Study On The Paleo-climatic Proxies In Karst Areas And The Paleo-climatic Changes Up To 140 Ka BP In Guilin Area

Based on the studies of the oxygen and hydrogen isotopes of the 5-yearrainwater in Guilin area and the drip water, the oxygen and carbon isotopes of thepresent carbonate, and the luminescence and the Mg/Sr, Mg/Ca ratios of No. 1stalagmite from Panlong cave and the Mr/Sr ratios of No.4 stalagmite from Fengyucave, some conclusions have been drawn as follows:1. The oxygen isotopic characteristics are mainly controlled by the monsoontypes and the precipitation cloud cluster origins and types. Theδ¹⁸O values of thesummer monsoon rainwater (mainly controlled by tropical storm) are much lowerthan those of the winter monsoon rainwater (mainly controlled by south-west warmand wet current), averagely, theδ¹⁸O values of the summer monsoon rainfall is -7.68‰(SMOW), and that of the winter monsoon rainwater is -3.48‰(SMOW). Inaddition, in the view of the general characteristics, theδ¹⁸O values of the rainwaterdecrease with the increase of the air temperature (γ=-0.72).2. There is a quite good normal correlation between theδ¹⁸O values of the dripwater and the averageδ¹⁸O values of the rainwater at the same period (γ=0.95),which shows that the characteristics of theδ¹⁸O values of the water for formingspeleothems can reliably reflect theδ¹⁸O characteristics of the rainwater, and then,the climate characteristics.3. The study of the present speleothems shows that when the hydrodynamiccondition is good, the deposit rate is high, and the speleothem is formed in isotopicequilibrium with the drip water, the oxygen isotopes can be as a reliable climaticproxy.4. Under a certain conditions, theδ¹³C values of speleothems can be as anenvironmental proxy, namely, the smaller theδ¹³C values, the larger the proportionof C₃ plants, and the more plentiful the rainfall. On the contrary, the vegetation maybe dominated by C₄ plants or the environment may be a stony desert environmentcaused by climatic changes or human activities.5. The luminescence spectral characteristics can be a reliable climatic proxy,that is, the higher the temperature and the larger the rainfall, the stronger theluminescence of the speleothem.6. Under a certain conditions (relatively stable geological and hydrogeologicalconditions), the Mg/Ca and Mg/Sr ratios of speleothems can be as a temperatureproxy, that is, the higher the temperature, the larger the ratios. But the Mg/Ca andMg/Sr ratios of speleothems depend on not only temperature, but also the contactaction time between karst water and carbonate rocks. The larger the rainfall, thelonger the action time, and the smaller the Mg/.Ca and Mg/Sr ratios.On the basis of the studies above, the high resolution paleoclimatic studies up to140,000 years with 4 stalagmites from Panlong cave, Shuinan cave, Xiangshuiyancave, and Fengyuyan cave have been made, which gives high resolution paleo- climatic records in Guilin area as follows:1. The paleo-monsoon climate since 10800 yr BPâ… : Since 1200 yr BPSince 100 yr BP, the air temperature has been rising, but there was a little dropduring 40 yr BP to 60 yr BP.; 100 yr BP to 180 yr BP, somewhat cold; 180 yr BP to 240 yr BP, somewhat warm; 240 yr BP to 340 yr BP, somewhat cold; 340 yr BP to 440 yr BP, somewhat warm; 440 yr BP to 580 yr BP, somewhat cold; 580 yr BP to 720 yr BP, warm; 720 yr BP to 1200 yr BP, cold.â…¡. 1200 yr BP to 10800 yr BP1200 yr BP to 4500 yr BP: The climate pattern was similar to the present one,but there were several cold and dry periods, that is, 3100 yr BP to 3600 yr BP, and2000 yr BP to 2200 yr BP;4500 yr BP to 7000 yr BP: The summer monsoon was strong, and the periodwas much warmer and more humid than the present.7000 yr BP to 8000 yr BP: It was warm and humid, but the summer monsoonwas weakened.8000 yr BP to 10,800 Yr BP: The temperature rised, and the rainfall increased.The summer monsoon gradually grew in strength.2. Younger Dryas event: It might happen fromll,300 yr BP to 10,800 yr BP inGuilin area.3. 32,500 yr BP to 12,500 yr BP: It was cold and dry, and the winter monsoonwas strong.4. 42,000 yr BP to 32,500 yr BP: It was warm and humid.5. 90,000 yr BP to 42,000 yr BP: The winter monsoon was stronger than thesummer monsoon.6. 125,000 yr BP to 90,000 yr BP: It was warm and humid, but there were severalclimatic fluctuations, that is, 102,000 yr BP to 90,000 yr BP, relatively warmand humid; 111,000 yr BP to 102,000 yr BP, relatively cold and dry; and125,000 yr BP to 111,000 yr BP, warm and humid.7. Before 125,000 yr BP: It was dry and cold.Finally, by correlating the records above with the records from other regions inChina and the global records, and combining with the formation mechanism of themodern monsoon climate, we hold that the paleo-monsoon circulation changes mightbe caused by the changes of the global ice amount.