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mon, dating back to at least 850 million years ago.
7
Indeed, these fer-
ruginous waters may also link with the common occurrence of anoxic
and often ferruginous conditions described in the last chapter during
the middle parts of the Proterozoic Eon. However, and critically, Simon
and I, together with Guy, also found that just after the Gaskiers gla-
ciation, the deep ocean waters of the Avalon Peninsula became well
oxygenated. This is, as far as I know, the first evidence for oxygenation
of the deep ocean during Earth's long history.
8
And this may not have
been just a local event. Further evidence for deep water oxygenation at
about this same time was found in rocks from western Canada by Yanan
Shen, working with Andy Knoll and Paul Hoffman from Harvard, and
also by our group.
Therefore, the Ediacaran Fauna of the Avalon Peninsula emerged
into an ocean undergoing oxygenation. Indeed, other lines of evidence
seem to support such a conclusion. For example, Tim Lyons's group
has very recently found that concentrations of molybdenum in black
shales increased dramatically around 630 million years ago in the late
Neoproterozoic Era, somewhat predating our evidence for deep ocean
oxygenation (
ig. 10.3)
. This increase in molybdenum abundance implies
higher seawater molybdenum concentrations, which in turn implies less
efficient removal of molybdenum from seawater. The most efficient re-
moval pathway for molybdenum occurs under anoxic conditions, par-
ticularly during interaction with dissolved sulfide. Piecing this together,
an increase in molybdenum abundance in black shales implies less an-
oxic removal of molybdenum in the oceans. This, in turn, can be rea-
sonably tied to an increase in ocean oxygenation. The same is true for
molybdenum isotopes (see endnote 14,
chapter 9
), which show a big
jump about this same time, as detected by my postdoc Tais Dahl and
then PhD student Emma Hammarlund (
ig. 10.3
). Tais and Emma also
interpret this jump as reflecting an increase in the removal of molyb-
denum under oxic conditions, and consequently an increase in ocean
oxygenation.
Different geochemical indicators provide somewhat different answers,
but an increase in ocean oxygenation seems apparent in the late Neo-
proterozoic Era, beginning perhaps as early as 630 million years ago and
expressing itself in local indicators of oxygenation by 580 million years
ago. All of this is timed roughly with the emergence of macroscopic
