233

Geomagnetic Field Effects on Living Systems

 

It has been reported that human health efects of environmental MFs including extremely low-

frequency (ELF) MFs (1–300 Hz) have been discussed on the basis of the RPM (reviewed by Juutilainen

et al., 2018). Te RPM appears to be involved in sensing and responding to the static GMF (~50 μT) by ani­

mals. Evidence from numerous studies suggests that cancer-related biological processes can be afected

by MFs ≥ 100 μT, 50–60 Hz. It is unreasonable to assume that the plausible mechanism by which these

efects occur is the modifcation by RP reaction on a specifc target molecule (such as CRY) involved in

the biological regulatory mechanism. Terefore, the results in MFs ≥ 100 μT do not directly explain the

epidemiological relationship between childhood leukemia and MFs ≥ 0.4 μT ELF (Juutilainen et al., 2018).

It remains unclear how it could explain human health efects of ELF-MFs < 1 μT (Juutilainen et al., 2018).

6.3 Change of the Geomagnetic Field

6.3.1 Pole Shift

In 1905, a French physicist, Bernard Brunhes found some rocks, in an ancient lava fow at Pontfarin in

the commune of Cézens (part of the Cantal département) in France, are magnetized reversely to the

present GMF (Brunhes, 1905a,b, 1906). His observations made it clear that the GMF, providing MF

values (total intensity, inclination and declination), is dynamic with frequent and aperiodic reversals of

N and S poles (Howell, 1990; Dunlop, 1997). About 20 years later afer the report by Brunhes, a Japanese

geophysicist, Motonori Matuyama (a professor at Kyoto Imperial University at that time) in 1926 mea­

sured the paleomagnetic feld of basalt samples from “Genbudo cave” (see also https://www.facebook.

com/genbuguide/photos/a.427376960700781/3883154781789631/) in Hyogo Prefecture and Yakuno in

Kyoto Prefecture in Japan, together with the Korean Peninsula and Manchuria, and he found that the

orientation of the paleomagnetic feld was divided into two groups (Matuyama, 1929). Tat is, the frst

group, including basalt samples from Yakuno, close to the current geomagnetic direction (declination:

northward, inclination: downward), and in contrast, the second group, including basalt samples from

Genbudo cave, indicating the opposite direction (Matuyama, 1929).

Tere is also evidence that the GMF was in two states. It has been reported that the estimated age

is 1.65 Ma from Genbudo cave samples, and 0.3–0.4 Ma from Yakuno samples using the K-Ar method

(Furuyama et al., 1993). Since Genbudo cave samples are estimated to be in the early Quaternary, and

Yakuno samples are much closer to modern times, Matuyama reported that the GMF reversal from the

second group to the frst group was in the relatively short period of the Quaternary (Matuyama, 1929).

Tus, he found that the GMF takes these two states of “reverse” polarity and “normal” polarity, and the

period required for the “GMF reversal transition” is relatively short. His discovery had a great impact on

our understanding of the mechanisms of GMF reversals. At that time, radiometric dating did not exist

and it would not have been easy to date rocks. It seems plausible that great insight was needed to obtain

these results. In the 1950s, the phenomenon of the self-reversal of “thermoremanent magnetization” of

the rock itself was discovered (Nagata et al., 1952, 1953), so there was a time when the discussion was

continued to deny the GMF reversal. Te GMF reversal was only established in the frst half of 1960 by

showing that rocks of the same era had magnetization in the same direction globally.

In 1964, an American geophysicist, Allan Verne Cox and co-workers published a groundbreaking

paper entitled “Reversals of the geomagnetic feld” (Cox et al., 1964). Tey measured the paleomagne­

tism of rocks collected from all over the world and at the same time dated these rocks as evidence of

controversial topics of GMF reversals. As a result, rock data collected from all over the world showed

that rocks of the same period have the same normal or reverse paleomagnetic polarity regardless of their

locations. Tese results suggest that the GMF reversals are not largely dependent on the self-reversal

of thermoremanent magnetization of the rock itself, and that the dipole polarity of the GMF reversed

many times in the past. Since then, advances in measurement technology have increased the accuracy of

restoring the past GMF intensity using volcanic rocks of various ages, and more reliable methods have

been developed for measuring the past long-term GMF intensity (Kono and Nagata, 1967; Kono, 1971).