Marine Carbonate System and Ocean Acidification

The reactions among dissolved carbon dioxide, bicarbonate, and carbonate ions form the chemical machinery that buffers the ocean's pH, stores most of its inorganic carbon, and is now being pushed out of balance by rising atmospheric carbon dioxide.

Definition

The marine carbonate system is the coupled set of chemical equilibria linking carbon dioxide, carbonic acid, bicarbonate, and carbonate ions in seawater; ocean acidification is the ongoing decline in seawater pH and carbonate saturation caused by the ocean's uptake of anthropogenic carbon dioxide.

Scope

This topic covers the equilibria of the seawater carbonate system, the measurable parameters of pH, total alkalinity, dissolved inorganic carbon, and the partial pressure of carbon dioxide; the buffering capacity of seawater; the saturation state of calcium carbonate minerals; and the causes and biological consequences of ocean acidification.

Core questions

• How do the carbonate equilibria distribute inorganic carbon among its chemical species?
• What do pH, alkalinity, dissolved inorganic carbon, and carbon dioxide partial pressure each measure?
• How does the carbonate system buffer seawater against changes in pH?
• Why does rising carbon dioxide lower seawater pH and the saturation state of calcium carbonate?

Key theories

Carbonate equilibria and buffering

Dissolved carbon dioxide reacts to form carbonic acid that dissociates into bicarbonate and carbonate, a system that resists pH change and partitions carbon among species according to pH and temperature.

Acidification and calcium carbonate saturation

Added carbon dioxide raises hydrogen-ion and bicarbonate concentrations while lowering carbonate ion, reducing the saturation state of aragonite and calcite and stressing calcifying organisms.

Mechanisms

When carbon dioxide dissolves in seawater it forms carbonic acid, which dissociates and releases hydrogen ions; these react with carbonate ions to form bicarbonate, lowering pH and depleting the carbonate ions on which corals, mollusks, and many plankton depend to build calcium carbonate shells and skeletons.

Clinical relevance

Ocean acidification threatens coral reefs, shellfish, and pteropods at the base of marine food webs, with consequences for fisheries and coastal economies; the carbonate system also governs how much carbon dioxide the ocean can absorb, central to the global carbon budget.

History

Revelle and Suess showed in 1957 that seawater chemistry limits how fast the ocean can take up fossil-fuel carbon dioxide. From the 1990s, sustained measurements documented falling open-ocean pH, and the term ocean acidification entered wide use in the 2000s as its biological risks became clear.

Key figures

• Roger Revelle
• Richard Feely
• Scott Doney

Seminal works

• zeebeWolfGladrow2001
• doney2009

Frequently asked questions

What is total alkalinity?

Total alkalinity is a measure of seawater's capacity to neutralize acid, dominated by bicarbonate and carbonate ions; together with another carbonate parameter it lets chemists calculate the full carbonate system.

Why does ocean acidification harm shell-building organisms?

Acidification reduces the concentration of carbonate ions and lowers the saturation state of calcium carbonate, making it harder and more energetically costly for corals, mollusks, and some plankton to build and maintain their shells.