Sashan L. Manikam , Andrew N. Green , Burghard W. Flemming , Andrew Cooper , Gaynor Deacon
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引用次数: 0
Abstract
Antecedent conditions and geology influence bedform dynamics and other sea-bed bottom current interactions on the continental shelf. However, the nature of this influence has not been well constrained, particularly on current-swept shelves. A variety of bedforms (narrow and broad sediment ribbons, comet marks and subaqueous dunes) are revealed by high resolution multibeam bathymetry and side scan sonar surveys of the mid-to outer “Wild Coast” shelf along the east coast of South Africa. The bedforms demonstrate a relationship to the geological framework of the shelf.
The various bedforms occur in conjunction with bedrock-incised valleys and topographic depressions filled with transgressive sediment and bounded laterally by prominent rocky outcrops. These sediment sources are siphoned and mobilised by the Agulhas Current to create the contemporary shelf bedforms. Mechanisms of bedform development are associated with the interplay between the loci of sediment supply and associated grain size on the one hand and the current velocity and pathway of the Agulhas Current on the other.
Sediment ribbons form parallel to the flow path of the Agulhas Current in narrow or broad forms that can be differentiated primarily by sediment grain size. With increased sediment availability from nearby bedrock-bound depocenters these transition along shelf to 2D dunes. A cross-shelf transition to 3D dunes on the outer shelf is marked by increased current velocity, grain size and decreased sediment availability outside of rock-bound depocenters. Rock outcrops affect current velocity and flow paths sufficiently to favour localised deposition over sediment stripping from the shelf. Comet mark forms develop from interactions between availability of finer sediment and perturbations of current flow paths by rock outcrops. This creates conditions that favour localised sediment scouring over deposition.
With variability in sediment size, grain size and current flow, dunes fluctuate between active (sharp crested), inert (round crested) and degraded (flat crested) states. A lack of available sediment to replenish outer shelf dunes sees prolonged stasis, eventual dune splitting and breakdown under progressive current influence. Mid-shelf dunes have a greater capacity to recover from periods of stasis and short-term degradation due to greater availability of sediment.
Dune migration rates are slower than previous regional estimates. Reasons for this are inconclusive and may be multifaceted. Possible reasons include inertial constraints, prolonged downcurrent reformation periods and longer dune relaxation associated with coarser grain sizes and a paucity of sediment on the outer shelf coupled with the Agulhas Current core migrating further in and offshore. This may explain the stability of the large barchan dunes and reduced movement of other sharp crested dunes relative to the regional migration rates.
期刊介绍:
Continental Shelf Research publishes articles dealing with the biological, chemical, geological and physical oceanography of the shallow marine environment, from coastal and estuarine waters out to the shelf break. The continental shelf is a critical environment within the land-ocean continuum, and many processes, functions and problems in the continental shelf are driven by terrestrial inputs transported through the rivers and estuaries to the coastal and continental shelf areas. Manuscripts that deal with these topics must make a clear link to the continental shelf. Examples of research areas include:
Physical sedimentology and geomorphology
Geochemistry of the coastal ocean (inorganic and organic)
Marine environment and anthropogenic effects
Interaction of physical dynamics with natural and manmade shoreline features
Benthic, phytoplankton and zooplankton ecology
Coastal water and sediment quality, and ecosystem health
Benthic-pelagic coupling (physical and biogeochemical)
Interactions between physical dynamics (waves, currents, mixing, etc.) and biogeochemical cycles
Estuarine, coastal and shelf sea modelling and process studies.