238

Bioelectromagnetism

 

FIGURE 6.3 Te GMF fuctuations in the Chiba composite section (Simon et al., 2019). Tey were indicated using

10Be-derived dipole moment and virtual geomagnetic pole (VGP) latitude over the M–B transition. Te black line

corresponds to a three-point running mean of the VGPs. Te “polarity switches” that explain most of the angular

deviation span 2 ka (773.9–771.9 ka), but the “transitional feld” prevails until 765.2 ka (two instability clusters, IC1–

IC2). Te largest VGP deviations correspond to the frst low dipole intensity interval (Dip1) interval (773.7–767.4 ka).

Te loading bar shows dipole moment percent relative to present value. Te results are compared with a schematic

reversal path illustrating the three-phase succession of reversal (precursor, transit, and rebound) described by Valet

et al. (2012) from reversal volcanic sequences. MBT indicates Mid-Brunhes Transition. (Reproduced with permis­

sion from Simon et al. (2019), Copyright 2019, Elsevier. It is licensed under the Creative Commons Attribution 4.0

International.)

disturbances from impacts should be considered improbable during the Phanerozoic when impact fre­

quency and meteorite sizes were lower than in the Precambrian (French, 1998). Recently, a meteorite

impact event was revealed just before the M–B transition. In this meteorite impact, no crater was found

for a long time. However, in 2020, a joint research team of from Singapore’s Nanyang Technological

University, Tailand’s Ministry of Natural Resources and Environment, and the University of Madison-

Wisconsin, etc., discovered a crater in Southern Laos, Indochina, fnally confrming the existence of this

meteorite impact event that happened ~790 ka (Sieh et al., 2019). Comparing the timing of this meteorite

impact with the GMF reversal record of the Chiba composite session reveals an interesting fact (Simon

et al., 2019).

As shown in Figure 6.3, the GMF fuctuations in the Chiba composite section are presented by Simon

et al. (2019).

Tat is, the virtual geomagnetic pole (VGP) latitude and dipole moment variations, both of which

are an indicator of GMF fuctuations in the Chiba composite section, were compared through time

using the chronology of Suganuma et al. (2018). It is estimated that this meteorite impact around 790 ka,

“immediately before the decline in the GMF intensity,” which is the process of the beginning of GMF

reversal, before the timing of the reversal. During a meteorite impact, small glass particles called

“microtektites” (Glass, 1990) are scattered over a wide area. If this microtektite is also contained in the

ancient strata of the Chiba composite section, it is considered that the timing of this meteorite impact

event can be determined directly from the strata of the Chiba composite section. Currently, the search

for microtektites has begun in the strata of the Chiba composite section (Suganuma et al., 2018). In the