Study On The Upper Junction Of Thermoluminescence Dosage In Loess Deposition

There is a large area of loess in the Loess plateau, China, which is an important area of aeolian loess deposits. Loess deposits, ice cores and deep-sea sediments are the three pillars of paleoclimate, paleoenvironmental evolution studies. The loess deposited in the Loess plateau records the paleoclimate change for 240Ma. The good corresponding between the environmental records deduced from loess deposits and deep-sea sediments during the counterpart period suggests that they are entirely able to reflect the characteristics of glacial-interglacial climate cycles in Quaternary period. Meanwhile, loess is closely related to the development of the prehistoric civilization. Many palaeoanthropology archaeological sites in stone age were buried in loess sediments. The loess deposits of Middle Pleistocene and the early stage of Late Pleistocene seem no effective method to determine the geological age. For discontinuous loess sequence, approximately stratigraphic layers are difficult to divide. This problem restrict the further development of the studies on environmental change and paleoanthropologic evolution. It is of practical significance to solve the problem of the luminescence dating method which can be used for the blind area from Loess Layer 2 to Brunhes/Matuyama boundary (B/M boundary).Thermoluminescence technique has been a focus as a technique to develop potential of thermoluminescence dating. The aim of this work is to investigate the upper limit of luminescence dating for loess deposits. The luminescence properties of quartz and feldspar which are common minerals in loess sediments have been investigated. The basic characteristics and general principles of luminescence process of minerals are concluded based on pure quartz and orthoclase. A set of loess samples were collected from loess sediments in Luo Chuan Loess plateau. Two apparent age sequences have been obtained from these samples with whole-rock materials and poly-mineral components. The equivalent doses of whole-rock have been calculated with regenerative method. The equivalent doses deduced from several regions in the range of 300-400℃ increases with the stratigraphic depth.The equivalent doses in different depths can be divided into three parts which can represent the position of the sample in the section to some extent. The apparent TL age calculated from equivalent dose which is deduced from the range of 355-395℃ can be used in stratigraphic division and geological age determination.The thermoluminescence apparent ages of poly-mineral components obviously increase with stratigraphic depth. The direct upper limit of thermoluminescence dating for loess desposits is 129ka. It can be directly used for loess sediments since last interglacial. Although the apparent TL ages increase the depth below Loess Layer 2, the apparent TL ages are systematically underestimated. The estimation can be corrected with the law of mid-term fading of TL signal. To be compared with real geological ages, a set of correction coefficients have be calculated. The correction degree of these correction coefficients increase with depths of the strata. These correction coefficients can be correctly used in stratigraphic division for loess deposits and obtained the geological ages of the correspondent layers.The indirect upper limit of thermoluminescence dating for loess deposit is 576 ka with regenerative method using single aliquot. For the samples collected below Loess Layer 6, the indirect upper limit of multiple-aliquots-regenerative method is 754 ka. Different indirect upper limits of luminescence dating can obtained from different methods. Validation and application of these methods are attempted in other two loess sections, and the preliminary results indicate that these methods enlarge the upper limit of thermoluminescence dating.The relatively luminescence intensity of poly-mineral components of loess samples collected from Luo Chuan Section has a tendency to increase with depth. The relative luminescence intensity can be used to roughly determine that the samples collected from which layer in the Loess Strata. By comprehensive consideration, the relative luminescence intensity and apparent TL ages of the whole-rock and poly-mineral components of loess samples can be used in determining the relative layer of loess stratas. It is of practical significance to roughly divide the discontinuous layers of loess stratum. The correction coefficients obtained from a known-age loess sequence can be applied in to discontinuous loess section in related region. This work aims to preliminarily explore the upper limit of the age determination of loess samples with thermoluminescence dating. It provides us a new idea and view for indirectly improving the upper limit for the thermoluminescence dating of loess sediments. It also gives us a practical way to determine the ages of old loess layers.