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排序方式: 共有1399条查询结果,搜索用时 125 毫秒
991.
This paper presents the result of measurements of floodplain sedimentation using sediment traps. The study was carried out on two embanked floodplains along the Rivers Rhine and Meuse in The Netherlands during a 3 day flood in January 1993. Raster maps of sediment accumulation were made by interpolating the measurements from the traps using block kriging. The sediment maps show clear patterns in sediment accumulation, together with the estimated interpolation errors. Average sediment accumulation ranges between 0·57 and 1·0 kg m?2. High sediment accumulation is found on the levees (4 kg m?2 or more) and on low lying areas (1·6 kg m?2); sediment accumulation decreases with distance from the main channel. The sedimentation patterns are related to floodplain topography and sediment transporting mechanisms. Sediment transport by turbulent diffusion as well as by convection can be recognized. Also, flood duration and the process of sediment settling out in ponding water in closed depressions are important. The applied method allows comparison of the results with raster-based sedimentation models. 相似文献
992.
High concentrations of mercury were measured in sediment and animals collected in the immediate vicinity of a closed-down chemical factory. Sediment contained up to 22 ppm (dry wt) of mercury, deposit-feeding bivalves between 1.4 and 4.4 ppm (wet wt), suspension-feeding bivalves between 0.9 and 1.9 ppm and predatory fish between 0.3 and 0.8 ppm. Outside the ‘hot spot’ area, the mercury concentration in sediment and mussels (Mytilus edulis) rapidly decreased with increasing distance from the former factory. Mercury concentration in flounders (Platichthys flesus) also decreased with distance from the source; the decline was, however, much less marked. 相似文献
993.
Prof. Dr. Hans Füchtbauer Dr. Detlev K. Richter 《International Journal of Earth Sciences》1983,72(1):53-66
Deep fissures in shallow-water limestones filled with deep-water sediments are described from many places especially in the Triassic and Jurassic of the Tethys and in the Middle to Upper Devonian of the Variscian Mountains. Mass flows occur in the same formations and areas. On the island of Hydra (Greece) it was possible to show that both phenomena are linked by internal breccias of well-fitted limestone fragments. They are indicative of slope or near-slope positions and are very probably caused by flexure-like deformations of shallow-burial limestones. Seismic shocks producing overpressure may be instrumental in the disintegration of the semi-lithified limestones thus providing the material for mass flows. These events are associated with a tectonical subsidence as shown by deep-water matrix in the brecciated shallow-water limestones. They occur in areas and periods of submarine rifting, especially in late Devonian and Triassic time. At least four Triassic brecciation events can be traced from Greece to the Alps and correspond presumably to major transgressions in the cratonic basin of middle Europe. 相似文献
994.
Sediments of shallow lakes contain besides plankton pigments as well pigments of purple non-sulfur and purple sulfur bacteria.
A thin-layer chromatographic method was developed for the separation of the most important pigments of purple bacteria beside
plankton pigments. The new method was examined in three different lakes for a stratigraphic analysis of bacteria pigments
beside plankton pigments and the results were discussed subsequently.
相似文献
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998.
Claire L. Schelske Hans Züllig Mary Boucherle 《Aquatic Sciences - Research Across Boundaries》1987,49(1):42-50
Biogenic silica concentrations were determined from core samples of laminated sediments collected from Lake Zürich and Lake St. Moritz and used to calculate rates of biogenic silica sedimentation. In Lake Zürich biogenic silica sedimentation increased from 65 g SiO2·?2·yr?1 in 1894 to nearly 900 g SiO2·m?2·yr?1 in 1896 and in Lake St. Moritz biogenic silica sedimentation increased about 6-fold during the 1930s. Both periods of increased biogenic silica flux followed increased loadings of domestic sewage to the lakes. In Lake Zürich the period of increased flux only lasted for a few years whereas in Lake St. Moritz high fluxes were maintained to the top of the core. In Lake Zürich increased production and sedimentation of diatoms could be maintained only until the silca reservoir in the water mass was depleted whereas in Lake St. Moritz (0.10 year residence time and a mean depth of 25 m) diatom production and sedimentation was maintained at a high level by rapid replenishment of silica from tributary inputs. Although historical patterns of biogenic silica sedimentation differed for the two lakes, it is hypothesized that increased biogenic silica deposition in both lakes occurred because diatom production was stimulated by phosphorus enrichment of the water mass and that the silica biogeochemistry of both lakes was affected as a result. 相似文献
999.
Phengite geobarometry based on the limiting assemblage with K-feldspar,phlogopite, and quartz 总被引:37,自引:0,他引:37
Hans -Joachim Massonne Werner Schreyer 《Contributions to Mineralogy and Petrology》1987,96(2):212-224
Following and extending the early work of Velde (1965) the pressure-temperature dependence of the compositions of potassic white micas coexisting with K-feldspar, quartz, and phlogopite in the model system K2O-MgO-Al2O3-SiO2-H2O was investigated up to fluid pressures of 24 kbar by synthesis experiments. There is a strong, almost linear increase of the Si content per formula unit (p.f.u.) of phengite, ideally KAl2–xMgx[Al1–xSi3+xO10] (OH)2 with pressure, as well as a moderate decrease of Si (or x) with temperature. The most siliceous phengite with Si near 3.8 p.f.u. becomes stable near 20 kbar depending on temperature. However, contrary to Velde's assumption, these phengites coexisting with the limiting assemblage are invariably not of an ideal dioctahedral composition (as given by the above formula) but have total octahedral occupancies as high as about 2.1 p.f.u.The stability field of the critical assemblage phengite — K-feldspar — phlogopite — quartz ranges, in the presence of excess H2O, from at least 350° C to about 700° C but has an upper pressure limit in the range 16–22 kbar, when K-feldspar and phlogopite react to form phengite and a K, Mg-rich siliceous fluid.For the purpose of using these phase relationships as a new geobarometer for natural rocks, the influence of other components in the phengite (F, Fe, Na) is evaluated on the basis of literature data. Water activities below unity shift the Si isopleths of phengite towards higher pressures and lower temperatures, but the effects are relatively small. Tests of the new geobarometer with published analytical and PT data on natural phengite-bearing rocks are handicapped by the paucity of reliable values, but also by the obvious lack of equilibration of phengite compositions in many rocks that show zonation of their phengites or even more than one generation of potassic white micas with different compositions. From natural phengites that do not coexist with the limiting assemblage studied here but still with a Mg, Fe-silicate, at least minimum pressures can be derived with the use of the data presented. 相似文献
1000.
Dr. sc. nat. Hans Züllig 《Aquatic Sciences - Research Across Boundaries》1984,46(2):297-300
The isolation of the carotenoid Okenone from the sediments of several shallow lakes gives evidence of anaerobic zones existing during the sedimentation 13 000 years before. 相似文献