Brunhes-Matuyama境界年代値の再検討

摘要

従来，Brunhes-Matuyama（B-M）境界の年代値は，深海底堆積物の天文年代および陸上火山岩の~(40)Ar／~(39)Ar年代から約780kaとされてきた．しかし，ここ数年，有意に若い年代値が相次いで報告されている．本稿では，これらの研究のレビューに加えて，新たに深海底堆積物における磁気層序極性反転層準の年代と堆積速度の関係を調べ，これまで深海底堆積物から求めていたB-M境界年代値にはlock-in depthに起因する年代差が生じていることを示す．この結果は，B-M境界年代値が約770～773kaに，その酸素同位体比変動曲線上の位置がMIS19の後期に修正される可能性を示すが，従来の~(40)Ar／~(39)Ar年代とは整合的ではない．今後，この問題を解決するためには，~(40)Ar／~(39)Ar法の適切な標準試料年代や地磁気様相の複雑さも考慮しながら，B-M境界の正確な年代値について深海底堆積物，氷床コア，および火山岩試料から多角的に取り組むことが望まれる．%The Matuyama-Brunhes (M-B) boundary age has been estimated from the astronomical ages of marine sediments and the ~(40)Ar/~(39)Ar ages of volcanic rocks. Although the currently used age for the M-B boundary is 780 ka, recent studies have questioned conventional estimates of the boundary age. In this article, clear evidence is presented for the existence of errors in paleomagnetic dating due to the effect of the post-depositional remanent magnetization (PDRM) lock-in depth, based on a comparison between previously published marine isotope ages for the M-B boundary and sedimentationrnrates. These findings indicate that the age of the M-B boundary should be revised to ca. 770-773 ka and that the boundary most likely lies in the late Marine Isotope Stage (MIS) 19 rather than in the middle of MIS 19. This new age for the M-B boundary is consistent with that obtained from the EPICA Dome C ice core using an EDC3 age model. In contrast, an age offset for the M-B boundary is recognized between marine sediments and recent ~(40)Ar/~(39)Ar ages. To resolve this discrepancy, additional data are required from marine sediments, volcanic rocks, and ice cores.