Abstract
We present a novel paleomagnetic record for the lower Matuyama chronozone, which includes the Réunion subchronozone and the lower Olduvai polarity reversal, from a continuous section of a 168-m-thick on-land marine succession in the southernmost part of the Boso Peninsula, central Japan. In this section, the Réunion subchronozone and the lower Olduvai reversal are observed at 38.6–44.6 m and 142.0 m, respectively. The average sedimentation rates between the lower and upper Réunion boundaries and between the upper Réunion boundary and lower Olduvai boundary are calculated as 25 cm/ky and 57 cm/ky, respectively. The virtual geomagnetic pole (VGP), observed in the Boso Peninsula, at both the upper and lower Réunion boundaries passed across the equator within a similar longitudinal band over Africa. Immediately below the upper boundary, between 43.0 and 43.5 m, the VGP settled in a cluster area around China. Relative paleointensity (RPI) values for the entire Réunion interval are generally lower than the average for the entire interval from the Réunion to the lower Olduvai subchronozone. Conversely, the VGP for the lower Olduvai reversal boundary did not pass across the equator within a narrow longitudinal band but settled in several cluster areas; i.e., the southern Indian Ocean, North America, and the southern South Pacific Ocean off South America. The VGP then moved rapidly between the clusters. The locations of VGP cluster areas in the lower Olduvai reversal seem to coincide with areas where a vertical component of the present geomagnetic non-axial dipole (NAD) field is dominant. During the reversal, the RPI declined rapidly and recovered slowly as the VGP moved rapidly between cluster areas. Our new paleomagnetic data are one of the most detailed records for those geomagnetic reversals from marine sediments, and will, therefore, help to understand the dynamics of the geomagnetic reversals.
Highlights
Geomagnetic reversals are one of the most significant fluctuations in the Earth's magnetic field, during which the magnetic intensity decreases by an order of magnitude as the polarity transitions (e.g., Merrill and McFadden 1999; Valet and Fournier 2016)
Studies based on several high-resolution reversal records suggest that the behavior of the geomagnetic field during polarity transitions could be related to the present non-axial dipole (NAD) field (e.g., Channell et al 2003, 2004; Hoffman et al 2008)
Paleomagnetic records from volcanic rocks, such as lava flows, can provide stable magnetic signals associated with the absolute intensity of past geomagnetic fields, the resulting records are intermittent because volcanic eruptions do not occur continuously (e.g., Jarboe et al 2011; Valet et al 2012)
Summary
Geomagnetic reversals are one of the most significant fluctuations in the Earth's magnetic field, during which the magnetic intensity decreases by an order of magnitude as the polarity transitions (e.g., Merrill and McFadden 1999; Valet and Fournier 2016). The details of paleointensity variations during polarity transitions are often not recorded in standard deep-sea cores because the sedimentation rates are generally relatively low, i.e., approximately a few centimeters per thousand years. Such a low sedimentation rate prevents the accurate recording of rapid geomagnetic variations due to the smoothing of the magnetic signal associated with the post-depositional remanent magnetization (pDRM) process (e.g., Roberts and Winklhofer 2004; Suganuma et al 2011; Valet et al 2016). For a more detailed description of geomagnetic field characteristics during polarity transitions, on-land marine successions with sufficiently high sedimentation rates, such as reported by Okada et al (2017) and Simon et al (2019), are crucial
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