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The Middle Atlantic Shelf of North America is a broad sand plain, characterized by a subdued ridge and swale topography. Some ridges extend into or merge with the shore face. 相似文献
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This paper examines the distribution of unconsolidated sediment in the KwaZulu-Natal Bight located along the east coast of South Africa. Results show that there is a general shelf-wide sediment distribution of coarser grain sizes between depths of 60 and 100?m, punctuated by a broad swathe of mud offshore of the Thukela River. Seasonal changes in sediment distribution patterns are small, being restricted to seaward fining on the inner shelf off the fluvial sources. Sediment distribution reflects a partitioning between sediment populations that are current- influenced and relict (palimpsest) populations associated with submerged shorelines. Wave ravinement during the deglacial transgression, the reworking of submerged shorelines during sea-level stillstands and, to a lesser extent, the Agulhas Current system, are the dominant controls on sediment distribution. 相似文献
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Fine-structure (centimeters to meters) in vertical profiles of acoustic volume scattering strength is a common and ecologically significant characteristic of the coastal marine water column. The processes that create these structures modify the availability of food and exposure to predation for secondary producers at and below spatial and temporal scales that characterize their daily ambits. Thin acoustic scattering layers may persist for weeks at a particular coastal location, but they may also appear and disappear in only a few hours. These layers are usually evidence of mesozooplankton and micronekton having aggregated at peaks, gradients, or boundaries in the vertical distribution of various water column properties that characterize a marine ecosystem. The behaviors of both predators and prey are implicated in the generation of complex, time-dependent patterns of fine-structure. Specifically, diel vertical migration to layers of prey, isolumes, isotherms, isopycnals, or chemoclines can be responsible for the nighttime formation of thin acoustic layers. Physical processes such as horizontal shear, internal waves, water mass intrusions, tidal forcing, wind mixing of the upper water column, and horizontal advection modify acoustic scattering patterns by changing the vertical distributions of organisms that scatter sound. 相似文献
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P. Pravec P. Scheirich L. Šarounová G. Hahn G. Esquerdo Z. Krzeminski B.D. Warner M.C. Nolan L.A.M. Benner A. Galád W. Holliday Yu.N. Krugly R. Whiteley D.R. DeGraff S. Larson W.R. Cooney Jr. J. Zhu R. Dyvig V. Reddy Š. Gajdoš G. Masi D. Higgins B. Knight R. Behrend G. Burki C. Demeautis N. Waelchli A. Klotz M. Rieugné V. Cotrez G. Kober 《Icarus》2006,181(1):63-93
Photometric data on 17 binary near-Earth asteroids (15 of them are certain detections, two are probables) were analysed and characteristic properties of the near-Earth asteroid (NEA) binary population were inferred. We have found that binary systems with a secondary-to-primary mean diameter ratio Ds/Dp?0.18 concentrate among NEAs smaller than 2 km in diameter; the abundance of such binaries decreases significantly among larger NEAs. Secondaries show an upper size limit of Ds=0.5-1 km. Systems with Ds/Dp?0.5 are abundant but larger satellites are significantly less common. Primaries have spheroidal shapes and they rotate rapidly, with periods concentrating between 2.2 to 2.8 h and with a tail of the distribution up to ∼4 h. The fast rotators are close to the critical spin for rubble piles with bulk densities about 2 g/cm3. Orbital periods show an apparent cut-off at Porb∼11 h; closer systems with shorter orbital periods have not been discovered, which is consistent with the Roche limit for strengthless bodies. Secondaries are more elongated on average than primaries. Most, but not all, of their rotations appear to be synchronized with the orbital motion; nonsynchronous secondary rotations may occur especially among wider systems with Porb>20 h. The specific total angular momentum of most of the binary systems is similar to within ±20% and close to the angular momentum of a sphere with the same total mass and density, rotating at the disruption limit; this suggests that the binaries were created by mechanism(s) related to rotation near the critical limit and that they neither gained nor lost significant amounts of angular momentum during or since formation. A comparison with six small asynchronous binaries detected in the main belt of asteroids suggests that the population extends beyond the region of terrestrial planets, but with characteristics shifted to larger sizes and longer periods. The estimated mean proportion of binaries with Ds/Dp?0.18 among NEAs larger than 0.3 km is 15±4%. Among fastest rotating NEAs larger than 0.3 km with periods between 2.2 and 2.8 h, the mean proportion of such binaries is (66+10−12)%. 相似文献
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