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Bioelectromagnetism

simulation methods incorporating the efects of turbulence is progressing, the development of experi­

mental equipment that imitates the geodynamo is also progressing (Glatzmaier and Olson, 2005). Tis is

an experiment to clarify the dynamo mechanism by rotating a huge container containing liquid sodium

(Glatzmaier and Olson, 2005). Geodynamo models are a powerful tool for testing various other hypoth­

eses for the Earth’s core. However, it is a matter of concern that many geodynamo models might be using

unphysical basic state buoyancy profles, prescribing either uniform heat fux throughout the core, or

even worse, heat fux that increases from the inner core boundary to the CMB (Sreenivasan, 2010).

More recent studies on the geodynamo simulations have revealed in detail that convection of the

liquid iron (ferrofuid) in the outer core (~3,000 km below the surface), and electrical properties of iron

are responsible for the geodynamo that generates the GMF (Yong et al., 2019). Molten iron moves at

a speed of ~1 mm/s, and when it cuts the GMF lines, it produces a voltage that reinforces the original

MF (Gubbins, 2008). Fluid motion is driven by buoyancy resulting from a density gradient caused by

the slow cooling of the whole Earth (Gubbins, 2008). Te core solidifes from the center to the outside,

and the light elements of the liquid separate and rise, and the heat fow promotes convection (Gubbins,

2008).

What is interesting is that even if the direction of the GMF is reversed, there is almost no change in

the direction of the molten iron fow in the outer core. Even if the GMF is reversed, the overall molten

iron fow does not reverse. GMF reversal can occur irregularly without giving an external trigger. It

seems likely that the GMF reversal may occur due to small things such as the slight disturbance of the

rotation speed of the Earth.

6.3.2 Chibanian

A geological layer indicating the latest GMF reversal of 774 ka, during the Matuyama–Brunhes (M–B)

reversal near the terminal Matuyama reverse, was found in a clif wall along the Yoro River in Tabuchi,

Ichihara City, Chiba Prefecture in Japan (Kazaoka et al., 2015). Tis clif wall in Chiba composite sec­

tion of the Boso Peninsula was with an exposed layer of marine deposits and mineral debris (Kazaoka

et al., 2015). Tis layer contains the volcanic ash of Mt. Kiso Ontake erupted 774 ka, located in central

Japan, and is accordingly named Ontake-Byakubi tephra bed (Takeshita et al., 2016). Te Byakubi tephra

zone (Byk A–E) is located within thick and massive siltstones in the Tabuchi section, and represents a

set of fve individual tephra beds (Kazaoka et al., 2015). Te most remarkable is the Byk-E bed, which

varies from 1 to 3 cm in thickness and consists of white, glassy, fne-grained ash (Kazaoka et al., 2015).

Te traces of the last GMF reversal 774 thousand years ago (ka) in Chibanian stratum have been shown

by Kazaoka et al. (2015) and Suganuma et al. (2018) (see also https://www.facebook.com/town.otaki/

photos/pcb.1093824400728659/1093822697395496). A layer contains volcanic ash from Mt. Kiso Ontake

(Ontake-Byakubi Tephra Bed) erupted 774 ka. Te layer was found in a clif wall along the Yoro River in

Tabuchi, Ichihara City, Chiba Prefecture in Japan. Te Byakubi (Byk) zones observed in this layer dur­

ing and afer the during the M–B reversal are classifed as follows: Byk-B, Fine sand grain scoria with

normal polarity; Byk-C, Medium sand grain scoria with unstable polarity; Byk-E, Tephra bed, White silt

grain volcanic ash with reversal polarity. Te M–B boundary, the primary marker of the GSSP, is located

~0.8 m above the Byk-E tephra bed. Te thick siltstones are interpreted to have been deposited in warm

oceanic conditions based on the pelagic gastropod assemblages (Ujihara, 1986). Te M–B boundary is

located ~0.8 m above the Byk-E tephra bed (Kazaoka et al., 2015; Suganuma et al., 2018). As shown in

Figure 6.2, lithofacies across the Lower–Middle Pleistocene boundary in the Tabuchi section are photo­

graphed by Kazaoka et al. (2015).

Geologically the Chiba composite section is located in the middle part of the Kokumoto Formation

as a member formation of the Kazusa Group. Te geological epoch, the Middle Pleistocene from ~774

to 129 ka was named “Chibanian” (Chiba era) afer the Japanese Prefecture Chiba, home to the city of

Ichihara (Te Geological Society of Japan, 2010; Suganuma et al., 2018; Suganuma, 2020). Te quater­

nary part of the international chronostratigraphic chart is shown by the Geological Society of Japan