Sedimentary Features of the Yangtze Derived Mud Deposits on the Inner Shelf of the East China Sea

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Title: Sedimentary Features of the Yangtze Derived Mud Deposits on the Inner Shelf of the East China Sea
Author: Velozzi, David Micheal
Advisors: Dr. J. Paul Liu, Committee Chair
Abstract: The purpose of this study is to document the nature of sedimentation on the inner shelf of the East China Sea (ECS), with particular interest in the fate of the Yangtze River sediment. The Yangtze River has discharged about 500 million tons of sediment annually. High-resolution Chirp Sonar seismic data and shallow cores reveal a broad mud wedge up to 100 km wide extending 800 km southward along the Zhejiang-Fujian coasts. The mud wedge is thickest (~ 40 m) between the 20 m and 30 m isobaths and progressively thins offshore reaching water depths up to 60 and 70 meters. The mud wedge has a clinoform morphology with gently dipping topset, steeply dipping foreset, and gently dipping bottomset beds. 14C age dates indicate the clinoform began to form around 7000 yrs BP after sea-level reached the middle Holocene Highstand stage and decelerated to its present position. Sediment accumulation rates were estimated from the vertical distribution of excess 210Pb in sediment cores collected at 8 stations along the inner shelf. Near the mouth of the Yangtze River sediment accumulation rates have historically been about 1 to 5 cm y-1, due to winter resuspension and erosion events. The highest apparent accumulation rates in the mud wedge (2.1 cm yr-1) were found just south of the Yangtze estuary (30.5&deg;N). Apparent accumulation rates generally decrease alongshore toward the south and offshore toward the east (~ 1.6 - .33 cm yr-1). This pattern is consistent with the dispersal of Yangtze River-derived sediments by the Chinese coastal current and tidal systems. A calculated sediment budget using Pb-210 apparent accumulation rates (100-yr time scale) indicates that 35% of the sediment annually accumulating in the inner shelf mud deposit can be accounted for in the sediments north of 30&deg;N. This portion of the inner shelf mud deposit accounts for 55 % of the annual sediment discharge from the Yangtze River. Surface sediments of the inner shelf mud wedge are composed of clayey-silts and silty-clays. Coarser grained sandy-silts and silty-sands identified from cores on the thin bottomset deposits are a mixture of the fine Yangtze River sediment and coarse relict sands. Clay mineral assemblages of the mud wedge are composed primarily of illite (generally more than 70 %), and subordinately of chlorite, kaolinite and smectite. Smectite concentrations (< 5 %) are a good indicator that the sediments are derived from the Yangtze River.
Date: 2005-07-20
Degree: MS
Discipline: Marine, Earth and Atmospheric Sciences
URI: http://www.lib.ncsu.edu/resolver/1840.16/2657


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