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

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

Multiproxy sedimentological and geochemical analyses across the Lower–Middle Pleistocene boundary: chemostratigraphy and paleoenvironment of the Chiba composite section, central Japan

“…Especially in terms of the bottom-water oxygen level, there are several evidences suggesting that the bottomwater oxygen level lowered at~764 and~760 ka. First, this interpretation is consistent with the lower S-ratio and magnetic susceptibility in the same intervals (Okada et al 2017;Simon et al 2019;Haneda et al 2020a) (Fig. 7).…”

“…h Radiolarian concentrations (Suganuma et al 2018). i S-ratio -0.3T record (Haneda et al 2020a). j Magnetic susceptibility record (Haneda et al 2020a).…”

“…i S-ratio -0.3T record (Haneda et al 2020a). j Magnetic susceptibility record (Haneda et al 2020a). k High-resolution δ 18 O records from planktonic (G. bulloides) and l benthic foraminifers from CbCS (Haneda et al 2020b).…”

“…The Lower to Middle Pleistocene is an important geological period that is characterized by gradual changes in the climate systems that affected the evolution of biota (Maasch 1988;Mudelsee and Schulz 1997;Mudelsee and Stattegger 1997;Raymo et al 1997;Head and Gibbard 2005;Clark et al 2006;Head et al 2008;Elderfield et al 2012;Head and Gibbard 2015). The Chiba composite section (CbCS) in the Kokumoto Formation of central Japan is a thick and continuous marine succession that straddles the Lower-Middle Pleistocene boundary and the wellrecognized Matuyama-Brunhes paleomagnetic polarity boundary (M-B boundary) (Niitsuma 1976;Okada and Niitsuma 1989;Suganuma et al 2015;Nishida et al 2016;Okada et al 2017;Suganuma et al 2018;Haneda et al 2020a). Because the M-B boundary serves as the primary guide for the Lower-Middle Pleistocene stage boundary (Head and Gibbard 2005), the Chiba section, which is one of the main sections of the CbCS (see Section 2 for a detailed description) that also contains many key tephra layers and thus chronostratigraphic tie-points, has recently been determined as the Global Boundary Stratotype Section and Point (GSSP) for the base of the Middle Pleistocene Subseries (Suganuma et al 2021).…”

Formal ratification of the Global Boundary Stratotype Section and Point (GSSP) for the Chibanian Stage and Middle Pleistocene Subseries of the Quaternary System: the Chiba Section, Japan†

“…Because the section is located at the easternmost margin of the Eurasian continent open to the Pacific Ocean, the marine sedimentary record preserves the interplay between terrestrial and marine paleoclimatic and paleoenvironmental changes influenced by the mid-latitude westerly jet stream, East Asian monsoon, and North Pacific Gyre, ranging from subtropical to subpolar settings. The M-B boundary occurs immediately above a widespread tephra bed, the Ontake-Byakubi-E (Byk-E), found throughout the Chiba composite section (Suganuma et al, 2015;Hyodo et al, 2016;Okada et al, 2017;Haneda et al, 2020a) (Fig. 4).…”