Geoscience Reference
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gundi event built on the already higher oxygen levels of the GOE to
produce oxygen levels similar to those at present. There isn't much evi-
dence yet to back this up, but if oxygen became so high, did things
settle back down after the Lomagundi event? hat indeed was normal
at this time in Earth history?
Many years ago I began to ponder exactly this point. The Lomagundi
isotope excursion hadn't really entered the discussion yet, but the GOE
had. Prevailing wisdom, stemming mainly from the contributions of
both Dick Holland and Preston Cloud, was that the GOE ushered in
oxygen levels high enough to ventilate the bottom of the ocean with
oxygen. Thus, the idea that oxygen rose to high levels during and after
the GOE isn't new. This oxygenation, in turn, oxidized iron from the
oceans. Thus, the GOE called an abrupt end to iron-rich conditions in
the deep oceans and hence the deposition of banded iron formations.
High post-GOE oxygen levels could, therefore, explain an important
geological observation; that is, the near cessation of BIF deposition fol-
lowing the GOE (
ig. 9.2
). Indeed, this view was presented in the last
chapter.
hile this idea of high post-GOE oxygen levels seemed reasonable
enough, there were some parts of the idea that didn't sit well with me.
For one, I had recently used a simple model of ocean chemistry, first
introduced by Jorge Sarmiento at Princeton University, to explore how
deep-water oxygen levels in the ocean might respond to changes in at-
mospheric oxygen concentrations. The model is very simple and only
approximates the workings of the real ocean, but the result was pretty
shocking. In order to fully oxygenate the oceans after the GOE, as Hol-
land and others had suggested, atmospheric oxygen levels would need
to rise to some 40% to 50% of today's value. Maybe Dick would have
accepted this conclusion easily, but I couldn't. For one, as we will ex-
plore in the next chapter, there was accumulating evidence for a late
Precambrian (some 600 million years ago) rise in atmospheric oxygen
levels. This would be difficult to accommodate if oxygen already rose to
near modern levels at the GOE. There is also a weaker argument ex-
plored in the next two chapters.
5
This argument is based on the idea
that the widespread evolution of large(ish) mobile animals, much later
in geologic history, occurred as oxygen levels rose to accommodate their
rather high energetic demands. This was would be equally inconsistent
