Event deposits of intense typhoons in the muddy wedge of the East China Sea over the past 150 years

摘要

Previous studies have considered that the wedge-shaped muddy body on the inner shelf of the East China Sea was formed by the deposition of resuspended fine-grained sediments from the Changjiang River estuary and its surrounding areas, which was transported southward by the Zhejiang-Fujian coastal currents. However, the occasional coarse-grained sedimentary layers in the muddy body have long been ignored. This study focuses on two sediment cores through the muddy wedge of the inner shelf. Grain sizes of the sediments with 0.25 cm interval and elemental compositions with 0.1 cm interval of the sedimentary cores were analyzed systematically, and the high-resolution sedimentary records of the cores were reconstructed by combining Pb-210 and Cs-137 isotopic dating. The event deposits resulting from intense typhoons were discerned by evidences of grain size anomalies and anomalous characteristics of elements. The possible mechanisms of the event deposits are discussed as well. The typhoon event layers (ELs) are a few mm to 8 cm in thicknesses, being in conformity with the underlying layers, having the sequences of fining upward in grain size. The ratio of Zr/Rb and content of Sr in the ELs increase sharply, are in agreement with the changes of sediments. The Core C0702 recorded 15 ELs, and Core DH6-1 recorded three in the past 150 years, the ELs were compared with the modern hurricanes. The cumulative thicknesses of the ELs account for 17.5% (C0702) and 7.6% (DH6-1) of the core total lengths. A concept model was developed to illustrate the sedimentary process of the formation of EL: coastal sediment was eroded and resuspended during intense typhoon period, transported by outward current in plume form and deposited on the inner shelf. The results suggest that summer typhoon plays an important role in the formation of the muddy wedge on the inner shelf of the East China Sea.