| Cave drip elements are crucial when act as environment proxies,which are designed to reflect surface climate and material sources,as well as to rebuild palaeoenvironment and paleoclimate.It is recognized by scholars worldwide.There are few researches on underground environment of rocky desertification so far,so it has great potential to use drip elements to explore surface environment of rocky desertification.This study took objects on six caves of different rocky desertification degrees in three areas,they are Xuanmu cave and Shijiangjun cave in Anshun,Chaoyin cave and Hanjiachong cave in Bijie,Daxiao cave and Rongfa cave in Huajiang.An one-year dynamic monitoring was conducted on drips in these caves and a rainfall response monitoring was taken twice on dips in Shijiangjun cave.Influence of cave overlying soil and bedrock to features of drip elements was analyzed and then discussed seasonal variation characteristics of drip elements and their environment significance.With the focus of rainfall response characteristics and hydrogeochemistry process of drip elements,this paper discussed environmental effect of rocky desertification of drips for the first time,so as to provide scientific basis for environmental monitoring of rocky desertification.Research results are as follows:(1)Geochemical characteristics of surface soil in rocky desertification area are mainly affected by soil erosion and weathering and pedogenesis of parent rock.Soil pH increases with rocky desertification degree increasing under the same lithology.Besides,influenced by parent rock lithology,soil pH in limestone area is higher than that in lime dolostone area,Mg content of soil is lower than that in lime dolostone area.Influenced by weathering and pedogenesis of parent rock in research areas of Anshun and Huajiang,the higher the degree of surface rocky desertification is,the more Mg contents in soil.Furthermore,due to the long-term chemical corrosion of drips,cave sediments in sedimentation process present higher pH and elements contents than surface soil,also,there has a distinct positive correlation between sedimentary and each element in drips.(2)Surface soil and development characteristic of bedrock are main factors that influence ionic concentration in drips.Ionic concentrations decrease with higher degree of overlying surface rocky desertification on cave.It is found that by comparison,Mg2+ concentration in drips are distinctly different due to bedrock lithology discrepancies,Mg/Ca and Mg/Sr of drips in limestone area are far below those in lime dolostone area.(3)Drip dropping rate in all the six caves have response variation to rainfall,presents as the dropping rates becomes fast with rainfall increasing and slow down with rainfall decreasing.It is found that by comparison,in same research area,the severer the rocky desertification degree is,the more sensitive the drip dropping rate response to rainfall with larger seasonal variation.In Xuanmu and Chaoying caves that with potential rocky desertification and Daxiao cave with slight rocky desertification,drips pH present low in rainy season and high in dry season,electric conductivities present opposite to that.While in Shijiangjun cave with moderate rocky desertification,Hanjiachong and Rongfa caves with intense rocky desertification,drips pH present high in rainy season and low in dry season,electric conductivities present opposite to that as well.(4)Influenced by surface rocky desertification differences,ionic concentration of drips have significant difference in seasonal variation.In Xuanmu and Chaoying caves with potential rocky desertification and Daxiao cave with slight rocky desertification,ionic concentration in drips present high in rainy season and low in dry season,and drip elements are more contributed by overlying soil.While in Shijiangjun cave with moderate rocky desertification,Hanjiachong and Rongfa caves with intense rocky desertification,ionic concentration in drips present a contrary variation that is low in rainy season and high in dry season,and drip elements are more contributed by overlying bedrock.Moreover,in caves of Xuanmu,Shijiangjun,Daxiao and Rongfa,drips are obviously affected by prior calcite precipitation.(5)Rainfall monitoring shows that: In Shijiangjun cave,influenced by development of surface rocky desertification,the residence time of rainfall in surface soil layer is short while its migration time in bedrock is relatively long.Drips are rapidly response to rainfall.The overlying water-soil-rock effect of drip points get strengthen owe to rainfall,result in that the dissolved ionic concentration and pH in drips increase with rainfall response.By correlation analysis,it is found that sources of Ca,Mg,Sr of drips are the same in Shijiangjun cave and drips are significantly affected by prior calcite precipitation,their geochemical characteristics are mainly contributed by bedrock on cave ceiling.Due to the differences of development degree of overlying soil,fissure and conduit in bedrock,seepage path,water-soil-rock effect of each drip point,there has significant differences in hydrodynamism type and spatial-temporal variation of drip elements in rainfall response. |
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