Climate And Environmental Evolution Since The Late Glacial In The Baiyangdian Area:Pollen And Grain Size Records From Gaoyang Profile

The study of climate and environment evolution in the North China Plain since the Lateglacial is the basis for understanding modern environmental changes in the region and provides a scientific and theoretical basis for social development,disaster risk assessment and response to extreme precipitation and flooding events in the context of future global warming in the region.Baiyangdian Lake,is located in the middle of the North China Plain,is the largest freshwater lake in the North China Plain.The Baiyangdian area,belongs to the East Asian monsoon climate zone,is very sensitive to climate and environmental changes.Since the Quaternary period,Baiyangdian area had received huge thick deposits of lake and river phases,which is a good vehicle for studying paleoclimate and paleoenvironment.A large number of studies have been conducted on the climatic and environmental evolution of Baiyangdian area since the Holocene,but the chronological resolution of the existing studies is mostly low,and the effectes of sediment phase changes and sediment interruptions on the sediment sequence has not been fully taken into account.The existence of extensive and continuous large lakes in the middle Holocene Baiyangdian area and the vegetation landscape in the hinterland of the North China Plain during the Holocene are still controversial.To address and answer the above questions,the paper selects the Baiyangdian(BG-2019)section as the study object,establishing a credible age framework for the section based on six AMS¹⁴C dating and six OSL dating to explore the regional vegetation succession,climatic environment evolution and the evolution of Baiyangdian Lake in the study area since the Lateglacial through grain size analysis and pollen analysis.The main conclusions are as follows:1)Using AMS¹⁴C dating and OSL dating for the lacustrine and fluvial sedimentary layers respectively,the established age framework indicates that there are sedimentary interruptions or stratigraphic deficiencies in the BG-2019 profile,and intervals of interrupted deposition were10,270?8000 cal.a B.P.and 7130?5400 cal.a B.P.2)The profile BG-2019 has two intervals of lacustrine facies,two intervals of fluvial facies,one interval of fluvial-lacustrine delta facies,one interval of swamp facies.The ages of the various facies are as follows.Lacustrine facies:13,710?10,270 cal.a B.P.and 5400?4630 cal.a B.P.;fluvial facies:4630?3700 cal.a B.P.and 3470?3230 cal.a B.P.;fluvial-lacustrine delta facies:8000?7130 cal.a B.P.;swamp facies:3700?3470 cal.a B.P..3)Pollen analysis shows that the vegetation landscape of the Baiyangdian Basin differed substantially between the Lateglacial and the middle Holocene.During the Lateglacial,the climate was cold and dry,and vegetation dominated by Artemisia,Chenopodioideae,Poaceae and Asteraceae developed in the plain,while the western mountains had a limited forest cover.During the middle Holocene,when the climate was warm and humid,grassland vegetation dominated by Artemisia,Chenopodioideae and Poaceae developed in the plain;and Ceratopteris and aquatic plants flourished in and around the lake;coniferous-broadleaved mixed forest dominated by Pinus and Quercus developed in the western mountains,with an increased forest cover.During the middle-late Holocene,when the climate was relatively mild and dry,grassland vegetation dominated by Artemisia,Chenopodioideae and Poaceae persisted in the plain,coniferous-broadleaved mixed forest dominated by Pinus developed in the western mountains,with a high degree of forest cover.4)Baiyangdian Lake was a small lake during the Lateglacial.The lake expanded eastward and northward during the early Holocene,but was more extensive during the middle Holocene,although with discontinuous lacustrine facies.During the late Holocene the lake began to shrink.