Study On Sr And Nd Isotopes Of Caihe Red Clay At Jingning County,Gansu Province

The Loess Plateau aeolian sedimentary sequence in China include Quaternary loess-paleosol sedimentary sequence and Neogene period red clay sedimentary sequence.As a continental deposit,it is the best evidence for studying global climate change.With the progress of mod ern science and technology,research on the loess paleosolis becoming more and more mature.The domestic and foreign scholars gradually shifted their attention to the accumulation of newly deposited red claysince the huge thick Mesozoic of the lower part of the Loess Palaeogene.It is believed that the deposit of red clay in this layer is consistent with the overlying loess-paleosol,and it is a continuous accumulation.It also records the changes of paleoclimate and geological structure in the interior of A sia since the late Cenozoic.It is one of the best carriers to study the formation period of East Asian winter and summer monsoon,the change of strength,the process of drought in the interior of Asia and the influence of the tectonic uplift of the Qingha i-Tibet Plateau on the change of the interior environment of Asia.In this paper,the magnetic susceptibility of Caihe red clay was studied in Jingning County,Gansu Province,and the origin and field of Ca i He red clay was traced.Combined with the Sr-Nd test,the source area of the red clay in Caihe was tracked.The contents and conclusions of the specific research are as follows:?1?.Based on the magnetic susceptibility test of the red clay in Caihe of Jingning County,the chronological framework of the sedimentary sequence was established.By comparing the magnetic susceptibility curves of several typical sections such as Qinan and Zhuanglang,the trend of summer monsoon in the Loess Plateau since early Miocene has been studied.The results show that the variation of magnetic susceptibility in the Caihe profile is higher in the upper and lower parts and lower in the central part.The magnetic susceptibility of the upper 14m is basically 30×10^-8m³/kg,with a large fluctuation range.The middle part is basically around 20×10^-8m³/kg,maintaining stability.The lower part is slightly higher than the middle part,and the change is relatively large.The age of red clay accumulation in Ca ihe is about9.7Ma¹4.12Ma,and the total time span is 4.42Ma.The frequ ency susceptibility of Caihered clay is verydifferent from that of other red clay on the Loess Plateau.It's low frequency and high frequency magnetic susceptibility values are very low.The negative frequency susceptibility of Caihe red clay can not be explained from the angle of high frequency magnetic susceptibility.In order to get a reasonable explanation,it also need to systematically study the magnetic properties of Caiheredclay.?2?.The Sr-Nd isotope test of the Caihe red clay in Jingning County,Gansu Province is carried out.The source and process of dust in the red clay since the Miocene were studied.And it provide new evidence for revealing the characteristics of climate environment in the middle Miocene of Asia.The results showed that the Nd isotopic values of the 20 samples of the Caihe red clay fluctuated from-11.1 to-8.7,with an average value of-9.8.The ratio of ^{8 7}Sr/^{8 6}Sr is 0.719458,the lowest is0.710993,the fluctuation range is very large,and the average value is 0.717238.The Sr-Nd isotopes of loess in different sections of the Loess Plateau are basically consistent with the ^{8 7}Sr/^{8 6}Sr values and Nd values of Caihe red clay.It shows that the source characteristics of Caihe clay are consistent with the provenance characteristics of the Loess Plateau.The Sr-Nd isotope of Caihe red clay profile has fluctuated from14.1Ma to 9.7 Ma,and the value of?_Nd has decreased to 2.4 units,^{8 7}Sr/^{8 6}Sr value increased from 0.716539 to 0.719458.Based on the variation trend of Sr-Nd isotopes along the age of Caihe red clay,combined with the Sr-Nd isotopic variation of the Qinan-Lingtai comprehensive section made by predecessors,it is believed that the uplift of the Qinghai-Tibet Plateau is the fundamental reason for the change of Sr-Nd isotopes in the red clay of Caihe.Following the uplift of Qinghai-Tibet Plateau during the Miocene,the Qilian mountains on the northern margin at this stage?at least between 14-9.7Ma?also gradually increased.The uplift of the mountain provides material source for the dust,which is transported to the Alashan plateau,providing sufficient dust for the Badain Jaran Desert and the Tengger Desert,and then deposited on the Loess Plateau,forming the accumulation of red clay.