Geoscience Reference
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constant rate. In this case, we watch the water level drop to a new level
where the total rate of the water leaving by the drain and the siphon
again equals the rate of input. Finally, our system has a float connected
to the spigot. The float functions a bit like a toilet float (though not so
sensitive), and the higher the water level, the slower the water flows
from the spigot. The float helps to define a maximum water level in the
tub. With this float mechanism, as the water level rises, the input rate is
reduced. This is another negative feedback acting to stabilize the water
level in the tub.
This simple system, of course, is a metaphor for how atmospheric
oxygen is controlled, and as we shall see below and in subsequent chap-
ters, it contains all of the principles we need to understand oxygen regu-
lation. As with the concentration of atmospheric oxygen, this system
reaches a dynamic equilibrium when inputs match outputs. A dynamic
equilibrium is possible because of the negative feedbacks in the sys-
tem. This system also contains a water loss mechanism, the siphon, which
operates independently of any feedback. This water removal pathway
influences the water level, but does not necessarily destabilize the sys-
tem. In other words, a dynamic equilibrium is still possible with the
siphon turned on, though at a lower water level compared to when the
siphon is off.
Positive feedbacks are also possible. These are destabilizing feedbacks.
An example of a positive feedback in our water control system is if the
float was rigged so that the flow of water into tub increased as water
level rose. With this positive feedback, the tub would overflow. As we
shall see, there are possible positive feedbacks on oxygen control, but
these are not the overriding controls on oxygen concentration. Oxygen
concentration is stabilized by negative feedbacks. We will now look at
some of these.
Let's begin with the oxygen removal pathways; we'll start with the
easiest one first. This is the degassing of oxygen-reactive gases from the
mantle. We are concerned in particular with gases like H
2
, CO (carbon
monoxide), SO
2
, and H
2
S, which are directly analogous to the siphon
in our bathtub example. These gases are sinks for O
2
and are added to
the atmosphere independently of any feedbacks in the operation of the
oxygen cycle. Their rate of addition depends on tectonics and the internal
