Record And Mechanism Study Of Stalagmite δ¹⁸O Since The Middle And Late Holocene In Naduo Cave,Guizhou Province

Global warming and climate anomalies have attracted worldwide attention.The study of global climate change has been paid more and more attention by all countries and fields in the world.Paleoclimate research is an important way to understand past global change,environmental evolution,simulate and predict future climate development.Among all kinds of high resolution palaeoenvironmental record carriers,the karst cave stalagmites have become an important carriers of late Quaternary palaeoenvironment research because of their high precision dating and reliable palaeoenvironmental records.The formation process of stalagmite shows that it is mainly affected by precipitation,ground and cave environment,and the signal of climate environment may also be affected by other factors during the transmission process of karst system.Therefore,there are still many uncertainties and multiple solutions in the interpretation of the environment by using these substitute indicators.The climate and environmental information recorded by stalagmites is preserved by the process of"soil penetration-dissolved bedrock-drip deposition"with precipitation as the transport carrier.How to eliminate the interference of"noise"and interpret the climate and environment information preserved by stalagmite accurately and reasonably?Continuous monitoring of atmospheric precipitation,cave dripping,and new secondary chemical sediments is needed to study how information about contemporary climate and environmental changes is transmitted and preserved in the secondary chemical sediments of caves.It has great scientific significance to reconstruct paleoclimate and paleoenvironment with stalagmite records.In this paper,this study monitored the modern process of secondary carbonate deposition in Naduo Cave,Guanling County,Guizhou province from January 2013 to December 2016.Once a month,the water temperature,p H,EC,HCO₃^-,Ca²⁺,cave temperature and humidity,and CO₂concentration of cave air were measured on site,and the samples were tested for hydrogen and oxygen stable isotopes,anions and cations.In the meantime,the same geochemical index and physical index of the atmospheric precipitation,soil infiltration,surface karst spring water monitoring was also measured and analyzed,In order to understand the hydrodynamic geochemical process of Naduo Cave and to explore the hydrogeochemical characteristics of cave water in response to changes in the external environment.The dynamic changes of atmospheric precipitation in the process of three-dimensional migration from soil penetration,dissolved bedrock,to drip-deposition were monitored,the interaction between"water-rock-soil-gas-organism"in the cave system were discussed,and the significance ofδ¹⁸O recorded in the secondary calcium carbonate sediments in the cave was revealed.In addition,a stalagmites sample collected in Naduo Cave is used to reconstruct the history of local climate and environmental changes of 0.55～5.07 ka BP based on the data of 13 ²³⁰Th ages and 642 groups of oxygen and carbon stable isotopes,combined with the significance ofδ¹⁸O in the new calcium carbonate deposits.The research results are as follows:（1）By analyzing the local atmospheric precipitation,the cave environment of Naduo Cave and the hydrogeochemical characteristics of drip water,it is found that drip water has a good inheritance and response to local precipitation.The p H,EC and HCO₃^-of drip water are affected by the changes of physical conditions such as temperature,precipitation and CO₂concentration,and it also has obvious seasonal variation characteristics.The concentrations of Ca²⁺,Mg²⁺and Sr²⁺were affected by the seasonal effect of CO₂,precipitation dilution,precalcite deposition（PCP）,piston action,water source difference and other factors,and the seasonal changes were also obvious.The Mg/Ca value is mainly affected by the external precipitation.When the precipitation is low,the migration time of karst water in the overlying strata is longer,which is prone to PCP effect,leading to the increase of Mg/Ca.There is a difference in the dissolution rate between dolomite and calcite.When the dissolution of calcite reaches saturation,the dissolution of dolomite continues.At this time,the content of Ca²⁺in water is basically stable,while the concentration of Mg²⁺continues to increase,and Mg/Ca becomes higher.（2）Based on the data of precipitationδ¹⁸O andδD,the equation of Local Meteoric Water Line（LMWL）is obtained,eq.δD=9.04δ¹⁸O+21.65（R²=0.98）.The values ofδ¹⁸O andδD of water drips are uniformly distributed near the LMWL,mostly to the lower right of the LMWL,indicating that precipitation is the main source of water drips in the cave,and that the value ofδ¹⁸O is affected by evaporation.