The prodelta environment of a fjord: suspended particle dynamics |
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| Authors: | JAMES P M SYVITSKI K W ASPREY D A CLATTENBURG GORDON D HODGE† |
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| Institution: | Atlantic Geoscience Centre, Geological Survey of Canada, Bedford Institute of Oceanography, Box 1006, Dartmouth, N.S., B2Y 4A2 Canada;The University of British Columbia, Department of Geological Sciences, Vancouver, B.C., Canada |
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| Abstract: | The dynamics of suspended particles within a fjord's estuarine circulation are investigated and the results compared with larger non-enclosed prodelta environments. In the upper prodelta, the seaward-flowing river plume flows over the ambient marine water depositing much of the initial riverine suspended load. Sedimentation is dominated by coarse silt and fine-grained sand particles with coarseness determined by the tidal and fluvial stage. Particles less than 10 μn have similar settling velocities regardless of size because they settle in flocs: the settling velocity at a water depth of 5 m is 30 m day-1 and increases with depth so that at 30 m the particles settle at 100 m day-1. For larger particles, the downward settling velocity enhancement due to flocculation decreases with increasing grain size. Hydraulic sorting allows the preferential settling of feldspar and quartz over mica. Particle dynamics in the lower prodelta are dependent on the character of the freshwater wedge that thins seaward of the upper prodelta. The vertical flux of particles is controlled by biogeochemical interactions such as pelletization of fine particles and flocculation (which occurs within rather than below the surface layer in contrast to the upper prodelta). The pellets are produced by indiscriminate filter feeding zooplankton. Across the lower prodelta the suspensate character, recognized in the composition of both flocs and pellets, changes from a dominance of mineral grains to that of autochthonous organic matter. The interaction of bacteria with the suspended particles increases with depth and seaward distance. At depth, the mucoid filaments form stable interconnecting webs. Particle concentration in the surface layer decreases at a rate proportional to the negative one-half and three-halves power of the distance seaward over the upper and lower prodelta, respectively. This relationship is hypothesized as being universal for large marine deltas dominated by buoyancy flow dynamics, regardless of the levels of initial riverine particle concentration or their composition. |
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