5:15 PM - 6:30 PM
[MIS18-P05] Reconstruction of history of evolution of the South Asian Monsoon based on Pb isotope variations from the Maldives sediments collected from IODP Exp. 359
Keywords:South Asian Monsoon, Pb isotope ratio, carbonate sediments
The South Asian Monsoon (SAM) is one of the most intense climatic elements mainly affecting around India. It has been suggested that an abrupt onset of monsoon winds in the Indian Ocean has been established around 13 Ma based on the presence of drift sediment from Maldives. In this study, we analyzed Pb isotope ratios in marine sediments from the Maldives in order to understand the monsoon-wind-driven current and its intensification, and its relation to the development of the SAM.
In this study, the core of Site U-1468 drilled during the cruise of the International Ocean Discovery Program Exp.359, which has an age from 25 Ma to the present, was used for the analysis of Pb isotope ratios. The core is characterized by coral reefs around Maldives with dead bodies piled up and rich in carbonate components. Therefore, the sample was chemically separated into carbonate and non-carbonate components and Pb were purified on each type of sample. The isotope ratios were analyzed using TIMS (Finnigan MAT262) and MC-ICP-MS (NEPTUNE), and the contents of Pb in 5% of samples were measured by ICP-MS (Aglient 7700x).
As a result of the measurements of Pb isotope ratios of Maldives sediments, carbonate component showed a peak in both 206Pb / 204Pb and 207Pb / 204Pb around 13 Ma, while 208Pb / 204Pb showed no clear variation or trend throughout the period. Especially 206Pb / 204Pb was unusually high (22 – 24) which has not been reported around Maldives. As recently ancient marine carbonates like Archean carbonates was reported to have very high 206Pb / 204Pb (> 20), Dharwar Craton can be considered to be one of candidates of such a high 206Pb / 204Pb ratio detected in this study. In this case, monsoon-wind-driven current has been intensified and carbonate components with high 206Pb / 204Pb has been transported into at least around the Maldives. However, since this peak was seen only at 13 Ma, monsoon current might be intensified temporary in the course of evolution of the SAM. Otherwise, chemical weathering might be intensified due to heavy monsoon rain around this time.
In this study, the core of Site U-1468 drilled during the cruise of the International Ocean Discovery Program Exp.359, which has an age from 25 Ma to the present, was used for the analysis of Pb isotope ratios. The core is characterized by coral reefs around Maldives with dead bodies piled up and rich in carbonate components. Therefore, the sample was chemically separated into carbonate and non-carbonate components and Pb were purified on each type of sample. The isotope ratios were analyzed using TIMS (Finnigan MAT262) and MC-ICP-MS (NEPTUNE), and the contents of Pb in 5% of samples were measured by ICP-MS (Aglient 7700x).
As a result of the measurements of Pb isotope ratios of Maldives sediments, carbonate component showed a peak in both 206Pb / 204Pb and 207Pb / 204Pb around 13 Ma, while 208Pb / 204Pb showed no clear variation or trend throughout the period. Especially 206Pb / 204Pb was unusually high (22 – 24) which has not been reported around Maldives. As recently ancient marine carbonates like Archean carbonates was reported to have very high 206Pb / 204Pb (> 20), Dharwar Craton can be considered to be one of candidates of such a high 206Pb / 204Pb ratio detected in this study. In this case, monsoon-wind-driven current has been intensified and carbonate components with high 206Pb / 204Pb has been transported into at least around the Maldives. However, since this peak was seen only at 13 Ma, monsoon current might be intensified temporary in the course of evolution of the SAM. Otherwise, chemical weathering might be intensified due to heavy monsoon rain around this time.