The a period of time. The cycle consists

The Marine Carbonate System – Meg Thompson



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Carbon is the fourth most abundant element in the
Universe, but on Earth the majority of it is stored in rocks. The remainder is
transferred in the carbon cycle, through reservoirs, which store carbon for a
period of time. The cycle consists of a slower and faster elements. The
interaction between these systems is known as the global carbon cycle, as
summarised in Figure 1. The slow cycle takes place over a geologic time period
and involves reservoirs such as rocks, oil, the ocean, atmosphere and
lithosphere. The cycle serves as a thermo-regulator and the exchange between
oceanic carbon and atmospheric carbon stabilises atmospheric  (carbon dioxide) concentrations over a
geologic period of time. However, during this process, there can be significant
fluctuations which take place more quickly.


Figure 1: Geocycle.

(2017). Illustrating the global carbon cycle, including the
fast and slow carbon cycles. Arrows indicate  transferral or reactions.


The oceans are a significant sink for anthropogenic ,
so the marine carbonate system plays a huge role in regulating the amount of
carbon in the atmosphere. The ocean is responsible for absorbing  to maintain a steady atmospheric concentration,
but having absorbed ~30% of total anthropogenic  emissions since the industrial revolution
(Sabine, 2004) the marine carbonate cycle has been significantly altered.


The cycle itself can be expressed by a series of equilibria
reactions, as illustrated below.  absorbed by the ocean is usually rapidly
hydrated to form carbonic acid (equations 1 and 2). The carbonic acid then
dissociates to form bicarbonate and   ions (equation 5). Some of these bicarbonate
ions dissociate to form carbonate ions and more  ions (equation 6).






intermediate reaction (4)





Equations obtained from Pinet,
P. (1992). 


Factors Impacting the
Marine Carbonate System


The marine carbonate system is founded on the concept
of equilibria and is therefore highly sensitive to a range of factors.


The pH of the ocean is one factor which affects the
equilibrium of the marine carbonate system. Increasing atmospheric  is a significant cause of ocean acidification.

The increased atmospheric concentrations leads to more absorption by the oceans
where it is converted into particulate organic carbon, illustrated by equation
1. Whilst this increases the buffering capacity of the ocean the series of
equilibria results in the dissociation of  to produce  ions (and carbonate ions), increasing the
acidity of the ocean system (Summerhayes and Thorpe, 1996), as shown by
equation 5. It is estimated that since the industrial revolution, acidity has increased
by 0.1 pH unit reduction (European
Commission, 2013). This is a negative feedback reaction because
increased acidity impedes further  absorption. In addition to direct absorption
from the atmosphere, rainwater contains dissolved gases, notably  and  which
dissociate in water to increase the acidity so precipitation is also
responsible for increasing the acidity of an ocean.  


Temperature is another factor influencing these equilibria
reactions. An increase in water temperature results in outgassing of  from the oceans. This is because a position of
equilibrium changes with temperature according to:





K = equilibrium constant at given temperature

R = the universal gas constant (8.134 x

T = temperature (Kelvin)


This means that the equilibrium reaction referred to
in reaction 1 is an exothermic reaction. Therefore, an increase in temperature
of the ocean results in reaction 1 shifting to the left, resulting in outgassing
of because
solubility of  decreases with temperature. This
results in a positive feedback cycle because the released  serves as a greenhouse gas, contributing to
further atmospheric and oceanic warming.


The rate of carbon fixation is also a significant
factor. Primary productivity and the formation of thermodynamically unstable organic
compounds through redox reactions in photosynthesis are the most significant
process of carbon fixation. In a system that has high levels of primary
productivity, the uptake of  from
the oceans into organic compounds is higher than in a body of water where
primary productivity is low. The result is a body of water undersaturated in with
respect to water. As a consequence, ocean surfaces are able to accept a greater
volume of  gas and
the cycle speeds up.


Calcifying organisms are similarly responsible for the
uptake of carbon, as they use carbonate ions which are released when carbonic
acid dissociates to form exoskeletons and supportive structures. The
polymorphic calcium carbonate structures formed by calcifying organisms are susceptible
to dissolution unless the surrounding solution is saturated in carbonate ions. Increasing
atmospheric concentrations of  are
resulting in decreased carbonate ion saturation, because when carbonic acid
dissociates there is a greater proportion of ions
compared to bicarbonate ions. To restore equilibrium, ions
bond with carbonate ions to form more bicarbonate ions, as shown in equation 6.

