A record of the upper Olduvai geomagnetic polarity transition from a sediment core in southern Yokohama City, Pacific side of central Japan

Chie Kusu,Atsushi Nozaki,Ryuichi Majima,Makoto Okada,Hideki Wada

doi:10.1186/s40645-016-0104-7

Abstract

A detailed paleomagnetic record of the upper Olduvai polarity transition was obtained from a 106.72 m-long sediment core drilled in southern Yokohama City, located on the northern Miura Peninsula, on the Pacific side of central Japan. The core spans the upper part of the Nojima Formation and the lowermost part of the Ofuna Formation, both of which correspond to the middle Kazusa Group (Lower Pleistocene forearc basin fill). The record was reconstructed using discrete specimens taken throughout mudstone and/or sandy mudstone sequences in the Nojima Formation. In this record, the virtual geomagnetic pole (VGP) fluctuation accompanying the polarity transition was determined to occur between depths of 66.99 and 63.60 m. These depths have been dated at 1784.4 and 1779.9 ka, respectively, and the duration of the polarity transition is estimated to be 4.5 kyr using an age model based on a δ18O record from that core. The VGP paths during the transition do not appear to show any preferred longitudinal bands. However, the VGP positions cluster in five areas: (A) eastern Asia near Japan, (B) the Middle East, (C) eastern North America (North Atlantic), (D) off southern Australasia, and (E) the southern South Atlantic off South Africa. The primary locations of the observed VGP clusters coincide with the areas on the Earth’s surface that possess a strong downward flux of the vertical component of the present geomagnetic non-axial dipole field. The relative paleointensity rapidly decreased approximately 1 kyr before the beginning of the polarity transition and gradually recovered to its initial level in 12 kyr.

Highlights

Studies of geomagnetic reversals, which are among the most conspicuous phenomena associated with the Earth’s magnetic field, provide valuable information about geodynamo processes
Our results from Cores M and I show that the fluctuating patterns in the records of magnetic susceptibility (Fig. 8i), in situ paleomagnetic inclinations (Fig. 8j), and relative paleointensities (Fig. 7c) coincide well with each other, and the Characteristic remanent magnetization (ChRM) inclinations of Core M are in close agreement with the value expected for a geocentric axial dipole (Fig. 6c)
(1)In Core M, the reversal boundary was observed between 65.69 and 65.67 m depth, the polarity transition was observed between 66.99 and 63.60 m depth, and the duration of the polarity transition is estimated to be 4.5 kyr, based on an age model derived from δ18O measurements in the planktonic foraminifer G. inflata from the same core

Summary

Introduction

Studies of geomagnetic reversals, which are among the most conspicuous phenomena associated with the Earth’s magnetic field, provide valuable information about geodynamo processes. Late Cenozoic volcanic records from intervals spanning polarity transitions suggest that VGPs were clustered mainly within two regions near the southern portions of South America and Western Australia (Hoffman 1991, 1992). These regions lie within the two preferred longitudinal bands identified in the sedimentary records. The two preferred bands coincide with the centers of the radial magnetic flux of the present geomagnetic field on the Earth’s surface when the axial dipole component is removed (Hoffman 1992). The two preferred bands coincide with the centers of the radial magnetic flux of the present geomagnetic field on the Earth’s surface when the axial dipole component is removed (Hoffman 1992). Laj et al (1991) reported that the two preferred

Results

Discussion

Conclusion