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
Highlights
The Matuyama–Brunhes (M–B) geomagnetic reversal is the last polarity reversal and one of the most studied paleomagnetic events (e.g., Opdyke et al 1966; Harrison 1974)
We identified magnetic polarities based on virtual geomagnetic pole (VGP) latitudes, where VGP latitudes > 45° were defined as normal polarity, latitudes < − 45° were defined as reversed polarity, and latitudes of − 45 to + 45° were defined as intermediate polarity
Okada et al (2017) revealed that some samples from the Chiba composite section became unstable to thermal demagnetization at temperatures above 300 °C, and the following alternating field (AF) demagnetization between 30 and 50 mT
Summary
The Matuyama–Brunhes (M–B) geomagnetic reversal is the last polarity reversal and one of the most studied paleomagnetic events (e.g., Opdyke et al 1966; Harrison 1974). A remarkably short transition has been identified in exposed lacustrine sediments in the Sulmona Basin of the Central Apennines, Italy (Sagnotti et al 2014, 2016) and marine sediments of the Valle di Manche section from Calabria, Southern Italy (Macri et al 2018) In these studies, the polarity switch at the M–B boundary, dated at ca. These complex behaviors and the lack of truly continuousinstantaneous recordings have resulted in an incomplete picture of centennial-to-millennial-scale field behavior during the M–B geomagnetic reversal
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