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Bioelectromagnetism

 

 

layer and let in a food of UV radiation, devastating the unusual creatures of the Ediacaran Period and

triggering an evolutionary event that led to the Cambrian explosion of animal groups.

Scientists have long argued over what caused the Cambrian explosion in the frst place. Potential

explanations have included rising levels of atmospheric O2 because of photosynthesis, allowing for the

development of more complex animals; the rise in carnivorous species and new predatory tactics, such

as the fat and segmented, armor-crushing creatures known as anomalocaridids; and the breakup of the

supercontinent Rodinia, which may have created new ecological niches and isolated populations as the

continents drifed apart.

A geologist Joseph Meert of the University of Florida in Gainesville and his colleagues propose a dif­

ferent hypothesis that these evolutionary changes might have been connected to rapid GMF reversal,

which occurs about once every Myr. However, in the Ediacaran, such reversals were a lot more common

(Meert et al., 2016). Certain minerals in rocks can preserve a record of the direction of the GMF when

the rock is formed. While studying these magnetic records 550 million years ago (550 Ma), Ediacaran­

aged sedimentary rocks in the Ural Mountains in western Russia, the team discovered evidence to sug­

gest the reversal rate then was 20 times faster than it is today. Meert et al. (2016) estimated that the GMF

underwent a period of hyperactive reversals.

Why was the GMF so hyperactive during the Ediacaran–Cambrian interval? Labrosse et al. (2001)

estimated the initiation period of inner core nucleation to be 1.0 ± 0.5 Ga, and thus inner core nucleation

began during the Late Ediacaran or alternatively, the inner core grew past some critical size leading to

instabilities in the geodynamo. Reversal frequency may be directly related to plate tectonics, variability

in core heat fux, or whole mantle convection processes (Biggin et al., 2012; Pétrélis et al., 2011; Driscoll

and Olson, 2011). Here, “plate tectonics” is thought to result in a high reversal rate following intervals

in Earth history when the Earth’s continents are asymmetrically distributed about the equator (Biggin

et al., 2012). Moreover, an increase in “core heat fux” drives the modeled geodynamo from a superchron

state to reversing behavior (Biggin et al., 2012).

As shown in Figure 6.17, Gallet et al. (2019) reviewed and analyzed the period between ∼550 and

∼500 Ma, which might be characterized by a single or more episodes of hyperactivity in the reversing

process.

Duan et al. (2018) recently proposed a frequency of ∼7 reversals per Myr between ∼524 and 514 Ma,

but the stratigraphic sequence from which the data were obtained was fragmentary; it, therefore, seems

possible (but not proven) that the reversal frequency was actually higher during this period (Gallet

et al., 2019). Te Lower Cambrian paleomagnetic data obtained at Khorbusuonka section in northeast­

ern Siberia by Gallet et al. (2003), also attest to a very high GMF reversal frequency during the second

stage (Tommotian) and the third stage (Atdabanian) of the Cambrian, in addition to the occurrence

of a major “true polar wander (TPW)” event near the end of the Lower Cambrian, in agreement with

Kirschvink et al. (1997). At this stage, it is only possible to afrm that the reversal frequency was high

(>5 reversals per Myr) during the Lower Cambrian, but the hyperactivity character (>15 reversals per

Myr) cannot be established (Gallet et al., 2019).

Tus, there have been suggested episodes of TPW or rapid plate motion in the Ediacaran–Cambrian

(Kirschvink et al., 1997; Meert and Tamrat, 2004). TPW episodes would support the argument that links

TPW to MF hyperactivity (Biggin et al., 2012). It is also an intriguing observation that an overall trend

in reversal frequency shows a ramp up into a hyperactive mode (Ediacaran and Jurassic) followed by a

decay in reversal frequency leading into the superchrons (Ordovician Reversed Superchron, Permian-

Carboniferous Reversed Superchron and Cretaceous Normal Superchron) (Biggin et al., 2012). Te

schematic model of MF behavior shows a ramp-decay process leading from hyperactive intervals into

superchrons (Meert et al., 2016). Te schematic model of MF behavior showing a ramp-decay process

leading from hyperactive intervals into superchrons is shown by Meert et al. (2016). However, whether

or not this ramp/decay is a real refection of geodynamo activity remains to be seen.

Previous research has suggested that the Earth’s protective MF would be weaker across such periods

of frequent reversal, compromising its ability to shield life from harmful solar radiation and GCRs