Japan Geoscience Union Meeting 2018

Presentation information

[JJ] Oral

M (Multidisciplinary and Interdisciplinary) » M-IS Intersection

[M-IS10] Paleoclimatology and paleoceanography

Thu. May 24, 2018 3:30 PM - 5:00 PM A07 (Tokyo Bay Makuhari Hall)

convener:Yusuke Okazaki(Department of Earth and Planetary Sciences, Graduate School of Science, Kyushu University), Atsuhiko Isobe(Research Institute for Applied Mechanics, Kyushu University), Akihisa Kitamura(静岡大学理学部地球科学教室, 共同), Masaki Sano(Faculty of Human Sciences, Waseda University), Hitoshi Hasegawa(Faculty of Science and Technology, Kochi University), Akira Oka(Atmosphere and Ocean Research Institute, The University of Tokyo), Michinobu Kuwae(Center for Marine Environmental Studies), Chairperson:Kuwae Michinobu(愛媛大学沿岸環境科学研究センター)

4:15 PM - 4:30 PM

[MIS10-32] Dating of stalagmites from Kyusendo cave, Kumamoto, using U–Th and radiocarbon dating methods

*Narumi Ishizawa1,2, Yusuke Yokoyama1,2, Yosuke Miyairi2, Takahiro AZE2, Shoko Hirabayashi1,2 (1.Graduate school of science, The University of Tokyo, 2.Atmosphere and Ocean Research Institute, The University of Tokyo)

Keywords:stalagmite, U-Th dating, radiocarbon dating

Recently, stalagmites are used as a good archive for paleoenvirnment. One of the advantages of using stalagmites is precise dating by U–Th methods. In Japan, however, there was no facility for U-Th dating. Although radiocarbon (14C) age is easier to date than U–Th age, it has “dead carbon fraction (DCF)” problem. This means the fraction of 14C–depleted carbon from the soil and host rock (Genty and Massault, 1999; Genty et al., 2001). Because of DCF, results of 14C dating are older than their true age. There are little study about DCF variation in Japan and the driving factors of DCF remain less understood. For precise dating for stalagmites, it is important to solve each problem and combine U–Th and 14C dating.
In this study, stalagmites KST4, 5 and 6 from Kyusendo cave in Kumamoto, Japan were used because U concentrations of the stalagmites were suitable for U–Th dating. Three stalagmites were dated using U–Th methods for the first time in Japan as follows; 41 ka–9 ka of KST4, 25 ka–1.3 ka of KST5, 45 ka–25 ka of KST6. 14C age of stalagmites and drip water were also measured. From drip water, modern DCF was determined. 14C age were corrected with modern DCF and the corrected age of the modern stalagmite showed good agreement with U–Th age.
Availability of U–Th dating methods in Japan would contribute to the activation of Japanese stalagmite research. In the future, by measuring stable isotopes of stalagmites, the paleoclimate system in the area of East Asian summer monsoon could be revealed. In addition, we are going to analyze trace elements and DCF fluctuation and examine the availability as environmental proxy.

Genty, D., Massault, M., Baker, A., Vokal, B., & Proctor, C. J. (1999, September). Reconstitution of bomb 14C time history recorded in four modern stalagmites by AMS measurements: Importance for carbon transfer dynamics. In 8th International Conference on AMS (pp. 6-10).
Genty, D., Baker, A., Massault, M., Proctor, C., Gilmour, M., Pons-Branchu, E., & Hamelin, B. (2001). Dead carbon in stalagmites: carbonate bedrock paleodissolution vs. ageing of soil organic matter. Implications for 13 C variations in speleothems. Geochimica et Cosmochimica Acta, 65(20), 3443-3457.