Fiche de révision : Ocean Acidification and Carbon Cycle

📋 Course Outline

1. Ocean acidification and carbon cycle
2. Carbon dioxide dissolving in seawater
3. Carbonic acid and pH decline
4. Diffusion and ocean CO2 uptake
5. Oceans as a carbon sink
6. Impacts on marine organisms

📖 1. Ocean acidification and carbon cycle

🔑 Key Concepts & Definitions

• Ocean acidification : Ocean acidification is the chemical increase in seawater acidity driven by added carbon dioxide from the atmosphere.
• Carbon cycle exchange : Carbon cycle exchange is the natural back-and-forth movement of carbon between the atmosphere and the ocean.
• Fossil fuel burning : Fossil fuel burning is a source of rising atmospheric CO2 that overloads the natural carbon exchange with the ocean.

📝 Essential Points

• Rising atmospheric CO2 adds more carbon to seawater than the ocean can process naturally in the overloaded carbon cycle.
• Carbon cycle exchange is balanced under natural conditions but becomes unbalanced when fossil fuels increase CO2 inputs.
• Ocean acidification reduces seawater pH by increasing hydrogen ion concentration through CO2-driven chemistry.

💡 Memory Hook

More CO2 in air → more CO2 dissolves → more H+ → lower pH.

📖 2. Carbon dioxide dissolving in seawater

🔑 Key Concepts & Definitions

• CO2 dissolution : CO2 dissolution is the transfer of carbon dioxide gas from the air into seawater.
• Diffusion-driven uptake : Diffusion-driven uptake is the process by which dissolved CO2 increases at the ocean surface when air CO2 rises.
• Dissolved CO2 reacts : Dissolved CO2 reacts with water in seawater to start the acidification pathway.

📝 Essential Points

• When CO2 dissolves in seawater, it reacts with water (H2O) to form carbonic acid (H2CO3).
• Higher air CO2 concentration increases the rate at which CO2 diffuses into the sea.
• Absorption at the ocean surface occurs through diffusion as CO2 moves toward a more balanced concentration.

💡 Memory Hook

CO2 falls into seawater, then immediately starts the H2CO3 pathway.

📖 3. Carbonic acid and pH decline

🔑 Key Concepts & Definitions

• Carbonic acid : Carbonic acid (H2CO3) is the intermediate formed when dissolved CO2 reacts with water in seawater.
• Hydrogen ions H+ : Hydrogen ions (H+) are the charged particles released when carbonic acid breaks down in seawater.
• Bicarbonate HCO3- : Bicarbonate (HCO3-) is the other product formed when carbonic acid dissociates.

📝 Essential Points

• Carbonic acid quickly breaks down to release hydrogen ions (H+) and bicarbonate (HCO3-).
• The more hydrogen ions (H+) in seawater, the more acidic it becomes.
• Increasing hydrogen ions leads to a pH decline, shown by lower pH values in the notes.

💡 Memory Hook

H2CO3 → H+ + HCO3- ; more H+ means lower pH.

📖 4. Diffusion and ocean CO2 uptake

🔑 Key Concepts & Definitions

• Diffusion : Diffusion is the movement of particles from low concentration regions toward higher concentration regions to balance concentrations.
• Low to high concentration : Low to high concentration describes the direction diffusion pushes CO2 and other particles during mixing.
• Ocean surface absorption : Ocean surface absorption is the location where CO2 enters the ocean as diffusion occurs.

📝 Essential Points

• Diffusion is driven by concentration differences, with particles moving from low to high concentration to reduce imbalance.
• Atmospheric CO2 directly increases ocean CO2 levels because higher air concentration speeds CO2 diffusion.
• Absorption occurs on the ocean surface through diffusion rather than by a separate reaction step.

💡 Memory Hook

Concentration imbalance drives diffusion; CO2 follows the gradient into the surface ocean.

📖 5. Oceans as a carbon sink

🔑 Key Concepts & Definitions

• Carbon sink : A carbon sink is a system that absorbs and stores carbon dioxide from the atmosphere.
• CO2 to carbonic acid : CO2 to carbonic acid is the chemical pathway by which absorbed CO2 becomes carbonic acid in seawater.
• Hydrogen ions increase : Hydrogen ions increase is the outcome of the acidification pathway that raises seawater acidity.

📝 Essential Points

• The ocean acts as a carbon sink by absorbing CO2 from the atmosphere.
• Absorbed CO2 reacts with water to form carbonic acid, which increases hydrogen ion concentration.
• By increasing hydrogen ions, the absorbed CO2 lowers seawater pH, even though natural cycling would otherwise balance exchange.

💡 Memory Hook

Ocean sink: CO2 in → carbonic acid → H+ up → pH down.

📖 6. Impacts on marine organisms

🔑 Key Concepts & Definitions

• Calcium carbonate shells : Calcium carbonate shells are hard structures that organisms build using carbonate chemistry in seawater.
• Exoskeletons : Exoskeletons are external hard coverings that some marine organisms maintain using calcium carbonate.
• Marine ecosystem impact : Marine ecosystem impact is the biological consequence of chemical changes caused by ocean acidification.

📝 Essential Points

• Increased acidity makes it difficult for marine organisms to build and maintain calcium carbonate shells and exoskeletons.
• Ocean acidification changes the seawater chemistry in a way that affects the formation and maintenance of calcium carbonate structures.
• The overloaded carbon cycle links fossil fuel CO2 inputs to reduced biological ability to maintain protective structures.

💡 Memory Hook

Lower pH = trouble building CaCO3 shells and exoskeletons.

⚠️ Common Pitfalls & Confusions

1. Mixing up the diffusion direction: particles move from low concentration toward higher concentration to balance concentrations.
2. Assuming CO2 directly lowers pH without the intermediate steps: CO2 forms H2CO3, then produces H+.
3. Forgetting that carbonic acid quickly breaks down rather than accumulating as the main pH-changing substance.
4. Confusing balanced natural cycling with present conditions: fossil fuel burning overloads the system and unbalances exchange.
5. Thinking absorption is a separate process from diffusion: ocean surface absorption here happens through diffusion.
6. Linking organism impacts to acidity without the mechanism: impacts come from difficulty maintaining calcium carbonate structures under lower pH.

✅ Exam Checklist

1. State the full acidification pathway from CO2 dissolving to H2CO3 formation and dissociation into H+ and HCO3-.
2. Explain why more hydrogen ions in seawater means the water becomes more acidic and pH declines.
3. Describe diffusion using the low-to-high concentration idea and the goal of balancing concentrations.
4. Connect higher atmospheric CO2 to faster CO2 diffusion into seawater via increased air concentration.
5. Identify what happens at the ocean surface for CO2 absorption and name the process involved.
6. Define how the ocean acts as a carbon sink and what chemical change accompanies CO2 absorption.
7. Explain how the natural carbon cycle exchange is balanced and how fossil fuel burning disrupts it.
8. State the organism-level impact mentioned: difficulty building and maintaining calcium carbonate shells and exoskeletons.
9. Use the cause→effect chain: rising atmospheric CO2 leads to higher ocean CO2, which leads to more H+, which leads to lower pH.
10. Recall the specific products of carbonic acid breakdown: hydrogen ions (H+) and bicarbonate (HCO3-).