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841.
842.
Land reclamation as a cause of fine-grained sediment depletion in backbarrier tidal flats (Southern North Sea) 总被引:4,自引:0,他引:4
Grain size distribution patterns along the East Frisian coast (German Wadden Sea) were evafuated with the aim of identifying
potential effects of man-made structures (e.g. dikes) on the textural composition and sediment budget of the area. It was found that the distribution of various grain size
fractions and settling velocities revealed a well-defined cross-shore energy gradient. Adjacent to the mainland dike the sediment
is dominated by the 3.0–3.5 phi sand fraction (>70% by weight). In comparison to the sedimentary sequences normally observed
in unobstructed backbarrier depositional environments, the sediments along the land-ward margin are in this case conspicuously
depleted in grain sizes <3.5 phi, the corresponding size fractions contributing <20% by weight. This suggests that the dike
interrupts the normal energy gradient, thereby inhibiting the deposition of sediments with settling velocities <0.5 cm s−1. This interpretation is confirmed when comparing individual cross-shore transects. The shorter the transect between the barrier
island shore and the dike, the greater the fine sediment depletion. Since many faunal assemblages are grain-size specific,
it is further suggested that the backbarrier ecosystem must have undergone significant modifications as a result of the substantial
reduction in mud flats since the onset of dike construction some 1000 years BP. It is predicted that in the wake of the persisting
and possibly accelerating sealevel rise this process of fine sediment depletion will continue and also affect progressively
coarser sediments. 相似文献
843.
Tae Soo Chang Burghard W. Flemming Elke Tilch Alexander Bartholomä Ralf Wöstmann 《Facies》2006,52(3):329-340
The late Holocene stratigraphic evolution of a back-barrier tidal basin in the East Frisian Wadden Sea, southern North Sea, was investigated on the basis of a conceptual model relating the rate of sea-level rise to the rate of sediment supply. For this purpose, more than 20 vibra-cores and box-cores were evaluated, complemented by 14C ages of in situ peats and historical charts. In spite of interspersed short regressive events, the late Holocene stratigraphy generally reveals upward-coarsening grain-size trends indicative of transgressive deposition in the course of sea-level rise rather than erosion and redeposition by migrating channels. A particular feature is the general absence of down-core bioturbation traces, which stands out in sharp contrast to the intensely burrowed modern surface layer. Thus, in the Wadden Sea, high sediment turnover in the course of rapid transgression evidently obliterates most bioturbation traces and other tidal signals such as minor regressive deposits, thereby emphasising the importance of preservation potential. 相似文献