Paleomagnetism reveals the emplacement age of tsunamigenic coral boulders on Ishigaki Island, Japan

Abstract

ABSTRACTWe use temperature-dependent viscous remanent magnetization to estimate the emplace-ment age of tsunamigenic coral boulders along the shorelines of Ishigaki Island, Japan. The boulders consist of the hermatypic coral Porites , and the time of their deposition by tsuna-mis has been established using radiocarbon dating. Recently deceased corals at reef edges around Ishigaki Island record the Earth’s current magnetic field (present Earth field, PEF) as a remanence parallel to the field in the skeleton. Since the time when the coral skeletons were emplaced on the shorelines as boulders by destructive tsunami waves, a new viscous magne-tization was partially overprinted in the boulder parallel to the PEF. The results of thermal demagnetization indicated that the boulders were rotated at least once, and their emplace-ment ages determined from L. Neel’s relaxation theory for single-domain magnetite agree well with the radiocarbon ages, although there are traces of multidomain magnetites. New applica-tion of Neel’s theory to tsunamigenic coral boulders gives us an opportunity to ascertain the age and transportation mode of individual tsunamigenic coral boulders in this area.