[M-IS04] Evolution and variability of the Asian Monsoon and Indo-Pacific climate during the Cenozoic Era

[MIS04-05] Sedimentary features and isotope records of the upper Eocene travertine-lacustrin carbonate in the SE Tibet and implication for hte monsoon climate

★Invited Papers

*佐久間 杏樹¹、狩野 彰宏¹、柿崎 喜宏¹、多田 隆治^2,3、Zheng Hongbo³ (1.東京大学大学院理学系研究科地球惑星科学専攻、2.千葉工業大学、3.雲南大学)

キーワード：始新世後期、トラバーチン、湖成炭酸塩、年縞堆積物、炭素・酸素同位体、アジアモンスーン

The Asian monsoon is one of the largest climatic systems in the world but estimates of its onset vary from the late Eocene to the Quaternary (e.g. Licht et al., 2014). We investigated the sedimentary environment of the late Eocene Jiuziyan Formation, a terrestrial limestone unit that occurs a few localities in the Jianchuan basin, Yunnan Province in China. Previously, this has been characterized as lacustrine carbonate and the transition to the palustrine deposit was interpreted as the appearance of wetter climate during the late Eocene (Sorrel et al., 2017). Our observations of macro- and microfacies revealed sedimentary fabrics indicating rapid CaCO₃ precipitation, such as dendritic calcite and calcified reed stems, which are unlikely to develop in a simple lacustrine setting. These fabrics, together with clearly higher δ¹³C values (-0.7 to +6.9 ‰) and lower δ¹⁸O values (-14.6 to -10.5 ‰) than those of lacustrine marlstone of the overlying Shuanghe Formation shown by Wu et al. (2018), indicate that the limestone was at least partly travertine, carbonate formed from thermogenic (endogenic) spring water. In addition, the occurrence of centimeter-scale lamination coupled with cyclic changes in δ¹³C and δ¹⁸O is almost identical with the modern travertine reported from Baishuitai in northern Yunnan Province (Sun and Liu, 2010). The centimeter-scale lamination is annual and the cyclic changes in δ¹³C and δ¹⁸O imply seasonal temperature and precipitation change, which have comparable amplitude to the record of the modern travertine at Baishuitai. Our results do not contradict late Eocene wetting and additionally suggest the development of a high-CO₂ aquifer in the underground, which was likely associated with a tectonic activity in the southeastern Tibetan Plateau. This might be related to the shift from the dry condition of the Baoxiangsi Fm. underlying the Jiuziyan Fm. to the wet condition of the Shuanghe Fm.