Geochemical Characteristics And Depositional Environment Of Typical Travertine Profiles In Jiuzhaigou

In recent years,global warming has become a hot research topic.In order to predict future climate change,paleo-climate and paleo-environmental research has also become the focus of climate research,Ice cores,loess,tree rings,and pollen have become the main objects of this research.With the continuous follow-up of research and the advancement of testing technology,calcium carbonate sediments have begun to enter the attention of researchers.Tufa has become a popular research object of global climate and environmental changes in recent years,which is because of its wide distribution,fast deposition speed,and sensitivity to climate and environmental changes.The occurrence of the 8.8 earthquake provided an opportunity for the study of tufa in Jiuzhaigou.The earthquake caused serious damage to the tufa in the scenic area,which dropped its viewing.In order to understand the climatic and environmental information recorded by tufa deposition,clarify its deposition mechanism,and provide a theoretical basis for tufa restoration,this article conducts research on tufa in the Huohuihai Dam in Jiuzhaigou,Sichuan,finds out the geological background of Jiuzhaigou through field geological surveys,at the same time,we were summarized the characteristics of tufa output in the field.According to hand specimens and field survey images,the tufa structure of the Huohuahai Dam is dominated by porous tufa,dense tufa,and detrital tufa.In order to further explore its geochemical characteristics and depositional environment,the mineral composition,mineral content,major elements,trace elements,carbon and oxygen isotopes of tufas in different layers were determined,and the climate and environmental information recorded during the deposition process were analyzed.In this paper,a preliminary analysis of the geochemical characteristics of the tufa and the ¹⁴C dating results of the Huohuahai Dam tufa has been carried out.The results show that the ¹⁴C age of the typical tufa in the study area is between 750 and 210 BP,which belongs to the late Holocene,and the bottom time may extend to the middle and early Holocene,or even earlier.The mineral composition of the tufa was tested by X-ray diffraction,and the mineral composition of the tufa of the sparkling sea dam was mainly composed of calcite and quartz.The black charcoal tufa was mainly composed of muscovite,and also contained muscovite and kaolin.The mineral composition of stone,albite,chlorite and montmorillonite indicates that the mineral composition of dam tufa is mainly derived from carbonate rock and silicate rock.Combined the indoor geochemical experiments with the characteristics of the field,the tufa of the Huohuahai Dam has the characteristics of alternating light and dark laminae.Among them,there are three typical black charcoal tufa layers.The structure is loose and the density is low.The test results show that in terms of isotopes,the black calcified layer has low?¹³C and?¹⁸O,while the light-colored laminar layer is characterized by large thickness,high?¹³C and?¹⁸O.In terms of elements and composition,The depositional environment of the sparkling sea dam tufa is typical anoxic marine salt water deposits.The overall element abundance of black laminar tufa is higher than that of gray laminar tufa.The high content of TFe₂O₃,K₂O,P₂O₅,A1₂O₃,etc.indicates black carbon tufa patterns is greatly affected by terrestrial materials during the deposition process.At the same time,the rare earth elements in dam tufa were analyzed and found that the total content of the rare earth elements differed greatly,LREE/HREE>1,with light rare earth enrichment characteristics,rare earth elements were fractionated to a certain extent,Eu showed negative anomalous characteristics,which proved the provenance supply of the dam is stable;the positive Ce anomaly proves that the depositional environment of the dam is hypoxic.These characteristics indicate that the dark layer is formed in summer and autumn,and the light layer is formed in winter and spring.Finally,based on the geochemical characteristics of tufa and the results of ¹⁴C dating,the tufa of the Huohuahai Dam is divided into three depositional units:a)During the deposition of the3rd unit,at least 10 main sequences composed of alternating deep and shallow carbonates appeared.It is characterized by low-intensity floods and presents the characteristics of enhancement to the second unit;b)Unit 2 shows new flooding activity.The maximum flood recorded by the Huohuahai Tufa Dam brought nearly 50cm of sediment to the center of the lake(the top of Unit 2),and there is no indication that there is more flooding in the lower part of Unit 2;c)A new flood activity record appears in Unit 1,which is slightly lighter than Unit2,and there are two typical deposition records of heavy rainfall activities.Plants on the top are developed,flowing water is continuous,and tufa is still being deposited.The increase in flood frequency in the past 500 years is consistent with other data on climate and environmental changes in my country during the Little Ice Age(15-19th century).The low values of?¹⁸O and?¹³C in the black charcoal tufa layer indicate that the rich flood period is mainly caused by a strong monsoon rain event in the history.Through the analysis of the geochemistry and ¹⁴C dating results of the tufa of the Huohuahai Dam in Jiuzhaigou,it can be found that the typical tufa layer during the tufa deposition process can be used to reconstruct the sediment input changes in the watershed and the lake development history.Greatly improve our understanding of the changes in the paleoclimate and paleoenvironmental information of the Tufa Institute.