11:00 AM - 1:00 PM
[MIS02-P05] Reconstruction of the evolution of the South Asian Monsoon using Pb and Sr isotope ratios from the Maldives sediments
Keywords:Maldives sediments, South Asian Monsoon, Pb isotope, Sr isotope
The South Asian Monsoon (SAM) is one of the most intense climatic elements mainly affecting around India. According to previous researches, the SAM initiated in a weaker state at ca. 20 Ma and then got intense to modern pattern and strength around 13 Ma based on the presence of drift sediments from the Maldives and/or Nd isotope records from Malta and the Maldives. In this study, we analyzed Pb and Sr isotope ratios, which have possibilities to constrain monsoon intensification in detail, in marine sediments from the Maldives in order to understand how their variation relates to the development of the SAM.
Here, we used two cores drilled during the cruise of the International Ocean Discovery Program Exp.359 for the analysis of Pb and Sr isotope ratios. One is the core of Site U-1468, which has an age from 25 Ma to the present, and the other is Site U-1467, which is mostly composed of drift deposits from 13 Ma to the present. 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 fractions. Pb and Sr were purified on each type of sample. The isotope ratios were analyzed using TIMS (Finnigan MAT262).
As a result of the measurements of Pb isotope ratios, 206Pb/204Pb and 207Pb/204Pb ratios in carbonate fractions showed two higher peaks around 22 Ma and 13 Ma, while the peak was not seen in 208Pb/204Pb ratios. Especially, 206Pb/204Pb in the Maldives was unusually high (22 - 24) compared with previous records around the Maldives. Recently, ancient marine carbonates like Archean carbonates were reported to have very high 206Pb/204Pb (> 20), thus Dharwar Craton can be considered to be one of candidates of such a high 206Pb/204Pb ratio detected in this study. As for Sr isotope ratios, 87Sr/86Sr of non-carbonate fractions was much higher than carbonate fractions from 22 to 19 Ma and from 8 Ma to the present, whereas both carbonate and non-carbonate fractions showed the almost same ratio between about 13 and 10 Ma. Considering different behaviors of Pb and Sr in the ocean, variations of these isotope ratios can assess the SAM intensification and it is suggested that water circulation and monsoon-wind might be intensified between 22 - 19 Ma and 13 - 10 Ma in the course of evolution of the SAM.
Here, we used two cores drilled during the cruise of the International Ocean Discovery Program Exp.359 for the analysis of Pb and Sr isotope ratios. One is the core of Site U-1468, which has an age from 25 Ma to the present, and the other is Site U-1467, which is mostly composed of drift deposits from 13 Ma to the present. 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 fractions. Pb and Sr were purified on each type of sample. The isotope ratios were analyzed using TIMS (Finnigan MAT262).
As a result of the measurements of Pb isotope ratios, 206Pb/204Pb and 207Pb/204Pb ratios in carbonate fractions showed two higher peaks around 22 Ma and 13 Ma, while the peak was not seen in 208Pb/204Pb ratios. Especially, 206Pb/204Pb in the Maldives was unusually high (22 - 24) compared with previous records around the Maldives. Recently, ancient marine carbonates like Archean carbonates were reported to have very high 206Pb/204Pb (> 20), thus Dharwar Craton can be considered to be one of candidates of such a high 206Pb/204Pb ratio detected in this study. As for Sr isotope ratios, 87Sr/86Sr of non-carbonate fractions was much higher than carbonate fractions from 22 to 19 Ma and from 8 Ma to the present, whereas both carbonate and non-carbonate fractions showed the almost same ratio between about 13 and 10 Ma. Considering different behaviors of Pb and Sr in the ocean, variations of these isotope ratios can assess the SAM intensification and it is suggested that water circulation and monsoon-wind might be intensified between 22 - 19 Ma and 13 - 10 Ma in the course of evolution of the SAM.