Moving the Waters - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 122

The main episodes of this form of ocean suffocation seem to have
lasted in the order of 100,000 years. They have been linked to the
warmest times within the Cretaceous—spikes of global heat that, in
some cases, have been more or less firmly linked to external forces
such as rare, prolonged volcanic outbursts that would have temporar-
ily increased carbon dioxide levels in the atmosphere.
There seems to be some kind of pattern here. The warmer the cli-
mate, the less efficiently oxygenated are the oceans. In the relatively
recent Cretaceous though (only the order of a hundred million years
ago), anoxic oceans were relatively rare events and are linked with
peaks of extreme warmth. This pattern can be taken even farther back
in time, to more than half a billion years ago during the Cambrian,
Ordovician, and Silurian periods. In those times, anoxia was a much
more frequent occurrence—and could sometimes encroach on to
shallow waters. Deep-water anoxia, then, was normal, and not a rare
catastrophe. In such oceans life was simply adapted to being confined
to shallow water conditions and the surfaces of deep oceans. Getting
on to the deep ocean floor was a bonus, only intermittently enjoyed
by the animals of those days.
Further back still, in the Precambrian, anoxic deep-sea floors were
very much the norm. Clearly, oxygen levels in the oceans climbed
only slowly and irregularly, from the initial appearance of free oxygen
in the atmosphere 2.5 billion years ago.
The physical and chemical structure of the oceans has been con-
stantly changing, as our watery planet has evolved from youth to
maturity. This had profound implications for the life that evolved
within it—as we shall see in the next chapter.