Optical Dating Of The Topmost Gray Mud Layer At The Shaling Sand Hill In Xingzi County,Jiangxi Province

Several sand hills were developed around Poyang Lake.A detailed chronological study of the sand hill is very important to understand the environmental evolution of Poyang Lake during the Late Quaternary.By field investigations,it is found that there are several gray mud layers in the sand hills near Poyang Lake.Especially,the height of some mud layer is close to,or even higher than the highest water level of Poyang Lake since1949.In this study,we selected the topmost gray mud layer in the Shaling sand hill of Xingzi County and carry out detailed optical dating analysis,including:(1)water content determination for optical dating;(2)quartz OSL ages by using single aliquot regenerative dose(SAR)protocol;(3)K-feldspar IRSL ages by using multi-elevated-temperature post-IR IRSL(MET-p IRIR)protocol.The results were as following:(1)At present,the average water content of the two samples from the topmost mud layer of the sand hill in Xingzi County is measured at～13.5%.To further determine the effect of the water content on the precision and accuracy of the final OSL age,numerical simulation studies were carried out in this study.The results show that the OSL age can increase up to～28%if the water content increase from 1%to 26%.The relative error of final OSL age increases by 0%to 46%,if relative error of water content increased from 5%to 50%(the effect depends on the absolute value of water content and the relative error of water content).The above numerical simulation studies show that water content has significant effect on the precision and accuracy of the final OSL age.In order to further improve the reliability of the OSL dating results,we carry out a new way to obtain the value for the water content of the topmost gray mud layer of Poyang Lake,i.e.by experimental and numerical simulation methods.This new value(14.4±0.9%)is theoretically reliable and is reasonable for the reduction for the error of water content.Accordingly,the precision of the final OSL age can be improved.(2)The over-dispersion(OD)values for the D_eresults of the samples were greater than 20%,indicating that some quartz grains may suffer from insufficient bleaching problem.If minimum age model is applied for the two quartz samples,the topmost mud layer at Xingzi Town is dated between 17.1±2.0 ka and 22.1±2.3 ka.If central age model is applied for the two quartz samples,the topmost mud layer at Xingzi Town is dated between 23.6±1.5 ka and 25.3±1.5 ka.(3)By using MET-p IRIR method,it is found that the two K-feldspar samples do not exhibit D_e-temperature plateau.However,when the annual dose rate errors and D_eerrors were incorporated to calculate the final OSL ages,the two K-feldspar samples show age-temperature plateau,i.e.the K-feldspar 200°C IRSL and the 250°C IRSL ages are consistent within error.In this study,the K-feldspar IRSL signals at 200°C were used for the final age determination of the MET-p IRIR method.Thus,the K-feldspar ages of the topmost mud layer were constrained between 23.7±1.2 ka and 25.0±1.3 ka.Based on above results,it is found that the topmost mud layer in Shaling sand hill formed in MIS 2 stage(glacial stage).Such result is inconsistent with traditional view of the mud layers in the sandhill around Poyang Lake,i.e.although the mud layers in the sandhill around Poyang Lake were thought to be formed a humid climate environment,the chronological data in this study do not support such view.In the furture,in addition climate factors,other factors should be considred in order to better understand the formation of mud layers in sand hills around Poyang Lake.