273

Geomagnetic Field Effects on Living Systems

FIGURE 6.17 Synthesized sketch of the evolution in GMF reversal frequency since the end of the Precambrian.

(Modifed from Gallet et al. (2019).) CNS, Cretaceous Normal Superchron (~83–125 Ma); PCRS, Permian–

Carboniferous (Kiaman) Reversed Superchron (~267–313 Ma); ORS, Ordovician (Moyero) Reversed Superchron

(~460–490 Ma). (Reproduced with permission from Gallet et al. (2019), Copyright 2019, Elsevier.)

(Meert et al., 2016). On top of this, the duration of each individual reversal episode—thought to take

an average of 7–10 kyr—would likely see the feld temporarily weakened even more before growing

back in the opposite direction. Tis weakened shielding would have allowed more energetic particles

into the upper atmosphere, which would have begun to break down the O3 layer that protects the

Earth from potentially harmful UV-B radiation. A 10%–30% decrease in global O3 levels can cause

considerable disruption to living organisms exposed to increased UV-B fux (Meert et al., 2016). As an

example, when 40% of O3 coverage is lost, the biologically efective amount of UV radiation reaching

the Earth’s surface will double (Cockell and Blaustein, 2001; Pavlov et al., 2005). It is estimated that O3

depletion during a reversal could reach up to 30% at higher latitudes and persist for the 7–10 kyr dura­

tion of the reversal (Winkler et al., 2008; Vogt et al., 2007). Organisms with the ability to escape UV

radiation would be favored in such an environment (Meert et al., 2016). Tis escape from dangerous

levels of UV light, therefore, might explain many of the evolutionary changes that occurred during the

Late Ediacaran and Early Cambrian (Meert et al., 2016). Creatures with complex eyes to sense the light

and the ability to seek shelter from the radiation, for example, by migrating into deeper waters during

the daytime, would have been more successful (Meert et al., 2016). Te growth of hard coatings and

shells would aford additional UV protection, as would the capacity to burrow deeper into the seafoor

(Meert et al., 2016).

In turn, these morphological and functional changes may have opened up new environments to sur­

vive. Te development of shells, for example, helps creatures colonize intertidal areas, protected not

only from UV-rays, but also stronger radiation, and the risk of drying out. Similarly, the breakdown of

the bacterial mats by early burrowing would have opened up the upper seafoor further for life. Looking

forward, the researchers are now hoping to examine other Ediacaran sediments from around the globe

to verify the rapid reversal signal, along with hunting for biological or chemical evidence for high doses

of UV radiation in the fossil record.