CLIMATE-DOMINATED (?) SEDIMENTATION ON THE OPEN-COAST TIDAL FLATS OF KOREA

Most classical studies of tidal flats have focussed on estuarine and back-barrier areas where the intertidal zone is protected from wave action by barriers. By comparison, open-coast tidal flats that have little or no protection from wave action represent an important, but unappreciated, depositional setting. The west coast of the Korean peninsula is bordered by such tidal flats along much of its length. Detailed studies reveal that sedimentation on these broad flats is profoundly influenced by seasonal variations in climate. In areas close to the mouth of large rivers (e.g., the Han and Keum rivers), the delivery of suspended sediment is greatest in May and June when rainfall is greatest. However, the impact of this non-uniform sediment delivery attenuates rapidly with distance from the river. In most parts of the coast, the seasonal cycle of wave action dominates the sedimentary regime. In all but the more sheltered areas, the following annual cycle occurs: summer- wave action is minimal (except for the short-term influence of typhoons) and mud accumulates, aided by drying during exposure; autumn- increased wave action leads to a slowing of mud deposition and may ultimately cause erosion of the summer mud layer; winter- intense wave action deposits sand, that contains hummocky cross stratification and surge-generated, landward-migrating climbing ripples in the most exposed areas; and spring- a gradual decrease in the intensity of waves brings about a return to mud deposition. Under present conditions, the resulting deposits in exposed areas are dominated by wave-generated structures and resemble shoreface deposits! Decadal and longer-term climatic variations are expected to have a pronounced affect on the nature of these tidal flats. A decrease in the frequency and/or severity of winter storms should allow increased preservation of mud, causing the deposits to more closely resemble those of "typical (i.e., sheltered) tidal flats. Such variations in ancient deposits would be difficult to interpret. Climatic changes that alter the sediment texture will also have a marked impact on the widespread aquaculture carried out on the modern tidal flats. Changes in the quantity of suspended sediment delivered by local rivers will also have an impact, but this is more likely to be restricted in geographic extent.