Geoscience Reference
In-Depth Information
O
2
Volcanic
outgassing
Weathering
Org-C and pyrite burial
Figure 5.2. the major processes controlling the concentrations of atmospheric oxygen.
Oxygen reacts with volcanic gases and organic carbon and sulfur compounds during
weathering, while it is liberated to the atmosphere through organic carbon and pyrite burial
in sediments. modified from Canfield (2005).
evolution of oxygen production. After oxygen production by cyano-
bacteria began, these volcanic gases became oxygen sinks. Sulfide and
SO
2
react with oxygen to form sulfate, whereas H
2
reacts with oxygen
to form water. A simple drawing summarizing these points and show-
ing the oxygen sources and sinks in the geological O
2
cycle is shown in
Knowing all of this is critical for understanding how oxygen is cycled,
but it still doesn't explain how oxygen concentrations are controlled.
Indeed, can we identify processes in the oxygen cycle that actually regu-
late oxygen levels? The short answer is “yes.” The trick is to recognize
that the rates of many of the processes producing and consuming oxy-
gen have a tendency to lead to stable oxygen levels. This may seem
rather obscure, but don't worry, I'll illustrate the principle before going
into more details on the specifics of how it works.
Imagine a very deep bathtub with a spigot letting water in and a sin-
gle drain (
ig. 5.3)
. The deeper the water in the tub, the faster it leaves
by the drain. Turn on the tap and let water flow in. The water in the tub
will reach a level where the water leaves the drain at the same rate that
