[S-EM22] 地磁気・古地磁気・岩石磁気

[SEM22-03] 韓国白翎島の玄武岩からの古地磁気強度測定

*山本 裕二¹、安 ヒョンソン² (1.高知大学 海洋コア総合研究センター、2.韓国地質資源研究院)

We report new absolute paleointensity (API) data from basaltic rocks probably aged ~ 4-5 Ma in Baengnyeong Island, Korea, together with paleodirectional and rock magnetic results. Paleodirectional analysis obtained an overall mean direction of D = 347.3 and I = 38.3 (a₉₅ = 4.9, k = 113.4) corresponding to a virtual geomagnetic pole at 342.1E and 70.2N. Comprehensive rock magnetic analyses identified Ti-poor titanomagnetite with, in part, multi-domain (MD) particles as a main carrier of remanent magnetization. The Tsunakawa-Shaw (TS) method yielded 12 qualified API estimates with a high success rate, efficiently removing possible MD influences, and resulted in a mean value of 13.1 uT with good precision (1.7 uT, standard deviation). The Thellier method of the IZZI protocol with pTRM checks, coupled with the use of a bootstrap approach instead of the "conventional best-fitting" in API determination, gave 6.6-19.7 uT as a 95% confidence interval of its mean API estimate, which supports the reliability of our TS-derived API mean estimate; but it is not considered in the final mean value because of the relatively large uncertainty. The virtual dipole moment corresponding to the TS-derived API mean, 2.9 (+/- 0.4) × 10²² Am², is somewhat lower than the expectations of the past few Myr averages. Combined with a global API database, our new data implies a larger dispersion in the dipole moment during the early Pliocene than previously inferred.