Studies to attain high accuracy in radiocarbon dating with AMS of sedimentary samples older than 30,000y.B.P.

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Radiocarbon (^<14>C) dating with accelerator mass spectrometry (AMS) has enabled us to measure ages of geological samples as old as 40 to 60 ky.B.P. with a reasonable accuracy. This study has been organized to test the reliability of ^<14>C dates of 40 to 60 ky.B.P. The samples are collected in geological and geographical fields, mainly selected from well-defined horizons, i.e., those horizons identified by tephrochronology.With this investigation, the followings have been clarified, concerning the age dating of old samples with a Tandetron accelerator mass spectrometer:(1)^<14>C dates are consistent with sedimentary horizons, for ages from 40 to 60 ky.B.P.(2)For ^<14>C dates beyond 60ky.B.P., a saturation of ^<14>C dates comes up, and ^<14>C dates are no more consistent with sedimentary sequences. The appearance of the plateau in ^<14>C dates is expected from the observation that dead graphite, which is too old to contain ^<14>C, showed apparent ^<14>C dates of 60 to 70 ky.B.P (average value: 65,080 y.B.P.).^<14>C dates are compared with those dates measured by some other dating methods in this investigation. The ^<14>C dates of charcoal collected from Tokyo pumice flow deposits are compared with a fission-track date of obsidian collected from Tokyo pumice deposits, and the ^<14>C dates of charcoal from the Ontake volcanic deposits with a K-Ar age of lava covering the deposits. The comparisons showed a good agreement. Details of the experiments are given elsewhere. Consistency tests among different dating methods are one of the important themes to be performed intensively.