Abstract
The Ko-f tephra is one of the prominent tephra layers from Hokkaido-Komagatake volcano, southwestern Hokkaido, Japan. It consists of Plinian pumice fall and pyroclastic flow deposits. Previous studies have estimated that the Ko-f tephra was erupted at approximately 3.0 kyr BP, on the basis of a radiocarbon date of 2750±110 yr BP(GaK-2836) for paleosol sample just above the Ko-f tephra. However, we suspect that this age is not acceptable, because 14C dates of paleosol just above a tephra layer are often far younger than its eruption age. In order to determine the eruption age of the Ko-f tephra accurately, we performed 14C dating of paleosol samples just below the tephra (Samples A and B) and charcoal fragments below and above the tephra (Samples C and D, respectively), using a Tandetron accelerator mass spectrometer at Nagoya University.The obtained 14C dates of paleosol, 5480±90 yr BP (NUTA-5783) for Sample A and 5530±80 yr BP (NUTA-5785) for Sample B, are identical within measurement errors. This agreement indicates that the organic matter in both samples was put in a closed system simultaneously when the Ko-f tephra deposited. Thus, it seems that these 14C dates represent the eruption age of the Ko-f tephra. Consequently, the age of the Ko-f is revised to be 5.5 kyr BP. This age is also supported by two 14C dates of charcoal fragments, 5730±80 yr BP (NUTA-5384) for Sample C and 5050±100 yr BP (NUTA-5842) for Sample D. From two soil 14C dates, we can calibrate to calendar year of about 6.3 cal kyr BP.
