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Chemistry of Ocean Acidification

The oceans act as an important carbon sink, absorbing more carbon dioxide than they release into the atmosphere. About a quarter of all anthropogenic (man made) carbon dioxide released into the atmosphere since the industrial revolution has been absorbed by the world's oceans. This absorption has benefited humankind by significantly reducing greenhouse gas levels in the atmosphere, thereby minimizing global warming, but this has had a dramatic, and worrying, effect on ocean chemistry.

What happens when carbon dioxide is absorbed by the ocean?

This equation shows the reaction that occurs within the ocean when carbon dioxide is absorbed.

Carbon dioxide (CO2) + carbonate ions (CO32-) + water (H2O) --> bicarbonate ions (2HCO3-)

When carbon dioxide reacts with seawater it produces carbonic acid. This reaction converts carbonate ions into bicarbonate ions.

The increase in the dissolution of anthropogenically sourced carbon dioxide is causing our ocean chemistry to become unbalanced, so much so that it is profoundly affecting the seawater carbonate system. As the partial pressure of carbon dioxide increases the concentration of the bicarbonate ions also increases, while the amount of carbonate ions and the pH of the surface ocean waters decreases. This change in ocean chemistry and decrease in pH is known as ocean acidification. Surface ocean pH is estimated to have decreased from approximately 8.25 to 8.14 between 1751 and 2004 and may further drop by 0.3 to 0.5 units by 2100.

As the carbonate ion concentration of the ocean declines, the capacity of the ocean to absorb further carbon dioxide emissions also diminishes. Hence with continuing carbon dioxide emissions, the fraction of each mole of carbon dioxide emitted that is taken up by the ocean declines and a greater fraction will remain in the atmosphere, producing a positive feedback on global warming.

Marine organisms such as corals, shellfish, and marine plankton use the carbonate ions to build themselves protective carbonate skeletons and shells by precipitating calcium carbonate (CaCO3). The two common polymorphs of calcium carbonate are aragonite and calcite, with calcite being much more stable and less soluble than aragonite. The formation of these minerals is directly influenced by the degree of calcium carbonate saturation, which in turn is controlled by the concentration of carbonate ions present in seawater. The saturation state is used to express the degree of calcium carbonate saturation.

In areas where saturation state > 1 seawater is supersaturated with respect to the mineral calcium carbonate, so it does not readily dissolve. This is a more favorable environment for organisms that produce calcium carbonate shells and skeletons and so most calcifying organisms live in such waters. When saturation state < 1 seawater is undersaturated and corrosive to calcium carbonate producing organisms as calcium carbonate will more readily dissolve.

Aragonite and calcite have different saturation states due to their differences in solubility. Under normal conditions, calcite and aragonite are stable in surface waters since the carbonate ion is at supersaturating concentrations. The depth at which these calcium carbonate minerals stop being formed and start to dissolve is known as the saturation horizon. Aragonites saturation horizon is nearer to the surface than the calcite saturation horizon as aragonite is more soluble than calcite. Organisms, therefore, that produce aragonite shells may be more vulnerable to changes in ocean acidity than calcite producing organisms as their shells and skeletons are more prone to dissolution. The increase in the pressure of carbon dioxide and the lowering of seawater pH causes a decrease in the concentration of carbonate ions and the saturation state of calcium carbonate. This raises the saturation horizons of both forms closer to the surface. This change has proved to have a detrimental affect on calcifying marine organisms vulnerable to dissolution. Corals, benthic macroalgae, planktonic algae and protists, mollusks and sea urchins have all shown calcification decreases of up to 57% at the carbon dioxide levels expected in 2100. A huge decrease in the survival of these organisms will drastically affect many biological aspects of the oceans, including important food chains.

The calcium carbonate shell material produced by marine plankton is much denser than its soft body parts. Its presence may play an important role in accelerating the rate of sinking of carbon. When the organic matter and its associated carbon, produced by plankton, are continually transported to the deep sea, a chemical gradient in the ocean is created, with lower concentrations of dissolved carbon dioxide at the surface than at depth. This ensures that carbon dioxide continues to be dissolved. However, if ocean acidification reduces the calcification by plankton at the ocean surface then the time that the organic matter was suspended in the warm upper ocean would increase and therefore increase the likelihood of it being consumed by bacteria. The consequence would be an increase in the rate of recycling of both nutrients and carbon dioxide back to the surface. While this may result in an increase in primary production by plankton due to greater nutrient availability, the faster rate of return of dissolved carbon would drive atmospheric carbon dioxide higher.

Dicynodon Illustration courtesy of John Sibbick.
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