RESPONSE OF SEDIMENTARY REGIME IN MIDDLE BOHAI SEA AND OFF THE DELTA TO THE CHANNEL MIGRATION AND DELTAIC COURSE SHIFTS OF THE YELLOW RIVER SINCE 1855

The lower river course of the Yellow River migrated from the Yellow Sea into the Bohai Sea in 1855. The deltaic course shifted frequently since 1855 and swept the whole Yellow River delta. Both the channel migration and deltaic course shift would result in dramatic changes of transport and deposition patterns of the huge river sediment in different parts of the Bohai Sea during different periods. Based on data of the sedimentary sequence, grain-size, element component, and 210-Pb profile in 8 cores from the middle Bohai Sea and off the delta, the sedimentary records of the channel migration in 1855 and major deltaic course shifts of Yellow River were discerned. The migration in 1855 impacted profoundly the sedimentary regime in the middle Bohai Sea, with finer sediment, lower primary production and neutral oxidation-reduction conditions after 1855. The variations of sediment grain size off the delta were in good corresponding to the deltaic course shift. The two latest eastward course shifts in 1976 and 1996 to the Laizhou Bay were well recorded by the grain-size changes and 210-Pb profiles. The lower parts with relative coarse sediments of the cores off the active river estuary were formed in coastal environment, while the upper parts with fine sediments were formed in deltaic environment. The sedimentary grain size sequences of the cores off the abandoned estuary became coarse upward, which were converse to those in the active estuary. The curves of 210-Pb were periodicity or anti-decay form, which matched with the deltaic course shifts well.