The eruption of Tsumaya pyroclastic flow is one of the three successive activities which took place within Aira caldera, southern Kyushu. The activities started with an eruption of Osumi pumice fall, then followed the eruption of Tsumaya pyroclastic flow, and then ended with the eruption of large scale Ito pyroclastic flow. According to the occurrence of the deposit, it is interpreted that the eruptions occurred within such a short period that the dating method cannot distinguish each eruption. Many
14C ages related with the activity of Aira caldera are shown in Table 2. These ages show widely scattered values, from the youngest age (2, 460 Y.B.P.) to the oldest age (38, 900 Y.B.P.). Younger ages among them are considered to be due to contamination of modern carbon to the dated samples. In order to check the possible contamination of modern carbon to the charcoal samples alkali treatment by KOH solution were carried out on a sample in Tsumaya pyroclastic flow deposit. Then, the two dissolved fractions and the final residue were dated by
14C dating method (Table 1). These results agreed with the age of about 22, 000 Y.B.P. within the range of errors. This shows that no contamination of modern carbon occurred to the dated sample in this case. From discussions on the nature of
14C dating samples and from comparison with the ages by ionium dating and fission track dating, it is suggested that the possible age of the activity of Aira caldera is about 22, 000 years before present.
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