桂林凉风洞洞穴空气及滴水水化学对区域环境的响应
Response of cave air and hydrogeochemistry of drip water to local climate in the Liangfeng cave,Guilin City
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摘要: 针对广西桂林凉风洞洞穴空气(CO2、δ13CCO2)和滴水的物理、化学指标(水温、电导率、pH、Ca2+、HCO3-)开展一年监测。结果表明:受到洞穴上覆土壤层CO2的影响,洞穴空气CO2和δ13CCO2分别呈现出明显夏季高、偏轻,冬季低、偏重的季节性变化规律;各个监测点CO2和δ13CCO2存在差异的主要因为是受到洞穴内部结构的阻隔作用,以及土壤层CO2在岩溶表层带不同滞留时间的影响,但是δ13CCO2变化相对于CO2对于外界环境的响应更加灵敏,且不同季节洞穴的通风模式差异使得洞穴空气CO2影响因素存在差异,夏季土壤CO2为主要影响因素、冬季大气CO2为主要影响因素。与此同时,洞穴滴水水化学指标也均表现出明显季节性变化特征。虽然雨季携带进入管道或裂隙的CO2增加,但是与表层岩溶带“老水”混合使其整体上呈现为缓慢上升。桂林地区季节性干旱使得洞穴顶部入渗水补给量减少,导致对围岩的溶解量相对降低,使其成为水化学指标快速下降的主要影响因素。此外,每个滴水点的水化学指标变幅因其径流路径的不同存在差异。Abstract: he hydrogeochemistry of cave drip water is an important environmental index in cave systems, to which the monitoring may be an essential solution for paleoclimate reconstruction. In this study, we collected samples from three drip points in the Liangfeng cave of Guilin City, southwest China, from January to December 2017. During the monitoring period, we measured the hydrogeochemical properties of the drip water (electrical conductivity, pH, Ca2+, HCO3-), and CO2 concentration was simultaneously collected at the same monitoring points. Then these monitoring proxies were compared with regional atmospheric temperatures and precipitation amounts. Results show that the hydrogeochemical index exhibits seasonal variations as water transforms from precipitation to cave drip water. Under the influence of the CO2 in the overlying soil layer on the top of the cave, the cave CO2 and δ13CCO2 present obvious seasonal variation patterns of high and light in summer, and low and heavy in winter, respectively. Differences in monitoring site CO2 and δ13CCO2 values are caused by the barrier of inner structure of the cave and retention time difference in epikarst. However, δ13CCO2 amplitude relative to the CO2 response is more sensitive to outside atmosphere environment, and cave CO2 the major influence factors are obviously different in different seasons. The major influence factor is soil CO2 in summer, while atmospheric CO2 is the major influence factor in winter. The hydrochemical index of drip water shows obvious seasonal variation. Although the precipitation amount increases in summer which bring more soil CO2 into the pipes or fissures of the cave, the mixing of precipitation with "old water" in the surface karst zone makes it rise slowly as a whole. The decrease of seepage water supply caused by seasonal drought in the Guilin area leads to the relative decrease of the dissolution of surrounding rock, which makes it become a hydrochemical index. In addition, the variation of the hydrochemical index of each drip point is mainly due to its different runoff paths.
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Key words:
- Liangfeng cave /
- cave CO2 /
- δ13CCO2 /
- hydrogeochemistry characteristic
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