Depth-controlled changes in grain size and carbonate content on a shelf-edge sand bank

Abstract

Analysis of sediment samples from a shelf-edge sand bank in the Celtic Sea, has shown that the overall mean grain size of the sediments may be closely related to the proportion of carbonate material. This has been found to vary from 80% on top of the bank to 20% on its flanks. While the mean grain size of the quartz sand fraction of these sediments has been found to be independent of depth, it has also been found that the coarsest carbonate fragments occur on top of the bank. It is suggested that the observed changes in carbonate content and size can be explained by depth-controlled variations in tidal current amplitude and enhanced winnowing of fine quartz sand and the finer shell fragments from the carbonate deposits on the top of the bank. The importance of this mechanism is illustrated by reference to tidal current measurements and from considerations of the effects of wave-induced currents at the seabed.