The ESR Signal Variation Characteristics And Its Chronological Significance For Lava Baked Lacustrine Sedimentary

Electron Spin Resonance(ESR)dating is a new dating method developed rapidly in Quaternary chronology in recent decades and has received extensive attention from the Quaternary scholars.According to the principle of ESR dating,the problem of whether the quartz ESR signal is reset to zero or not is the basis for its successful application in quaternary sediments dating.For lacustrine sediments,the main influencing factors of quartz ESR signal reset to zero include sunlight bleaching and river mechanical action.Therefore,it is important that the quartz ESR signals were bleaching to zero or stable residual value before last deposition for lacustrine sediments for ESR dating.Datong volcanic group is in the Datong Basin in Shanxi province,the northern part of China.Many researchers have studied in this area.According to the researches,the temperature of Datong volcanic lava flows was up to 1100 °C,and it was enough to bleach the quartz ESR signals to zero.The volcanic erupted in this area and the lava flows covered on the lacustrine sediments layers.This area is in the loess accumulation area without depositional hiatus,so we can consider that the baked layer and the unbaked layer were deposited in the same age within the ESR dating error.And then,we try to evaluate the resetting conditions of quartz ESR signal in the lacustrine sediments before last deposition through comparing the ages of the baked layer and the unbaked layer.On the other hand,until now many studies on ESR dating are mainly focused on the determination of sediment age and dating reliability,but there are less studies on the heated characteristics of quartz E' center,Ge center,Al center,Ti-Li center,which were directly related to dating.Especially for the signal characteristics of samples subjected to special effects such as high temperature and high dose radiation.In fact,the essential characteristics of the ESR signal directly determine the dating range,experimental procedure and application of the method.In addition,the characteristics of ESR signal can be used for source tracing,judging heating time and heating history,and has broad application prospects in Quaternary geology studies.Therefore,it is great significance to research the characteristics of quartz ESR signals.In this paper,we plan to compare the ESR age between lacustrine sediment closed to baked layer by volcanic lava and after baked by volcanic lava,and get to evaluate the restetting characteristics of Ti-Li center and Al center by sunlight bleaching and river mechanical action in lacustrine sediment before last deposition,and summarized the effects of high-temperature baking and sunlight bleaching on quartz ESR signals.The results show that:High-temperature baking significantly enhances the sensitivity of the quartz Ti-Li center ESR signal.There is no significant change in the sensitivity of the quartz Al center ESR signal.The sensitivity of the E' center signal increases slightly.There is no Ge center signal before high-temperature baking,and Ge center signal appeared after high temperature baking.The results of the sunlight bleaching experiment showed that after the high-temperature baking effect,the unbleaching part of Al center was reduced,and rest to zero fastly.The increases in the sensitivity of the ESR signals will reduce the relative error of the signal measurement.Therefore,the samples which have been baked at high temperatures in the past,the ESR dating may have better results.The dating result of quartz Al center and Ti-Li center ESR ages show that the time of Datong volcanic eruption is about 400 ka,which is equal with K-Ar dating result within the error.In this study,CT01,CT02,and CT03 were baked by lava,the high temperature heats the ESR signals of the three samples to zero.However,the CT04 was not baked by lava or not change the characteristic of ESR signal for not enough high temperature,so we suggested the Ti-Li center of CT04 was bleached to zero and the Al center of CT04 was bleached to a steady residual value by sunlight bleaching and river mechanical action before last deposit.