Questions: Ocean Carbonate Equilibrium and Acidification

Question 1

Ocean pH has dropped from 8.2 to 8.1 since pre-industrial times. A student says this 0.1-unit change is negligible for marine life. What is wrong with this reasoning?

• A. pH is a logarithmic scale, so a 0.1 drop represents approximately a 26% increase in hydrogen ion concentration
• B. The student is correct — a 0.1 pH change is too small to affect marine organisms
• C. A 0.1 pH drop represents a 10-fold increase in hydrogen ion concentration
• D. The change is negligible in absolute terms but significant over geological timescales

Answer: (A)

pH is defined as the negative logarithm (base 10) of [H+]. A drop from 8.2 to 8.1 corresponds to a factor of 10^0.1 ≈ 1.26 increase in [H+] — roughly 26% more hydrogen ions. This is biologically significant even if the number sounds small. Option C would only be true for a drop of exactly 1.0 pH unit.

Question 2

A pteropod is transferred from warm tropical surface water to cold polar surface water with the same pH but lower carbonate saturation (Ω < 1). Its aragonite shell begins dissolving. What is the direct mechanism?

• A. Lower pH in polar water directly attacks the carbonate in the shell
• B. When saturation state Ω < 1, the surrounding water is thermodynamically undersaturated with respect to aragonite, so the mineral spontaneously dissolves to approach equilibrium
• C. Cold temperatures reduce the metabolic rate needed to maintain the shell
• D. Higher dissolved CO2 in cold water reacts directly with the shell surface

Answer: (B)

Shell dissolution is driven by saturation state (Ω = [Ca²⁺][CO₃²⁻] / Ksp), not pH directly. When Ω < 1, the surrounding seawater is undersaturated and thermodynamically favors dissolution of CaCO3. Low pH contributes by depleting CO₃²⁻ ions, reducing Ω, but the proximate cause of dissolution is thermodynamic undersaturation — not acid attack. This distinction matters: waters can remain alkaline (pH > 7) yet become corrosive to shells.

Question 3

Ocean acidification is primarily a concern because seawater will eventually become acidic (pH below 7), making it inhospitable to most marine life.

• T. True
• F. False

Answer: False

Seawater is currently at pH ~8.1 and is not projected to drop below 7 in any realistic emissions scenario. The concern is not about the ocean becoming acidic in a chemical sense. The threat is the reduction of carbonate ion concentration and saturation state (Ω), which undermines shell-building organisms like pteropods, corals, and mollusks long before pH reaches 7. Aragonite undersaturation is already occurring in parts of the Southern Ocean and North Pacific.

Question 4

Polar and subpolar oceans acidify faster than tropical oceans because cold water absorbs more CO2 from the atmosphere.

• T. True
• F. False

Answer: True

Gas solubility increases with decreasing temperature — cold water can hold more dissolved CO2 than warm water. This means polar oceans absorb a disproportionate share of anthropogenic CO2, acidifying more rapidly. They are also naturally closer to carbonate undersaturation because deep, CO2-rich water upwells in these regions. The saturation horizon (depth below which carbonate dissolves) is therefore shoaling fastest in polar seas.

Question 5

Why can shell-forming organisms struggle and begin dissolving even though the ocean remains alkaline (pH > 8)? Explain using the concept of carbonate saturation state.

Model answer: Shell-forming organisms build structures from calcium carbonate minerals (aragonite or calcite). Whether these minerals dissolve depends not on pH directly, but on the saturation state Ω = [Ca²⁺][CO₃²⁻] / Ksp. As ocean CO2 absorption depletes CO₃²⁻ ions (Le Chatelier: added CO2 shifts the carbonate equilibrium toward HCO₃⁻, consuming CO₃²⁻), Ω falls below 1 even though pH remains well above 7. Below Ω = 1, CaCO3 is thermodynamically unstable and dissolves. Organisms must then expend extra metabolic energy to maintain shells against this dissolution pressure — or fail entirely if Ω drops too far.

The key is that alkalinity (pH > 7) and carbonate saturation are different properties. A water body can be alkaline yet chemically corrosive to calcium carbonate minerals. Ocean acidification primarily operates through the carbonate saturation pathway, not through making the ocean literally acidic.