Mechanism Study Of Oxygen Isotope In Karst Records Based On Modern Process Monitoring Of Jiguan Cave, Henan Province, China

With the continuous economic and social development, we are facing a series of unknown global environmental issues, such as the global warming, sea level rising, the scarcity of water resources, the deterioration of the environment and disasters and so on. To scientifically analyze the forming reasons and changing rules, predict its future development trend and propose reasonable solutions are the questions of Global Change Science must answer. This needs us to have a sufficient knowledge and understanding of the past global changes. Stalagmite has attracted more and more attention because of its obvious advantages which other geological records can not instead of, such as its wide distribution, rich proxies, small outside interference, the sensitive growth mechanism to external climate, the precise timing, the wide range of its record and the lower sampling costs and so on. The main proxies of palaeo-climate are oxygen and carbon isotopes, trace elements, deposition rate, stalagmite microlayer, etc. Oxygen isotope as one of these proxies has the most concern and study and has been used most widely. However, due to the diversity of influencing factors and the temporal and spatial differences, there is controversial explanation of the various indicators; therefore it is necessary to have a more comprehensive, deeper interpretation and argumentation to its mechanism through modern monitoring and regional research.In this paper, we select Jiguan Cave which locates in Luanchuan County in Henan province as the fixed monitoring points and take the oxygen isotope as the monitoring proxy. Through a comprehensive monitoring of three hydrological years(from October2009to October2012) of oxygen isotope composition of precipitation in Luanchuan area, cave drip water, modern spelethems, cave pool water and stream water, we have analyzed the evolution and information transmission process of the oxygen isotope and we also have a preliminary study of the mechanism of modern spelethems. At the same time, we do a simple analysis between the air temperature and rainfall of Luanchuan in recent50years with the record of oxygen isotope of cave stalagmite, and have the following preliminary conclusions:(1) The annual rainfall of Luanchuan is mainly concentrated in7-9months, the amount of precipitation in this three months account for more than65%of the annual precipitation. The δ18O composition of precipitation in Luanchuan has an obvious seasonal variation:relatively lighter in summer and fall, relatively heavy in winter and spring. This reflects the precipitation characteristics of the monsoon climate. The maximum of δ18O does not appear in winter which has the least rainfall, but in spring, before the arrival of summer and in this season rainfall begins to increase gradually. This may be related to the water vapor from ocean which brings more negative oxygen isotopic and hasn’t arrived yet in spring, and the ground surface temperature is higher, evaporation is stronger, leading to the heavy isotope enriched. At the same time, the minimum of δ18O does not appear in July which has the most rainfall, but in September. This may be concerned with in the process of forward and back of the water vapor from the ocean, heavy isotope falls several time which makes the remaining water vapor become lighter, even if late precipitation is relatively small, the δ18O is still more negative and this maybe indicate the significant circulation effect of oxygen isotopes. From the annual scale, there is a weak negative correlation between δ18O and rainfall, but still has a certain amount effect. There is no correlation between δ18O and temperature basically. But in summer which has most rainfall, there is a significant negative correlation between δ18O and temperature and rainfall.(2) Seasonal variation characteristics of δ18O in cave drip water are consistent with the rainfall of Luanchuan. The months that appear the maximum and minimum of δ18O in cave drip water and so does the rainfall. This suggests that cave drip water inheritances and reflects the seasonal δ18O variation characteristics especially the summer rainfall very well. Seasonal variation characteristics of δ18O in cave drip water could indicate the seasonal variation characteristics in precipitation. The two cave drip water points have the same variation range and average value and similar variation tendency basically, this indicates that the two cave drip water may be from the same water source and the differences are the transmission pipe lines.(3) Seasonal variation characteristics of δ18O in modern spelethems are consistent with the rainfall of Luanchuan and cave drip water. This suggests that the variation of δ18O in modern spelethems could reflect the variation of δ18O in cave drip water and rainfall; it also could reflect the amount of precipitation especially the δ18O variation of summer rainfall and the change of summer monsoon intensity to certain extent. Under the polarizing microscope we found that the crystal structure of modern spelethems are all calcite deposition, and the amount has a seasonal variation characteristic, much in fall and winter, small in spring and summer.(4) δ18O of the two pool water also has a seasonal variation characteristics: relatively light in summer and fall, relatively heavy in winter and spring. This suggests that the variation of δ18O in pool water also could reflect the variation of δ18O in precipitation. As YZT located close to entrances, air flow is fast, evaporation is strong, and the average δ18O in YZT is heavier than YCG. δ18O of the underground stream in Jiguan Cave does not have obvious seasonal variation characteristics.(5) From the inter-annual scale, there is an anti-phase relationship between temperature and rainfall roughly. Annual average temperature takes on a rising trend; this is consistent with the global warming in recent20years. The variation of temperature and rainfall of Luanchuan faithfully responded two global abrupt climate change event, which occurred in the mid-1960s and late1970s. This suggests that Luanchuan is a sensitive area to the Asian monsoon. The δ18O of stalagmite also record these two events very well. This suggests that short scale monsoon evolution study based on the stalagmite record is feasible. There is a roughly negative correlation between the δ18O record of DSY-1stalagmite and the recent50years rainfall of Luanchuan. This suggests that the δ18O record of DSY-1stalagmite could indicate the rainfall information of Luanchuan area; this is consistent with the result of monitoring.