FLUVIAL-LACUSTRINE SEDIMENTS AND HOLOCENE CLIMATIC AND HYDROLOGIC EVENTS IN THE RENQIU SECTION,CHINA

The histories of hydrologic and climatic events in central Hebei Plain,for the past 11 640 cal.aBP are reflected by the fluvial-lacustrine sediments using proxy records of grain size and magnetic susceptibility.We propose a model that explains variations in grain size and magnetic susceptibility of fluvial-lacustrine sediments as a record of Holocene climatic and hydrologic changes.The model is based on the site around Lake Baiyangdian and incorporates the processes of erosion and deposit that are correlative with regional climatic events.For the open lakes in the humid climatic regions of eastern China,the grain size is still reliable for the reconstruction of environmental changes.But as for the fluvial-lacustrine sediments in this region,the magnetic susceptibility has two kinds of mechanisms.Namely,in the fluvial sediments the magnetic mineral was much richer in coarse materials especially in the matter bigger than 125 μm.But to the lakes with great productivity,it positively correlates with the content of fine materials less than 10 μm in grain size. In the Holocene,this area has experienced three lake-spread episodes.The ages are 9 500~9 050,(8 110)~7 240 and 5 670~3 300 cal.aBP respectively.Once more,there were two major fluvial aggradation events at 9 050~8 110 and 6 700~6 240 cal.aBP when the lake contracted and the river course occupied the location of the lake basin.Both the lake extent and the river progradation were driven by climate changes.When the climate was warm and humid,the lake spread.In reverse,the river proceeded.