Environmental Engineering Reference
In-Depth Information
This balance is missing an important feature. For the past 700 million years atmospheric
O
2
has been roughly at a steady state. Setting
Δ
O
2
to zero would cause B to also be zero.
At about 0.1 to 0.2 Pg/y B is very small in comparison to GPP or R (each about 100 Pg/y)
but it is not zero. Clearly, we are missing a small global sink for O
2
that does not involve
the oxidation of organic matter. This sink is the microbial and abiotic oxidation of iron
sulfide (pyrites) in rock (
Garrels et al. 1976
). The oxidation of sulfide or other reduced
compounds that were produced during the anaerobic decomposition of organic matter
would not affect this long-term balance. Rather, these simply reflect intermediate states.
Sulfate is reduced to sulfide at the expense of the oxidation of organic matter; any sulfide
that is eventually reoxidized simply consumes the oxygen that would have been
consumed had the organic matter been directly oxidized by oxygen. Thus, the oxygen sink
in the long-term global balance has to be a reduced compound that is present on Earth
independent of modern primary production and respiration, such as mineral pyrite.
Summary of the Global Carbon Cycle
We have learned that the concentration of CO
2
in the atmosphere is the end result of a
number of linked physical and biogeochemical cycles and that oxygen, sulfur, and iron are
key regulators of this cycle. While the terrestrial system tends to be a small but consistent
net sink for CO
2
, largely as a result of biological processes, the ocean has at times been
a source and is presently a sink for CO
2
. The modern oceanic sink for CO
2
is largely a
physical chemical process. Given the complexity of the C cycle and its coupling to other
cycles, it is remarkable how slowly atmospheric CO
2
concentrations have changed in the
past, and that these changes have been in a relatively narrow range, at least during the
past 500 million years. The combustion of fossil fuel, a very small reservoir of C on Earth,
has led to increases in atmospheric CO
2
at rates much faster than anything in the geologi-
cal record. Because CO
2
is a greenhouse gas, the temperature of Earth increases as its
concentration in the atmosphere increases. It is not a warmer Earth that worries climate
scientists so much as the unprecedented rapidity of this temperature rise and the ability of
natural and human systems to adapt to this pace (
IPCC 2007; MEA 2005
).
In summary, in the pre-Anthropocene part of the Holocene (since the retreat of the last
glacier) CO
2
in the atmosphere increased slightly at very slow rates. Processes that
removed CO
2
from the atmosphere (photosynthesis, weathering, etc.) were in near-perfect
balance with those that added it back to the atmosphere (respiration, precipitation of
CaCO
3
, etc.). Over longer timescales we see that CO
2
has varied with the phases of the gla-
ciations. Although there are pronounced differences in atmospheric CO
2
over these longer
timescales, the rate of increase of atmospheric CO
2
is much slower than in the present.
THE CARBON CYCLE IN SELECTED
ECOSYSTEMS
In this section we will look at some key aspects of the C cycle at a smaller spatial scale:
in forest, river, and large-river-forest complex ecosystems. We will look mostly at the
ecosystem-level aspects of the C cycle.
