Environmental Engineering Reference
In-Depth Information
(e.g., NEP; see Chapter 2) is either exported to the sea or stored in biomass, soils, or fresh-
water sediments. Both the export and storage terms are part of terrestrial C sequestration,
and in both cases the material is organic C.
In the ocean, nearly all the GPP of phytoplankton is respired within the mixed zone
of the ocean. Additional decomposition occurs as particles sink through the deep-water
column. Only about 0.12 Pg C/y is stored (long term) as organic C in marine sediments,
which is less than 10% of the oceanic total CO 2 sink. This part of the oceanic sink is called
the biological pump because organisms are actively involved. The larger oceanic CO 2 sink is
predominantly a physical-chemical one, called the diffusion pump because it is caused by
physical and chemical processes. Rising atmospheric CO 2 concentrations create a gradient
causing CO 2 to diffuse into the ocean. There it meets two major fates. Downwelling ocean
water brings newly equilibrated surface water into the deep ocean where it is isolated
from the atmosphere for about 1000 years (the mixing time of the oceanic water column).
Seawater also has high concentrations of bicarbonate and carbonate. Dissolved CO 2 reacts
with CO 3 5 to form two ions of HCO 3 2 :
CO 2
CO 3 5 5
2HCO 3 2
ð
aq
Þ 1
H 2 O
ð
6
8
Þ
1
:
Seawater is well buffered, meaning that it can absorb a lot of CO 2 with little change in
pH and little change in the concentration of free CO 2 . Depending on location and tempera-
ture, surface seawater needs to absorb about 10 moles of CO 2 for the CO 2 concentration
to increase by 1 mole. So the total DIC increases much faster than does dissolved CO 2 . You
learned earlier that dissolved CO 2 is an acid. When the carbonate system is the dominant
buffer, as it is in the ocean, the pH is proportional to the ratio of CO 2 to the sum of bicarbon-
ate plus carbonate. As more and more CO 2 enters the ocean from the atmosphere, the oce-
anic pH also declines (
ocean acidification; Doney et al. 2009 ), as does the capacity of
seawater to absorb more atmospheric CO 2 ( Thomas et al. 2007 ). The pH in the ocean is now
measurably reduced as a result of anthropogenic CO 2 . There is great interest in ocean acidi-
fication ( Orr et al. 2005 ) and there are already some reports of negative biological effects
( Balch and Fabry 2008 ) as a result of increasing ocean acidification. On land there are also
abiotic sinks for CO 2 in the weathering of carbonate and silicate minerals but these are small
in comparison to storage in the organic products of terrestrial photosynthesis (see Raymond
and Cole 2003 ).
To summarize the present-day C cycle, atmospheric concentrations of CO 2 are rapidly
rising; both the ocean and the continents are net sinks for this excess CO 2 but the domi-
nant processes are different. Biological processes dominate on land and physical-chemical
ones dominate in the ocean. The astute reader will have noticed something odd about the
terrestrial C balance in that it is treated both as an anthropogenic C source (e.g., forest
clearing and burning) and as a C sink (forest regrowth). What is the rationale for this?
5
THE HOLOCENE PREINDUSTRIAL
GLOBAL CARBON BUDGET
From the start of the last interglacial period, some 10,000 to 12,000 YBP, until 200 years
ago, the concentration of atmospheric CO 2 was relatively constant ( Figure 6.2b ). Since the
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