Chapter 16: Problem 128

Carbon dioxide dissolves in water according to the equations: Carbon dioxide levels in the atmosphere have increased about 20% over the last century. Given that Earth"s oceans are exposed to atmospheric carbon dioxide, what effect might the increased CO2 have on the pH of the world"s oceans? What effect might this change have on the limestone structures (primarily CaCO3) of coral reefs and marine shells?

Short Answer

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Increased atmospheric CO2 levels will lower the pH of the world's oceans, leading to ocean acidification. Acidification endangers limestone structures like coral reefs and marine shells by promoting the dissolution of calcium carbonate, which can weaken or destroy these structures.

Step by step solution

Understanding Carbon Dioxide Dissolution

Carbon dioxide (CO2) gas dissolves in water to form carbonic acid (H2CO3). This process can be represented by the chemical equation CO2 (g) + H2O (l) → H2CO3 (aq). The carbonic acid can further dissociate into bicarbonate (HCO3-) and carbonate (CO32-) ions along with hydrogen ions (H+). The dissolution and dissociation process of carbon dioxide can be summed up by the reactions: CO2 (g) + H2O (l) → H2CO3 (aq) → HCO3- (aq) + H+ (aq) → CO32- (aq) + 2H+ (aq). The increase in hydrogen ions (H+) leads to a decrease in pH, making the water more acidic.

Effect on Ocean pH

With a 20% increase in atmospheric CO2 levels over the last century, more CO2 has dissolved in the oceans, leading to the formation of more carbonic acid. Through the dissociation of this acid, an increase in hydrogen ion concentration occurs which drives down the pH of ocean water, leading to increased ocean acidity or 'ocean acidification'.

Impact on Limestone Structures in Marine Environments

The decrease in ocean pH can affect marine calcifying organisms like corals and mollusks that use calcium carbonate (CaCO3) to build their shells and skeletons. Acidified ocean water is more prone to dissolving CaCO3, which can weaken the structural integrity of coral reefs and marine shells. This could lead to reduced calcification or even dissolution, affecting the survival of these organisms and the marine ecosystems that depend on them.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Carbon Dioxide Dissolution

The process of carbon dioxide (CO₂) mixing with water is fundamental to understanding ocean acidification. When CO₂ from the atmosphere comes into contact with ocean water, a series of chemical reactions occur. Initially, CO₂ dissolves in seawater to form carbonic acid (H₂CO₃). This reaction can be represented by the chemical equation:
CO₂ (g) + H₂O (l) → H₂CO₃ (aq).
Subsequently, the newly formed carbonic acid can dissociate into bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions, releasing hydrogen ions (H⁺) in the process. An increased presence of H⁺ in the water is what ultimately lowers its pH, contributing to a more acidic environment.

pH of World's Oceans

The pH level, which measures the acidity of a solution, is a crucial parameter for the health of ocean ecosystems. Over the last century, the 20% rise in atmospheric CO₂ has led to higher concentrations of carbonic acid in the oceans, and thus higher levels of H⁺ ions. The net effect is a drop in the pH of the world's oceans, an ongoing phenomenon known as ocean acidification. This reduction in pH has various implications for marine life, altering the fundamental biochemical cycles and processes within the oceanic environment.

Impact on Limestone Structures

Limestone structures such as coral reefs and the shells of shellfish are primarily composed of calcium carbonate (CaCO₃). These structures are crucial both as marine wildlife habitats and for their role in the carbon cycle. As ocean pH decreases, the waters become corrosive to calcium carbonate. This process, known as enhanced solubility, can lead to the weakening or dissolution of these limestone structures. The degradation of coral reefs and the thinning of shellfish shells represent not only a loss of biodiversity but also affect the broader oceanic ecosystems, including the services they provide to human communities.

Marine Calcification

Calcification is the process by which marine organisms, such as corals and mollusks, build their exoskeletons and shells by depositing calcium carbonate (CaCO₃). Ocean acidification, however, hampers this process. The additional H⁺ ions react with carbonate ions (CO₃²⁻), reducing their availability to form CaCO₃ with calcium ions (Ca²⁺). As a result, the rate of calcification decreases, and existing structures face increased risks of dissolution, which can be devastating for the organisms that rely on their shells or skeletons for protection and structure.

Carbonic Acid in Seawater

Carbonic acid formation in seawater is the immediate result of CO₂ dissolution: CO₂ (g) + H₂O (l) → H₂CO₃ (aq). While carbonic acid is relatively weak and exists in equilibrium within the marine environment, its presence is vital to the discussion on ocean acidification. The balance between carbonic acid and its dissociated ions (HCO₃⁻ and CO₃²⁻) is sensitive to changes in pH. Therefore, the increase in carbonic acid due to absorbing more CO₂ shifts this equilibrium and contributes to the overall decrease in ocean pH, affecting a wide array of marine processes.

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Short Answer

Step by step solution

Understanding Carbon Dioxide Dissolution

Effect on Ocean pH

Impact on Limestone Structures in Marine Environments

Key Concepts

Carbon Dioxide Dissolution

pH of World's Oceans

Impact on Limestone Structures

Marine Calcification

Carbonic Acid in Seawater

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