Question 1 · Explain
4 marksExplain how the physical (solubility) pump contributes to carbon sequestration in the oceans.
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Worked solution
To gain full marks, the explanation must sequentially link the atmospheric exchange, the role of water temperature, ocean currents/downwelling, and deep-ocean storage:
1. **Air-Sea Gas Exchange:** Atmospheric carbon dioxide (\(\text{CO}_2\)) dissolves into the surface waters of the ocean via diffusion to reach an equilibrium.
2. **Temperature Dependency:** Ocean temperature controls the rate of solubility. Colder water at high latitudes absorbs and retains larger quantities of \(\text{CO}_2\) compared to warmer equatorial water.
3. **Downwelling / Thermohaline Circulation:** High-latitude surface waters become cold and saline, making them dense enough to sink. This downwelling carries the dissolved inorganic carbon deep into the ocean basin.
4. **Deep Storage (Sequestration):** Once downwelled, the carbon is locked away in deep ocean currents and benthic waters, separating it from the atmospheric cycle for centuries.
1. **Air-Sea Gas Exchange:** Atmospheric carbon dioxide (\(\text{CO}_2\)) dissolves into the surface waters of the ocean via diffusion to reach an equilibrium.
2. **Temperature Dependency:** Ocean temperature controls the rate of solubility. Colder water at high latitudes absorbs and retains larger quantities of \(\text{CO}_2\) compared to warmer equatorial water.
3. **Downwelling / Thermohaline Circulation:** High-latitude surface waters become cold and saline, making them dense enough to sink. This downwelling carries the dissolved inorganic carbon deep into the ocean basin.
4. **Deep Storage (Sequestration):** Once downwelled, the carbon is locked away in deep ocean currents and benthic waters, separating it from the atmospheric cycle for centuries.
Marking scheme
Award 1 mark for each relevant, developed point up to a maximum of 4 marks:
- **1 mark:** For identifying that atmospheric carbon dioxide (\(\text{CO}_2\)) dissolves directly into ocean surface waters at the air-sea interface (diffusion).
- **1 mark:** For explaining that cold water (especially at high latitudes) has a higher solubility capacity, allowing it to absorb more carbon dioxide.
- **1 mark:** For explaining the process of downwelling, where cold, dense polar water sinks, transporting the dissolved carbon into the deep ocean.
- **1 mark:** For linking this deep transport to long-term sequestration, explaining that thermohaline circulation isolates this carbon from the atmosphere for centuries.
- **1 mark:** For identifying that atmospheric carbon dioxide (\(\text{CO}_2\)) dissolves directly into ocean surface waters at the air-sea interface (diffusion).
- **1 mark:** For explaining that cold water (especially at high latitudes) has a higher solubility capacity, allowing it to absorb more carbon dioxide.
- **1 mark:** For explaining the process of downwelling, where cold, dense polar water sinks, transporting the dissolved carbon into the deep ocean.
- **1 mark:** For linking this deep transport to long-term sequestration, explaining that thermohaline circulation isolates this carbon from the atmosphere for centuries.