日本地球惑星科学連合2015年大会

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口頭発表

セッション記号 M (領域外・複数領域) » M-IS ジョイント

[M-IS34] 古気候・古海洋変動

2015年5月27日(水) 16:15 〜 18:00 301A (3F)

コンビーナ:*山田 和芳(静岡県 文化・観光部 文化学術局 ふじのくに地球環境史ミュージアム整備課)、池原 実(高知大学海洋コア総合研究センター)、入野 智久(北海道大学 大学院地球環境科学研究院)、岡 顕(東京大学大気海洋研究所)、岡崎 裕典(九州大学大学院理学研究院地球惑星科学部門)、北場 育子(立命館大学古気候学研究センター)、北村 晃寿(静岡大学理学部地球科学教室)、佐野 雅規(総合地球環境学研究所)、中川 毅(立命館大学)、林田 明(同志社大学理工学部環境システム学科)、座長:岡崎 裕典(九州大学大学院理学研究院地球惑星科学部門)

17:39 〜 17:42

[MIS34-P17] 鍾乳石の年代決定のための蛍光縞の観察とU-Th年代結果の考察

ポスター講演3分口頭発表枠

*久持 亮1渡邊 裕美子1阿部 勇治2中井 俊一3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、2.多賀町立博物館、3.東京大学地震研究所)

キーワード:鍾乳石, 縞, 古気候

Stalagmites are recognized as a powerful tool to reconstruct paleoclimate. However, it is difficult to date stalagmites. Dating methods of stalagmites are mainly U-Th dating and laminae counting. U-Th dating sometimes shows stratigraphic inverted results, whereas laminae counting has a problem that we have little information about the annual laminae (for example, when and how they are formed? or what they look like?). In this study, I analyzed stalagmite samples collected at Taga Mine, Shiga Prefecture, Japan. I try to reveal the characteristics of laminae in stalagmites and investigate U-Th age.
Stalagmite samples have fluorescent laminae, which can be divided into three types: lamina A, lamina B and wavy lamina. Lamina A is normal type, lamina B is relatively ambiguous lamina and wavy lamina is of wavy shaped. According to FE-SEM image, both lamina A and wavy lamina consist of small particles (diameter is several 10~100nm). Lamina B can not be observed by FE-SEM probably because particles smaller than 10nm can not be resolved due to polishing scratches. Wavy lamina has many gaps filled with mud. This indicates that wavy lamina is formed when mud covers the stalagmites and prevents calcite growing.
In previous researches, main component of fluorescent laminae may be fulvic acid. Thus the small particles of FE-SEM images may be fluvic acid. To confirm this, I use micro-Raman spectroscopy, micro-FT-IR, SEM-EDX. However, all analyses can not reveal what the small particles are, probably because the small particles are too small to analyze. The result of EPMA indicates that laminae A contains mud. However, the strength of laminae A is not proportional to that of Si and Al peaks, hence Si and Al are not Principal component of lamina A.
I dated stalagmite samples by U-Th dating and I got stratigraphic inverted results: the upper parts of stalagmites are older than the lower parts of them. I calculate the quantity of contamination substance in stalagmite which can influence the U-Th age. In consequence, the stratigraphic inverted results of U-Th dating are probably because of contaminations by mud. On the other hand, humic substances in stalagmites possibly influence U-Th dating.