Optical Luminescence Dating Of Holocene Sediments In The Upper Hanjiang River And Weihe River Basin

The climatic change during the late Quaternary has been the focus of geomorphologists.The Holocene have a close relationship with the development of human society is the nearest period to modern time and the geological evidence is preserved relative completely,for example,the loess-palaeosoil sequences in Chinese Loess Plateau which recorded the information of global climate change is one of the most important mediums.In the Hanjiang and Weihe River basin,the loess-soil sequence along the river terraces of the river valley has often recorded the Holocene climate change information.However,how precisely dating these sediments is the key to obtain high-resolution climate change information.Climatic hydrological events are regional response to global climate change,thereby reconstruction the palaeoflood events ages and its palaeoenvironment in this area is good help to deeply investigate deposit formation?pedogenesis and response mechanism of climatic driving factors.By using optically stimulated luminescence(OSL)dating technology,a large number of ages of loess-paleosol and palaeoflood slackwater deposit(SWD)in the Hanjiang and Weihe River basin were systematically measured.Accordingly,the ages of the palaeoflood events were determined and the causes of climatic hydrological events were also studyed.Field investigations were carried along the upper reaches of the Hanjiang River,Beiluohe River,Weihe River and Qianhe River(the tributaries of Weihe River),13 typical Holocene loess-paleosol profiles was selected and 8 of these profiles with interbedded palaeoflood slackwater deposits(SWD).75 samples for OSL dating was taken from palaeosol,loess,SWD,slope sediments and pottery with the strict sampling procedures.Based on the standared experimental procedures,we have made a large number of experimental measurements,mainly including sample pre-processing,measurements of radioelements(e.g.U,Th,K)and soil moisture,and measurements of equivalent does(De)for these samples,and then calculated does rates(Dy),equivalent does(De)and OSL ages for all samples.In this paper,experimental of temperature setting conditions before measurements of equivalent does(De)was studyed.In addition,a large number of experimental studies and theory investigations have been made for facters that influence the De measurements,main including a preheat plateau test the rational combination of preheat temperature and cut-heat temperature),(De(t))plots experiments,sensitivity correction experiments and the experiments of anomalous fading of luminescence signals.Meanwhile,dating results and the facters that influence the De measurements was discussed.According to systematical experiment and analysis,the results show the age sequence of loess-palaosoil stratigraphy in the upper reach of Hanjiang River and Weihe River basin and the chronological framework of palaeoflood events occurrence,and then the relationship between regional palaeoclimatic and abrupt global climatic events was studyed.Based on the study we have obtained the following major conclusions:1.Preheat plateau test was performed using 20 ? steps from 180? to 300? with the program of blue-light stimulated luminescence(BLSL)of single-aliquot regenerative-dose protocol(SAR).The preheat plateau was clearly identified from 200 ? to 260 ?,and the De values were independent on the preheat temperature but obvious dependence on cut-heat temperature.For loess samples,the equivalent doses estimated by BLSL and post-IR OSL show an obvious plateau between 200? and 220? cut-heat,and the difference of equivalent doses estimateed in this cut-heat temperature range are at least.Therefore,preheat at 200?-260? for 10s and cut-heat at 200?-220? for 10 S are relatively suitable to estimate equivalent doses.For palaeoflood SWD samples,the equivalent doses estimated by BLSL and post-IR OSL also show an good plateau between 160? and 220 ? cut-heat,and the difference of equivalent doses estimated in the cut-heat temperature range from 160 ? to 220 ? are few.Accordingly,preheat at 200?-260? for 10s and cut-heat at 160?