Evolution Of Sedimentary Environment And Provenance Since The Late Pleistocene In The Central Bohai Sea

The Bohai Sea is the inland sea of China,which receives a large amount of terrigenous materials from the surrounding rivers every year.And its water depth is shallow with an average water depth less than 20 m,which is sensitive to environmental and climate change.Therefore,the Bohai Sea is an ideal area to study environment evolution and sediment provenance.A high-resolution sediment drilling borehole BH07 was collected in the central Bohai Sea using Kan 407 in 2009.Based on the data of foraminera,sediment color,grain-size,major and trace elements,magnetic susceptibility,carbonates,clay minerals and Sr-Nd-Pb isotopes of borehole BH07,the evolution history of the sedimentary environment since the late Pleistocene in the central Bohai Sea was reconstructed and the sources of the sediments were distinguished qualitatively and quantitatively.Four kinds of sedimentary facies were indentified in the borehole BH07 according to analysis of grain-size,structure,sediment color,elemental composition,micropaleontology and AMS 14C dating,named U ??U??U? and U? in descending order.U ?:shallow sea facies.It formed during the high sea level period of Holocene.The sediments were composed with yellowish brown and gray brown silt,and the foraminifera assemblage inferred a sedimentary environment with a depth of 50 m;U ?:tidal flat-coastal swampy facies.The grain size of sediments varied greatly,from clayey silt to silt,indicating that the sedimentary environment was variable during this period,responding to rapid sea level change.The sediments of U ? were deposited in the early Holocene,and AMS 14C dating of two peat layers were 10518±70 a B.P.(5 m)and 12986±60 a B.P.(8.29 m).U?:lake facies formed at the MIS 2 stage.The sediments were mostly silty sand,and grain size was fine and the parallel bedding was developed in part of the unit.U?:terrestrial fluvial facies.the sediments were brownish-yellow-brown sand,which showed strong oxidation conditions and also indicated that the well-ventilated condition during this period.Fine-grained sediments in the study area have been continuously affected by the Yellow River(Huanghe)material since the late Pleistocene.The material from Luanhe River became the major source of the sediments in the study area when the Yellow River diverted from the Bohai Sea to the Yellow Sea from the end of the Pleistocene(?13 ka B.P.).The Yellow River reentered the Bohai Sea since the high sea level stage during Holocene and,the material of Yellow River played a dominant role in fine-grained sediments once again31.3?17.7 ka B.P.,the provenance information of the whole rock indicated by the trace elements showed that the borehole sediments were mainly affected by the materials of the Luanhe River.17.6?13.1 ka B.P.,the influences of the Yellow River material gradually strengthened.The characteristics of elements in this area were similar to those of the Yellow River at about 13 ka B.P.(12.60 m),indicated that the Yellow River material had already played a controlling role in the borehole.The material sources in whole-grained size indicated by the element characteristics did not completely consistent with the provenance information indicated by the fine-grained sediments,indicating that the sediments in the study area are affected by sorting effects of hydrodynamic force.