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Geomagnetic Field Effects on Living Systems

FIGURE 6.2 Lithofacies across the Lower–Middle Pleistocene boundary in the Tabuchi section (Kazaoka et al.,

2015). Te white ash stratum of Byakubi-E (Byk-E) tephra bed is ~2 cm. 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. Te most remarkable is the Byk-E bed (1–3 cm in thickness) and consists of white, glassy, fne-grained ash.

(Reproduced with permission from Kazaoka et al. (2015) Copyright 2015, Elsevier.)

(2010). Te name of Chibanian was approved by the International Union of Geological Sciences, and was

certifed as a Global Boundary Stratotype Section and Point (GSSP) on January 17, 2020, (Suganuma,

2020). Other than this Chiba composite section, it is known that the formations that prove the latest

GMF reversal of 774 ka exist in Montalbano Jonico section (Bertini et al., 2015; Marino et al., 2015;

Maiorano et al., 2016; Simon et al., 2017), and Valle di Manche section (Capraro et al., 2015, 2017; Macri

et al., 2018) in southern Italy.

More recently, the M–B boundary is estimated to be dated to 772.9 ka (Suganuma et al., 2015, 2018;

Okada et al., 2017; Simon et al., 2019; Haneda et al., 2020), and the polarity switch is completed within

1.1 ± 0.4 kyr (1σ) (Suganuma et al., 2018). Te most detailed sedimentary record revealed that the average

stratigraphic position of the M–B boundary between the Chiba, Yoro-Tabuchi, and Yanagawa sections,

and the TB-2 sediment core drilled 190 m northeast of the Chiba composite section (Hyodo et al., 2016),

lies 1.1 ± 0.3m (1σ) above the Byk-E tephra bed, and the average age of the M–B boundary in the Chiba

composite section is estimated as 772.9 ± 5.4 ka (1σ) (Haneda et al., 2020). Te GSSP defning the base

of the Chibanian Stage and Middle Pleistocene Subseries is placed at the base of the regionally wide­

spread and signifcant Byk-E tephra bed in the Chiba section, with an astronomical age of 774.1 ka (Te

Geological Society of Japan, 2010; Suganuma et al., 2018; Suganuma, 2020). Te GSSP lies 1.1 m below

the directional midpoint of the M–B boundary (772.9 ka, duration 1.7 kyr) which therefore serves as its

primary guide (Suganuma et al., 2021).

Overall, there is no bias in the polarities of the GMF found so far, such as either normal or reverse

being long or short. According to the dynamo theory, the GMF reversal is thought to be due to “spon­

taneous convective instability of the liquid outer core.” In addition, since the convective velocity of

the outer core is ~20 km per year, it is estimated that a period of several hundred years is required for

GMF reversal (Anderson, 1989). However, the frequency of long-term GMF reversals has a tendency to

occur as follows (Biggin et al., 2012). In the last 800 ka, the GMF reversal has occurred only once in the

M–B reversal (Biggin et al., 2012). However, in the past 2.5 Ma (million years ago), GMF reversals have

occurred more than 11 times, i.e., fve times in ~1 Ma (Biggin et al., 2012).

Could GMF reversals be caused by meteorite or comet impacts? Extraterrestrial impacts such as

meteorite impacts may also trigger the GMF reversals. Tis “meteorite impact theory on the origin of

geomagnetic reversals” is a theme that has been discussed for a long time, but no clear evidence has been

presented so far, and it has been denied for a long time. Gilder et al. (2018) indicated that the impact events

producing large impact craters, e.g., the Manicouagan crater (Quebec, Canada), Nördlinger Ries crater

(Germany), Rochechouart crater (France), and Mistastin crater (Labrador, Canada), had no observable

efect on the geodynamo (Koch et al., 2012; Eitel et al., 2014, 2016; Hervé et al., 2015). Hence, geodynamo