[U-06] 宇宙・太陽から地球表層までのシームレスな科学の新展開

[U06-P07] 静岡県竜ヶ岩洞の滴下水中dead炭素率の季節変動

*南 雅代¹、加藤 ともみ²、堀川 恵司³、中村 俊夫¹ (1.名古屋大学年代測定総合研究センター、2.名古屋大学大学院環境学研究科、3.富山大学大学院理工学研究部)

キーワード：滴下水, 石筍, 放射性炭素

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.