礫岩質ビーチロックの正確な形成年代の決定法 (3)

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  • Physical and chemical analyses to obtain accurate formation age of conglomeratic beachrock (3)
  • レキガンシツ ビーチロック ノ セイカク ナ ケイセイ ネンダイ ノ ケッテイホウ(3)

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The formation age of beachrock has been determined based on the 14C age of biological origin carbonate (i.e. shell and coral) collected from beachrock. However, it indicates the death age of the organism and does not indicate an accurate age of the beachrock formation. Therefore, it is necessary and indispensable to measure directly the age of the cement material of the consolidated intertidal sediments. We sampled cement materials attached on the surface of cobbles or boulders of the conglomeratic beachrocks formed on the coasts of Sasebo, Nagasaki Prefecture, Shimauratou Island, Miyazaki Prefecture, Zamami Island, Aka Island, Tokashiki Island, Geruma Island, Iheiya Island, Bise and Tsuha coast of Okinawa Prefecture. The cement materials were scraped using a small electric hand drill with a stainless steel drill edge of 2−3 mm in diameter and a stainless steel brush of 1 cm in diameter. A sample of 20 mg powder was used for AMS14C dating and analysis of chemical composition by ICP-MS. Except for the Sasebo samples, the cement materials were rich in calcium carbonate; 69.1% for Hiinohama and 85.4% and 74.2% for Zamami samples. However, the carbonate content of the Sasebo sample was as low as 0.6%, and high in Al (6.95%) and Fe (4.91%). The δ13C values of the whole samples except for the Sasebo sample were in a range between −4‰ and +4‰, suggesting that the carbon of the cemented material originated from seawater. The 14C ages of three in situ shell samples (Claudiconcha japonica) lying under the boulders of conglomeratic beachrock of Sasebo were Modern (101.6−102.5 pMC). The formation ages of conglomeratic beachrock collected at Hiinohama of Shimaura Island indicated during 3270−3220 BP and during 2920−2510 BP, respectively. The 14C ages of the two cemented materials collected from Zamami Island were 1043±26 BP and 1020 ±26 BP. Other cemented materials of the beachrock from Aka Island yielded ages of 1531±26 BP and 1336 ±26 BP, being younger than those of shell and coral samples from the same beachrocks. Results of this study demonstrate that the pretreatment method in this study was valid to determine the formation age of the conglomeratic beachrock.

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