This reduces the concentration of carbonate ions, increasing the likelihood of


The detritus of these organisms creates calcite and
aragonite deposits when they die and when they do this they serve as a major
sink for carbon from the marine carbonate system. Calcification is becoming
increasingly difficult for marine organisms because of elevated atmospheric
concentrations and increased dissolution of calcite and aragonite. An increased
proportion is vulnerable to dissolution so the oceans capacity as a sink for
atmospheric  is decreased. The increased acidity and
temperature of oceans results in a rise of the saturation horizon and calcite compensation
depth, reducing the habitats of calcifying organisms  (The Effect of Ocean Acidification on Calcifying
Organisms in Marine Ecosystems: An Organism to-Ecosystem Perspective, 2010).

The carbonate compensation depth is the depth at which
carbonate accumulation equals carbonate dissolution, and the dramatic decrease
of carbonate accumulation rate at the compensation depth is illustrated by
Figure 2.


Consequently, a greater amount of carbon is being
stored in the marine system, whereas in history a greater proportion would eventually
be transferred to lithospheric reservoirs over a geological period of time.


Importance of the Marine Carbonate System


The oceans are a key factor in regulating the Earth’s
atmospheric temperature. The equilibrium maintained in the marine carbonate
system facilitates the removal of   from the
Earth’s atmosphere and transferral to the oceans. With 48% percent of anthropogenic
carbon emissions being absorbed by the ocean (,
2018), the marine carbonate system is vital in slowing
the effects of global warming, by removing the second largest contributor to
the greenhouse effect from the atmosphere. The absorption of infrared radiation
by and
subsequent heating of the atmosphere is a natural process and critical for
maintaining habitable atmospheric conditions. However, increasing levels of the
gas since the industrial revolution are contributing to an unsustainable rate
of global warming, and the oceans are unable to accept an amount of  to maintain a steady concentration of


As the solubility of  increases in colder waters, it is absorbed by cold
and dense polar waters and transferred to a depth below 500m by thermohaline
circulation. This forms a heat and carbon sink because residence times are
believed to be quite high at this depth. This creates negative feedback at the
surface because heat is transferred down theoretically warming in the surface
waters is slowed as well as in the lower atmosphere (Pinet, 1992).


The marine carbonate system is of importance to biotic
life cycles, because it is the main source of carbon for carbon fixation in marine
life and is required for photosynthesis. It enables biological productivity,
which in turn supports food webs on land. It also circulates carbonates to calcifying
organisms. Marine ecosystems can be devastated by the state of the marine
carbonate system because one of the factors defining the habitability of a
marine ecosystem is the acidity of the ocean, which is determined by the
concentration of  ions. The rapid reactions within the marine
carbonate system result in a relatively stable environment.




In conclusion, the marine carbonate system has
functions such as thermoregulation of the atmosphere, facilitating life in
marine ecosystems and serving as an important boundary between lithospheric and
atmospheric systems in the global carbonate cycle. Whilst many factors are
responsible for the natural fluxes of the marine carbon cycle, it is clear that
anthropogenic emissions are having significant effects on short term fluxes and
have the potential to affect long term fluxes. Perhaps the most concerning
affect it has is on calcifying organisms, because it results in the decreased
efficiency of a carbon sink, endangering marine and terrestrial food chains and
increasing the atmospheric concentrations.



European Commission (2013). Modelling the Carbonate System
to adequately quantify Ocean acidification. Luxembourg: European Union, p.2.

Pinet, P. (1992). Oceanography,
an introduction to the planet Oceanus. St. Paul: West Pub. Co.

PubMed (2012). A Cenozoic record of the
equatorial Pacific carbonate compensation depth. Macmillan Publishers Limited, p.609.

Sabine, C. (2004). The ocean sink for anthropogenic
CO2. Science, 305.

(2018). Carbon Cycle | Science Mission
Directorate. online Available at:
Accessed 10 Jan. 2018.

Summerhayes, C. and Thorpe, S. (1996). Oceanography.

New York: John Wiley.

The Earth Observatory.

(2011). The Carbon Cycle. online Available at:
( Accessed 6 Jan.


The Effect of Ocean
Acidification on Calcifying Organisms in Marine Ecosystems: An Organism
to-Ecosystem Perspective. (2010). Annual Reviews, p.134.

Tung, C. and Chen, A.

(2018). Carbonate Chemistry of the Oceans. In: Encyclopedia of Life
Support Systems, 1st ed. Paris: EOLSS, pp.1-5.

University of Augsburg,
Environmental Science Centre. Geocycle. (2017). image.