Study On The Climatic Implication Of Proxies In Travertine And Their Controlling Mechanisms

The information on climatic and environmental change in the past 2000a is very important to the investigation of the global change.The latter program requires information with higher time resolution so as to find out how climate and environment change,to reveal the regulation of the climate evolution,and to forecast the future climate change.Travertine and tufa in karst areas are one of the most important geological archives for the study of past climate changes because of their high resolution and sensitive record of climate and environment.However,before one uses the travertine or tufa to reconstruct the paleo-climate changes,one must understand the climatic implication of proxies in travertine or tufa and their controlling mechanisms.The climatic implication of proxies in travertine or tufa and their controlling mechanisms are generally complicated and of site-specific.Therfore,the direct application of previous studies at one site to another site may be problematic.Moreover,the previous studies focused mainly on the meteogene tufa records and paid little attention to thermogene travertine records.To understand the climatic implication of proxies in travertine and their controlling mechanisms,a case study at Baishuitai of Yunnan was conducted.The methods of one hydrologic year hydrochemical data logging,in-situ titrating and experiment and laboratory sample analysis were used to understand the spatial-temporal variations in geochemistry and stable isotopes of water and modern travertine at Baishuitai.The following conclusions were obtained through the study:(1) Two vapour sources of precipitation during summer and winter monsoon have been recognized at Baishuitai.One is from the Indian Ocean and Bengal Bay during summer,and the other from the inner continent during winter.The precipitation amount effect on rainwaterδ¹⁸O is significant at Baishuitai.(2) Theδ¹⁸O of travertine was lower in summer rainy season and higher in winter dry season.The remarkable seasonal variations inδ¹⁸O of travertine were due to the precipitation amount effect and the change of vapour sources of precipitation.The quantitative relationship between the travertineδ¹⁸O and the precipitation amount was that the travertineδ¹⁸O decreased by 0.7‰when the precipitation amount increased by 100mm.The temperature effect coefficient on the travertineδ¹⁸O,being -0.1‰/℃was smaller than that in O'neil Equation. (3) The lower travertineδ¹³C values in travertine-depositing canal and travertine-depositing pools during the warm and rainy seasons were related to the dilution of overland flows after rainfall,which was mainly influenced by soil water,and so had lowerδ¹³C values.(4) The deposition rate of travertine decreased in warm and rainy seasons because of the dilution of the rainfall and the overland flows after rainfall,which had lower concentration of [Ca²⁺]and[HCO₃^-].Obviously,the dilution effect to decrease the deposition rate of travertine is stronger than the temperature effect to increase the deposition rate of travertine.(5) The seasonal travertine laminas were distinguished with the color change.The thin brown porous lamina was formed in the warm and rainy season,while the thick dense white lamina was formed in the dry season.(6) The effect of aquatic plants to increase theδ¹³C values and the deposition rate of travertine was weaker than the dilution effect.Therefore,theδ¹³C values of travertine and the deposition rate of travertine were lower in warm and rainy season than in dry season.