Paleomagnetic dating of wave-emplaced boulders

Numerous studies have attempted to use boulder deposits in coastal zones for assessing the hazard due to tsunamis and/or storms. One critical problem is still related to the determination of boulder ages. Although the age of wave-emplaced boulders can often be obtained through 14C and U/Th dating, marine organisms that were killed during transport or shortly afterward are required for these dating methods to provide accurate ages. In addition, these approaches cannot determine ages of multiple movements of single boulders with complex transport histories. Paleomagnetic dating offers the potential to overcome current challenges for dating the dislocation coastal boulders. After dislocation, wave-emplaced boulders successively acquire a viscous remanent magnetization (VRM) that is parallel to the Earth’s geomagnetic field. Since the formation of the VRM is a function of site-specific ambient temperature and time, VRM can be used to determine ages for the emplacement of tsunami and storm boulders. This chapter summarizes state of the art sampling, measurement, and analysis strategies for VRM dating of coastal boulders. First case studies from Ishigaki Island, Beppu Bay, and the Sanriku coast (all Japan) illustrate the potential and current limitations of the method when applied to date the dislocation of tsunami and storm boulders.