-220? for 10s are suitable for palaeoflood SWD samples.With temperature increase,the thermal transfer is only 0.04Gy-2.5Gy for all the samples.In the dose recovery experiment,the ratios of recovered dose versus applied dose close to 1,suggesting that the applied dose was recovered well in this primary test condition.and recuperation of remaining aliquot is below 5%for all the samples.2.The measurement OSL ages of coarse particle and the size of particulate quartz with different excitation modes,the results indicates that the De from different kinds of sediments to difference.However,there is no obvious different for loess,soil and pottery shard samples,which have been sufficiently bleached at the time of deposition.For relative completely bleached palaeoflood SWD,the equivalent doses estimated by IRSL show overestimate,while the BLSL and Post-IR was a few difference.For partial bleaching of the palaeoflood SWD and loess,the equivalent doses estimated by IRSL?BLSL show overestimate and Post-IR show middle of them.Therefore,different excitation modes shows the degree of corrected sensitivity changes different.The sensitivity changes corrected by Post-IR signals is better than by IRSL signals and BLSL signals.3.Based on the bleaching experiments of particulate quartz,the results show that most of the loess samples?palaeosoil samples?and brick pottery samples were completely bleached with little residual signal while few loess samples contaminated by slope deposit show partial bleaching characteristics.And very few palaeoflood SWD were completely bleached at deposition.The residual OSL signals may lead to an age overestimation because of partial bleaching at the time of deposion.For these samples,we selected the aliquotes relatively better bleached for De determination,which can obtain relatively real ages.4.There is no significant differences between BLSL ages and Post-IR OSL ages for Holoence loess-paleosol samples.Howerer,the palaeoflood SWD show obviously differences,ages estimated by BLSL are older,ages by Post-IR OSL are close to real ages.Additionly,there is also significant differences between the upper reach of Hanjiang River and Weihe River,the former error is greater which range from7.5%to 21.6%while the latter ranges from-1.1%to 9.09%.There are similar characteristics with palaeoflood SWD for loess-paleosol samples partial bleached,the errs of ages estimated by BLSL are range from 3.5?19.4%and the the errs of ages estimated by Post-IR OSL are range from 1.6-5.1%.5.Basis on the OSL dating results,grain-size distribution and the magnetic susceptibility,the study provides an elementary OSL age framework for Holocene loess-palaeosol profiles in the upper reach of Hanjiang River and Weihe River.Investigations were carried along the upper reach of Hanjiang River and Weihe River Holocene profiles,the two regional river valley terraces widespread loess-paleosol and palaeoflood SWD.It was cold and wet during the late Pleistocene(11500 a B.P.)and frequent dust storms led to the formation of Malan loess layer Li.It is a climate shift from cold-wet to warm-humid during the early Holocene(between 11500-8500 a B.P.)and dust storm activity gradually decreased to form a transitional loess layer Lt.When entered the "climatic optimum" period until 3.1 ka B.P." there have developed paleosol of S0 in this area,but the "climatic optimum" is do not a continuous warmer and wetter period,which interrupted by two abnormal cooling-drought climate events taking place about respectively 6000?5000a B.P.and 4200?3800 a B.P..It tended to be cooler and drier during the late Holocene(since 3000 a B.P.),and dust storms were obviously inrensified to form modern loess L0.6.According to the OSL dating results,the Holocene palaeoflood events chronological framework is established in the upper reach of Hanjiang River and Weihe River.There are several palaeoflood events occurred respectively in 9500?8500?3200?2800?2500?2300?2300?1800 and 1200?1000 a B.P.in the upper reach of Hanjiang River.The palaeoflood episodes took palce at 2300?1800 a B.P.in the Weihe River and Qianhe River Valley(the tributaries of Weihe River)flooding episodes occurred between 6000?5000 a B.P.;the flooding episodes of Beiluohe River occurred in 8000?7500?6000?5000 and 4200?4000 a B.P.,respectively.These flooding episodes are corresponded to global cold-drought period in the early Holocene about 9000?8000 a B.P.,abnormal cooling-drought climate events about 6000?5000 and 4200?3800 a B.P.,and the transient period from" Climatic Optimum" to cooler and drier late Holocene period about 3500?2500 a B.P..Accordingly,the climatic highly instable and precipitation variable in the episodes of climate changes of being cold/warm and climatic transition may be the main reason of palaeoflood occurrence in these areas.