[M-IS34] 古気候・古海洋変動

[MIS34-P18] 大分県稲積洞における大気モニタリングと滴下水の化学分析結果

*進藤 辰郎¹、渡邊 裕美子¹、三島 壮智¹、大沢 信二¹、田上 高広¹ (1.京都大学大学院理学研究科地球惑星科学専攻)

キーワード：古気候学, 石筍, 滴下水, 鍾乳洞内大気モニタリング

10 month cave air monitoring and chemical analysis of drip water were conducted at Inazumi underwater cave, Oita, Japan, from February to December, 2014, to understand the processes and mechanism of recording paleoclimate in stalagmite. Cave air CO₂ was measured for the cave monitoring, and EC, pH, HCO₃^- and Ca²⁺ were measured for chemical analysis of drip water.

Special sampling techniques were designed for two sampling sites, SS2 and SS3, to highlight the relationship between cave air CO₂ and drip water chemistry. At SS2, the drip waters, before and after hitting on three handrails arranged in tandem in limestone cave (SS2-U, -M2, -M3 and -L, respectively), were sampled. At SS3, the drip waters, befor contacting with the cave air (BCWA) and after hitting on the artificial stalagmite, were sampled (BCWA, SS3-U, -M, -L, respectively).

At SS2, as the drip water hit on the handrails, HCO₃^-, Ca²⁺ and EC showed decreasing trend respectively and this trend became significant during winter when the cave air CO₂ was low. Only pH was negatively correlated with other drip water parameters the above. This suggests that the two conditions, 1) the cave air CO₂ is lower and 2) water film gets thinner as the drip water hits on the handrails, promote more CO₂ degassing and CaCO₃ precipitation.

At SS3, HCO₃^- and Ca²⁺ between BCWA and the drip water after contacted with the cave air showed almost the same concentration during summer, high cave air CO₂ season, however, marked significant margin between them during winter, low cave air CO2 season. This result suggests that once drip water contacts with lower cave air CO₂, more CO₂ degassing and CaCO3 precipitation are promoted.

S.I.cc, standing for Saturation Index of Calcium Carbonate, and ∆pCO₂, the difference of partial pressure of drip water (pCO_{2-drip water}) and of cave air (pCO_{2-cave air}), were calculated if CO₂ degassing and CaCO₃ precipitation takes place or not. S.I.cc showed positive value at SS2 and SS3 during almost all monitoring periods, indicating CaCO₃ is produced almost all the time. However, ∆pCO₂ showed frequently negative value at SS2 and SS3, indicating that CO₂ is not degassed from the drip water and the following CaCO₃ precipitation is not taken place and most of drip water sampled already degassed CO₂.

This research implied that CO₂ variation is a main forcing factor to control HCO₃^- and Ca²⁺, main components in drip water to form CaCO₃-made-laminae used for reconstructing paleoclimate; however, the calculation of ∆pCO₂ showed that CaCO₃ would not be produced from drip water. To solve the contradiction, another approach such as development of new sampling method will be necessary.