Question

How does carbon dioxide interact with the world ocean? [Section 18.5]

Short answer

Carbon dioxide (CO2) interacts with the world ocean by dissolving in the water, forming carbonic acid (H2CO3), which then dissociates into bicarbonate (HCO3-) and carbonate (CO3 2-) ions. As a result, the pH level in the ocean lowers, leading to ocean acidification, which affects the ocean's chemistry and marine life, particularly those with calcium carbonate shells. This can lead to impaired sensory systems, reduced shell formation and growth, and lower reproductive rates in certain species, ultimately impacting marine ecosystems, food chains, and biodiversity.

Step-by-step solution

Dissolving of CO2 in ocean water

Carbon dioxide is a gas that is present in the Earth's atmosphere. When it comes in contact with the ocean's surface, it dissolves into the water. This is due to the difference in the partial pressure of CO2 in the air and its solubility in water, which causes it to dissolve.

Formation of carbonic acid

Once CO2 has dissolved in the water, it reacts with the water molecules (H2O) to form carbonic acid (H2CO3). This reaction can be represented by the following chemical equation: \[CO_{2(aq)} + H_{2}O \rightleftharpoons H_{2}CO_{3(aq)}\] This reaction is reversible. Carbonic acid can decompose and return to CO2 and H2O.

Dissociation of carbonic acid into bicarbonate and carbonate ions

Carbonic acid tends to dissociate into bicarbonate ions (HCO3-) and hydrogen ions (H+): \[H_{2}CO_{3(aq)} \rightleftharpoons H^{+} + HCO_{3^{-}}\] Further dissociation of bicarbonate ions can produce carbonate ions (CO3 2-) and additional hydrogen ions (H+): \[HCO_{3^{-}} \rightleftharpoons H^{+} + CO_{3^{2-}}\]

Effects on ocean chemistry and pH

The increase in hydrogen ions, as a result of the above reactions, leads to a lowering of the pH level in the ocean, making the water more acidic. This is known as ocean acidification. This can affect the entire ocean chemistry, with potential negative consequences for marine life, particularly for organisms with calcium carbonate shells (such as corals, mollusks, and crustaceans) because increased acidity reduces the availability of carbonate ions.

Impact on marine life

Ocean acidification caused by increased CO2 concentration can negatively affect the physiology and behavior of marine organisms. For example, it can cause impaired sensory systems, reduce shell formation and growth, and lower reproductive rates in some species. This, in turn, can have significant impacts on the marine ecosystem, food chains, and overall biodiversity.

Key concepts

carbon dioxide dissolution

Carbon dioxide (CO2) is naturally present in the Earth's atmosphere. When the wind blows over the ocean surface, CO2 comes into contact with seawater and begins to dissolve. This happens because gases tend to move from areas of higher concentration to areas of lower concentration. In this case, the difference between the concentration of CO2 in the air and that in seawater causes the gas to dissolve. Once dissolved, CO2 becomes part of the ocean’s chemistry. This process is crucial as it regulates the concentration of CO2 in the atmosphere, playing a role in controlling global temperatures.

The solubility of CO2 in water depends on various factors, including temperature and salinity. Cooler waters tend to dissolve more CO2 than warmer ones. As the Earth’s temperature changes, so does the ability of oceans to absorb CO2, significantly influencing climate patterns and oceanic life.

carbonic acid formation

Once dissolved in water, carbon dioxide (CO2) reacts with water molecules (H2O) to form carbonic acid (H2CO3). This chemical reaction can be represented by the equation:

\[CO_{2(aq)} + H_{2}O \rightleftharpoons H_{2}CO_{3(aq)}\]

This process is reversible and dynamic, meaning carbonic acid can break down back into CO2 and water. This reversible nature allows for a continuous exchange between CO2 and water in the form of carbonic acid. Over time, the production of carbonic acid has increased due to higher levels of atmospheric CO2, primarily from human activities like burning fossil fuels and deforestation. This excess carbonic acid is a significant contributor to ocean acidification.

The ability of oceans to transform CO2 into carbonic acid is crucial for buffering the introduction of excess CO2. However, while this process may reduce atmospheric CO2 levels temporarily, it introduces long-term changes to ocean chemistry.

bicarbonate and carbonate ions

Carbonic acid is not very stable and will quickly dissociate into two types of ions: bicarbonate \((HCO_3^- )\) and hydrogen ions \((H^+)\). The reaction is as follows:

\[H_{2}CO_{3(aq)} \rightleftharpoons H^{+} + HCO_{3^{-}}\]

Furthermore, bicarbonate ions can further break down into carbonate ions \((CO_3^{2-})\) and additional hydrogen ions:

\[HCO_{3^{-}} \rightleftharpoons H^{+} + CO_{3^{2-}}\]

The presence of these ions is critical in the ocean. The additional hydrogen ions increase the ocean's acidity, seen as a decrease in pH levels. On the other hand, carbonate ions are essential for marine organisms forming calcium carbonate shells and skeletons. These organisms include corals and shellfish, which rely on abundant carbonate ions to maintain their hard structures.

The balance between bicarbonate and carbonate ions is crucial. Ocean acidification disrupts this balance, where more hydrogen ions lead to less availability of carbonate ions needed by marine life.

marine life impact

The rising levels of hydrogen ions due to increased carbonic acid formation result in ocean acidification—a serious concern for marine ecosystems. The decreased pH caused by this process affects various marine organisms, especially shell-building species such as mollusks, crustaceans, and corals.

These organisms depend on carbonate ions to create calcium carbonate structures. With lower availability of carbonate ions, marine creatures face challenges in forming and maintaining their shells and skeletons. This can lead to weaker shells, making them more susceptible to predation and environmental stresses.

Beyond physical impacts, ocean acidification can also alter the behavior and physiology of marine life. Impaired sensory functions and reduced reproductive rates are some observed effects in fish and other organisms. Consequently, these changes can disrupt marine food webs, affecting biodiversity and the balance of oceanic ecosystems.

Addressing ocean acidification requires global efforts to reduce CO2 emissions and protect marine habitats, ensuring that ocean life survives amidst changing chemical balances.