10Be analysis of the rock samples from the lake terrace of a high-altitude lake (Pumoyum Co, south Tibetan Plateau)
Dr Fumiko Nara1, Dr Takahiro Watanabe2, Dr Yoko Kokubu2, Dr Kazuho Horiuchi3, Professor Liping Zhu4
1Nagoya University, , Japan, 2Japan Atomic Energy Agency, , Japan, 3Hirosaki University, , Japan, 4Chinese Academy of Sciences, , Japan
The Tibetan plateau is the largest and the highest (ca. 4,500 m asl on average) plateau in the world. Because of its topographic features, the Tibetan Plateau has played an important role in global climate changes, especially Northern Hemisphere monsoon circulation. Furthermore, the Tibetan plateau is covered highly sensitive area to the Asian monsoonal activity. This means that the Tibetan plateau is an important key area to understand the hydrological response to the climate changes in the Asian region. Lake Pumoyum Co is located on the south Tibetan Plateau (altitude, ca. 5030 m asl; lake surface area, 281 km²; maximum water depth, ca. 65 m), and a fresh water lake formed by fault activity on the south Tibetan plateau. Precipitation variability in this region is strongly influenced by Indian monsoon activity. Therefore, reconstruction for the past precipitation variability in this region gives us important insight into the past Indian monsoonal activity. The researches on the past climate and precipitation changes during the mid-Holocene using the sediment cores from Lake Pumoyum Co have been carried out. On the other hand, the researches on the past lake volume (lake level) changes are still limited. The lake terraces are developed on the eastern lake shore, and it supposed that the large lake level changes would have happened in Lake Pumoyum Co. The in-situ terrestrial cosmogenic radionuclide (TCN) can be used to estimate the earth surface processes, such as the erosion rate and exposure age dating of rocks. Here we report the results of 10Be values as the TCN of the rock samples from the lake terraces around Lake Pumoyum Co on the Tibetan Plateau. Rock samples were collected on 2009 China-Japan Scientific Research expedition. The ¹⁰Be/⁹Be ratios were measured relative to the ICN 01-5-1 standard (¹⁰Be/⁹Be = 2.709 ± 0.030 × 1/10¹²). The concentrations of 10Be were measured by the JAEA-AMS-TONO-5MV (AMS; 15SDH-2, National Electrostatics Corporation) in the Tono Geoscience Center, Japan Atomic Energy Agency. The 10Be concentrations ranged from 3.78-10.8 × 10⁶ (atoms/g), but the ¹⁰Be values showed the decreasing trend following to the distance from the lake shore. This result indicates that ¹⁰Be values of the rocks at the shore of the Lake Pumoyum Co could be influenced from the erosion rate or tectonic process rather than the exposure date resulting from the past lake level changes.
Presenting author is Fumiko Nara. Since many years, she studies lake sediment in connection with climate and environmental changes. She is currently works at the Geochemical and Cosmochemical Laboratory, Department of Earth Planetary Sciences, Nagoya University, Japan. Fumiko does research especially in lake sediment geochemistry, geochoronology, and paleohydroclimatology.