Geoscience Reference
In-Depth Information
in the geologic record. Going back in time, I would bring a small drill.
With it, I would collect samples from the surface of shale in contact with
rain and weather to deep within into the rock where weathering fluids
had yet to reach. This would allow me to see how the organic matter was
affected as the weathering fluids penetrated and transformed the rock.
In the modern world, as revealed by Steven Petsch and discussed in
chapter 5,
organic matter concentrations are universally lowest near the
upper surface of a shale as the shale undergoes oxidative weathering
(
ig. 5.3)
. This oxidation requires oxygen, but it is not super fast, and
some organic matter remains, even at the upper surface that is exposed
to oxygen the longest.
1
So, what happens when oxygen concentrations
are really low, as they were before the GOE? I would expect that or-
ganic matter escaped oxidation as the ancient shales were lifted into the
weathering environment. If this was the case, then organic matter like
pyrite might also have been largely recycled from rock to sediment to
rock again, accumulating to higher and higher concentrations as more
CO
2
entered the surface environment from volcanoes.
2
Therefore, the
time machine will have told us something very important about the
dynamics of oxygen-sensitive species before the GOE. In the absence
of a time machine, we can only make reasonable suppositions and hope
they are correct. We suppose, therefore, that before the GOE (and out-
side of times of oxygen whiffs as discussed in the last chapter), large
proportions of the pyrite, organic matter, and many other oxygen-
sensitive species brought into the weathering environment escaped
oxidation and were cycled around and around from rock to sediment
to rock again.
3
Now comes the oxygen. An increase in atmospheric oxygen levels
would have caused efficient oxidation of pyrite to sulfuric acid,
4
and
organic matter to CO
2
with the potential liberation of any nutrients tied
up with the organic matter. One might view this as the geochemical
equivalent of stirring up a wasps nest! Dick Holland was, as far as I
know, the first to speculate on what happened next:
The large positive δ
13
C excursion in carbonate sediments between
ca. 2.22 and 2.06 Ga suggests that during this period, PO
4
- 3
was
exceptionally available for photosynthesis and carbon burial. Sev-
eral changes probably contributed to this greater availability. As
