Chapter 1
28
Figure 1.3: Global map of annual sea to air CO
2
exchanges (in µatm; Sarmiento and Gruber, 2006).
The atmospheric gaseous CO
2
is dissolved and transformed into DIC within the surface ocean
(right part of Figure 1.2): it becomes aqueous and is transformed in carbonic acid (H
2
CO
3
),
itself in equilibrium with carbonate (CO
3
2-
) and bicarbonate (HCO
3
-
) ions (Equation 1.1). This
last form is the most abundant in seawater, representing more than 94% of total DIC (Equation
1.2). In parallel, we can note that CO
2
absorption increases the ocean acidity by adding H
+
ions in solution. Indeed, sea surface pH increased by about 0.1 pH units since the beginning
of the industrial revolution (Caldeira and Wickett, 2003).
CO
2
(g) + H
2
O ↔ CO
2
(aq) + H
2
O ↔ H
2
CO
3
↔ H
+
+ HCO
3
-
↔ 2H
+
+ CO
3
2-
(Equation 1.1)
CO
2
(aq) + H
2
O + CO
3
2-
→ 2 HCO
3
-
(Equation 1.2)
At high latitudes, where temperature is low and CO
2
is more soluble (Figure 1.3), the DIC can
be transferred to the deep ocean through deep water formation. The depth at which DIC is
transported via the circulation is important as the DIC can be sequestered by ocean, i.e. not
exchangeable with atmosphere, for longer time scales, from weeks in surface to centuries at
3000 m depth.
Chapter 1
29
1.1.2.2.
The biological pump
In 1934, Redfield noticed that the growth, sinking and remineralization of phytoplankton
generate a difference of carbon concentrations between surface and deep waters. This
difference was named “soft-tissue carbon pump” by Volk and Hoffert (1985) better known
nowadays as the biological carbon pump (BCP). The biological pump is a suite of biologically
mediated processes (left part of Figure 1.2) that consist of surface production and subsequent
sinking and remineralization of organic matter (Figure 1.4). These steps are described in
details in section 1.2.1.
Figure 1.4: Simplified view of the biological carbon pump (S. Hervé, IUEM).
In surface waters, phytoplankton assemblages take up nutrients and DIC during
photosynthesis and convert it into particulate organic matter (POM) and biomineral compounds
(calcium carbonate, also named calcite - CaCO
3
, for coccolithophores or biogenic silica, also
named opal – BSi, for diatoms). Photosynthesis is a reductive chemical reaction transforming
CO
2
and H
2
O into organic molecules (e.g. sugars; Equation 1.3).
6CO
2
+ 6H
2
O + light → C
6
H
12
O
6
+ 6O
2
(Equation 1.3)
Chapter 1
30
This carbon fixation, also defined as primary production (PP) in surface waters, produces
particulate organic carbon (POC) which can then be exported to the deep ocean through the
sinking of organic particles.
This organic matter can be consumed by zooplankton through grazing, which then excretes
fecal pellets and DOC. POC can also be remineralized from the particulate to the dissolved
phase, and oxidized from the organic to the inorganic form (DIC) through bacterial activity or
zooplankton respiration. If this remineralization occurs in the surface ocean, the released CO
2
can be then transferred back to the atmosphere.
As a result, only a small fraction of POC (< 0.1 Pg C.year
-1
) reaches the seabed, trapping the
carbon for thousands to hundreds of thousand years (Sabine and Feely, 2007).
1.1.2.3.
The carbonate pump
Another important part of the biological pump involves the formation of PIC via the precipitation
of calcium carbonate by specific plankton species such as coccolithophores, foraminifera and
pteropods (e.g. Emilio et al., 1993). These calcifying species directly use DIC to synthesize
their carbonate shell, thereby releasing CO
2
(Equation 1.4). This process is known as the
carbonate counter pump (left part of Figure 1.2).
Ca
2+
+ 2HCO
3
-
↔ CaCO
3
+ CO
2
+ H
2
O (Equation 1.4)
The precipitation of carbonates results in an increase of the surface ocean pCO
2
(Frankignoulle
et al., 1993) on timescales of 100-1000 years (Zeebe, 2012). However, an opposite effect of
the carbonate counter pump is the increased organic carbon export flux to the deep ocean (0.1
Pg C.year
-1
; Sabine and Feely, 2007), ballasted by the calcium carbonate (Francois et al.,
2002).
1.1.2.4.
The microbial pump
The majority of the organic matter is remineralized by respiration, releasing CO
2
back to the
atmosphere. However, a large reservoir of dissolved organic carbon (DOC) exists, in which
95% is recalcitrant DOC (RDOC). Being resistant to biological decomposition, the RDOC can
persist in the ocean for at least 100 years and represents thus a reservoir for carbon. The
Chapter 1
31
origin of this RDOC is still misunderstood but some authors have suggested that RDOC may
be formed by the microbes ‘pumping’ the bioavailable DOC into this RDOC (Jiao et al., 2010;
Legendre et al., 2015).
Physical transfers of carbon via the solubility pump are one order larger in magnitude than the
carbon export via the biological pump (265 Pg C.yr
-1
compared to 11 Pg C.yr
-1
) but in obduction
regions, the carbon related to the solubility pump is released back to the atmosphere
(276 Pg C.yr
-1
; Levy et al., 2013).
Sarmiento and Gruber (2006) have compared the impact of the oceanic pumps on the surface-
to-deep gradient of DIC concentrations (ΔsDIC; Figure 1.5). The authors have estimated that
the biological carbon pump (ΔC
soft
) is responsible for 70% of the DIC increase in the deep-
waters whereas the solubility pump (ΔC
gas-ex
) and the carbonate counter pump (ΔC
carb
) account
for 10 and 20% respectively of the observed gradient. This finding clearly highlights that the
biological carbon pump is the most important process controlling the distribution of DIC in
ocean and therefore the atmospheric CO
2
concentrations.
Figure 1.5: Contribution of the different carbon pump components (the solubility pump ΔC
gas-ex
; the
carbonate counter pump ΔC
carb
; and the biological carbon pump ΔC
soft
) to DIC increase in the deep
ocean (Gruber and Sarmiento, 2006).
In the following section, the biological pump will be at the core of the different chapters.
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