Mid-late Holocene Environmental Evolution And Sea Level Change In A Typical Area On The West Coast Of Bohai Bay,North China

Bohai Bay is one of the three bays of Bohai Sea in China.The sediment is mainly silt and clay,and the sedimentation rate is fast.It is an ideal place to study sea level and climate change.However,the history of Holocene environmental change in the west coast of Bohai Bay is still controversial due to the complex sedimentary environment and low chronological resolution.Based on the west coast of Bohai Bay Haixing wetland HX-A drilling deposits sediment as the study object,on the basis of seven AMS ¹⁴C dating,through the grain-size,the geochemical elements and pollen and algae multi-index comprehensive analysis,high-resolution to reveal the middle-late Holocene sedimentary environmental evolution history and its impact on sea level change.The results show that:（1）Before 6700 cal a B.P.,has the highest Cl content and Sr/Ca ratio which.Ti,K indicating terrestrial environment and Fe/Mn ratio are the lowest.The sand content is more than 70%.End-member showed EM2 predominance indicating marine provenance components.The study area was dominated by marine sediments with the highest sea level before 6700 cal a B.P.6700-5000 cal a B.P.,Cl content and Sr/Ca began to decrease,Ti,K gradually increase,sandy silt content is significantly higher than before,mostly above 40%,sand content drops to below 60%,averaging～28%.End-member analysis showed that the proportion of EM1（lacustrine sedimentary component）increased significantly,EM2（Marine sedimentary component）still occupied a certain proportion（～30-40%）,and EM3（fluvial sedimentary component）began to appear,while the concentration of Pediastrum,Typha indicating freshwater increased significantly,and the concentration of Selaginella sinensis indicating fluvial action also started to appear,indicating that the study area was strengthened by fluvial action of terrestrial origin,and the relative sea level started to drop,forming a lagoon.5000-3500 cal a B.P.,Ti,K content further increased and became dominant in the later stage,Cl content and Sr/Ca ratio further decreased;sandy silt content increased to more than 60%.End-member analysis showed that EM1 component dominated and EM2component only appeared in individual periods,reflecting that the connectivity between the study area and the ocean further weakened at this stage,and the relative sea level further decreased and the lagoon gradually closed.After 3500 cal a B.P.,in most periods,Ti content reached the highest value in the whole borehole,Cl content and Sr/Ca decreased to the lowest value,EM1 component was dominant,EM2 almost disappeared,dinoflagellates almost disappeared,and a large number of Typha appeared,indicating that the study area was separated from the influence of the ocean during most of the period.（2）During～2500-2000 cal a B.P.,the content of Cl and Sr/Ca related to marine sediments increased slightly,while,the terrigenous indicators Ti and K,and the Fe/Mn ratio showed low peaks during～2500-2000 cal a B.P.A peak in the median grain size and an abrupt increase in grain-size.From the end member analysis,EM2 also occurred at this time,while EM3 decreased continuously.There was also a slight increase in the concentration of dinoflagellate cysts.The simultaneous decreases in the concentrations of Typha pollen and Pediastrum coenobia indicate the occurrence of a marine transgression event in the study area,which may be related to the emergence of the Roman Warm Period.（3）The three regressions,at 6700-6500,5000-4800,and 3500-3250 cal a B.P.may have been caused by weak monsoon events.The research area is mainly affected by the input of the Yellow River and surrounding rivers.When the climate is cold and dry,the monsoon weakens,precipitation decreases,and distribution is uneven,resulting in an increase in the flow rate of rivers,an increase in flood events,and a large amount of alluvial material brought by rivers,which forms deposits downstream,causing a decrease in relative sea level.