Study Of The Precipitation Mechanism Of Tufa At Lake Shore In Badain Jaran Desert

There is debate regarding the formation of tufa and their environmental significance in Badain Jaran Desert,and study on characteristics investigation and origin analysis of tufa are rare.Analyzing the precipitation mechanism of tufa could provide new evidence for the researches of the evolution of lakes or paleoclimate and the origin of groundwater in Badain Jaran Desert,especially while tufa is a product of the chemical action of water.This paper was supported by the Geological Survey Project of China?No.DD20160295?.Lake water and groundwater samples were collected from 7 lakes there are the distribution of tufa in Badain Jaran Desert,then the chemical composition were measured.Lake Cherigele was selected as a typical lake to analyze the precipitation mechanism of tufa while tufa samples were collected from a 3.8 meters deep drilling.We confirmed the sedimentary age and geochemical characteristics in tufa using SEM scanning,energy spectrum analysis,mineral testing and U-Th dating.The laboratory experiment and computer forward or reverse model were carried to study the impacts of several factors,including pH,PCO₂,temperature and the mixing ratio.Finally we clarify the material source and the precipitation mechanism of tufa.The achievements of this thesis are as follows.1:The geochemical characteristics of tufa in Lake CherigeleThe content of CaO is higher than 50%of the tufa samples and the mineral morphology including calcite like hexagonal columniation or triangular pyramid,acicular aragonite demonstrate the low temperature environment while tufa deposited.The ?13C value is correlated positively with ?180 value,in which shows the influence of degassing effect and evaporation.The dating results of tufa in the range of 597-4216a BP shows a confusion that is explained by the coexistence of both bottom-up and late stage filling growth pattern.Since 2000 years,the rate of tufa deposition is fastest.We also found the existence of modern tufa deposition in 2016 and the enhance of hydrodynamic condition is obviously helpful to tufa deposition.2:Hydrodynamic conditions of groundwater-lake systemThe ?D,?180 value and dynamic monitoring data showed that groundwater recharge lakes all the year round and evaporation is the discharge of lakes.3:Hydrochemical characteristics for precipitationResults indicates that the lake water is the type of Cl-Na,along with low concentration of Ca²⁺ and high concentration of CO₃²-+HCO₃^-and groundwater are the types of Cl-SO₄-Na and Cl-CO₃-Na,with low concentration of CO₃^2-+HCO₃^-and high concentration of Ca²⁺.Saturation index of calcite and aragonite in many water samples is more than 0,in which provides basic conditions for tufa deposition.4:The precipitation mechanism of tufa in Lake CherigeleThe precipitation mechanism of tufa in Lake Cherigele is comprised of Mixing between lake water and groundwater,Spring degassing,and Evaporation concentration.And the precipitation process is affected by temperature,pH,PCO₂.According to the quantity of calcium carbonate precipitation from computer model calculation,the order and ratio of contribution from four effects to tufa deposition are as follows:mixing effect?10?>degassing effect?6.2?>evaporation concentration?2.4?>temperature effect?1?.Low PCO₂ value and high pH value in the system are in favor of tufa deposition.The content of CO₂ in the escaping gas from spring is 9 times of that in air,in which demonstrates that CO₂ is mainly derived from the atmosphere and respiration and decomposition in soil.The calcium mainly comes from the dissolution of the ancient lacustrine sediments and aeolian sand.