Environmental Significance Of The Modern Karst Cave In The Western Of Henan Province

With the development of human society economy and the improvement of the level of industrialization,global climate change have attracted more people’s attention.As a geological carrier of paleoclimate records,stalagmites in caves have abundant surrogate indicators,such as,oxygen,carbon isotopes and various trace elements（Ca,Mg,Ba,Sr）and their ratios,which can provide a variety of paleo-environmental information,e.g.paleo-precipitation,paleo-vegetation evolution,paleo-hydrogeology and so on.Moreover,stalagmites have developed rapidly over the past 20 years due to its precise dating,wide distribution,abundant climatic proxies and weak external interference.However,the formation process of stalagmites is complicated,and various substituting indicators have multiple solutions and complexities.Especially the mechanism of oxygen and carbon isotope,and its environmental significance are still unclear.Therefore,it is necessary to explore the mechanism of carbon isotope change through modern monitoring of caves.The paper has carried out a 9-year continuous monitoring in Jiguan Cave,Luanchuan country,Henan Province,China.The variation characteristics of stable carbon isotope have been analyzed on different time scales and different spatial scales,and we also combined with the geochemical background,atmospheric circulation,groundwater hydrochemical characteristics,and the changes of cave CO₂ concentration,the control mechanism and environmental significance.The main Conclusions are as followed:（1）Theδ¹³C value of the overlying vegetation in the Jiguan Cave ranged from-28.1‰to-23.2‰,respectively,and theδ¹³C value of the overlying soil ranged from-18.5‰to-13.5‰,respectively.The average values ofδ¹³C_DIC in the two drip waters in the cave are-12.9‰and-12.1‰,respectively,and the average values ofδ¹³C_DIC for the modern sediments are-9.2‰and-10.6‰,respectively,indicating that carbon isotopes have a consistent isotope fractionation process in the migration process of vegetation,soil water,cave water,and modern sediment,in addition,modern sediments responding to changes to the external environment of the cave have been monitored.（2）Theδ¹³C_DIC of cave water shows an obvious seasonal and inter-annual variations due to the effects of soil CO₂ concentration,cave air CO₂ concentration,cave drop velocity,and water-rock interaction time of cave water migration in the pipeline.In general,the rainy season ofδ¹³C_DIC of cave water is lighter,and the dry season is heavier.In addition,in the rainy season,the average value ofδ¹³C_DIC is-11.8‰,and the average value of dry season is-10.63‰.On the change of day and night scale,theδ¹³C_DIC of cave water of is mainly affected by the degassing effect of CO₂,which shows theδ¹³C_DIC is lighter in daytime,and theδ¹³C_DIC of nighttime is heavier.（3）“Autumn Rain in Western China”is stronger than normal year when La Ni?a event happened.Owing to the long-distance（Indian Ocean）water vapor transport of La Ni?a event in 2011,and drip and modern sedimentδ¹⁸O on the autumn lighter than normal year,and the value was-10.1‰and-10.5‰,respectively.Similarly,the drip waterδ¹³C_DIC of the cave responded to the influence of“Autumn Rain in West China”,which was more negative than normal year in October 2011,and the value was-12.8‰.The analysis ofδ¹⁸O andδ¹³C of modern sediments formed by dripping water in Jiguan Cave was consistent with the fluctuations of atmospheric and drip water oxygen isotope.The oxygen isotope variation of atmospheric precipitation dominated by ENSO variability,and finally exhibiting in the cave sediments of the Jiguan cave.（4）The stalagmiteδ¹³C is mainly affected by the overlying vegetation and soil CO₂on the long-term scale.However,on a short time scale,hydrodynamic conditions and human activities of the CO₂ can affect the stalagmiteδ¹³C.Through modern monitoring,the stalagmiteδ¹³C on short-term scale can record the change of local ambient temperature and precipitation,especially for extreme weather events.The impact of atmospheric circulation based on modern monitoring forδ¹³C will also be reflected in stalagmites,and the impact of ENSO events should be considered when we reconstructed the change of paleo-enviorment via high-resolution carbon isotope of stalagmites.