Stratigraphic Sequences And Characteristics Of Sedimentary Geochemistry Since The Late Pleistocene In The Bohai Bay

The Bohai Bay and its coastal areas are one of the most attractive hotspots for the studies of marine geology and Quaternary geology in China.Although many achievements have been made,some fundamental geological questions still remain unsolved.For example,did the Yellow River flow through the Bohai Bay during the Late Pleistocene?How did the multi-stage delta lobes of the Yellow River Delta distribute in the Bohai Bay?Those questions have been attracting great attentions of marine geologists in China.Based upon?2000 km seismic profile data,two?30 m-long boreholes(cores BHZK2017-1 and BHZK2017-2)collected in the Bohai Bay during 2016?2017,and numerous AMS14C and OSL dating results and data of foraminifera,grain size and macro-elements and also previous achievements by studies of seismic profiles and boreholes,the stratigraphic sequences have been divided into four seismic units(SU1-SU4)and six sedimentary units(DU 1-DU6)in descending order,namely prodelta,tidal flat,floodplain,river channel,land-sea interaction facies and lacustrine/marsh facies,respectively.The general stratigraphic framework and sedimentary evolution of the Bohai Bay since the Late Pleistocene were analyzed accordingly.Our results revealed that the Yellow River likely flowed through the central basin of the Bohai Sea by nearly east-west direction on the northern part of the Bohai Bay during 21.8—9 cal ka BP.Moreover,four subaqueous delta lobes,formed during 1400 AD—present,1048—1128 AD,700 BC—11 AD and 1855—present,and 1855—present,respectively,were identified from the west to the south in the Bohai Bay since the Holocene;Those two northern subaqueous delta lobes were mainly related to the Haihe River,and the rest of the southern delta lobes were likely to be the Qikou superlobe and the modern Yellow River Delta superlobe,respectively.According to the chemical weathering parameters of CIA,n(Na)/n(K),A-CN-K and A-CNK-FM,our study showed that low degree of chemical weathering in cold and dry climates prevailed in the Bohai area in which was lack of an effective and intensive chemical weathering process.The evolution of paleoclimate can be divided into four phases:the degree of chemical weathering was relatively strong,and the climate gradually developed toward a warm and humid trend in the lacustrine/marsh facies between the late MIS3 and the MIS4 and even MIS5 phases,;The late MIS3 land-sea interaction facies(33.2 ± 3.3?39.4± 3.9 ka BP)and the MIS2 river channel facies(13.7 ± 1.4?21.8 ±2.2 ka BP)experienced the weakest geochemical weathering,and the climate gradually changed into a cold and dry condition;During the late MIS2 of floodplain facies,and the MIS 1 tidal flat facies(9990?3340 cal a BP)the chemical weathering was slightly enhanced compared to the river channel facies,the climate developed toward a warm and humid trend;the MIS 1 prodelta facies(3340 cal a BP?present),the degree of chemical weathering was still in a low level of chemical weathering,but the climate developed toward a cold and dry trend.Our study upon the sedimentary evolutions,paleochannel development and the subaqueous delta distribution since the Late Pleistocene would be conducive to the layout of coastal engineering programs and enhance the mitigation efforts of geological hazards in the Bohai Bay.