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Geomagnetic Field Effects on Living Systems

future, if microtektites are found, the relationship between meteorite impact event and GMF reversal

will be clear (Suganuma et al., 2018). Tis research approach could provide a major breakthrough in

solving the mystery of GMF reversal (Suganuma et al., 2018).

6.3.3 Superchron

Around 15 Ma, GMF reversals occurred very frequently but randomly, occurring on average about

once every 100 ka (0.1 Ma). Further back in the Cretaceous, there was a period of 42 Myr (~83–125

Ma), in which GMF reversals have never occurred and the stable state persisted (Biggin et al., 2012).

Te period, in which GMF reversals have not occurred and one polarity has been maintained for

tens of millions of years, is termed “superchron” (Eide and Torsvik, 1996). As other superchrons, the

Ordovician (Moyero) Reversed Superchron (ORS; ~460–490 Ma), and the Permian–Carboniferous

(Kiaman) Reversed Superchron (PCRS; ~267–313 Ma) are known even before the Cretaceous Normal

Superchron (CNS; ~83–125 Ma) (Biggin et al., 2012). Examining the trends of GMF reversals over the

past 600 Ma suggests that the frequency of GMF reversals gradually increases afer the superchron and

tends to peak thousands of years before the next superchron (Biggin et al., 2012). In this way, the fre­

quency of GMF reversals fuctuates slowly on timescales of thousands to hundreds of millions of years,

and it is assumed that superchrons occur at a rate of <200 Myr in their process (Biggin et al., 2012).

Te available data show that the GMF reversal frequency has varied significantly over time, from the

zero value during superchrons to epochs, when seven to ten reversals could occur for 1 Myr (Opdyke

and Channell, 1996; Biggin et al., 2012; Pavlov and Gallet, 2005, 2010). As shown in Figure 6.4, records

of geomagnetic polarity reversal frequency and dipole moment since the Cambrian are presented by

Biggin et al. (2012).

Including superchron, the frequency of GMF reversals is fuctuating greatly and several hypotheses

about the mechanisms have been proposed. One of the most plausible hypotheses is “mantle convection

theory” (Biggin et al., 2012). Te Earth has been cold since its birth. In the process, the mantle, which

occupies 80% of the Earth’s volume, is gently convected, transporting heat from the outer core to the

Earth’s surface. Since the mantle is a huge heat transporter for the Earth, when the mantle becomes

active, the heat fow to the Earth’s surface increases. As shown in Figure 6.5, the average reversal fre­

quency and eruption ages of large igneous provinces (LIPs) (ofset by +50 Myr) that have not yet been

subducted are presented by Biggin et al. (2012).

Since the Cretaceous, mantle convection has gradually become inactive, no extreme volcanic erup­

tions have occurred, and it is presumed that the Earth has gradually cooled. Intermittent upstream from

the lower part of the mantle, called “mantle plume,” is thought to play a major role in mantle convec­

tion changes over a long timescale. Mantle convection, including upstream of mantle plume, also has

periodic fuctuations, which are estimated to be 200 Myr. In particular, huge one is called “superplume”

(Larson, 1991), and it is thought that when the superplume rises from the bottom of the mantle, large-

scale magma and volcanic activity occur on the Earth’s surface.

Mantle plume heads leaving the CMB may refect enhanced heat fow out of the core potentially

increasing reversal frequency tens of Myr before the resulting eruption of the “large igneous provinces

(LIPs).” As shown by Figure 6.6, the easterly distribution of the Middle-Late Permian LIPs in Pangea is

presented by Isozaki (2009), which is modifed from Isozaki (2007a).

LIPs are ofen accompanied by large food basalt eruptions. Allowing for an average rise-time of 50

Myr produces a broad correlation that would associate GMF reversal hyperactivity in the mid-Jurassic

with widespread LIP emplacement in the mid-Cretaceous. In the period 0–50 Myr, mantle plume heads

that had lef the CMB would not yet have reached the surface.

When the supercontinent Pangea split in the Jurassic, multiple plumes rose side by side in the N-S

direction, and the crevices are connected to create a new ocean, the Atlantic Ocean. Apart from the LIPs

on the Atlantic coast, multiple plumes also hit the eastern part of Pangea in the Permian, forming each

LIP. Typical examples of food basalt are Siberia and Mt. Emei Scenic Area, Emeishan in China.