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81.
This is a reply to the discussion by Camenen and Larson (Coastal Eng., 58, 2011, 131–134) of “Measurements of sheet flow transport in acceleration-skewed oscillatory flow and comparison with practical formulations” by D.A. van der A et al. (Coastal Eng. 57, 2010, 331–342). The authors of the original paper (Van der A et al., 2010) thank the discussers for their interest in and comments on the work presented in the paper. 相似文献
82.
Dominic J. Benford Michael J. Amato John C. Mather S. Harvey Moseley Jr David T. Leisawitz 《Astrophysics and Space Science》2004,294(3-4):177-212
The Single Aperture Far-InfraRed (SAFIR) Observatory’s science goals are driven by the fact that the earliest stages of almost
all phenomena in the universe are shrouded in absorption by and emission from cool dust and gas that emits strongly in the
far-infrared (40μ–200μ) and submillimeter (200μ–1 mm). In the very early universe, the warm gas of newly collapsing, unenriched
galaxies will be revealed by molecular hydrogen emission lines at these long wavelengths. High redshift quasars are found
to have substantial reservoirs of cool gas and dust, indicative of substantial metal enrichment early in the history of the
universe. As a result, even early stages of galaxy formation will show powerful far-infrared emission. The combination of
strong dust emission and large redshift (1 < z < 7) of these galaxies means that they can only be studied in the far-infrared and submillimeter. For nearby galaxies, many
of the most active galaxies in the universe appear to be those whose gaseous disks are interacting in violent collisions.
The details of these galaxies, including the effect of the central black holes that probably exist in most of them, are obscured
to shorter wavelength optical and ultraviolet observatories by the large amounts of dust in their interstellar media. Within
our own galaxy, the earliest stages of star formation, when gas and dust clouds are collapsing and the beginnings of a central
star are taking shape, can only be observed in the far-infrared and submillimeter. The cold dust that ultimately forms the
planetary systems, as well as the cool “debris” dust clouds that indicate the likelihood of planetary sized bodies around
more developed stars, can only be observed at wavelengths longward of 20μ.
Over the past several years, there has been an increasing recognition of the critical importance of the far-infrared to submillimeter
spectral region to addressing fundamental astrophysical problems, ranging from cosmological questions to understanding how
our own Solar System came into being. The development of large, far-infrared telescopes in space has become more feasible
with the combination of developments for the James Webb Space Telescope (JWST) of enabling breakthroughs in detector technology.
We have developed a preliminary but comprehensive mission concept for SAFIR, as a 10 m-class far-infrared and submillimeter
observatory that would begin development later in this decade to meet the needs outlined above. Its operating temperature
(≤4 K) and instrument complement would be optimized to reach the natural sky confusion limit in the far-infrared with diffraction-limited
performance down to at least the atmospheric cutoff, λ {>rsim} 40 {μ}. This would provide a point source sensitivity improvement of several orders of magnitude over that of the Spitzer
Space Telescope (previously SIRTF) or the Herschel Space Observatory. Additionally, it would have an angular resolution 12
times finer than that of Spitzer and three times finer than Herschel. This sensitivity and angular resolution are necessary
to perform imaging and spectroscopic studies of individual galaxies in the early universe. We have considered many aspects
of the SAFIR mission, including the telescope technology (optical design, materials, and packaging), detector needs and technologies,
cooling method and required technology developments, attitude and pointing, power systems, launch vehicle, and mission operations.
The most challenging requirements for this mission are operating temperature and aperture size of the telescope, and the development
of detector arrays. SAFIR can take advantage of much of the technology under development for JWST, but with much less stringent
requirements on optical accuracy. 相似文献
83.
Peter Convey Mark I. Stevens Dominic A. Hodgson John L. Smellie Claus-Dieter Hillenbrand David K.A. Barnes Andrew Clarke Philip J.A. Pugh Katrin Linse S. Craig Cary 《Quaternary Science Reviews》2009,28(27-28):3035-3048
The evolutionary and biogeographic history of the contemporary Antarctic terrestrial and marine biotas reveals many components of ancient origin. For large elements of the terrestrial biota, long-term isolation over timescales from hundreds of thousands to tens of millions of years, and thus persistence through multiple glacial cycles, now appears to be the norm rather than the exception. For the marine biota there are some parallels with benthic communities also including ancient components, together with an incidence of species-level endemism indicating long-term isolation on the Antarctic continental shelf. Although it has long been known that a few ice-free terrestrial locations have existed in Antarctica for up to 10–12 million years, particularly in the Dry Valleys of Victoria Land along with certain nunataks and higher regions of large mountain ranges, these do not provide potential refugia for the majority of terrestrial biota, which occur mainly in coastal and/or low-lying locations and exhibit considerable biogeographic regionalisation within the continent. Current glacial models and reconstructions do not have the spatial resolution to detect unequivocally either the number or geographical distribution of these glacial refugia, or areas of the continental shelf that have remained periodically free from ice scouring, but do provide limits for their maximum spatial extent. Recent work on the evolution of the terrestrial biota indicates that refugia were much more widespread than has been recognised and it is now clear that terrestrial biology provides novel constraints for reconstructing the past glacial history of Antarctica, and new marine biological investigations of the Antarctic shelf are starting to do likewise. 相似文献
84.
85.
Porosity and permeability in sediment mixtures 总被引:1,自引:1,他引:0
Porosity in sediments that contain a mix of coarser- and finer-grained components varies as a function of the porosity and volume fraction of each component. We considered sediment mixtures representing poorly sorted sands and gravely sands. We expanded an existing fractional-packing model for porosity to represent mixtures in which finer grains approach the size of the pores that would exist among the coarser grains alone. The model well represents the porosity measured in laboratory experiments in which grain sizes and volume fractions were systematically changed within sediment mixtures. Permeability values were determined for these sediment mixtures using a model based on grain-size statistics and the expanded fractional-packing porosity model. The permeability model well represents permeability measured in laboratory experiments using air- and water-based permeametry on the model sediment mixtures. 相似文献
86.
