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991.
Recent 24 s deep seismic reflection records revealed five flat reflectors in the lithospheric mantle in Eastern China. With increasing depth, they are named M1 to M5 and can be seen on both field single-shot and stacked records. Reflector M1 corresponds to the Moho discontinuity, whereas M5 may be the reflection from the bottom of the current lithosphere, which is about 78 km deep according to geothermal measurements. The other three reflectors seem peculiar and might result from interactions between the lithosphere and deeper mantle. Based on lithological and geochemical data, it is suggested that the lithosphere has been thinned from about 150 km to about 60 km in the Late Mesozoic, and then has been thickened to about 78 km during the Cenozoic. The thinning process produced a granulite layer in the old lower crust caused by magmatic underplating, whereas an eclogite layer formed beneath owing to the subduction of the Paleo-Tethys and Yangtze Craton during the Permian and Early Mesozoic. Reflector M2 at about 12 s two-way traveltime (TWT) might result from the Paleozoic Moho, which represents the boundary between the previous granulite and eclogite facies. Reflector M3 at about 14 s might correspond to the bottom of the eclogite layer, beneath which the old lithospheric mantle remained. The old and the newly developed mantle may have different compositions, resulting in reflector M4. The multi-layered mantle reflectors demonstrate a mantle structure that possibly correlates with the lithospheric thinning process that occurred in Eastern China during the Late Mesozoic. The discovery of multi-layered mantle reflectors in the studied areas indicates a high heterogeneity of the upper mantle. Reflection seismology with improved technology, together with velocity and resistivity imaging and rock-physics measurements, can provide more details of the heterogeneity and related dynamic processes that occurred in the lithospheric mantle. 相似文献
992.
The coal-bearing, alternating marine and non-marine Longzhaogou Group in eastern Heilongjiang, northeastern China, has long been considered as Jurassic, or mainly Jurassic, in age. However, recent studies have demonstrated that the ammonites and dinoflagellate cysts are of Early Cretaceous age. This has now been confirmed by new radiolarian evidence. The radiolarian fauna recovered from the upper Qihulin Formation of the Longzhaogou Group consists of nine poorly preserved species referable to nine genera. Novixitus is a Cretaceous genus, and the specimens of Archaeodictyomitra sp. and Xitus sp. recovered resemble A. vulgaris Pessagno and X. spicularius (Aliev), respectively. 相似文献
993.
Chongxuan Liu John M. Zachara James P. McKinley 《Geochimica et cosmochimica acta》2003,67(16):2893-2912
The desorption of 137Cs+ was investigated on sediments from the United States Hanford site. Pristine sediments and ones that were contaminated by the accidental release of alkaline 137Cs+-containing high level nuclear wastes (HLW, 2 × 106 to 6 × 107 pCi 137Cs+/g) were studied. The desorption of 137Cs+ was measured in Na+, K+, Rb+, and NH4+electrolytes of variable concentration and pH, and in presence of a strong Cs+-specific sorbent (self-assembled monolayer on a mesoporous support, SAMMS). 137Cs+ desorption from the HLW-contaminated Hanford sediments exhibited two distinct phases: an initial instantaneous release followed by a slow kinetic process. The extent of 137Cs+ desorption increased with increasing electrolyte concentration and followed a trend of Rb+ ≥ K+ > Na+ at circumneutral pH. This trend followed the respective selectivities of these cations for the sediment. The extent and rate of 137Cs+ desorption was influenced by surface armoring, intraparticle diffusion, and the collapse of edge-interlayer sites in solutions containing K+, Rb+, or NH4+. Scanning electron microscopic analysis revealed HLW-induced precipitation of secondary aluminosilicates on the edges and basal planes of micaceous minerals that were primary Cs+ sorbents. The removal of these precipitates by acidified ammonium oxalate extraction significantly increased the long-term desorption rate and extent. X-ray microprobe analyses of Cs+-sorbed micas showed that the 137Cs+ distributed not only on mica edges, but also within internal channels parallel to the basal plane, implying intraparticle diffusive migration of 137Cs+. Controlled desorption experiments using Cs+-spiked pristine sediment indicated that the 137Cs+ diffusion rate was fast in Na+-electrolyte, but much slower in the presence of K+ or Rb+, suggesting an effect of edge-interlayer collapse. An intraparticle diffusion model coupled with a two-site cation exchange model was used to interpret the experimental results. Model simulations suggested that about 40% of total sorbed 137Cs+ was exchangeable, including equilibrium and kinetic desorbable pools. At pH 3, this ratio increased to 60-80%. The remainder of the sorbed 137Cs+ was fixed or desorbed at much slower rate than our experiments could detect. 相似文献
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995.
Kirt E. Moody 《Estuaries and Coasts》2003,26(3):759-764
Blue crabsCallinectes sapidus in lower Chesapeake Bay are subject to high rates of predation during the late summer of their first year of growth as they
migrate out of vegetated nursery habitats. Predators, potentially contributing to this pattern, were identified in video-recorded
field observations of tethered juvenile crabs (20–25 mm carapace width). Predators were also tested in large laboratory tanks
containing similarly-sized untethered crabs as prey. Seven different predators attacked tethered crabs in the field. Only
two predators, larger blue crabs and northern puffers,Sphoeroides maculatus, consistently succeeded in preying on crabs in both field and laboratory settings. These results confirm the importance of
cannibalism on juvenile blue crabs and identify puffers as a potentially overlooked source of predation pressure. 相似文献
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2002年11月30至12月4日,北京持续4天大雾天气,空气污染物在持续的稳定层结条件下,空气质量连续3天达5级以上。文中分析了大雾天气各主要污染物的变化特征,以及此次过程中天气形势的特点及演变,并对造成大雾日空气污染天气的物理量分布特征进行分析。结果表明:高空WNW气流、稳定性持续增加、逆温层结持续存在、低空风速较小、相对湿度大,导致局地污染物不能及时随大气扩散;1000~700hPa有弱的上升气流形成和维持,与500hPa高空下沉气流之间在低空的某层高度上形成稳定层结(逆温层),导致大雾及重污染的形成;850hPa为暖区,850~500hPa为冷平流,有利于大雾的形成和加重。 相似文献