Environmental Change Re-constructed From The Paleo-channel Sedimentary Records In Lake Cheng Hu, Central Taihu Plain, China

There is a paleo-channel along the direction of NNW-SSE in lake Chenghu, central Taihu Plain,China.This paper focuses on the paleo-channel sediment based on the study of the SC7 core(about 4 meters long) that drilled in the paleo-channel at the bottom of the lake center.Particle-size,foraminifera,organic and magnetic susceptibility are analyzed comprehensively.Combining AMS¹⁴C dating,the historical archives,the relation of the paleo-channel and the ancient Dongjiang River, the evolution of the ancient Dongjiang River,and the time and the reason of the lake formation are discussed in detail.Preliminary results are as follows:(1) The location,the direction,the cutting depth and the residual width of the paleo-channel are the same as historical archives on the ancient Dongjiang River.So are the paleo-channel connected with the sea,its formation time and developmental characters.So it's inferred that the paleo-channel at the bottom of the lake is the ancient Dongjiang River.(2) Combining the convection of the sedimentary environment from river to lake, based on the comprehensive analysis of particle-size,foraminifera,organic and magnetic susceptibility,the paleo-channel sedimentary environment is divided into four phases,that is,the ancient Dongjiang River sediment phase(5100～3500aB.P.); River - lake transition phase(3500～3200aB.P.);Lake sediment phase (3200～200aB.P.);Disturbance phase(after 200aBP).The ancient Dongjiang River sediment phase can be divided into three periods in detail:1) unaffected by the sea intrusion period(5100～4300aB.P.);2) affected seriously by the sea intrusion period (4300～4100aB.P.);3) affected weakly by the sea intrusion period(4100～3500aB.P.). Lake sediment phase can be divided into two periods in detail:1) small sallow lake period(3200～1100aB.P.);2) mature lake period(1100～200aB.P.).(3) The lower of the ancient Dongjiang River has been blocked by the sea intrusion after Shang Dynasty.Then small sallow lake,that is,the embryo of the lake Chenghu formed upon the paleo-channel.A number of floods made the small sallow lake expand and deepen.The lake Chenghu matured after the end of Tang Dynasty.