Dispersal mechanism of fine-grained sediment in the modern mud belt of the East China Sea

Abstract

Mud deposition is common on continental shelves worldwide, resulting from various dispersal behaviors of fine-grained sediments. The mud belt found in the East China Sea (ECS) is a product of a complex land-ocean sediment dispersal system, and has thus been the focus of oceanographic research for decades. However, its formation mechanisms, particularly the roles of different local currents on cross-shelf versus longshore transports, are still not clear. Therefore, we conducted a comprehensive review of the provenance and dynamical mechanism of the ECS mud belt, based on existing and new data from fields of geochemistry, sedimentology, and physical oceanography. Our results show that, during the summer, the Nearshore Kuroshio Branch Current (NKBC) intrusion onto the northwestern ECS shelf triggers the formation of transverse circulation and super rip current within the coastal area, facilitating the cross-shelf transport. Subsequently, the current displays a meandering trajectory off the southern Zhoushan Islands, leading to the formation of both cyclonic and anticyclonic eddies. During the winter, this northwards NKBC forms a coupled effect with the southwards Zhe-Min coastal current (ZMCC), enhancing the longshore transport. In winter,the Taiwan Warm Current (TWC) intrudes into the southwestern ECS and also forms a coupled effect with the southwards ZMCC. In summer, the ZMCC shifts northward and collides with the southward-directed Oujiang diluted water then forms a super rip current off the southern Oujiang River estuary. In the Taiwan Strait, we find there is a perennial Taiwan Coastal Current (TCC), which can transport Taiwan-derived sediments to the NKBC. These sediments are then carried further towards the northern ECS shelf. We conclude that the sources of the mud belt are mainly from the Yangtze River-derived sediments, and partially from Taiwan- and Minjiang River-derived sediments. The dispersion and sedimentation of Taiwan-derived sediments are controlled jointly by the TCC, NKBC, and anticyclonic eddy, and mainly occur in summer. The previously deposited Yangtze River-derived sediments in the coastal area are resuspended and then cross-shelf transported to form the depocenters during summertime. The diffusion and deposition of resuspended sediments from the Yangtze River estuary are dominated by the coupled effect of ZMCC, NKBC, and TWC in winter. Finally, with the Yangtze River sediment discharge decreasing, we predict that the ECS mud belt will increase in grain sizes in the coastal areas, and the contribution from small and medium-sized rivers will increase. On the other hand, the eastern portion of the mud belt, i.e. clinofrom's foreset, is expected to continue its gradual accumulation, the nearshore topset accumulation will slow down or experience erosion. These findings have scientific significance in enhancing our understanding of the fates of river-derived sediments to the sea, as well as the interactions between oceans and marginal seas.