Characteristics and Distribution of Surface Sediments in the Central East China Sea Shelf

Li Jin-duo^*

• Ningbo Marine Environment Monitoring Center Station, Ministry of Natural Resources, Ningbo, China

Based on grain-size analysis of the surface sediments on the central East China Sea (ECS) shelf, the spatial distribution, depositional settings, and transport patterns of these sediments were investigated in 2022. The results show that the study area can be divided into four Subareas: the nearshore clayey-silt or silt-dominated muddy deposition Subarea (Subarea I), the inner shelf silty-clay-dominated muddy deposition Subarea (Subarea II), the inner and outer shelf transition zone sand-silt-clay or clayey-sand-dominated mixed deposition Subarea (Subarea III), and the outer shelf sandy deposition Subarea (Subarea IV). Typical sediment grain size frequency curves and cumulative frequency curves reflect the sediment differentiation in Subareas I and II, as well as the sediment mixing and modification in Subareas III and IV. The spatial distribution of the sediments is affected by the hydrodynamic and environmental conditions. The main components of the surface sediments in this area are Yangtze River-derived terrigenous clasts. The contents of the sand, silt, and clay fractions obtained using the laser grain size analysis method were approximately 5% larger, 20% larger, and 25% smaller, respectively, than those acquired using the sieve and pipette analysis method, which was the reason that clayey-silt has been reported to occupy almost all of the muddy zone (Subareas I and II) in previous studies. Grain-size trend analysis images revealed the existence of generally north-to-south and west-to-east (i.e., coast-to-offshore) transport trends of the surface sediment in this area, indicating continuity of the provenance, environment, age, and genetic type of the sediments on the ECS shelf. Local regions exhibited different transport directions, which were influenced by the northward Taiwan Warm Current and upwelling. Mixed sediments experienced material redistribution and exchange with neighboring muddy and sandy deposits. The bidirectional northwest-southeast transport trends of some sandy sediments suggest a tidal genesis of the linear sand ridges and their effect on the sandy sediments in this region. Fine clay particles are spread over the entire ECS shelf, suggesting cross-shelf transport of modern fine-grained sediments.