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the late Neoproterozoic Era, the isotopic composition of molybdenum
remains much lower than we find today. This suggests that around the
time animals emerged, marine oxic conditions were not as widespread
as today. This would in turn imply lower oxygen levels, maybe even
much lower, than we have at present.
Not too low though. We also attempted to estimate levels of atmo-
spheric oxygen when we first described evidence for deep-water oxy-
genation from the rocks of the Avalon Peninsula, as explored above.
The argument went like this. In the modern ocean there is typically
a zone of minimum oxygen concentration developed in a depth range
from a few hundred meters to about 1500 meters. This is approximately
the depth range where the organisms represented by the Avalon fossils
lived. The oxygen minimum results from the convergence of two fac-
tors. The first is the settling of organisms from the surface ocean; these
consume oxygen as they decompose. The second is the sinking of cold,
oxygen-rich surface water from the polar regions of the oceans into the
ocean depths. This water forms the deep water of the oceans. Very little
organic matter survives the long transit from the ocean surface to these
depths, so little organic matter decomposition takes place, allowing
oxygen to persist. Consequently, in the modern ocean anyway, we have
high oxygen levels in the ocean surface, high levels at great depth, and
lower oxygen levels in between, which generates an oxygen minimum
zone. The magnitude of this oxygen minimum varies from place to
place, but in the modern world, the decrease in oxygen concentration
is, at a minimum, around 40 micromoles of oxygen per liter. In today's
oceans, the deep waters start with an oxygen concentration of about
325 micromoles per liter, 9 and a 40 micromole per liter decrease in con-
centration represents 12% of this modern deep water value. We assumed
then that the oxygen decrease where the organisms represented by the
Avalon fossils lived was at a minimum about 40 micromoles per liter
(possibly more). Finally, to give the organisms a bit to live on, we ar-
gued that some oxygen should be left after this 40 micromoles per liter
was used. In the end, we estimated that the organisms lived in a world
where atmospheric oxygen levels were 15% or more of present levels. 10
Less than this, and a lack of oxygen would have suffocated the organ-
isms at the depth they lived, but higher oxygen levels would have still
enabled life. So, if we were right, oxygen was something like 15% or
 
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