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in the Phanerozoic Eon. In the COPSE model, this feature arises be-
cause the evolution and spread of land plants later generates a funda-
mental shift in oxygen regulation, giving rise to higher levels of oxygen.
Thus, in the COPSE model, relatively low levels of atmospheric oxygen
are the stable state before the evolution of land plants. If this is true,
then the low levels of oxygen as predicted in the COPSE model in the
early Phanerozoic Eon could provide an of estimate of the maximum
average levels of oxygen late in the Neoproterozoic Era, as discussed
in the last chapter. Thus, distinguishing between the predictions of the
COPSE model and the GEOCARBSULF has important implications
for unraveling the histories of atmospheric oxygen in both the late Pro-
terozoic and the early Phanerozoic Eons.
Tais Dahl and Emma Hammarlund, whom we met in the last chapter,
may have figured out a way to differentiate between these two model
results.
13
If you recall, they analyzed the isotopic composition of molyb-
denum in sedimentary rocks deposited into ancient sulide-rich (eux-
inic) environments. This approach was mentioned in the previous chap-
ter (see also endnote 14,
chapter 9)
, but in short, the greater the value of
δ
98
Mo, the more Mo has been removed from the oceans under oxygen-
ated conditions, and the less has been removed into sulfidic environ-
ments like the Black Sea. In other words, to a first approximation, the
greater the value of δ
98
Mo, the more oxygenated the oceans. hat Tais
and Emma found was an increase in the value of δ
98
Mo at around 400
million years ago (
ig. 10.3
). Sound familiar? Yes indeed, this is about
the time of land plant expansion onto the continents and about the
time the COPSE model suggests a major rise in atmospheric oxygen
levels. This would be apparent support for the COPSE model results
of the early Phanerozoic Eon.
There is more. The rise in δ
98
Mo values is also timed with a rather
profound change in the size of fish in the oceans. Indeed, we observe
the emergence of fish like
Dunkleosteus
, a true monster up to 10 meters
long with a thick armored skull and jaws made for crushing. As we dis-
cussed above for dragonflies, it seems reasonable that larger and more
energetic fish would require more oxygen, and indeed, this seems to be
true for modern fish where, on average, smaller fish can tolerate lower
levels of oxygen than larger fish (
ig. 11.5)
. One sees a similar pattern in
the history of some other creatures of the sea. For example, eurypterids,
