3-Million Years Old Stalagmites And Their Significance In Geomorphic Evolution & Paleoclimate-A Report From Wulong,Chongqing,Southwest China

To explore the unknown world is always the yearning and motivation of human life,earth and human beings are closely related.Since ancient times,the study of the earth and the universe has never stopped in the history of mankind,for the study of history is to live for the future development to provide information and direction.The earth still maintained operation rules.The difference is that humans are now facing a series of issuses,such as:global population growth,shortage of resources,climate change,species extinction,disasters caused by natural and human factors,etc.To find the reasons of these problems and solutions,we need to study the inherent law of the earth,the understanding to comprehensive science and better with the earth together.Because of its unique growth environment and generation type,stalagmite have a place in paleoclimate research with many advantages,carbon and oxygen isotopes,various trace elements ratios,grayscale,deposition rate all affected by the climate outside the cave.In addition,stalagmites also record the development of caves,more and more scholars are concerned on the study of the evolution of regional geomorphology through sediments inside caves.However,due to the differences between caves,the changes of various proxy indicators in the infiltration process under rainwater,the subjective differences in the identification of the internal microscopic forms of caves,there is still much controversy about the interpretation of these indicators in stalagmites.Further studies of the indicators in stalagmites are required.This paper selects four stalagmites and six fixed dripping monitoring sites in Furong Cave,Wulong County,Chongqing City for the study.U-Pb dating technology is used for dating the stalagmite,the XRD?mineral composition analysis?experiment was conducted on the peak spectrum of mineral components of stalagmites,and the mineral morphology of modern deposits was observed with a microscope,oxygen isotopes in stalagmites and modern sediments were tested.By using experimental results to analysis the formation time and reason of Furong Cave,regional tectonic movement in the lower region of Wujiang River,the time of the Yangtze River pass through,and the information reflected by oxygen isotopes in stalagmites and modern sediments,and in combination with the responding mechanism of modern precipitation to analysis climatic state of the end of the Pliocene,we draw the following conclusions:?1?Four stalagmites were collected from Furong Cave,we dated them by the U-Pb method,the ages of four stalagmites respective are FR7:3.05±0.13 Ma,FR8:3.0857±0.0039 Ma,FR9:3.033±0.089 Ma,FR10:3.24±0.23 Ma,all ages of them are older than three million years,growing in the end of the Pliocene,which were rare at present.?2?Through the XRD mineral analysis of stalagmite and the observation of the mineral form of modern sediments,there are calcite stalagmites and aragonite-calcite stalagmites in the Furong Cave whether it was in the end of the Pliocene or modern.In this study,recrystallization of calcite occurred in some parts of two aragonite stalagmites,but most of them still maintain the morphological characteristics of the aragonite.This study is still cautious about whether the mineral phase transition of stalagmites can reflect the environment outside the cave and whether the oxygen isotopes of aragonite-calcite stalagmites can reflect the paleoclimate characteristics.?3?The stalagmite growth time can provide evidence for the formation time of the cave.This article combines age data,the altitude of the cave,the height difference between the hole and the ancient water surface,and the characteristics of the regional cave system.Comprehensive analysis of the results of regional tectonic uplift combined with the uplift of the Qinghai Tibet Plateau.It is believed that the formation time of Furong Cave was at least 3.24±0.23 Ma.It formed in the interval between the uplift of the crust rather than the late Pleistocene.?4?The age and elevation of the cave can show the history of regional landscape changes.As the lateral caves mostly formed during the structural stability period.This article believes that the formation of Furong Cave is related to the formation and evolution of the water system outside the cave.Therefore,this article tentatively excludes many unreasonable speculations.Analysis proves that the Three Gorges should incised before the formation of Furong Cave in the late Pliocene or earlier period?before 3 Ma B.P.?which supported the findings of some scholars.?5?Based on 6-year?¹⁸O and?D monitoring records of precipitation from Chongqing,as the obstruct of high altitude Tibet Plateau,we speculate that the north-south position migration of the westerlies and the changes of water vapor route are correlated with the?¹⁸O changes of precipitation in southwest China under the climate change scenario around the North Atlantic.?6?To provide evidence of the Tibet Plateau altitude in Pliocene,we found the Furong Cave stalagmite?¹⁸O records in late Pliocene is lighter than that in Quaternary,and speculate that the moisture component from North Atlantic by westerlies is heavier in late Pliocene,the North Tibet may get modern altitude in late 3.0 Ma B.P..But in the study of the uplift evolution history of Tibet Plateau,we should integrate other subject methods to investigate the mechanism and truth.