The Study On Trace Element Of Stalagmite And Paleoclimate Reconstruction In Chongqing Since The Last Glacial Period

The global climate change and its impact are the main concern of Quaternary research.The global climate and environment have changed of rapidly since the twentieth century,and a series of crucial environment problems have seriously endangerd to human living and development.So resolving its characters,formation, change mechanism of these environmental problems scientifically is an urgent issue of the entire world,in order to forecast their development trend,evaluate their social and economic impace and put forward some rational countermeasure.To improve the ability of forecasting cimate change in the future,we should have good knowledge of the history,range and mechanism of past global climate change as a foundation of enacting strategy to accommodate the global change.We can reconstruct paleoclimate and paleoenvironment change mode by studying speleothem records,which can provide the scientific foundation for sustainable development between human and nature in the Karst region.High resolution cave speleothem study has become one of the most important development directions in international paleoclimate research currently.Chongqing is exactly located in the cross area of east-Asian monsoon and southwest monsoon system,which is a typical site to discuss and study of Asian monsoon.There are large distribution of carbonate rock and abundant speleothems in caves of Chongqing area. Cave stalagmite has contained plentiful physical and chemical proxies that have possibility of recording climate change,but the climate explaination based on each proxy may have some difference because of spatio-temporal diversity.The carbon and oxygen stable isotopes have been considered as a more mature proxy currently,by which paleoclimate reconstruction work in Chongqing has been obtained primary achievement.Subsequently we started to develop trace elements research and to recognise important factors which are controlled by climate and environment change so as to choose some fungible climate proxy of trace element.It has very important significance of trace elements research for validating each other among multi-proxies. It also can provide a new way for reconstructing paleoclimate and plaeoenvironment in Chongqing aera and also supply a gap of trace element research in Chongqing area.The main contents in our research can be separated into two parts,including observation of Karst cave drip water and paleoclimate records achieved from stalagmite.Through the preliminary research in paper,we can get the following conclusions:(1) Geochemistry informationhe from cave drip water which has less disturbance of groundwater and is mainly made of precipitation infiltrating karst cave can reflect climate change outside and cave speleothems formed by it have more possibility to record exact climate information.In this paper,elements change of drip water from Xueyu Cave can reflect rainfall condition change outside quickly and exactly.While drip waters from Furong Cave lagged and most of which were mixed with old groundwater.The element changes from Furong Cave water are more complex,and reflect climate change less exactly than that from Xueyu Cave.(2) The content and ratio of trace elements from cave drip water have definite seasonal change.It is mainly ralated with two factors:(1) the degree of soil eluviation and rock erode;(2) the groundwater residence time.The effects of two factors are opposite,but one of them may become a leading factor at different time.According that we can also deduce climate information from trace elements change.Speleothems Mg/Ca composition from Furong Cave and Xueyu Cave are related to drip water Mg/Ca composition,which can be used to reflect rainfall condition change.(3) The trace element records of stalagmite in Chongqing can be used as climate and environment proxy.Mg/Ca ratio may be the best one.Compared with carbon and oxygen stable isotope research,we find that Mg/Ca ratio has better correlation with carbon stable isotope,which indicate that it may be used as a climate proxy of the past rainfall.It also accord with the conclusion from trace element research of cave drip water.Whereas it is noticeable that any other factors can also affect Mg²⁺ content in original solution,just as D_Mg(distribution coefficient change by temperature), vegetation,soil microbe activity and so on.So we consider that when using Mg/Ca ratio to recover paleoclimate,we should collate it with carbon and oxygen stable isotope in order to avoid wrong information. (4) The correlation among different trace element of one stalagmite can reflect whether the factors which control trace element change are the same.As a whole, Mg/Ca ratio has low relativity with Ba/Ca ratio,and we may consider that they are respectively controlled by rainfall and soil microbe.On the other hand,Sr/Ca ratio can keep positive relativity with the former two ratios,so it indicates that Sr/Ca ratio may be controlled by the former two factors.In a word,it is more credible to use Mg/Ca ratio as climate psychrometer which has less external influence and it is questionable to use Sr/Ca and Ba/Ca ratios as one climate proxy because they are sensitive to many factors just like vegetation,soil microbe activity.(5) When the oxygen isotope change is in consistent with carbon isotope,trace element change(Mg/Ca ratio) are also the same as stable istopes and paleoclimate information recorded by them are in accordance.Reversely,trace element and stable isotope record may have different paleoclimate information.We wonder that whether climate information of trace element record is true only in the condition of typical monsoon climate model? We still have more evidence to confirm it.(6) Mg/Sr ratio change is in accord with Mg/Ca of stalagmite record in Chongqing,which mainly affected by Mg²⁺ content change and it can be used as an proxy of past rainfall not temperature.Only when the original trace element content or ratio in karst water has no or little change can Mg/Sr ratio be used as paleo-temperature proxy.Otherwise we should consider fully various factors which have an effect on trace element change in order to make sure that these climate information records of trace element are true.(7) The Mg/Ca ratio of stalagmite record in Chongqing can be used as a proxy of past rainfall.However the precipitation and temperature conditions not always have isochronous change.If some extreme climate event happened in falling temperature but with on rainfall change,and it is possible that the Mg/Ca ratio may have no sensitive to these events.