Response Of The Variations Of Elements In Drip Water To Extreme Drought Event In Maomaotou Big Cave,Guilin City,China

Climate change has always been the hot topic of common concern among human societies,especially,with the increasing of frequency and intensity of extreme droughts under the background of global warming.Speleothems are ideal for reconstructing past climatic and environmental changes because they often grow continuously and year by year,yield data at a high temporal resolution,can be accurately dated,and easy to take.As an intermediary for external hydrological and climatic conditions,cave drip waters are the direct material for the formation of speleothems,which can preserve geochemical information,and thus provide a means to ground truth paleoclimate reconstructions using speleothems.This research chose Maomaotou Big Cave as the research area,which is located in Guilin City,Guangxi Zhuang Autonomous Region,and continuously monitored at seven sites for three years since January 2018.Combining with atmospheric temperature and precipitation of the local meteorological station,the temporal and spatial variations of monitored parameters including cave air temperature,CO₂concentration of cave,drip rate,drip volume and electric conductivity,single element ion concentration(Ca²⁺,Mg²⁺,Sr²⁺and Ba²⁺)and their ratios（Mg/Ca,Sr/Ca and Ba/Ca）and the potentially influencing factors were discussed.Furthermore,the response of variations of elements to changes in the external environment and the relationship between elements and extreme drought events were explored.The main conclusions are as follows:（1）The cave air temperature at each monitoring sites in Maomaotou Big Cave had a seasonal variation with a small fluctuation.CO₂concentrations showed obvious seasonal fluctuation,with higher in summer and lower in winter,showing a gradually decreasing trend.Precipitation played a major role in drip rates and drip volumes spatially.Based on the analysis of drip rates and drip volumes,drip sites are categorized into constant drip sites fed by seepage flows（M-1,M-2 and M-3）and seasonal drip sites fed by fast fissure flows（M-5,M-6 and M-7）.There was a large spatial difference in the EC（EC:Electric Conductivity）.M-5 and M-7 were affected by the precipitation and showed seasonal change of higher in rainy season and lower in dry season,M-6drip site,however,was the opposite.Due to the influence of long-term CO₂degassing,the EC in the pool site was lower than that of constant drip sites.（2）The element concentrations of drip waters(Ca²⁺,Mg²⁺,Sr²⁺and Ba²⁺)in Maomaotou Big Cave were relatively low,and influencing mechanisms were complex.And there were a significant differences in the seasonal and interannual variation,and the responses to the external environment were complex.Not all element concentrations were able to respond to extreme drought events.When summer-autumn-winter drought event occurred in 2019,only Mg²⁺concentration responded significantly,showing a remarkable increase.However,elements ratios（Mg/Ca,Sr/Ca and Ba/Ca）had strong regularity in response to the external environment changes.The Mg/Ca and Sr/Ca ratios of M-5 and M-7 drip site were mainly affected by PCP（PCP:Prior Calcite Precipitation）,showing the characteristics of higher in dry season and lower in rainy season.Among them,Mg/Ca ratios could respond to extreme drought events,showing a significant increase,and this response was not limited by the type of drip water,mainly due to the impact of strong WRI（WRI:Water-Rock Interaction）,PCP,and differential dissolution of bedrock.With the decrease of rain fall from October to December in the dry season,Sr/Ca ratios only at M-5 showed an increasing trend because of the effect of water-soil-rock interaction.The influence mechanisms of Ba/Ca ratios were relatively complicated and could mainly reflect changes in external hydrothermal conditions.（3）It is proposed that when the dry season in Guilin lasts for up to five months and the total precipitation is less thanσ=260 mm,it can be considered that an extreme drought event has occurred,which can cause a significant increase in Mg/Ca ratios in drip water and speleothems,and this relationship is not affected by the type of drip water.Based on this assumption,it is found that the occurrence of extreme drought events in Guilin area may be mainly influenced by the East Asian monsoon and ENSO on the interannual scale after analyzing tδ¹⁸O of stalagmite DY-2 in the cave,and the peak,variation trend and cycle of Mg/Ca ratios.