of potential systematic errors, owing to the homogeneity of the data. 相似文献