The incomplete understanding of the effect
that further acidification will have on marine organisms and inability to
predict the effectiveness of global climate change mitigation policies is a cause
for concern.

The Marine Carbonate cycle acts as a
governor for the global carbon cycle; the formation of CaCO3 in the
oceans is the major sink for dissolved carbon. This helps to reduce the CO2
content in the atmosphere and therefore, helps to regulate the temperature of
the planet. The difficulty arises as to the same calcifying organisms that have
constructed the deep-sea carbonate sink are now threatened by the continued
atmospheric release of fossil fuel CO2 which is increasing acidity
of the ocean’s surface.

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acidification is impacting many ocean species. Photosynthetic algae and
seagrasses benefit from higher CO2 conditions in the oceans as they
require it to live. However, studies have shown that the more acidic ocean
environment is causing waters to become increasingly less saturated causing CaCO3
to be dissolved away affecting the strength of the shells and skeletons
of marine organisms such as clams, oysters, corals and calcareous
plankton. Furthermore, building shells and bodies will be a more energetic
process as less carbonate (CO32-) is available to build
them as it is being dissolved.

and observational studies suggest that the absorption of CO2 by the
ocean has already decreased the pH of the ocean surface by 0.1 since 1750 [Orr
et al., 2005]. This rate of change is faster than at any time during the last
55 million years [Pearson and Palmer, 2000].

equation [10] increasing the concentration of H+ ions increase the acid
dissociation constant. The larger this value the weaker the pH and the stronger
the acid.

[10] Ka = [CO32-] [H+] /

 The acid dissociation constant, Ka, is
directly related to pH.

equations [3] and [4], carbonic acid (H2CO3) dissociates
into bicarbonate ions (HCO3-) and then into carbonate (CO32-)
ions respectively. The release of H+ ions in the dissociation of
carbonic acid these equations is what causes acidity to increase. Although this
presence of carbonic acid is necessary as part of the marine equilibrium, increasing
levels of CO2 in the oceans causes higher concentrations of H+
ions to be present:

dioxide (CO2) has the largest effect on the marine carbonate system.
Due to Anthropogenic emissions of CO2, atmospheric concentrations
have increased from 280ppm to approximately 390ppm and it is estimated that 27
± 5% of the recent emissions have been absorbed by the ocean rather than
remained in the atmosphere. It is the uptake of CO2 that leads to surface
ocean acidification.

of Carbon dioxide (CO2)


geological time, the rocks will be uplifted and then dissolved by rain water (see
figure 9. b7) in a process called dissolution which releases CO2
back into the atmosphere and the cycle will begin again.

Due to
modern neritic environments being over-saturated in carbonate, relatively
little carbonate (CO32-) dissolves in situ and alternatively
contributes to the formation of reefs and other calcareous structures. Biogenic
carbonate precipitation also takes place and is carried out by many marine
organisms to form calcareous shells and spines. The precipitation of CaCO3
results in a higher concentration of CO2 at the surface of the ocean
and causes a net transfer of CO2 from the ocean to the atmosphere.
Furthermore, when these marine organisms die, they sink to the bottom of the
ocean or laid down in shallow seas and are made into rocks through the process
of sedimentation. The process of precipitation is climatically important as it
removes carbonate from the cycle and stores it in a geological reservoir.

and Dissolution












the marine carbonate cycle is not a closed system to the oceans, Le Chatelier’s
principal can still be followed as a guide. Equation [6] is an exothermic
reaction and therefore, if you decrease the temperature of the sea the forward
reaction will be favoured and less CaCO3 and CO2 will be
created. This relationship creates many types of ocean environments at depth
and across the globe. Cooler seas, nearer the poles, and deeper seas contain
less life as ocean waters are less saturated in carbonate causing it all to

·       If you decrease the temperature of a system in dynamic
equilibrium the exothermic reaction will be favoured. The system counteracts
the change you have made by producing more heat.

·       If you increase the temperature of a system in dynamic
equilibrium the endothermic reaction will be favoured. The system counteracts
the change you have made by absorbing all of the extra heat.

Chatelier’s principles states:


process effects the marine carbonate cycle as it absorbs CO2 and
emits oxygen into the water in its place.

[7] 6CO2 + 12H2O à C6H12O6 + 6O2 +

Primary productivity is the measure of the
rate at which new organic matter is developed. Marine plants use the process of photosynthesis to make
carbohydrates. This involves dissolved carbon dioxide, sunlight and water.  The equation below summarises photosynthesis:


Many factors can impact the delicate
equilibrium of the Marine Carbonate system:

[6] CaCO3
+ CO2 (g) + H2O (l) ó
Ca2+ + 2HCO3-

Therefore, the overall equation is:

[5] CaCO3
ó Ca2+ +

[4] HCO3-
ó H+ +
CO32- (carbonate)

[3] H2CO3
ó H+ +
HCO3- (bicarbonate)

[2] CO2 (aq)
+ H2O ó H2CO3
(carbonic acid)

[1] CO2
(g) ó CO2

The Marine Carbonate System:

The marine carbonate cycle
is the absorption of atmospheric carbon dioxide (CO2) by seawater
and the resulting equilibrium that occurs in the oceans that give rise to a
complex chemical system. The
creation of this system occurred in the Mid-Mesozoic Revolution due to the
ecological success of calcareous plankton. Today the marine carbonate cycle
provides an important buffering system of ocean chemistry and controls the
circulation of CO2 between the oceans, atmosphere, lithosphere and
biosphere. The recent realisation of the ‘Greenhouse gas effect’, due to
increasing levels of CO2 in the atmosphere, has led scientists to
question the effect it is having on our oceans and ocean life.


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