About 100 Year Directional Variations in the Matuyama-Brunhes Transition Field Inferred from the Sedimentary Records in the Boso Peninsula, Japan.

Abstract

About 100 year variations are found in the field direction of the Matuyama-Brunhes transition. The analysed data are composed of the palaeomagnetic directions from the marine sediment in the Boso Peninsula, whichpreciselyrecordedthe Matuyama-Brunhes transition with a very high accumulation rate (3.7 mm/yr). The section is tentatively divided into two subsections from the directional change, where the main reversal and the rebound occurred in the early stage (subsection 1) and subsequently the field directions show moderate changes in the late stage (subsection II). Applying the Bayesian smoothing method to the directions spanning about 4100 years, the inclination, declination and angular changes in the transition field imply short-period components of the order of 102 years. After removing long-term trends, the maximum entropy method was employed for the detection of short-period variations. The results of the spectral analyses suggest the 60-80 year and the 120-150 year periodicity in the subsections of I and II, respectively. The main reversal and the subsequent rebound seem to have been accomplished for short intervals of about 200 and 300 years, respectively. These rapid variations of the order of 102 years are suggested to be a substantial feature of the transition field.

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

“…1c). These pioneering studies identified the M-B boundary approximately 2 m below the Byk-E tephra bed using alternating field (AF) demagnetization, which removes secondary remanences by exposing a specimen to an alternating magnetic field (Niitsuma 1971;Okada and Niitsuma 1989;Tsunakawa et al 1995Tsunakawa et al , 1999Aida 1997).…”

Further application of the deconvolution method of post-depositional DRM to the precise record of the Matuyama-Brunhes reversal in the sediments from the Boso Peninsula, Japan

“…Schneider, 1995;Tsunakawa et al, 1996). As the pDRM origin is inferred for the Boso sedimentary magnetism from the evidence of intensive bioturbation (Okada and Niitsuma, 1989;Tsunakawa et al, 1995), we have to restore more accurate variations from the measured remanence directions. Tsunakawa et al (1996) applied the deconvolution method (Tsunakawa, 1995) to the two directional datasets of about 900 years from the different sites, Yanagawa and Heizogawa.…”

“…(Hyodo, 1984) assumes that the geomagnetic field really has frequency components approximated by the finite Fourie transform. The details are discussed in Tsunakawa et al (1995). The deconvolved directions were analyzed with the spectral analysis to examine nearly 100 year variations.…”

“…This calculation is a very simple recursive type and basically has no assumptions of the frequency dependence in the original field, while the FFT method (Niitsuma, 1971;Okada and Niitsuma, 1989). The inclination in the left column and the declination in the middle were smoothed by the solid curves in the Bayesian procedure (Tsunakawa, 1992;Tsunakawa et al, 1995). Available data of the NRM/ARM ratio are also shown in the right diagram as relative intensity changes, which are calculated from the demagnetization results by 40 mT (Okada and Niitsuma, 1989) and smoothed with a broken line (Tsunakawa, 1992).…”

Matuyama-brunhes Geomagnetic Reversal Record and Associated Key Tephra Layers in Boso Peninsula: Extraction of Primary Magnetization of Geomagnetic Fields From Mixed Magnetic Minerals of Depositional, Diagenesis, and Weathering Processes

“…The M-B boundary was identi ed in the middle part of the Kokumoto Formation of the Kazusa Group in the central Boso Peninsula by Nakagawa et al (1969) and Niitsuma (1971Niitsuma ( , 1976. A series of paleomagnetic studies identi ed the M-B boundary approximately 2 m below the Ontake-Byakubi-E (Byk-E) tephra bed using alternating eld (AF) demagnetization (Niitsuma 1971;Okada and Niitsuma 1989;Tsunakawa et al 1995Tsunakawa et al , 1999. In contrast, recent paleomagnetic records using thermal demagnetization by Suganuma et al (2015), Okada et al (2017), and Haneda et al (2020) obtained for the Chiba composite section indicate that the M-B boundary is located slightly above the Byk-E tephra bed, which provides magnetostratigraphic bases for the proposal and the nal rati cation of the GSSP as well as the record of 10 Be (Simon et al 2019).…”