QCM : Ocean Acidification and Carbon Cycle — 12 questions

Questions et réponses du QCM

1. What best describes ocean acidification in the context of the carbon cycle?

A natural rise in seawater temperature caused by solar heating
A decrease in ocean salinity caused by river runoff
A chemical increase in seawater acidity driven by added atmospheric carbon dioxide
A temporary increase in dissolved oxygen caused by wave mixing

A chemical increase in seawater acidity driven by added atmospheric carbon dioxide

Explication

Ocean acidification is the increase in seawater acidity caused by extra carbon dioxide from the atmosphere. It is linked to carbon cycle imbalance, not to temperature, salinity, or oxygen changes.

2. Why does burning fossil fuels contribute to ocean acidification?

It converts dissolved carbon dioxide directly into oxygen
It increases seawater alkalinity and prevents gas exchange at the surface
It raises atmospheric CO2 and overloads the normal carbon exchange with the ocean
It removes carbon dioxide from the air faster than the ocean can absorb it

It raises atmospheric CO2 and overloads the normal carbon exchange with the ocean

Explication

Fossil fuel burning increases atmospheric CO2, which overwhelms the natural back-and-forth exchange of carbon between air and sea. That extra CO2 promotes acidification.

3. What happens first when carbon dioxide dissolves in seawater?

It turns into calcium carbonate crystals
It reacts with water to form carbonic acid
It separates into oxygen and hydrogen gas
It immediately becomes bicarbonate ions

It reacts with water to form carbonic acid

Explication

Dissolved CO2 reacts with water to form carbonic acid, which is the first step in the acidification pathway. Bicarbonate forms later after carbonic acid dissociates.

4. Why does higher atmospheric CO2 increase CO2 uptake by seawater?

It blocks diffusion and traps CO2 in the atmosphere
It causes seawater to stop reacting with dissolved gases
A larger air-sea concentration difference speeds diffusion into the ocean
It forces CO2 to move from the ocean back into the air

A larger air-sea concentration difference speeds diffusion into the ocean

Explication

When air contains more CO2, diffusion into seawater increases because the concentration difference is larger. This makes more CO2 enter the ocean surface.

5. What are the main products when carbonic acid breaks down in seawater?

Carbon dioxide and oxygen
Calcium ions and carbonate rocks
Hydrogen ions and bicarbonate
Water and methane

Hydrogen ions and bicarbonate

Explication

Carbonic acid dissociates into hydrogen ions and bicarbonate. The added hydrogen ions are what drive the pH decline.

6. How does an increase in hydrogen ions affect seawater pH?

It has no effect unless salinity changes
It lowers pH and makes the water more acidic
It turns dissolved carbon dioxide into solid carbon
It raises pH and makes the water more basic

It lowers pH and makes the water more acidic

Explication

More hydrogen ions mean greater acidity, so pH falls. This is the direct chemical reason acidification lowers seawater pH.

7. How is diffusion described in the movement of carbon dioxide into the ocean?

Particles move only from high concentration to low concentration and stop at equilibrium
Particles move from low concentration toward higher concentration to balance concentrations
Particles move only when sunlight is present
Particles move without regard to concentration differences

Particles move from low concentration toward higher concentration to balance concentrations

Explication

In this context, diffusion is described as movement from low concentration toward higher concentration to reduce imbalance. That is the mechanism by which CO2 enters surface seawater.

8. Where does ocean carbon dioxide absorption occur during diffusion?

Inside marine shells
At the ocean surface
Only in river mouths
Only in deep-sea vents

At the ocean surface

Explication

CO2 enters the ocean at the surface as diffusion occurs. The absorption is not a separate process from diffusion.

9. What is a carbon sink in relation to the oceans?

A region that only releases carbon dioxide into the air
A process that converts seawater into freshwater
A habitat that prevents any gas exchange with the atmosphere
A system that absorbs and stores carbon dioxide from the atmosphere

A system that absorbs and stores carbon dioxide from the atmosphere

Explication

A carbon sink absorbs and stores atmospheric CO2, and the ocean does this on a large scale. This storage is part of its role in the carbon cycle.

10. What chemical change accompanies the ocean acting as a carbon sink?

Absorbed CO2 reacts with water to form carbonic acid, increasing hydrogen ions
Absorbed CO2 turns directly into bicarbonate with no acidity change
Absorbed CO2 raises pH by removing hydrogen ions
Absorbed CO2 becomes calcium carbonate without any intermediate step

Absorbed CO2 reacts with water to form carbonic acid, increasing hydrogen ions

Explication

When the ocean absorbs CO2, it reacts with water to form carbonic acid, which increases hydrogen ions and lowers pH. That chemical change is central to ocean acidification.

11. What is the main biological effect of ocean acidification on many marine organisms?

It increases the salinity required for normal growth and reproduction
It makes it harder for them to build and maintain calcium carbonate shells and exoskeletons
It prevents them from absorbing any dissolved oxygen from seawater
It gives them extra energy to produce thicker shells and exoskeletons

It makes it harder for them to build and maintain calcium carbonate shells and exoskeletons

Explication

Lower seawater pH makes calcium carbonate structures harder to form and maintain. This is the key organism-level impact described for shells and exoskeletons.

12. Why does ocean acidification create a problem for shell-forming marine organisms?

It increases the amount of light available for shell growth
It raises seawater temperature enough to dissolve every hard structure immediately
It removes all carbon from seawater, leaving organisms without any dissolved nutrients
It changes seawater chemistry in a way that interferes with calcium carbonate structure formation

It changes seawater chemistry in a way that interferes with calcium carbonate structure formation

Explication

The chemistry change caused by acidification affects the formation and maintenance of calcium carbonate structures. The other options describe unrelated or exaggerated effects not supported here.

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Mémorisez les réponses avec 12 flashcards sur Ocean Acidification and Carbon Cycle.

Ocean acidification — definition?

Chemical increase in seawater acidity from CO2.

Carbon cycle exchange — role?

Balances carbon transfer between atmosphere and ocean.

Fossil fuel burning — effect?

Raises atmospheric CO2, overloads natural exchange.

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