[S-EM18] Paleomagnetism and rock magnetism applied to solving geological and geophysical problems

[SEM18-P10] Centennial-resolution terrestrial climatostratigraphy and Matuyama-Brunhes transition from a loess sequence in China

*兵頭 政幸¹、番匠 健太²、楊 天水³、加藤 茂弘⁶、時 美楠³、安田 裕紀⁴、福田 惇一¹、三木 雅子²、ブラダック バラージュ⁵ (1.神戸大学　内海域環境教育研究センター、2.神戸大学大学院理学研究科惑星学専攻、3.中国地質大学、4.北海道大学地震火山研究観測センター、5.ブルゴス大学物理学科、6.兵庫県立人と自然の博物館)

キーワード：地磁気逆転、カラブリアンーチバニアン境界、東アジアモンスーン、レスの磁気学

Terrestrial records of the last geomagnetic reversal often have few age constraints. Chronostratigraphy using suborbital-scale paleoceanic events during marine isotope stage 19 may contribute to solving this problem. We applied the method to an 8-m-long, high-resolution paleomagnetic record from a loess sequence in China, and revealed millennial-to-sub-centennial scale features of the Matuyama-Brunhes (MB) transition. All samples were subjected to progressive thermal demagnetization with 14-15 steps up to 650-680°C. As a result, 96 % of the samples yielded a high-quality remanent magnetization. The MB transition terminated with a 75-cm-thick zone with nine polarity flips. The polarity flip zone, dated at about 779-777 ka, began between the warm events “I” and “J” and terminated at the end of the cooling event coincident with the lowest axial-dipole strength interval. Most polarity flips occurred within 70 years. The virtual geomagnetic poles (VGPs) in the upper polarity flip zone clustered in the SW Pacific region, where the MB transitional VGPs from lavas of the Hawaiian and Canary Islands and lacustrine deposits of Java also clustered. These sites were probably dominated by dipolar fields. The absence of transitional fields across polarity flips implies a short time span for averaging fields due to a thin loess-magnetization lock-in zone. The reverse-to-normal polarity reversal dated at about 778 ka in Lingtai occurred at the end of the SW Pacific VGP zone, an important key bed for MB transition stratigraphy. The reversal is a good candidate for the main MB boundary. We found an excursion at about 766 ka spanning about 1 ka.