Questions: The Carbonate System and Ocean Buffering

Question 1

As rising atmospheric CO₂ increases the amount of CO₂ dissolving in the ocean, carbonate ions (CO₃²⁻) are progressively consumed. What is the most direct biological consequence of this?

• A. Photosynthesis by marine plants accelerates because more dissolved carbon is available
• B. The saturation state of seawater with respect to calcium carbonate decreases, threatening the ability of corals and shellfish to build their shells
• C. The ocean becomes truly acidic (pH < 7), killing most marine life directly
• D. Bicarbonate concentration falls, removing the buffer and causing rapid pH collapse

Answer: (B)

Calcium carbonate (CaCO₃) saturation depends on the product of Ca²⁺ and CO₃²⁻ concentrations. As CO₂ is added and the equilibrium shifts toward bicarbonate, CO₃²⁻ is consumed, reducing saturation. When saturation falls below 1 (undersaturation), seawater actively dissolves CaCO₃ shells and skeletons. Organisms like corals, oysters, and foraminifera that build CaCO₃ structures face increasing difficulty calcifying and even dissolution of existing shells. The ocean isn't becoming truly acidic — current pH is ~8.0 — but the trend is toward conditions hostile to calcifying organisms.

Question 2

At the ocean's typical pH of about 8.1–8.2, which carbonate species constitutes the majority of dissolved inorganic carbon?

• A. Dissolved CO₂ — it is the form in which carbon enters from the atmosphere
• B. Carbonic acid (H₂CO₃) — the intermediate that drives the buffering reactions
• C. Bicarbonate (HCO₃⁻) — roughly 90% of dissolved inorganic carbon at ocean pH
• D. Carbonate (CO₃²⁻) — it is the most stable form in alkaline water

Answer: (C)

At pH 8.1–8.2, the equilibrium strongly favors bicarbonate. The first pKa of carbonic acid is about 6.35, so at pH 8.1 the first deprotonation is essentially complete — nearly all of the carbonic acid has lost a proton to become HCO₃⁻. The second pKa is about 10.33, so at ocean pH the second deprotonation is only partial, leaving carbonate at about 9% and dissolved CO₂ at only ~1%. Understanding this distribution is essential because bicarbonate is the form that absorbs most of the CO₂ the ocean takes up.

Question 3

The ocean has absorbed roughly 30% of human CO₂ emissions since industrialization without experiencing catastrophic pH collapse, demonstrating that the carbonate buffer has effectively neutralized the added acid.

• T. True
• F. False

Answer: True

True in one sense and nuanced in another. The buffer has indeed prevented catastrophic pH collapse — without it, ocean pH would have dropped far more than the ~0.1 pH units observed so far. However, 'effectively neutralized' overstates the case: each mole of CO₂ absorbed does lower pH slightly and consumes carbonate ions, weakening the buffer for future additions. The Revelle factor quantifies how this works: as the buffer weakens, each additional unit of CO₂ causes more pH change. The ocean is doing its job, but at a cost to its own future buffering capacity.

Question 4

Ocean acidification describes the process by which the ocean's pH drops below 7, making it truly acidic and immediately lethal to most marine organisms.

• T. True
• F. False

Answer: False

Ocean acidification refers to a measurable decline in pH — from pre-industrial ~8.2 to current ~8.1, with further decreases projected — but the ocean remains alkaline (pH > 7). 'Acidification' means 'becoming more acidic,' not 'becoming acid.' The biological threat is not acute toxicity from true acidity but the reduction in carbonate ion concentration and CaCO₃ saturation, which impairs calcification by corals, mollusks, and other organisms. This is a slow chemical shift with serious ecological consequences, not a sudden lethality event.

Question 5

Explain why the ocean's capacity to absorb additional CO₂ decreases over time as more CO₂ is added, even though the buffering system continues to function.

Model answer: Each CO₂ molecule added to seawater reacts to form bicarbonate, consuming carbonate ions in the process. As carbonate ion concentration falls, the buffer becomes less effective — there are fewer carbonate ions available to absorb the next increment of CO₂. This weakening is quantified by the Revelle factor: as total dissolved inorganic carbon increases, the same increase in dissolved CO₂ causes a larger change in partial pressure of CO₂, meaning less CO₂ can be absorbed per unit of atmospheric pressure difference. The buffer is not destroyed, but it becomes progressively less capable of absorbing CO₂ without large pH changes.

This is the crucial distinction between 'the buffer is working' and 'the buffer capacity is unlimited.' A buffer works by consuming its reserve components — in this case, carbonate ions — to neutralize added acid. Once those reserves are depleted, the buffer fails. The ocean's carbonate reserve is enormous, so the buffer won't fail catastrophically in the near term, but each increment of anthropogenic CO₂ leaves the system with a smaller reserve and greater sensitivity to future additions. This is why early CO₂ additions were absorbed cheaply and later additions are increasingly costly in terms of pH change.