[U-06] Evolution of New Seamless Science: From Space, Sun to the Earth Surface

[U06-P07] Seasonal variation of dead carbon fraction in dripwater in the Ryugashi Cave, Shizuoka Prefecture, Japan

*Masayo MINAMI¹, Tomomi KATO², Keiji HORIKAWA³, Toshio NAKAMURA¹ (1.Center for Chronological Research, Nagoya University, 2.Graduate School of Environmental Studies, Nagoya University, 3.Department of Environmental Biology and Chemistry, Toyama University)

Keywords:dripwater, stalagmite, radiocarbon

Stalagmite is a cave deposit precipitated from dripwater. Dripwater contains some dead carbon derived from carbonate-dissolved CO₂ through interaction with cave bedrock limestone, which will make the ¹⁴C ages of the stalagmite older, and so a correction of the dead carbon fraction (DCF) is needed for ¹⁴C dating of stalagmites. In this study, we investigated seasonal variation in ¹⁴C in dripwater in the Ryugashi Cave, Shizuoka Prefecture, to examine the DCF stability in a stalagmite. The results show that ¹⁴C concentration in dripwater was different depending on the site in the Ryugashi Cave, and that the ¹⁴C showed similar seasonal variations at all sites: lower in fall and winter, while higher in spring and summer, though the extent of the seasonal variations was different by site. The ¹⁴C concentration in dripwater tended to be higher (DCF tends to be lower) in dripwater with lower drip rate, indicating that the ¹⁴C in dripwater was correlated with the drip rate, and also correlated with rainfall amount around the Ryugashi Cave.
A growing stalagmite collected from a site in the Ryugashi Cave showed a roughly constant DCF (around 12%) compared with the ¹⁴C with the IntCal13 calibration curve, though the DCF was slightly fluctuated in detail. The results indicate that high-resolution ¹⁴C measurement can be performed on stalagmites in the Ryugashi Cave, and further that the DCF fluctuation observed for stalagmites could give information on change of paleo-rainfall amount. Based on the scenario that the increase in rainfall amount brings the increase in drip rate of dripwater, followed by the increase in soil-derived carbon fraction in dripwater, further followed by the ¹⁴C increase (DCF decrease) in dripwater, the reconstruction of precipitation could be performed using DCF variation in a stalagmite.