Geoscience Reference
In-Depth Information
At about this time, Simon Poulton joined my lab as a postdoc. Simon
worked as a PhD student with my old friend Rob Raiswell from the
University of Leeds,
7
and like Rob, Simon knows his way around a
pub. Also like Rob, he is a creative and resourceful scientist. So, to
explore this Canfield Ocean problem, Simon and I focused on the last
episode of major BIF deposition before its resurgence much later in the
Neoproterozoic Era, which we will discuss in the next chapter. Our
focus, therefore, became an episode of major BIF deposition between
about 2.1 and 1.9 billion years ago as observed in several places around
the world, but with particularly good representation in northern Min-
nesota and southern Ontario. This became our target.
Wait a minute, you might say, no matter what the model, I thought
the GOE was supposed to have ushered in the end of BIFs, and here
you're talking about BIFs some 300 to 400 million years AFTER the
GOE? hat gives? Good question, and before I can get to our work on
the Canfield Ocean model, we must contend with these BIFs.
Indeed, these BIFs are quite telling. For one, deposition began either
just after or near the end of the Lomagundi isotope excursion. Also, at
least some of them deposited in very shallow water. Imagine taking a
swim at the beach and coming home covered in rust! This is a critical
point. You can imagine the scenario like this: dissolved iron, accumu-
lated in the deep oxygen-free ocean, was transported up onto the conti-
nental shelf by a process called upwelling and was then carried across
the shelf into shallow water.
8
This sounds simple enough, but if you re-
fore, if oxygen concentrations were high like today, then the Fe
2+
simply
could not be transported from the deep ocean, across the continental
shelf, and further to the beach. The only logical way I can think of for
Fe
2+
to persist over extended transport in the surface ocean is if oxygen
levels were very low, perhaps only 0.1% of present-day levels or maybe
even lower.
9
Indeed, some recent geochemical evidence provides inde-
pendent evidence for low oxygen at this time. This evidence comes from
the behavior of chromium isotopes. I won't go into it here, but you can
learn more by following this endnote.
10
Let's review. There was a substantial rise in oxygen defining the GOE.
This may, in turn have led to the Lomagundi isotope excursion, which
as we argued, was associated with high rates of organic matter burial and
