渝东南近100年石笋灰度变化及气候环境意义

以采自重庆市酉阳自治县天坑洞的石笋TK22-1为研究对象,利用~(230)Th测年、~(210)Pb测年和石笋纹层等计年法建立了近100年来高分辨率、精确定年的石笋灰度序列,并与器测气象资料对比,重建了渝东南近100年的气候环境变化历史,进一步明确石笋灰度指示的气候环境意义。研究结果显示,石笋TK22-1灰度值与当地年平均温度呈显著的正相关关系,与年平均湿度、降雨天数呈显著的反相关关系,这表明温度和湿度等气候条件是驱动石笋灰度值变化的主要因素。渝东南岩溶槽谷区属于生态环境脆弱区,薄层土壤对降水的储存能力较弱,水分是主导土壤湿度变化的主要因素之一。当降水量和地表湿度较大时,雨水在土壤中滞留时间较少、下渗速度加快,水岩作用减弱,土壤水中溶解的有机质和杂质浓度降低,最终在石笋表面结晶出较为纯净的方解石矿物,进而使石笋灰度值降低;反之亦然。当温度较高时地表蒸发量较大,地表及土壤中湿度较小,水在土壤和石灰岩地层中滞留时间长,土壤中有机质和CO_2的释放量增加,洞穴滴水中有机质和杂质浓度升高,导致石笋中方解石结构疏松、杂质较多,石笋灰度值较高;反之亦然。重庆东南岩溶槽谷区近100年期间的干旱事件与石笋TK22-1灰度序列有较好的对应关系,干旱事件发生的年代对应着石笋灰度峰值,即当地降水减少,发生伏旱时,石笋灰度值升高,进一步表明土壤湿度和水岩作用是影响石笋灰度值变化的主要因素。

Changes of stalagmite grayscale in the last 100 years and significance of climate and environment in the southeast of Chongqing

One stalagmite (TK22-1) derived from the Tiankeng Cave in Youyang Autonomous County, Chongqing is used as the research object in this article. We established high-resolution and age-accurate grayscale sequence of the last 100 years with accurate ²³⁰Th dating, ²¹⁰Pb dating, and layer counting. We reconstructed the history of climate and environmental change of the last 100 years in Chongqing to make the significance of climate and environment that the stalagmite grayscale indicated more explicit by comparing with observed meteorological data. The results show that grayscale of stalagmite (TK22-1) is positively associated with the local mean annual temperature, while it is negatively associated with the local mean annual humidity and annual rainfall days. This indicates that climate conditions such as temperature and humidity are the main factors driving the grayscale to change. Karst trough valley area in the southeast of Chongqing is in the ecological environment fragile zone because the thin layer soil has weak storage capacity for precipitation. Water is one of the main causing leading soil humidity to change in this area. With high precipitation and high surface humidity, the retention time of rainwater in soil decreases and infiltration rate speeds up, eventually weakens the water-rock interaction and decreases the concentration of organic matter and impurities dissolved in the soilwater. Thus the purer calcite mineral crystallizes in the surface of stalagmite, and the grayscale decreases, and vice versa. With high temperature and high evaporation, the surface humidity and soil humidity decrease, which prolongs the retention time of water in soil and epikarst zone. The content of organic matter and CO₂ releasing from soil increases. Meanwhile, the concentration of organic matter and impurities in the cave drip water increases, resulting in the loose structure of calcite and more impurities in stalagmite, and the grayscale increases and vice versa. There is a good correspondence between drought events occurred during the past 100 years in karst trough valley area of the southeast of Chongqing and the stalagmite (TK22-1) grayscale sequence. The grayscale peaked when drought events happened. With the local precipitation decreasing, summer droughts occurred, and the grayscale increased, which indicated that soil humidity and water-rock interaction were the main factors affecting the stalagmite grayscale change.