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This lesson examines the slow (geological) carbon cycle, covering the formation of fossil fuels, carbon sequestration in rocks, volcanic outgassing, and chemical weathering as a long-term carbon sink. This material is central to Edexcel A-Level Geography (9GE0), Paper 1, Topic 6, and addresses the Enquiry Question: "How does the carbon cycle operate to maintain planetary health?"
The slow carbon cycle refers to geological processes that transfer carbon between the lithosphere, atmosphere and oceans over timescales of thousands to millions of years. The total flux through the slow cycle is small — roughly 0.1–0.3 GtC per year in each direction — but over geological time, these slow processes have shaped Earth's climate and atmosphere.
The slow carbon cycle involves four main processes:
flowchart TD
A[Atmosphere CO₂] -->|"Chemical weathering<br>~0.3 GtC/yr"| B[Rivers carry<br>dissolved bicarbonate<br>to oceans]
B --> C[Marine organisms use<br>bicarbonate to build<br>CaCO₃ shells]
C -->|"Death and<br>sedimentation"| D[Ocean floor<br>sediments]
D -->|"Burial, compaction<br>& lithification<br>millions of years"| E[Sedimentary rocks<br>limestone, chalk]
E -->|"Subduction &<br>metamorphism"| F[Magma / volcanic<br>CO₂ release]
F -->|"Volcanic outgassing<br>~0.1 GtC/yr"| A
E -->|"Uplift &<br>exposure"| G[Surface weathering<br>cycle repeats]
G --> A
Fossil fuels — coal, oil (petroleum) and natural gas — are forms of geological carbon derived from ancient organisms. Their formation requires specific conditions and immense timescales.
Coal formed primarily during the Carboniferous period (~360–300 million years ago), although coal deposits of other ages also exist.
Stage-by-stage process:
| Stage | Name | Carbon Content | Characteristics |
|---|---|---|---|
| 1 | Peat | ~60% | Soft, waterlogged, partially decomposed plant matter |
| 2 | Lignite (brown coal) | ~65–70% | Soft, crumbly, high moisture content |
| 3 | Bituminous coal | ~75–90% | Hard, black, most commonly mined type |
| 4 | Anthracite | ~90–95% | Very hard, high energy density, low impurities |
The coalification process involves increasing temperature and pressure driving off water, volatile gases and oxygen, progressively concentrating the carbon.
Exam Tip: The reason coal formed mainly in the Carboniferous is that the organisms that decompose lignin (the tough structural molecule in wood) had not yet fully evolved. Modern forests rarely form coal because fungi and bacteria decompose dead wood efficiently.
Oil and gas formed from marine microorganisms — mainly phytoplankton and zooplankton — in ancient oceans.
| Condition | Oil Formation | Gas Formation |
|---|---|---|
| Temperature | 60–160°C ("oil window") | >160°C ("gas window") or biological decomposition |
| Depth | ~2–4 km | >4 km (thermogenic) or shallow (biogenic) |
| Source material | Marine plankton, algae | Same, or coal at higher temperatures |
| Timescale | 10–100+ million years | 10–100+ million years |
Global fossil fuel reserves represent a transfer of carbon from the biosphere/oceans to the lithosphere over hundreds of millions of years:
| Fossil Fuel | Estimated Proven Reserves (GtC) | Current Annual Extraction (GtC/yr) | Years of Supply at Current Rates |
|---|---|---|---|
| Coal | ~560 | ~3.9 | ~140 |
| Oil | ~160 | ~3.3 | ~50 |
| Natural gas | ~100 | ~2.3 | ~43 |
| Total | ~820 | ~9.5 | — |
These figures are approximate and vary with new discoveries, technological advances and changes in demand. Unconventional reserves (tar sands, oil shale, shale gas) could extend supply but at higher economic and environmental cost.
Limestone (CaCO₃) is the most important geological carbon store. It forms primarily from the shells and skeletons of marine organisms:
When these organisms die, their shells sink to the ocean floor. Over millions of years, the accumulated shell material is compacted and lithified into limestone. The reaction is:
Ca²⁺ + 2HCO₃⁻ → CaCO₃ + H₂O + CO₂
Carbonate rocks store approximately 60,000,000 GtC — the single largest carbon store on Earth. The chalk of southern England (Cretaceous, ~100–66 million years ago) is a classic example.
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