Calcium carbonate (CaCO3) is an important part of the carbon dioxide cycle (DiVenere, 2017). As such, it acts as a partial buffer for the earth’s climate. The formation of calcium carbonate takes carbon dioxide out of the atmosphere and sequesters carbon, primarily as part of limestone. However, under current conditions, with more carbon dioxide in the atmosphere and more carbon dioxide in rainwater making it acidic, calcium carbonate may end up doing more harm than good. Therefore, it is important to understand how calcium carbonate functions in order to predict its influence on climate change. The calcium carbonate cycle, its solubility, the effects of temperature, pH, and partial pressure will be explored.
Calcium Carbonate and the Carbon Dioxide Cycle
Calcium carbonate has relatively low solubility in regular water, but becomes more soluble in rainwater and seawater that have high levels of carbon dioxide (“Carbonate chemistry,” 2012). Carbon dioxide in water forms carbonic acid, a weak acid (DiVenere, 2017). Carbonic acid breaks down to form hydrogen and bicarbonate ions. Bicarbonate ions and calcium form calcium carbonate (and carbon dioxide and water). Calcium carbonate is the primary ingredient in limestone (DiVenere, 2017).

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Most calcium carbonate is formed by sea creatures such as coral and shellfish, in shallow warm waters (King, 2017). When they die, their skeletons and shells form layers of limestone on the lake or sea floor. Calcium carbonate can also directly precipitate from marine or acidic fresh water, especially from hot springs. Marine limestone becomes bedrock as the seas move over thousands to millions of years. Caves are created when parts of the underlying limestone are dissolved in water. Calcium carbonate can also precipitate from cave rock, creating stalagmites and stalactites (King, 2017).

Note that calcium carbonate locks in carbon until the rock dissolves. This “weathering” of limestone releases carbon dioxide into the atmosphere (DiVenere, 2017). Several factors can affect the solubility of calcium carbonate. These include temperature, pH, and the partial pressure of carbon dioxide gas. Each therefore will be examined in turn.

Effect of Temperature on the Solubility of Calcium Carbonate
The solubility of calcium carbonate increases with decreased temperature (“Carbonate chemistry,” 2012). This explains its deposits around hot springs. It also has led to its use in construction: Heating limestone yields lime plus the release of carbon dioxide gas. Lime has been mixed with sand and water to make mortar. As mortar dries, the lime mixes with carbon dioxide in the air to reform calcium carbonate, which holds the sand together to create the hardened material at the finish (“Carbonate chemistry,” 2012).

More importantly to climate change issues, the decreased temperatures during glacial periods resulted in carbon dioxide being sequestered in large calcium carbonate deposits, but now that carbon dioxide may be released with warmth and acidity (Smith, 2013). Over eons, as temperatures have fluctuated, so has the usefulness of the ocean and lake waters as carbon source or sink.

Effect of pH on the Solubility of Calcium Carbonate
The solubility of calcium carbonate also increases with decreased pH/increased acidity (“Carbonate chemistry,” 2012). More acidic rainwater has resulted in the increasing weathering of limestone, creating more sinkholes and releasing more carbon dioxide into the atmosphere, creating increasingly acidic rainwater in addition to hazards associated with living atop limestone. Adding an acid to limestone will cause the rock to “fizz,” which signals the release of carbon dioxide (“Carbonate chemistry,” 2012).

In addition, in the ocean at the shallow benthic layer where coral are forming calcium carbonate, increased ocean acidification has resulted in increased weathering of the sediment (Eyre, Andersson, & Cyronak, 2014). The limestone sediment dissolves, releasing calcium ions and carbon dioxide. This changes the balance of carbon sequestered in the ocean versus carbon released by the ocean, increasing the risk of carbon dioxide in the atmosphere. This is also a risk to coral, as it will result in smaller sizes of coral reefs, with more difficulty accumulating the calcium carbonate that they need (Eyre, Andersson, & Cyronak, 2014).

Effect of Partial Pressure on the Solubility of Calcium Carbonate
Carbon dioxide dissolves in the upper levels of water to create an equilibrium between carbon dioxide in the air and carbon dioxide in the water (DiVenere, 2017). More carbon dioxide in the water will result in more carbonic acid, which can form bicarbonate ions and therefore increase levels of calcium carbonate; but also with increased acidity, limestone on the sea floor will become increasingly weathered, releasing calcium ions and carbon dioxide. Therefore, the ocean acts as a buffer to increasing amounts of carbon dioxide in the atmosphere, but as carbon dioxide content in the atmosphere rises, so does carbon dioxide content in the ocean, so that eventually it will release carbon dioxide rather than absorb it (DiVenere, 2017). There is an upper limit to how helpful such waters can be in protecting against the effect of greenhouse gases on climate change.

In evaluating effects of carbon dioxide on calcium carbonate in water, it is helpful to distinguish between the upper levels of water near the surface, or pelagic; versus the lower levels of water near the deep lake or sea floor, or benthic (Smith, 2013). Historically, more carbon dioxide has dissolved into the water than has been released by it. At the pelagic level, calcium carbonate production takes place, but most of that dissolves back into the water. Even so, some calcium carbonate accumulates, and there is a net carbon dioxide sink. At the benthic level, calcium carbonate escapes being dissolved, there is as higher level of calcium carbonate production associated with carbon dioxide production, leading to more carbon dioxide efflux. Currently, however, with rising carbon dioxide in the atmosphere and therefore rising partial pressure of carbon dioxide gas, the calcium carbonate ocean cycle is being pushed toward increased release of carbon dioxide gas. Note that flux of calcium carbonate happens quite slowly. However, there is more carbon stored in limestone than in all other components of earth’s crust, the atmosphere, hydrosphere, and biosphere combined. Therefore, the effect of calcium carbonate on climate change needs to be weighed carefully (Smith, 2013).

Conclusion
Calcium carbonate is an important factor affecting the carbon dioxide cycle and therefore climate change. Its ability to act as a buffer has been affected by the higher levels of carbon dioxide in the atmosphere. While calcium carbonate can precipitate directly into water, most sequestration of carbon in calcium carbonate occurs in warm shallow waters via the biological actions of sea creatures such as coral. These eventually become part of the sediment at the bottom of the ocean that creates layers of limestone. As temperatures warm, calcium carbonate becomes less soluble, making it more likely that it will form and sequester carbon dioxide. However, with increased acidity, calcium carbonate becomes more soluble, making it dissolve and release carbon dioxide. More carbon dioxide in the air and in rainwater has led to weathering of limestone on land and in shallow waters, causing increased release of carbon dioxide. Increased partial pressure of carbon dioxide in the atmosphere will lead to more carbon dioxide in the upper layers of the water. However, weathering of limestone in shallow waters will increase carbon dioxide from that source. On balance, at the present time more limestone is dissolving and releasing carbon dioxide gas than carbon dioxide is being pulled from the atmosphere via calcium carbonate.

    References
  • Carbonate chemistry. (2012). Science Learning Hub.
  • DiVenere, V.J. (2017). The carbon cycle and earth’s climate. Columbia University.
  • Eyre, B.D., Andersson, A.J., & Cyronak, T. (2014). Benthic coral reef calcium carbonate dissolution in an acidifying ocean. Nature Climate Change, 4(11), 969-976.
  • King. H. (2017). Limestone. Sedimentary Rocks.
  • Smith, S.V. (2013). Parsing the oceanic calcium carbonate cycle: A net atmospheric carbon dioxide source, or a sink? Waco, TX: L&O e-books.