The role of fluid mud in the formation of extensive mud sheets during summer on the Duuri macrotidal flat, west coast of Korea

Abstract

On the Duuri macrotidal flat along the west coast of Korea, mud sheets exceeding 20 cm in thickness form during summer, and are completely eliminated in the course of the following winter. This cyclical seasonal deposition/erosion of mud on the extensive intertidal flat of Korea has to date not been plausibly explained. To elucidate the responsible mechanism, vertical profiles of suspended sediment concentrations (SSCs) were obtained from March to August 2008 using an ASM-IV, which comprises a vertical array of OBS sensors mounted within a 1.44-m-long pole. The measurements reveal that a high-concentration mud layer begins to develop in mid-May, and reaches concentrations over 10 g/l (fluid mud) by August. SSCs are observed to be generally higher at spring tide than at neap tide. The fluid mud (SSCs > 10 g/l) clearly responds to wave action: the calm period being characterized by a two-layer profile, whereas a gradational one-layer profile during the rough summer season. The fluid mud sheet blanketing the exposed flat is supposed to be semi-consolidated mainly by organic processes and solar radiation during low-water slack tide. The differential response of the fluid mud to the variable seasonal wave-energy conditions thus plays a key role in the rapid and widespread formation of ephemeral mud flats in the study area during summer. The available evidence, however, suggests that the summer mud sheet has a very low potential for preservation in the rock record.