Geoscience Reference
In-Depth Information
with or even help to explain the rather sudden appearance of animal-
like forms? Our focus was on oxygen. There has been a long prejudice,
starting with a paper published in 1959 by J. Ralph Nursall from the
University of Alberta, that the evolution of animals, at least that of large
forms with relatively high oxygen demand, was spurred by an increase
in the oxygen content of the atmosphere. One can discuss long and
hard the relationship between animal evolution and oxygen, and we'll
entertain some of that discussion below, but Simon and I reasoned that
looking for signs of changes in ocean chemistry would be a good place
to start. These rocks on Avalon Peninsula seemed like a good place to
begin.
Rather remarkably, throughout the southern part of the Avalon Penin-
sula, one can sample rocks more or less continuously from a time begin-
ning before the Gaskiers glaciation, through the Gaskiers, and extending
to some 20 million years later. Importantly, one can sample with con-
stant reference to the emergence of Ediacaran life, because fossils are
abundant and well preserved throughout these rocks. Furthermore, Guy
and Jim are the perfect trip leaders. They've worked on these ancient
sediments for years and probably know them better than anyone. They
know the fossils, where to find them, and where on the seafloor these
organisms lived. They led many lively discussions on how the organ-
isms fed, their possible animal affinities, and how they were preserved.
We approached the famous Mistaken Point location at sunset. This is
the best-known Ediacaran fossil site on the Avalon Peninsula. Guy likes
to arrive here at sunset because in many cases the fossils here, and
throughout the peninsula, are only faint impressions. Difficult to see
at high noon, they emerge in all their glory as they cast small shadows
in the light of the setting sun.
Simon and I collected hundreds of samples.
5
Back in the lab, we con-
ducted the same type of chemical analyses, the so-called Fe speciation,
which I described in the last chapter for the transition between the
Gunflint BIF and the overlying Rove Formation. We found that before
and during the Gaskiers glaciation, the deep waters of the ocean in this
part of the world were anoxic and ferruginous.
6
Indeed, you can see this
just by looking at the glacial deposits of the Gaskiers, which are blood
red with iron (
plate 9)
. Subsequent work by Simon, myself, and many
others showed that these ferruginous deep water conditions were com-
