Environmental Engineering Reference
In-Depth Information
For the global ocean B is estimated at about 0.12 Pg/y; rivers deliver about 0.5 Pg/y of
organic C (combining POC and DOC;
Cole et al. 2007
and references therein). Thus, total
oceanic NEP would be negative and equal to about
0.38 Pg/y. So the modern ocean is a
biological source of CO
2
to its water column. The very rapid rise in atmospheric CO
2
over
the past 100 years, along with the reactions that this CO
2
has with the carbonate system,
allows the ocean to be a net sink of atmospheric CO
2
while being simultaneously slightly
net heterotrophic (
del Giorgio and Williams 2005
). Note that this net dominance of respira-
tion over primary production is very small in comparison to either GPP or R, both of
which is near 50 Pg/y.
The Global Carbon Balance at Longer Timescales
There is considerable variation in atmospheric CO
2
and CH
4
over long periods of time.
The primary evidence for this variation comes from the analysis of bubbles of air trapped
in glacier ice (
Figure 6.2
). There are excellent records going back to about 400,000 YBP
at multiple sites around the globe (
Figure 6.2c
). Over this time frame (excluding the
modern Anthropocene period), atmospheric CO
2
varied between about 190 and 280 ppm.
While this variation is small compared to the changes during industrial times, it is intrigu-
ing. Atmospheric CO
2
was lowest during glacial times and highest during periods
between glaciations (interglacials). Since the mass of both terrestrial vegetation and soil
organic matter was also lowest during glacial times, the expansion and contraction of the
terrestrial biosphere should have had an effect on atmospheric CO
2
opposite to what was
actually observed. The explanation for the interglacial CO
2
pattern probably reflects
changes in the oceanic C cycle. Decreased delivery of terrestrial C during glacial times
deprived the ocean of external supplies of both inorganic and organic C, but this was
probably too small and too short-lived to explain the atmospheric pattern.
Martin et al.
(1990)
, in a very influential paper, argued that the low atmospheric CO
2
during glacial
periods was driven by high net primary production in the southern ocean, which was
stimulated by iron-rich dust.
Martin et al. (1990)
calculated that the drier terrestrial soils
during glacial periods would have supplied 50 times more iron in windblown dust than
during the interglacial. The present Southern Ocean, while rich in nitrogen and phos-
phorus, has very low supplies of iron that limit primary production. Martin and collea-
gues intriguing hypothesis led to a series of open-ocean iron additions, which
demonstrated that the Southern Ocean phytoplankton could be greatly stimulated by iron
(
Coale et al. 2004
). These experiments are at least consistent with Martin's hypothesis.
At the timestep of tens of millions of years, the coarse variation in atmospheric CO
2
and
the rest of the global C cycle is easier to visualize as being linked to the cycles of oxygen
and sulfur. Prior to about 2.5 billion years ago there was effectively no free oxygen in the
atmosphere. The oldest fossils of cyanobacteria, the first organisms capable of oxygenic
photosynthesis, are about 3.5 billion years old. The time lag between the possible advent
of oxygenic photosynthesis and free oxygen in the atmosphere has multiple causes. It took
considerable time for cyanobacteria to become abundant, and reduced compounds (e.g.,
Fe
1
2
and sulfides) were so abundant that they consumed the photosynthetically produced
oxygen for many years (
Kennedy et al. 2006
). By 2 billion YBP the atmosphere had about
1% of its present level of O
2
; by 0.7 BYA, 10%; and by 0.35 BYA, 100% (
Berner and
