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

中文翻译：

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日本中部千叶组合区的Matuyama-Brunhes地磁逆转的完整序列

摘要

Matuyama-Brunhes（MB）地磁逆转的地质记录促进了沉积和火山序列年龄模型的发展，并有助于破译地球磁场的动力学。但是，由于M–B地磁反转过程中的复杂磁场行为，其地磁场的结构仍存在争议。在这项研究中，我们对千叶复合剖面（CbCS）进行了古岩磁分析，CbCS是日本中部连续和扩展的海相演替，以重建M–B地磁逆转的完整序列。我们定义了M–B边界的平均地层位置，并基于CbCS和相邻钻芯TB-2中的三个剖面来估计其年龄。CbCS中M–B边界的平均地层位置确定为1.1±0。在广泛的火山灰床上（比克-E特非拉火山）上方3 m。假设与5 kyr的轨道调谐和0.4 kyr的地层不确定性相关的时间误差，则CbCS的M–B边界为772.9±5.4 ka（1σ）。从古磁方向计算得出的虚拟地磁极在783和763 ka之间显示出一些短时波动，同时地磁场强度指数也随之降低。磁场不稳定性终止后，场强恢复并变得高于M–B边界之前的水平，且法向极性方向稳定。CbCS中的古磁记录在轴向偶极子衰变和场恢复之间表现出场不对称性，提供了完整的M–B逆转序列，这表明非轴向偶极子场在ca期间占据了主导地位。由于主轴向偶极子分量的耗尽，跨M–B边界的长度为20 kyr。我们的结果可能提供了M–B地磁逆转的最详细的沉积记录，并提供了有价值的信息，可进一步了解地磁逆转的机理和动力学。

图形概要