Environmental Engineering Reference
In-Depth Information
Kothavala 2001
). At the coarse timestep atmospheric oxygen has been roughly at its pres-
ent level, near 20% (or 0.2 atm), for the past 350 million years (
Figure 6.3
). The production
of organic matter by oxygenic photosynthesis consumes CO
2
and releases O
2
, so the O
2
in
the atmosphere represents the net balance between the production and consumption of
oxygen. The graph in
Figure 6.3
suggests that for the past 500 million years (Phanerazoic
time), the processes that produce and consume O
2
have been about equal. During the
rapid rise in O
2
from about 2 to 0.7 billion YBP, the production of O
2
exceeded its con-
sumption. The mass balance is simplified by recalling that (1) at the scale of the entire
Earth, there is no significant import or export of either organic matter or oxygen; (2) we
can use O
2
as a proxy for organic matter, because it is produced and consumed mostly by
the same processes (e.g., photosynthesis and respiration). Thus, the change in the amount
of O
2
is approximated by:
Δ
O
2
5
GPP
R
NEP
ð
6
12
Þ
5
:
NEP
B
ð
since I and E are 0
Þ
ð
6
13
Þ
5
:
This means that for the past 2 billion years or so, to a first approximation, the accumula-
tion of atmospheric O
2
is equal to the burial (or preservation in rock) of organic matter. In
fact, the net preservation of organic matter and the accumulation of O
2
in the atmosphere
are in some sense the same process. If organic matter had not been protected from decom-
position, there would not be oxygen in the atmosphere. The long-term sink for organic mat-
ter on Earth is in marine sediments and, more important, in uplifted sedimentary rocks
derived from marine sediments. There is a surprisingly large amount of organic matter at
low concentration in sedimentary rocks (
Falkowski and Godfrey 2008
). While this organic
matter is potentially available to bacteria when it is released from the rocks (
Petsch et al.
2001; Schillawski and Petsch 2008
), its release requires special conditions that are rare. The
long-term preservation (hundreds of millions years) of organic matter in these rocks caused
Paul Falkowski to write, “Were it not for the continents, the atmosphere would never have
oxidized” (
Falkowski and Godfrey 2008
). What he means here is that the really long-term
sink for organic C (and hence the net production of oxygen) is in uplifted sediments now on
continents that have escaped from the cycle of subduction and volcanism.
FIGURE 6.3
Rough diagram of the long-term (4 bil-
lion year) record of oxygen in Earth's atmosphere.
Arrows show the appearance of major groups of the
biota. (Modified from
Berner and Kothavala 2001
.)
0.25
Mammals
0.20
Land plants
0.15
Eucaryotes
0.10
Cyanobacteria
0.05
0.00
1
2
3
4
Billions of years before present
