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Bioelectromagnetism

 

the polarity of the GMF is reversed, the GMF strength generally decreases when the reversal is repeated

frequently (Brown et al., 2007; Valet and Plenier, 2008; Ferk and Leonhardt, 2009).

It is important to investigate the carbon isotope composition of limestone in seawater in order to

estimate the history of biological production. Across the P–T boundary and the G–L boundary, the mid-

oceanic paleo-atoll carbonates recorded secular change in stable carbon isotope composition. Musashi

et al. (2001) and Isozaki et al. (2007b) documented the secular change in carbonate carbon isotopic ratio

(δ¹³Ccarb) of mid-Panthalassa across the P–T boundary and the G–L boundary, respectively. Besides the

boundary negative shifs both at P–T boundary and G–L boundary properly predicted from previous

studies (e.g., Holser et al., 1989; Wang et al., 2004), a unique high productivity interval in the Capitanian

(late Guadalupian) was newly detected on the basis of the appreciable length of high positive δ¹³Ccarb

(between +5‰ and +6‰) interval (Isozaki et al., 2007a).

As shown in Figure 6.10, the schematic diagram indicating the late Guadalupian Kamura event docu­

mented by high positive δ¹³Ccarb values at Kamura in Japan is presented by Isozaki et al. (2007a), which

is modifed from Isozaki et al. (2007b).

B: Two possible paths (broken lines) for the Guadalupian secular change of δ¹³Ccarb values were

shown by Korte et al. (2005); the lower for the Tethyan domain, the upper for the Delaware basin in

Texas. Te Capitanian Kamura event recorded much higher positive δ¹³Ccarb values between +5.0‰

and +7.0‰ in Kamura, suggesting the positive excursion of the global context in the late Guadalupian.

G–LB, Guadalupian–Lopingian boundary; P–TB, Permian–Triassic boundary; Road, Roadian; Wor,

Wordian. (Reproduced with permission from Isozaki et al. (2007a), Copyright 2007, Elsevier.)

Such high positive values over +5.0‰ are quite rare in the Phanerozoic record except for several unique

events in the Paleozoic (e.g., Veizer et al., 1999; Saltzman, 2005). It was found in Japan that the frst major

isotopic change occurred in the late Guadalupian, and Isozaki et al. (2007a, b) named this Capitanian

episode “Kamura event” afer the discovery site, Kamura section, Takachiho-cho, Nishiusuki-gun,

Miyazaki Prefecture, in Kyushu. Te Kamura event emphasized its signifcance of global cooling and

relevant extinction of large fusulines and gigantic bivalves in low-latitude Panthalassa (Isozaki et al.,

2007a). In the fusuline-tuned section, the waning history of the Kamura cooling event was clearly docu­

mented in high resolution, whereas the earlier history including the onset timing was not yet revealed,

owing to the absence of continuous exposure in the previously studied section. Tis lef a big chasm in

the understanding of the major environmental change in the late Guadalupian, in particular, the cause

and processes of the Kamura cooling event.

Te main extinction occurred not at the G–L boundary per se but in a much lower horizon in the

midst of the positive δ¹³Ccarb excursion interval. Tus an appreciable time has elapsed between the end-

Guadalupian extinction and the following radiation of the Lopingian fauna in shallow mid-Panthalassa

as shown by Isozaki et al. (2007a). It is noteworthy that a strange condition has appeared in the middle

of the superocean around the Wordian–Capitanian boundary because the Kamura event may mark the

frst episode of large isotopic excursion in the Permian as shown by Isozaki et al. (2007a).

Wei et al. (2014) hypothesized that GMF reversals cause O2 level drops and subsequent mass extinc­

tions. As shown in Figure 6.11, temporal evolution of reversal rate, O2 level, and marine diversity over

the Phanerozoic are presented by Wei et al. (2014).

Because of several data gaps in this database, the relative reversal rate from an older database

(dashed line, McElhinny, 1971) is also plotted to show the trend of reversal rate for a reference. Te

reversal rate (Ogg et al., 2008), atmospheric O2 level (Berner, 2009), and marine diversity (Alroy, 2010)

show a strong correlation in support of their hypothesis. During the second, third, and fourth mass

extinctions identifed by the diversity drops, the reversal rates increased and the O2 level decreased. In

contrast, when the reversal rate remained at zero or very low, namely during the superchrons (Merrill

and Mcfadden, 1999), the diversity increased, and the O2 level also increased for three out of four

superchrons. However, during frst and ffh mass extinctions, the reversal rate also increased despite

that the O2 level just remained at low level or had no discernable change. Tese features suggest that

some mass extinctions might be explained by their hypothesis: increasing GMF reversals continually