Geoscience Reference
In-Depth Information
Table 12.1
Summary of effects and feedbacks. A positive (negative) feedback tends to increase (decrease) atmospheric CO
2
(see Box 12.1)
Process
Causality
Sign of
feedback
Magnitude
Level of
understanding
Direct feedbacks
CO
2
buffer factor
Decrease of ocean uptake capacity
Positive
Large
High
Indirect group 1 feedbacks
Photosynthesis
Enhancement of biological production and export by CO
2
fertilization
Negative
Medium
Medium
Export: stoichiometric ratio
Increase of C:N ratio, thereby enhancing marine productivity
in terms of carbon
Negative
Small to medium
Low
Calcii cation
Decrease of marine calcii cation (BUT mixed experimental
evidence)
Negative
Small to medium
Low to medium
CaCO
3
dissolution
Increase in dissolution of CaCO
3
in particles and sediments,
thereby increasing ocean alkalinity
Negative
Small (short-term)
— large on long
timescales
Medium
Remineralization: ballast
effect
Decrease of CaCO
3
production, leading to reduced org.
matter export
Positive
Small to medium
Low
DMS production
Enhancement or reduction of DMS production
Unknown
Unknown
Low
Indirect group 2 feedbacks
N
2
i xation
Enhancement of biological i xation of N
2
, increasing
N inventory, enhancing biological production
Negative
Medium
Medium
Oxygen denitrii cation
Reduction in penetration depth of organic matter, shallow
remineralization creating higher O
2
demand in low-O
2
regions, causing expansion of these regions, enhancing
denitrii cation, lowering N inventory, lower biological
production
Positive
Medium
Low
Nitrii cation
Reduction in nitrii cation
Unknown
Small
Low
Nitrous oxide production
Decrease in oxygen concentrations, enhancing
N
2
O production
Positive
Medium
Low
12.2
The marine carbon cycle
order to maintain this downward gradient against
homogenization by mixing and transport (Volk and
Hoffert 1985). Three pathways have been identii ed
as gradient makers ('pumps'): the solubility pump,
the carbonate pump, and the soft tissue pump. The
latter two are often collectively referred to as the bio-
logical pump. Ocean acidii cation, by modifying the
carbonate chemistry of the surface ocean and envi-
ronmental conditions for marine biota, interacts
with all three pumps ultimately altering the air-sea
balance of CO
2
.
The marine carbon cycle is an essential component of
the earth system, participating in the long-term stabi-
lization of atmospheric CO
2
levels. The oceans are by
far the largest reservoir for carbon, with the excep-
tion of sedimentary rocks (Falkowski
et al.
2000 ; see
Chapter 2). They interact with the atmosphere on a
variety of timescales ranging from hours (daily cycle
of biological production) over seasons (mixed-layer
dynamics), to several centuries (large-scale ocean cir-
culation), and up to several thousands of years (inter-
action with marine sediments through the mechanism
of CaCO
3
compensation). A crucial element of this
interaction is the fact that the surface ocean is sub-
stantially depleted in dissolved inorganic carbon (
C
T
)
relative to the deep ocean, requiring processes that
transfer carbon from the near-surface to depth in
12.2.1
The solubility pump
The solubility pump refers to the physico-chemical
process driving the uptake of CO
2
and its downward
mixing and transport along with the large-scale
