[M-IS16] 津波堆積物

[MIS16-P08] Unlocking information about reworking movements of tsunami boulders from paleomagnetic dating

*佐藤 哲郎¹、佐藤 雅彦²、山田 昌樹³、斉藤 央岳³、佐竹 健治²、中村 教博⁴、後藤 和久²、宮入 陽介²、横山 祐典² (1.早稲田大学、2.東京大学、3.信州大学、4.東北大学)

キーワード：津波石、年代測定、残留磁気

Fields of reworked boulders are amongst the one of most impressive sedimentary evidence of catastrophic events, such as tsunamis and/or landslides. Tracking their reworking history is important for reconstructing past events; however, the multiple movements of massive boulders remain unresolved. The boulder field on Ishigaki Island has been formed by repeated tsunamis. Although the individual movement histories of the tsunami boulders contribute to retrodict the history of different magnitude tsunamis, their radiocarbon dates indicate an initial tsunami age that detached boulders from the reef. As an attempt to overcome the current challenges for dating the reworking of singular boulders, paleomagnetic dating method is implemented. Reworked boulders are expected to acquire a new magnetic component (viscous remanent magnetization) parallel to the current geomagnetic field. The remanent magnetization gradually grows since the reworking event, which helps to determine the passage of time. In this study, the paleomagnetic dating method was verified by comparison to the radiocarbon ages of un-reworked boulders detached by the recent AD 1771 Meiwa tsunami. In the classical theory, which is applicable to assemblages of fine magnetite particles, samples appear to have anomalously old ages. To estimate the reworking ages of the anomalous boulders, we used an alternative dating protocol which is applicable to admixture of magnetic particle sizes. With this approach, the estimated ages are consistent with their radiocarbon ages. Moreover, the reworking movement was estimated using an additional boulder, whose, using our alternative dating method, radiocarbon age indicated that an older AD 800 tsunami moved it onshore, whereas the remanent magnetization age unveiled a reworking of the boulder attributed to the Meiwa tsunami. This is evidence that the boulder has been moved by repeated tsunamis.

Refences
Tetsuro Sato, Masahiko Sato, Masaki Yamada, Hirotake Saito, Kenji Satake, Norihiro Nakamura, Kazuhisa Goto, Yosuke Miyairi, Yusuke Yokoyama, Two-step movement of tsunami boulders unveiled by modified viscous remanent magnetization and radiocarbon dating, Scientific Reports, 12, 13011, doi: https://doi.org/10.1038/s41598-022-17048-8.