HR: 10:20h
AN: PP32B-01    [Abstracts]
TI: Study on Mg/Sr paleo-thermometer in speleothem: Mg source effect
AU: * Li, H
EM: hli1960@mail.ncku.edu.tw
AF: Department of Earth Sciences, National Cheng-Kung University, No. 1 University road, Tainan, 70101, Taiwan
AU: * Li, H
EM: hli1960@mail.ncku.edu.tw
AF: College of Geographical Sciences, Southwest University of China, No. 2 Tiansheng Road, Beibei, Chongqing, 400715, China
AU: Liu, Z
EM: michelle-lily@163.com
AF: College of Geographical Sciences, Southwest University of China, No. 2 Tiansheng Road, Beibei, Chongqing, 400715, China
AU: Li, J
EM: jxljy@swu.edu.cn
AF: College of Geographical Sciences, Southwest University of China, No. 2 Tiansheng Road, Beibei, Chongqing, 400715, China
AB: The partitioning coefficient (K_Mg) of Mg/Ca between cave dripwater and calcite is a temperature (T) dependent, whereas K_Sr of Sr/Ca has little T influence. Using Sr/Ca variation in a stalagmite to normalize Mg/Ca variation for removal of Mg source change in dripwater, Mg/Sr in stalagmites may be a proxy of paleo- temperature. One of the assumptions for this paleo-temperature indicator requires that Mg/Sr ratio in the source water should remain constant through time. We have investigated Mg/Ca and Sr/Ca ratios in cavewaters and modern calcite deposits in Western Guizhou, China for verifying the assumptions. Cavewater samples from Zhengjia, Shijiangjun and Zhijin Caves in central western Guizhou, China have average Mg/Sr ratios of 1278± 411, 1155± 207 and 14.0± 2.7, respectively. The modern speleothem deposits from the caves exhibit the same feature: high Mg/Sr ratios in samples from Zhengjia and Shijiangjun Caves, and low in samples from Zhijin Cave. This is mainly because high Mg/Ca in dolomite limestone in the area of Zhengjia and Shijiangjun Caves. The above results show that bed-rock compositions affect the Mg/Sr ratios of the cave water. In a stalagmite from Shijiangjun Cave, the Mg²⁺ concentration sharply increased while the Sr²⁺ concentration strongly decreased in the upper 6-cm part where high detrital content of CaCO₃ deposition appeared. The large variations and high values of Mg/Sr ratios in this part of the stalagmite do not represent either the temperature effect on CaCO₃ deposition or influence of bed-rock composition. Instead, such variations may reflect the changes in overlying environment caused by climate variability and human activity. This means that Mg/Sr ratio in source water experienced large changes when the overlying conditions changed. The Mg/Sr in such a stalagmite cannot be used for paleo-temperature reconstruction. Based on the measurements of Mg/Ca and Sr/Ca ratios in both waters and modern carbonates from Zhijin Cave, we have calculated the partitioning coefficients for Mg/Ca and Sr/Ca between calcite and water. The K_Sr value ranges from 0.04 to 0.134, averaging 0.073± 0.035. The average K_Mg value is 0.021±0.010. The values provide us the fundamental data for using Mg/Sr ratio as a proxy of paleoclimate reconstruction.
DE: 0330 Geochemical cycles (1030)
DE: 1030 Geochemical cycles (0330)
DE: 1806 Chemistry of fresh water
DE: 1875 Vadose zone
DE: 4875 Trace elements (0489)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting