Distribution and cycling of fine-grained sediment in the Eastern Scheidt, Southwest Netherlands

Abstract

Surface and core samples of sediment were taken from 3 types of mud depositional areas in the Eastern Scheldt in 1985-1986, before the completion of the Storm Surge Barrier. Total accumulation rates of clay and silt were highest (100-200 Gg yr-1) in abandoned channels. Clay and silt were also deposited in large quantities (30-80 Gg yr-1) in salt marshes, and were cycled in large amounts (200-500 Gg y-1) in mussel banks by deposition of faeces and pseudo-faeces. In the latter habitat no net accumulation occurred, because equal amounts were resuspended by wave agitation and dredging. The balance of sediment input and accumulation indicated a net seaward flux of 80-1800 Mg day-1 of clay + silt and 3-80 Mg day-1 of organic carbon through the mouth of the Eastern Scheldt. Most of the sediment probably came from eroding Holocene deposits elsewhere in the Eastern Scheldt, and smaller amounts from fluvial influx. Analysis of their elemental composition indicated that a major source of the sediments was formed by sandy deposits with clayey laminae of subrecent Dunkirk age, and a minor source was older, more clayey Calais deposits. The sedimentation rates were highest (5-15 cm yr-1) in abandoned channels near dams and dikes. In such environments, seasonal variations in sedimentation rates were clearly reflected in cyclic changes of porosity and organic carbon concentrations in the sediment. Similar seasonal changes were observed in the sediment at the surface of sandy intertidal flats.