Environmental Engineering Reference
In-Depth Information
atmosphere is dynamic, this near constancy implies a near steady state for CO
2
. That is,
the inputs to and outputs from the atmosphere were very close to balanced for these
10,000 years. Since some organic C accumulated in sediments and on land, the question
remains: Where did the CO
2
come from to keep atmospheric concentrations from falling?
Geological sources resulting from plate tectonics (including volcanism and diffuse
degassing) supply the long-term (hundreds of thousands of years) source, which balances
long-term storage. While individual volcanic eruptions can occasionally put large amounts
of CO
2
into the atmosphere, the long-term average tectonic input is small.
Williams et al.
(1992)
estimate that 65 Tg of C as CO
2
(0.07 Pg) is emitted each year. This estimate is close
to but smaller than the long-term burial rate of C in the oceans of 0.12 Pg/y. Clearly,
tectonic inputs were too small to balance both sedimentation in the ocean and NEP on
land. Further, some estimates of long-term tectonic inputs are an order of magnitude lower
than those of Williams (Morner and Etiope 2002).
NEP on land is positive; that is, GPP
R (see Chapter 2). The fates of this NEP are
increases in terrestrial biomass or organic matter in soils, or export in rivers to lakes or
the ocean. Including the export term in rivers (a major fate for terrestrial NEP), terrestrial
NEP (at 1 to 4 Pg C y
2
2
) is much larger than volcanism. The inference is that NEP on
land, during this period, was supported by the degassing of CO
2
from the oceans. Since
the oceans were not declining in DIC (nor was the pH increasing), this degassed marine
CO
2
should be looked at as part of a terrestrial-marine loop. That is, much of the NEP on
land over this 10,000-year period was exported to the ocean (some 0.5 Pg/y) where it
was decomposed in surface waters and returned to the atmosphere as CO
2
.Thiscycle
implies that the overall ocean was “net heterotrophic” (R
.
GPP) and is a source of CO
2
to the atmosphere during the pre-Anthropocene era following the last glaciations about
12,000 YBP.
The present-day ocean is still most likely net heterotrophic (
del Giorgio and Williams
2005
) but is clearly no longer a net source of CO
2
to the atmosphere. It is a major sink
(
Quay et al. 1992
). This apparent paradox is easy to resolve by looking at the organic C
balance of the ocean.
The organic C inputs to the ocean are gross primary production (GPP) in the ocean, and
import from land (I) largely from rivers. The outputs are respiration (R) and long-term
burial in the sediments (B), and any long-term trended change in the standing stock of
POC (particulate organic carbon) or DOC (dissolved organic carbon) in the water column
(
.
Δ
S). Since inputs and outputs must be equal:
1
Δ
ð
Þ
GPP
1
I
5
R
1
B
S
6
:
9
Since
S in the water column is negligible in comparison to the other terms, we can
dismiss it in comparison to the other terms and rearrange:
Δ
GPP
R
B
I
ð
6
10
Þ
5
:
We learned in Chapter 2 that GPP
NEP, which is a measure of the net biological CO
2
or O
2
balance for an ecosystem, in this case the global ocean. Thus,
R
5
NEP
B
I
ð
6
11
Þ
5
:
