Effects Of ENSO Circulation Conditions On The Modern Process Of Oxygen Isotope In Karst Cave

Since the second half of last century,the global temperature has risen rapidly and extreme weather events occurred frequently.How will the climate change has become an important topic for the scientific circle and the public,and global change is a significant factor influencing the stability,prosperity and sustainable development of human society.Past is the key to the present,it is clear that the laws of climate change and the driving factors in the historical period are the basis for making accurate prediction for future climate change.The karst cave sediment-stalagmite has been developed quickly in the past 20 years because of its precise dating,widely distributed,rich climatic index and weak external disturbance.The scientific community has not reached a consensus on the understanding of oxygen isotope,which is the main proxy for climatic interpretation.And the fact that stalagmites formed in same period recorded the similar fluctuation pattern in Chinese monsoon region makes the previous interpretation of δ¹⁸O as the change of temperature,rainfall or monsoon strength skeptical.Recently,the proposal of "circulation effect" perhaps explains the consistency of oxygen isotope record at the regional scale,but it need to be testified by modern cave monitoring.Continuous monitoring of modern cave is an important means to link modern climate records with paleoclimate.In particular,it needs further study to verify whether "circulation effect" of oxygen isotope is recurrent.In this paper,a consecutive monitoring of the Jiguan Cave,Luanchuan in western Henan Province is carried out for a period of seven years（covering a complete process of El Ni?o → La Ni?a → El Ni?o）.During the period,the local precipitation was collected in each event,and the collection and oxygen isotope analysis of different types of water（drip,pool and underground）and modern sediments were carried out at intervals of 1 or 2 months.Study area is located in the central region of typical pivotal part of China’s geographic north-south boundary,which is mighty sensitive to the Asian monsoon.The results of impact of ENSO circulation conditions on the oxygen isotope of atmospheric precipitation and the feedback of cave are expressed as follows:（1）During the monitoring span,the ranges of istotopes in precipitation at Luanchuan are-16.08‰⁸.08‰,-118.29‰⁴5.31‰ for δ¹⁸O and δD respectively.There is an obvious seasonal pattern in the isotopes: light in wet season and heavy in dry season.There is not a significant correlation between δ¹⁸O and rainfall amount and air temperature.Combined with LMWL of singular year and multi-years,it is concluded that the rainfall amount and air temperature are not dominant factors to determine the δ¹⁸O of precipitation.The results show there is a significant interannual variation in the annual rainfall weighted average of precipitation δ¹⁸O: positive in El Ni?o year and negative in La Ni?a year.Using HYSPLIT Model to simulate the vapor trajectories in several important wet seasons,results show the vapor trajectories from the Pacific were apparently significant than those of the Indian Ocean during major vapor transport height（850hPa）in El Ni?o year,while in La Ni?a year,the Indian Ocean is the main source of marine vapor.The oceanic Nino index（ONI）,calculated by nino3.4 SSTA running average,has a good correlation with the annual weighted average δ¹⁸O of precipitation（r=0.72）.The multiple linear regression shows the sum of variance contribution of ONI,temperature and precipitation is 73%,and ONI is the most important factor.（2）Based on the influence of fissure and aquifer storage in the overlying bedrock,the variations of δ¹⁸O in drips are weaker and there is not an evident seasonal pattern.The δ¹⁸O of drips are comparatively heavy in the initial stage,then keep stable at a negative value for four years,and become positive again in 2015.Moreover,the mean value of δ¹⁸O is lighter than that of precipitation,which indicates the evaporation of rainwater is not obvious during the infiltration.And there is a “threshold effect” in the infiltration,so drips are mainly recharged by the lighter rainfall in the wet season when the vapor is dominantly from distant ocean,which are similar to the precipitation that is affected by ENSO circulation.The cave underground oxygen isotope has a similar change pattern compared with drips,but it is not same to the pools whose isotopes are heavier in dry season and lighter in wet season because of evaporation and ventilation.（3）The δ¹⁸O of modern calcites in Jiguan Cave exhibits a significant seasonal pattern in 2010 and 2011: negative in summer and fall,but positive in winter and spring.The interannual change is consistent with drips and precipitation.In other words,they inherit the interannual variation pattern of δ¹⁸O in drips,which means the change of δ¹⁸O in precipitation caused by ENSO circulation could be recorded in modern calcites.（4）The precipitation formed by vapor from different sources can be recorded in the isotopes of rainwater at single heavy rain scale.The δ¹⁸O of precipitation from local vapor is little heavier than that of formed by distant marine vapor.And evaporation can weaken temperature effect,meanwhile,the slope,intercept of LMWL and excess deuterium all decrease.Rainfall can be reflected by drip and underground.The cave drip has the fastest response to the rain,and the δ¹⁸O of drip became heavier when dripping rate increased and then lightened slightly.The underground has a similar pattern,but with a slight lag.Pools responded differently,the one near entrance could reflect the different stages of rainfall via the fluctuation of δ¹⁸O,but the other in the deep did not have an evident response to external rainfall.The δ¹⁸O of drip and underground are significantly lighter than that of precipitation,indicating they are mainly recharged by the infiltration of precipitation from distant marine vapor in summer monsoon.