A full sequence of the Matuyama–Brunhes geomagnetic reversal in the Chiba composite section, Central Japan

• Published in: Progress in Earth and Planetary Science
• Main Authors: Haneda, Yuki, Okada, Makoto, Suganuma, Yusuke, Kitamura, Takahiro
• Other Authors: Japan Society for the Promotion of Science, the Center for the Promotion of Integrated Sciences (CPIS) of Sokendai, NIPR through an Advanced Project, Geoscience Research Grant from the Tokyo Geographical Society
• Format: Article in Journal/Newspaper
• Language: English
• Published: Springer Science and Business Media LLC 2020
• Subjects: General Earth and Planetary Sciences; Byk; Geomagnetic Pole
• Online Access: http://dx.doi.org/10.1186/s40645-020-00354-y; https://link.springer.com/content/pdf/10.1186/s40645-020-00354-y.pdf; https://link.springer.com/article/10.1186/s40645-020-00354-y/fulltext.html

Abstract Geological records of the Matuyama–Brunhes (M–B) geomagnetic reversal facilitate the development of an age model for sedimentary and volcanic sequences and help decipher the dynamics of the Earth’s magnetic field. However, the structure of the geomagnetic field during the M–B geomagnetic reversal remains controversial due to its complex field behavior. In this study, we conducted paleo- and rock-magnetic analyses of samples from the Chiba composite section (CbCS), a continuous and expanded marine succession in Central Japan, to reconstruct the full sequence of the M–B geomagnetic reversal. We define an average stratigraphic position of the M–B boundary and estimate its age based on three sections in the CbCS and a neighboring drill core, TB-2. The average stratigraphic position of the M–B boundary in the CbCS is established at 1.1 ± 0.3 m above a widespread volcanic ash bed (the Byk-E tephra). Assuming a chronological error associated with orbital tuning of 5 kyr and stratigraphic uncertainty of 0.4 kyr, the M–B boundary in CbCS is at 772.9 ± 5.4 ka (1σ). The virtual geomagnetic pole, which is calculated from the paleomagnetic directions, shows several short fluctuations between 783 and 763 ka, with concomitant decreases in geomagnetic field intensity index. After termination of the field instabilities, the field intensity recovered and became higher than before the M–B boundary, with a stable normal polarity direction. The paleomagnetic records in the CbCS exhibit a field asymmetry between the axial dipole decay and field recovery, providing a full sequence of the M–B reversal, suggesting that the non-axial dipole field dominated several times during periods ca. 20 kyr long across the M–B boundary, due to depletion in the main axial dipole component. Our results provide probably the most detailed sedimentary record of the M–B geomagnetic reversal and offer valuable information to further understand the mechanism and dynamics of geomagnetic reversals. Graphical abstract