High-resolution 10Be and paleomagnetic recording of the last polarity reversal in the Chiba composite section: Age and dynamics of the Matuyama–Brunhes transition

Abstract

We present new magnetic (direction and relative paleointensity) and beryllium isotope (10Be, 9Be and 10Be/9Be ratio) results covering the last geomagnetic reversal, i.e., the Matuyama–Brunhes transition (MBT), from the Chiba composite section (CbCS), east-central Japan. The very high sedimentation rates (>90 cm/ka) of the studied site, a candidate site for the global boundary stratotype section and point (GSSP) of the Lower–Middle Pleistocene boundary, allow the acquisition of a dataset of exceptional resolution. Coupled cosmogenic 10Be and magnetic results measured on the same samples demonstrate that the magnetization acquisition conserved the timing of rapid geomagnetic features, allowing accurate paleomagnetic interpretations. A polarity switch (PS) capturing most of the angular deviation occurring between 771.9 and 773.9 ka was followed by a directional instability rebound (IC1) until 768.5 ka. This sequence with 5.4 ka duration was characterized by a weak dipole field as low as 2.3 ± 0.3 × 1022 Am2. Four rapid episodes of directional instabilities preceding and following the PS-IC1 phase completed the reversal sequence. The asymmetry observed between the long-term dipole decay and sharp recovery, and rapid oscillations (<1 ka) demonstrate the complex transitional field behavior during dipole moment low before reestablishment of the full polarity state. Our observations reinforce the fact that most reversal records do not integrate the full field behavior associated with the geodynamo action. Although this poses problems for understanding the underlying physical processes that produce reversals, it does not hamper stratigraphic correlations among most geologic records.