The Mechanism
Atmospheric carbon dioxide, , dissolves in seawater and becomes part of the ocean's dissolved inorganic carbon pool. This pool includes carbon dioxide, hydrogencarbonate ions and carbonate ions.
During calcification, reef-building corals and molluscs combine calcium ions with carbonate ions:
The resulting calcium carbonate forms hard structures. Coral polyps deposit it as external skeletons that accumulate into reefs, while molluscs use it to construct shells.
| Stage | Carbon process |
|---|---|
| Ocean uptake | Atmospheric dissolves in seawater and enters the dissolved inorganic carbon pool. |
| Calcification | Corals and molluscs convert dissolved carbon into solid . |
| Sedimentation | Dead skeletons and shells accumulate on the seabed. |
| Long-term storage | Burial, compaction and cementation can eventually form limestone, storing carbon over geological timescales. |
A common misconception is that all carbon in a coral reef is stored in living tissue. In fact, much of it is stored in the reef's calcium carbonate framework. However, calcification is not identical to simple removal of atmospheric carbon because carbonate chemistry can also release some .
Ocean acidification reduces carbonate-ion availability, making calcification more difficult and threatening reefs such as Australia's Great Barrier Reef. This can weaken carbon storage alongside biodiversity, coastal protection and tourism services.
Exam Technique
For an explain question, identify the ocean as a carbon store, describe calcification, write the balanced ionic equation, and trace shells or skeletons to sediment and limestone. Avoid claiming that corals perform photosynthesis themselves or that all captured carbon remains permanently stored.