Structure of the footwall of a listric fault system revealed by 3D seismic data from the Niger Delta
We use three‐dimensional (3D) seismic reflection data to analyse the architecture of the footwall of a listric fault, in a gravitationally driven extensional system, in the north‐western Niger Delta. In contrast to conventional listric normal fault models with a single master listric fault plane the level of detachment switches from a deeper to shallower level. The footwall evolves through the generation of new master detachment faults and detachments, which transfers hanging wall rocks into the footwall. New detachments form by branching off pre‐existing detachment levels, cutting‐up through stratigraphy to the next mechanical weakness, separating discrete sections of extended strata. As a consequence a deeper, older array of seaward‐dipping, tilted extensional fault blocks is now located in the footwall beneath the master listric detachment fault. The structural complexity located below the master detachment fault highlights extensional episodes on separate detachment faults that are not captured in conventional listric models. We speculate that changes in the level of the detachment are caused by mechanical weaknesses controlled by lithology, pore pressure and episodes of sediment loading related to deltaic progradation. 相似文献
87.
Modeling multiscale heterogeneity and aquifer interconnectivity 总被引:2,自引:0,他引:2
A number of methods involving indicator geostatistics were combined in a methodology for characterizing and modeling multiscale heterogeneity. The methodology circumvents sources of bias common in data from borehole logs. We applied this methodology to the complex heterogeneity within a regional system of buried valley aquifers, which occurs in the western glaciated plains of North America and includes the Spiritwood Aquifer. The region is conceptualized as having a hierarchical organization with three facies assemblage types (large-scale heterogeneity) and two facies types within each assemblage (small-scale heterogeneity). We statistically characterized the sedimentary architecture at both scales, formulated indicator correlation models from those characterizations, and used the models to simulate the architecture in a multiscale realization. We focused on the interconnectivity of units creating higher-permeability pathways. Higher-permeability pathways span the realization even though the proportion of higher-permeability facies is less than the percolation threshold. Thus, geologic structures as represented in the indicator correlation models create interconnectivity above that which would occur if the higher-permeability facies were randomly placed. This amount of interconnection among higher-permeability facies within the multiscale realization is consistent with that suggested in prior hydraulic and geochemical studies of the regional system. 相似文献
88.
A Dominic Fortes, Ian G Wood and Lothar Oberauer think through the possibilities for an entirely new – and currently technologically impossible – means of examining the core of the Earth. 相似文献
89.
Dominic A. Hodgson Elie Verleyen Angela H. Squier Melanie J. Leng Wim Vyverman 《Quaternary Research》2005,64(1):83-99
Little is known about the response of terrestrial East Antarctica to climate changes during the last glacial-interglacial cycle. Here we present a continuous sediment record from a lake in the Larsemann Hills, situated on a peninsula believed to have been ice-free for at least 40,000 yr. A mutli-proxy data set including geochronology, diatoms, pigments and carbonate stable isotopes indicates warmer and wetter conditions than present in the early part of the record. We interpret this as Marine Isotope Stage 5e after application of a chronological age-depth model and similar ice core evidence. Dry and cold conditions are inferred during the last glacial, with lake-level minima, floristic changes towards a shallow water algal community, and a greater biological receipt of ultraviolet radiation. During the Last Glacial Maximum and Termination I the lake was perennially ice-covered, with minimal snowmelt in the catchment. After ca. 10,500 cal yr B.P., the lake became seasonally moated or ice-free during summer. Despite a low accumulation rate, the sediments document some Holocene environmental changes including neoglacial cooling after ca. 2450 cal yr B.P., and a gradual increase in aridity and salinity to the present. 相似文献
90.
Felix V. Kaminsky Sergei M. Sablukov Ludmila I. Sablukova Dominic M.DeR. Channer 《Lithos》2004,76(1-4):565-590
In Venezuela, kimberlites have so far only been found in the Guaniamo region, where they occur as high diamond grade sheets in massive to steeply foliated Paleoproterozoic granitoid rocks. The emplacement age of the Guaniamo kimberlites is 712±6 Ma, i.e., Neoproterozoic. The Guaniamo kimberlites contain a high abundance of mantle minerals, with greater than 30% olivine macrocrysts. The principal kimberlite indicator minerals found are pyrope garnet and chromian spinel, with the overwhelming majority of the garnets being of the peridotite association. Chrome-diopside is rare, and picroilmenite is uncommon. Chemically, the Guaniamo kimberlites are characterized by high MgO contents, with low Al2O3 and TiO2 contents and higher than average FeO and K2O contents. These rocks have above average Ni, Cr, Co, Th, Nb, Ta, Sr and LREE concentrations and very low P, Y and, particularly, Zr and Hf contents. The Nb/Zr ratio is very distinctive and is similar to that of the Aries, Australia kimberlite. The Guaniamo kimberlites are similar in petrography, mineralogy and mantle mineral content to ilmenite-free Group 2 mica kimberlites of South Africa. The Nd-Sr isotopic characteristics of Guaniamo kimberlites are distinct from both kimberlite Group 1 and Group 2, being more similar to transitional type kimberlites, and in particular to diamondiferous kimberlites of the Arkhangelsk Diamond Province, Russia. The Guaniamo kimberlites form part of a compositional spectrum between other standard kimberlite reference groups. They formed from metasomatised subcontinental lithospheric mantle and it is likely that subduction of oceanic crust was the source of this metasomatised material, and also of the eclogitic component, which is dominant in Guaniamo diamonds. 相似文献