甘肃武都万象洞方解石现代沉积控制因素分析

ANALYSIS OF MODERN CALCITE DEPOSITION CONTROLLING FACTORS IN WANXIANG CAVE FROM WUDU,GANSU

对洞穴次生碳酸盐沉积机理的认识是解释洞穴石笋气候环境指示意义的关键。近两个水文年的洞穴现代过程监测结果显示,万象洞内温湿度全年基本保持恒定;洞穴滴水的pH值夏季偏低、冬季偏高,呈现出一定的季节变化特征,电导率和HCO_3^-浓度及主要阴阳离子含量年内变化不显著;洞内CO_2分压夏季偏高,冬季降低接近于当地大气的CO_2分压水平。对不同滴水点的对比研究表明,滴水中Ca^(2+)达到一定浓度是方解石沉积的必要条件。此外,万象洞夏季CO_2分压的升高对方解石形成产生明显的抑制作用。万象洞石笋沉积的主要受控于滴水饱和度以及洞穴CO_2分压。

Secondary carbonate deposition mechanism is the key point for understanding the stalagmite environmental significance.Wanxiang cave is a large cave system which is rare in North West China,and there are abundant stalagmites in the cave which can be well dated,recorded valuable palaeoclimate information.Previous research focused on stalagmite crystallogphy and its responds to climate,the relationship between Asian monsoon changes and Chinese dynasty subrogation,optical characterization and micro-layers reflected stalagmite deposition rate,and crystal characterization is sensitively response to environment and climate.However,there is debate regarding the detail processes and rules of this response.Hence,in this paper monthly in-situ monitoring work which was lasted nearly 2 years（including local precipitation,cave air temperature and relative humidity,drip rates,pH,EC,HCO3- concentration,and cation ＆ anion concentration of the drip water at each monitor sites）was carried out in Wanxiang cave.Wanxiang cave（33°19＇N,105°00＇E;1200m above the sea level）is located in the northwest margin of the Asian summer monsoon,the local climate is sensitively response to Asian monsoon changes.The New Wanxiang cave was discovered in 2007,and the entrance is 10m far from Wanxiang cave. 5 drip sites（X1,X2,X3,X5,and X6）in Wanxiang cave and 1dirp site（X4）in New Wanxiang Cave were selected as the monitor sites,and modern calcite deposition was found at all of the monitor sites,demonstrates that calcite deposition were active recently at these drip sites.Every month,two bottles of drip water were collected at each drip site for cations and anions test using ICP-OES Optima 2100 DV and 761Compacp-IC respectively.Moreover,cave and drip water conditions（including cave temperature,relative humidity,cave air CO2,and drip rate,EC,pH,HCO3- concentration of the drip water）at each drip site were tested monthly.Through the monitor of the cave modern process in Wanxiang cave,the characteristics of cave drip water chemistry,cave conditions and stalagmites deposition situation were obtained.Monitor results showed that the pH of the drip water is lower in summer and higher in winter,indicating an obviously seasonality; the monthly variations of the drip water EC and HCO3- concentration is not notable; The temperature and relative humidity of Wanxiang cave are stable in the whole year; the cave air pCO2 is higher in summer,achieves about 1500 ppm at the bottom of the cave.In winter,the pCO2 dropped to the atmosphere level outside of the cave.Modern deposition at monitor site X4 was actively all through the year,and its concentration of Ca2＋ and HCO3- is higher than other drip sites.There are no modern deposition at drip site X2,X3,X5 and X6 during the whole monitoring,the low concentration of Ca2＋ and HCO3- could caused it.Modern deposition appears in winter from November at drip site X1 when the cave pCO2 is low,and significantly reduced in the next summer from April.Hence,enough Ca2＋ concentration in drip water is an essential condition for calcite deposition,and the cave air pCO2 rising in summer obviously suppressed the calcite deposition.Stalagmite deposition in Wanxiang cave was mainly controlled by the drip water concentration and cave air pCO2.