（3）The ENSO activity was significant during the monitoring period from January2013 to December 2016,and precipitationδ¹⁸O recorded the transition between La Ni（?）a and El Ni（?）o:the precipitationδ¹⁸O value during La Nina is negative,and the precipitationδ¹⁸O value during El Ni（?）o is positive.When El Ni（?）o occurs,the subtropical high in the western Pacific strengthens,the share of water vapor from the far source of Indian Ocean decreases,and the near source water vapor from the Pacific Ocean brought by the southeast monsoon increases,which make the precipitation¹⁸O enriched,δ¹⁸O biased.When La Nina occurs,the subtropical high in the western Pacific weakens,and the far source water vapor in the Indian Ocean brought by the southwest monsoon increases,and the¹⁸O in the water vapor is continuously"washed out"along the way,resulting in a negative value ofδ¹⁸O in the precipitation air mass arriving.（4）Due to the mixing effect of karst groundwater and the lag effect of dripping water,theδ¹⁸O andδD of drip water in Naduo Cave were relatively higher in the rainy seasons and lower in the dry seasons,contrary to theδ¹⁸O andδD of atmospheric precipitation.The drip waterδ¹⁸O andδD were generally positive during La Ni（?）a and negative during El Ni（?）o,contrary to the responses of atmospheric precipitationδ¹⁸O andδD to ENSO activities.（5）The variation trend ofδ¹⁸O at the 7 drip water sites in Naduo Cave is consistent,and it has obvious recurrence characteristics.It is indicated that theδ¹⁸O of drip water in Naduo Cave records the same weather signals and seasonal changes outside the cave,which can accurately reflect the variation characteristics ofδ¹⁸O of precipitation,and in turn reflects the variation characteristics ofδ¹⁸O of precipitation.（6）The oxygen and carbon isotopic sequences of 642 groups of stalagmite ND3were established by combining with the age model,and the average resolution was 6～7years.The change of ND3 stalagmiteδ¹⁸O from 5.07 ka BP to 0.55 ka BP can be roughly divided into three stages:positive change—stable fluctuation—negative change.In the stage of positive change,δ¹⁸O value fluctuates from 5.07 ka BP to 2.00 ka BP in the range of-10.8‰～-7.4‰,with an average of-9.9‰.ND3 stalagmiteδ¹⁸O since 5.07ka BP was gradual,the response to abate caused by solar radiation cooling in the north Atlantic,and the cause of the change of the temperature gradient between the north and south hemisphere ocean,and the Intertropical Convergence Zone（ITCZ）Moving southward,resulting in a low latitudes in the northern hemisphere monsoons.The stable stage of theδ¹⁸O values lasts from 2 ka BP to 0.96 ka BP,with the fluctuation range from-10.2‰～-7.0‰and the mean value-8.7‰,recording the 100-year-scale weak summer monsoon events occurring at 1.9 ka BP,1.5 ka BP and 1.0 ka BP.The transition of the ND3 stalagmiteδ¹⁸O from"positive change to stable fluctuation"recorded the"2-kyr shift"of the Asian monsoon anomaly.In the last stage,during the period of 0.96ka BP～0.55 ka BP,the stalagmiteδ¹⁸O continued to be negative,with a range of-10.6‰～-7.2‰and an average of-9.0‰,entering the"medieval warm period".（7）The values of ND3 stalagmiteδ¹⁸O range from-7.0‰to-10.8‰,with a variation of 3.8‰and an average value of-0.9‰.The values of ND3 stalagmiteδ¹³C fluctuate between-1.9‰～-12.3‰,with a variation of 10.5‰,and with an average value of-8.9‰.The variation trend ofδ¹⁸O andδ¹³C was consistent from 5.07 ka BP to2.00 ka BP,but the high-frequency oscillation ofδ¹³C occurred during the period from2.00 ka BP to 0.55 ka BP,indicating two strongly skewed phases.According to correlation analysis,δ¹⁸O andδ¹³C were significantly correlated（correlation coefficient r=0.308,n=318,P<0.001）during the 5.07～2.00 ka BP period.However,during the period of 2～0.55 ka BP,theδ¹⁸O andδ¹³C showed that there was no significant correlation（P>0.05）.Theδ¹⁸O andδ¹³C indicate that the meaning of climatic environment changes asynchronously during the period of 2.00～0.55 ka BP.During the period of 5.07～2.00 ka BP,the influence of human activities is weak,theδ¹⁸O andδ¹³C indicate similar climatic and environmental meanings,both of which change in the same direction（positive correlation）.In other words,whenδ¹⁸O is positive/negative,it indicates that summer monsoon weakens/intensifies and precipitation decreases/increases,leading to vegetation declining/flourishing,biological activity weakening/intensifying,soil CO₂decreasing/increasing,and finallyδ¹³C is also the positive/negative.However,since the 2.0 ka BP,the intensity of human activities,the transformation and influence of surface vegetation have increased,and the native vegetation has been destroyed in large amounts.Therefore,δ¹³C indicates the change of summer monsoon intensity that is not synchronized withδ¹⁸O.