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anomaly, a huge amount of oxygen is required to perform the oxida-
tion,
15
producing a huge oxygen sink.
Let's see where we stand. Geochemical evidence suggests increased
oxygenation of the oceans by 580 million years ago, and maybe some-
what earlier, and in approximate concert with the expansion of the Edi-
acaran Fauna. Perhaps, as Andy Knoll and Lou Derry suggested, this
was driven by high rates of organic matter burial associated with the
breakup of supercontinent Rodinia. Sometime soon after this, how-
ever, the Shuram-Wonoka anomaly suggests a huge oxygen sink and
the probability of significant oxygen drawdown in the atmosphere and
oceans. We don't know how far oxygen might have decreased through
this anomaly, but apparently not so low as to affect the respiration of
early animals.
hat happened next? Did oxygen rise again to pre-Shuram-Wonoka
levels? Some of the geochemical evidence as outlined above might sug-
gest this, but the carbon isotope record isn't revealing much. In fact,
Lou's model only showed a transient increase in oxygen liberation
around 580 million years ago. After this, organic carbon burial and oxy-
gen liberation reverted to background levels. Also, to confuse things
even further, the carbon isotope record reveals another big
13
C deple-
tion (
ig. 10.4)
just at the Cambrian-Precambrian boundary, in concert
with the great expansion of animal life. Taken at face value, animal life
increased in magnitude and diversity as oxygen release to the atmo-
sphere waned. here is much here we don't understand.
Well, what about animals? As mentioned earlier in this chapter, there
has been a prejudice dating back to Nursall that animal evolution, at
least macroscopic motile animals, was enabled by an increase in at-
mospheric oxygen concentrations. Indeed, in 1982 Bruce Runnegar of
UCLA tried to put some numbers on this. He took the Ediacaran fossil
Dickensonia
as his example, which he viewed at the time as an example of
an ancient annelid worm. Most would probably disagree with this view
now, but for the purpose of the calculation it really doesn't matter very
much what
Dickensonia
was. hat is important is the reasonable assump-
tion made by Bruce that
Dickensonia
obtained its oxygen by passive dif-
fusion across its outer surface. With this assumption, Bruce concluded
that at least 10% of present-day oxygen levels would be required to main-
tain
Dickensonia
's respiration. Ignoring the possible oxygen gymnastics
