Geoscience Reference
In-Depth Information
sphere at all. Through this discussion we will try and identify the main
processes controlling the oxygen concentration.
Big deal, you might think. Any third grader knows that oxygen comes
from photosynthesis. That's why we have oxygen in the air, so what's
the problem? True enough, but it takes a really clever third grader to
think of the following experiment. Put a plant in a closed container and
watch how much oxygen accumulates during the day.
2
Write it down.
Now, watch during the night. You will probably see that during the
night, the plant uses nearly as much oxygen during respiration as it
produced during the day.
3
We're nearly back were we started—but not
exactly, and that's the point.
Look again at the measurements. The oxygen value is likely a little bit
higher, and if we follow this experiment over some weeks, we might in-
deed see some oxygen accumulate. This is because the plant has grown.
Recall that oxygen is a byproduct of photosynthesis and that oxygen
production is balanced by the production of plant material. The more
plant material created, the more oxygen produced. In our experiment,
if ALL the plant material is used up again during respiration, then no
oxygen accumulates, but if the plant grows, this growth represents un-
respired plant material. Simply stated, if oxygen is not used to respire
the plant material, it will accumulate in the jar. In this way, plant growth
can be equated to oxygen accumulation. It's as simple as that; or is it?
Let's try the following calculation. There is currently in the atmo-
sphere 3.7 × 10
19
moles of oxygen. That's a big number. If we cooled
this down to liquid, we would form a layer of liquid oxygen about 6 cm
(2.5 in.) deep over the whole surface of the planet. From a combination
of satellite imagery and ground-based measurements it is estimated that
the net rate of primary production on Earth, which is roughly the same
as the combined growth of plants, algae, and cyanobacteria, is about
8.8 × 10
15
moles of carbon per year. If we compare this rate of net pri-
mary production to the mass of oxygen in the atmosphere, we calculate
that the oxygen in the atmosphere could be generated in a mere 4200
years. This calculation might imply that the oxygen context of the at-
mosphere is unstable over rather short time intervals, and that slight
imbalances between oxygen production and respiration might give us
Mount Everest-like oxygen concentrations on short order, or alterna-
tively, very high levels.
