The Response To Climate And Environment Change Of Trace Element Ratio From Speleothem In Central Western Guizhou, China

We have collected water and carbonate samples from Zhengjia, Shijiangjun and Zhijin caves in central western Guizhou province. The samples have been analyzed for elements including K⁺, Na⁺, Ca²⁺, Mg²⁺, Si²⁺, Ba²⁺, Fe³⁺ and Mn²⁺. We have analyzed the variation of trace elements concentration and Mg/Ca, Sr/Ca, Mg/Sr ratios of two soda straws (SJJ-5, SJJ-6) from Shijiangjun cave. The average Mg/Sr ratios of the cave waters in Zhengjia, Shijiangjun and Zhijin Caves are 1278±411, 1155±207 and 14.0±2.7, respectively. The geochemical feature of cave waters from Zhengjia and Shijiangjun Caves is enriched in Mg²⁺ and depleted in Si²⁺; whereas waters from Zhijin Cave depleted in Mg²⁺ and enriched in Sr²⁺. This feature indicates that not only the bed rocks compositions but also vegetation and soil coverage above the caves affect the Mg/Sr ratios of the cave water. Based on the measurements of Mg/Ca and Sr/Ca ratios in both waters and modern carbonates from Zhijin Cave, we have calculated the co-precipitation coefficient constants for Mg/Ca and Sr/Ca between calcite and water. The K_Sr value changes from 0.04 to 0.134, averaging 0.073±0.035. The average K_Mg ratio is 0.021±0.010. The values provide us the fundamental data for using Mg/Sr ratio as a proxy of paleoclimate reconstruction.There is opposition relationship of Mg and Sr concentration, positive relationship between Ba and Sr concentration, Fe and Mn concentration of soda straw SJJ-5, SJJ-6. The changing of Mg/Ca and Mg/Sr ratio are controlled by the concentration of Mg. The concentration of Sr affects Sr/Ca ratio. Mg/Sr ratio changed obviously during some period. We think that it is not affected by the change of temperature. It may be affected by variation of the soil and vegetation above the cave. The relationship of Mg/Sr ratio andδ¹³C orδ¹⁸O is oppositive. We conclude that the climate was warm and humid when Mg/Sr ratio increased andδ¹⁸O decreased. Based on the variation of Mg/Ca, Sr/Ca, Mg/Sr ratios, we surmise the climate was warm and humid, cold and dry in turn when soda straw SJJ-5 was depositing.δ¹⁸O andδ¹⁸C increased so that we imagine the precipitation decreased gradually since the deposition of SJJ-6 soda-straw. It induced the vegetation above the cave reducing and the soil on the surface being swept. The process reflected in the variation of Mg/Sr ratio and it decreased gradually. We conclude that Mg/Sr ratio of soda straw can reflect the environment changing outside of the cave.Mg/Ca,Sr/Ca and Mg/Sr ratios of the soda straws are similar with those of carbonate samples which are from the top of stalagmites in Shijiangjun cave. It is explained that they deposited at the same phase. The average ratio of K_Mg and K_Sr is 0.00547±0.00629 and 1.751±0.271 in SJJ-5 soda straw, respectively. The average ratio of K_Mg and K_Sr of SJJ-6 soda straw is 0.00202±0.00304, 1.731±0.173, respectively. K_Sr ratios of SJJ-5 and SJJ-6 soda straw are much more than those of modern deposition from the top of stalagmite from Zhijin cave, while they are close to the K_Sr ratios of deposition from the top of stalagmite from Shijiangjun cave. We consider that the speleothem samples collected from Shijiangjun cave are not depositing today.We have argued the geochemical features and Mg/Ca, Sr/Ca, Mg/Sr ratio of the samples from cave water and speleothem in central western Guizhou, calculated K_Mg,K_Sr ratio of the deposition from Shijiangjun cave and compared Mg/Ca, Sr/Ca, Mg/Sr ratio withδ¹³C,δ¹⁸O. We conclude that the climate and vegetation at central western Guizhou province changed largely and trace element ratio can reflect and record the environment change. The ratios including Mg/Ca, Sr/Ca, Mg/Sr are important indexes to study paleoenvironment evolution at at central western Guizhou province.