Geoscience Reference
In-Depth Information
or “holes” in the shale), water flow dynamics, and pyrite and organic
carbon oxidation kinetics. The results, however, are clear. Given the
normal range of erosion rates and soil porosity, organic matter will com-
pletely oxidize during weathering with atmospheric oxygen levels of
25% of today's levels or higher. With lower oxygen levels, however, or
with much higher erosion rates such as one might find in mountainous
terrains, for example, oxygen concentration should influence the ex-
tent of organic matter oxidation. Therefore, atmospheric oxygen could
have provided a negative feedback on organic matter oxidation dur-
ing times in the past when oxygen levels were considerably lower than
today or when erosion rates were considerably higher. A negative feed-
back of oxygen concentration on pyrite oxidation is also possible, but
only at extremely low oxygen concentrations; this will be explored in
The discussion up to now has focused on the efficiency of organic
matter and pyrite oxidation, but the amount of organic matter available
for oxidation must also play a role. Quite simply, the more organic mat-
ter and pyrite available for oxidation, the greater the oxygen sink. The
amount available will depend on several factors. One is the uplift rate
of the continents as this controls the rate at which rocks become ex-
posed to weathering. Another factor is the concentration of organic car-
bon and pyrite in the rocks being weathered, and this will depend, to a
large degree, on which rocks become available. Bob Berner has thought
deeply about this, and he has conceived of a way in which atmospheric
oxygen concentrations are regulated based on this principle. Bob sug-
gests that there is a tight coupling between the burial of organic carbon
and pyrite into sediments, which represents an oxygen source, and the
subsequent availability of these same sediments for weathering and oxi-
dation, which represents an oxygen sink.
The reasoning goes like this: imagine that for some reason there is a
period where sediments are formed that are very rich in organic carbon
and in pyrite. The burial of these sediments would represent a strong
oxygen source to the atmosphere. However, these sediments, at least
many of them, should be quite prone to weathering and oxidation on
relatively short time scales.
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This is because these sediments (and the
rocks they form) are subject to exposure during sea-level changes. They
are also subject to uplift and erosion as a result of a number of tectonic
