230TH/234U DATING OF HOLOCENE MOLLUSK SHELLS FROM JEJU ISLAND, KOREA BY MULTIPLE COLLECTORS INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY

The 230Th/234U ages of fossil mollusk shells collected from the Sinyangri and Hamori Formations, the youngest stratigraphic units on Jeju Island, Korea, were estimated using multiple collectors inductively coupled plasma mass spectrometry. Seven aragonite shells yielded 230Th/234U ages ranging from 3434±40 yr to 4980±33 yr (2 sigma mean), in concordance with radiocarbon ages for samples containing little 232Th. Our data suggest that the 230Th/234U method can be a potentially useful tool for dating Holocene mollusks, provided that there is no evidence for severe recrystallization, cementation, and considerable amount of common thorium. An early uptake of uranium immediately after burial and subsequent maintenance of closed system for uranium and thorium can be assumed for our mollusk samples. This study, together with previous radiocarbon and optical dating results, demonstrates that the Hamori Formation where human footprints were recently discovered deposited during the middle Holocene.