Chapter 1
25
(Levitus et al., 2005). This large amount of heat, which is mainly stored in the upper layers of
the ocean, can be transported by ocean currents inducing an important effect on regional
climates. At a global scale, the large-scale Meridional Overturning Circulation (MOC; also
referred to as thermohaline circulation) induces variations at seasonal to decadal time scales
(e.g., Vellinga and Wood, 2002). Life in the sea is dependent on the biogeochemical
status of
the ocean and is also influenced by changes in the physical state and circulation. Changes in
ocean biogeochemistry can directly feed back to the climate system, for example, through
changes in the uptake or release of radiatively active gases such as carbon dioxide. The ocean
is thus a primordial reservoir to investigate in order to better understand the global carbon
cycle (Figure 1.1) as it strongly
participates to the CO
2
regulation.
1.1.2. Marine
carbon cycle
The ocean is the major carbon reservoir (Figure 1.1), storing ~ 38 Teratones of carbon, under
four chemical forms, namely dissolved inorganic carbon (DIC), dissolved organic carbon
(DOC), particulate inorganic carbon (PIC) and particulate organic carbon (POC) resulting in a
complex internal cycle as shown in Figure 1.2. Exchanges with the atmosphere are a key
component of the marine carbon cycle and are promoted by the high solubility of gaseous CO
2
in the surface ocean. This gas exchange depends on many factors such as biology,
temperature, wind speed, precipitation, waves or sea-ice cover (Nightingale et al., 2000;
Shutler et al., 2016; Wanninkhof and McGillis, 1999;
http://www.esa.int/spaceinvideos/Videos/2016/02/Carbon_flux) and aqueous CO
2
in the
surface ocean can exchange back to the atmosphere. More importantly, aqueous CO
2
can
also undergo various transformations that contribute to its redistribution into the ocean interior.
This carbon redistribution occurs on different timescales.
The fast marine carbon cycle represents a rapid CO
2
interchange between the surface ocean
and the atmosphere (time scales of days; Heinze et al., 2015), and is mediated through
phytoplankton photosynthesis. A fraction of the organic matter produced by this process can
Chapter 1
26
be then consumed by zooplankton or bacteria and the respired carbon can return to the
atmosphere. Another short term exchange of carbon between ocean and atmosphere results
from
the diffusion of CO
2
across the air-sea
interface, in both directions (timescale of minuts)
The thermohaline circulation, can store carbon for centuries (Kuhlbrodt et al., 2007; IPCC
Report, 2007) as dissolved CO
2
in the surface ocean is transferred to the deep ocean via the
subduction of dense water masses in high latitudes and stays in the deep ocean for years to
centuries before the water is mixed back to the surface where warmer waters release the CO
2
back to the atmosphere.
Finally, the long-term marine carbon cycle is regulated by the quantity of particulate organic
carbon reaching the sediments. Only a minor fraction of this organic matter, produced in
surface waters, is buried in the deep-sea sediments over geological timescales (million years;
IPCC Report, 2007; Heinze et al., 2015).
Four ocean carbon pumps are recognized to deplete the ocean surface of carbon relative to
the deep ocean: the solubility pump, the biological pump, the carbonate pump and the recently
proposed microbial pump (Honjo et al., 2014; Jiao et al., 2010; Legendre et al., 2015;
Sarmiento, 2002;
Sigman and Boyle, 2000; Turner, 2015;
Volk and Hoffert, 1985).
Chapter 1
27
Figure 1.2: The four ocean carbon pumps: The solubility pump, i.e., the dissolution of atmospheric CO
2
in surface waters (
1), followed by deep mixing of the CO
2
-rich water and sequestration (
2); The
carbonate pump, i.e., the bio-precipitation of CaCO
3
(or PIC) in the upper water column which is
accompanied by the release of CO
2
(
3), followed by the sinking of bio-mineral particles to depth where
their carbon is sequestered (
4); The biological pump, i.e., the photosynthetic uptake of carbon by
phytoplankton and its transformation by the food web in the euphotic zone, including respiration (6) and
loss to the atmosphere (
7), followed by transfer of particulate organic carbon (POC) into deep waters
where it is sequestered (
8). During the downward transit from 100 to 1000m, CO2 is released in the
water column by dissolution
of part of sinking CaCO
3
(
5) and remineralization of part of the POC that is
transferred to depth (
9). The production of recalcitrant DOC (RDOC) and semi-refractory DOC (SRDOC)
with a life time ≥ 100 years (i.e., DOC
>100
) presumably by microbial activity, will sequester ocean carbon
because their lifetimes are ≥ 100 years (
10). The small numbers in full circles identify arrows in the
figure. (Modified from Legendre et al., 2015).
1.1.2.1.
The solubility pump
The solubility pump starts with the dissolution of atmospheric CO
2
into seawater which is the
difference between the fugacity of CO
2
in the seawater and the atmosphere. The disequilibrium
between oceanic and atmospheric CO
2
is shown in Figure 1.3 representing the CO
2
partial
pressure difference across the air-sea interface (ΔpCO
2
).
ΔpCO
2
indicates if CO
2
is absorbed
into the ocean from the atmosphere (under-saturation, negative values) or if CO
2
is released
from the ocean to the atmosphere (over-saturation, positive values).