Late Pleistocene Sedimentary Records Indicative Of The Environmental Changes In The Chenghu Area Of The Central Taihu Plain In China

The bottom of Chenghu in Suzhou is mainly composed of the'hard clay'(a kind of sediments consisting of silt and clay),which is similar to other lakes in Taihu Plain. The wusong elevation of the bottom of Chenghu is about 1 meter.The hard clay is representative of the Hard Clay Plain developed in Last stage of Late Pleistocene.In order to study the paleo-geographic basis before the lakes formed in the region,we drilled a 18 meters core(named SC6)in the central Chenghu.The core is composed of the hard clay in the upper layer and the costal or shallow sea sediments(not drilled the bottom)in the lower layer.Based on the AMS¹⁴C,the age of the core is from 40ka to 10ka BP.By the analysis of the comprehensive environmental indexes,including sedimentology,environmental-magnetism,micropaleontology,element-geochemistry and mineralogy,it comes to 3 main conclusions as follows:First,in Medium-term of Late Pleistocene,the Chenghu Area suffered the different degree transgressions by the analysis of the foraminiferal.In the earlier stage (about 40～35ka BP),the scale was small and the average of the foraminiferal is 330 per 50g.The foraminiferal assemblage is characterized by Florilus decorus-Ammonia beccarii-Elphidium advenum.The deposition thickness is about 10 meters.In the later stage(35²9ka BP),the scale was large and the average is 3130 per 50g.The foraminiferal assemblage is characterized by Florilus decorus-Ammonia beccarii-Elphidium advenum.The deposition thickness is 2 meters.Second,the hard clay in the bottom of Chenghu is of aeolian origin by the detailed analysis of the particle-size and the contrast among the hard clay,the loess in northern China,Xiashu loess in Zhenjiang.The deposition thickness is 6 meters and the age is from 29ka to 10ka BP.Third,by AMS¹⁴C we can see that the age of SC6 is equivalent to the interstade (MIS-3)and the later period(MIS-2)in last glacial.Its environmental change of the paleo-coastal belt is a response to global climate change.This is as follows:The sediments of 810-1800cm belong to the earlier period of the medium-term of Late Pleistocene.The value of the magnetic susceptibility is high.In the warm climate,the Yangtze River Delta Plain widespreadly suffered the transgression.This transgression arrived at the Chenghu Area.The foraminiferal assemblage is characterized by Ammonia beccarii-Elphidium magellanicum-Florilus decorus.In addition,some layers conserve small amount of ostracodas.It reflects that during the period,the Chenghu Area has been estuarine facies sedimentary environment.The clay mineral is characterized by high content of illite,very low content of smectite and the steady changes in the content of all kinds of the clay minerals,which is similar to the sediment from the Yangtze River.The sediments of 600～810cm belong to the later period of the medium-term of Late Pleistocene.The value of the magnetic susceptibility begins to fall.In the warm-humid climate,the Chenghu Area suffered the large scale transgression.The foraminiferal assemblage is characterized by Florilus decorus-Ammonia beccarii-Elphidium advenum.It reflects that during the period,the Chenghu Area has been shallow sea sedimentary environment.The characters of the particle-size,the clay minerals and elemental-geochemistry show that during the period,the climate has been unsteady.Until 29ka BP,the seawater took off this area,and ended the history of the transgression.The sediments more than 600cm belong to last stage of Late Pleistocene,which is approximately line with MIS-2 stage.During the period,the climate began to dry and cold.The Chenghu Area located in the channel of the strong northwest-wind, received the dust deposition and formed the hard clay.It mainly consists of fine silt and clay,and the mean particle-size is from 6.33 to 7.39Φ.The degree of sorting is poor.The average of the magnetic susceptibility is 7.89×10^-8m³kg^-1.The assemblage of the clay minerals is illite-1.4nm mineral-kaolinite-smectite.The main chemical components are SiO₂,Al₂O₃ and Fe₂O₃,the sum of which is 83.31%.The weathering index,CIA,SiO₂/(MgO+CaO),(Al₂O₃+Fe2O₃)/(RO+Na₂O)and Rb/Sr,gradually increases from the lower to the upper part,and the weathering index,SiO₂/ Al₂O₃ and (CaO+Na₂O+MgO)/TiO₂,gradually decreases.It infers that the degree of the temperature and humidity has gradually enhanced.