Geological Carbon and Fossil Fuels

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

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:

1. Weathering of silicate and carbonate rocks (removes CO₂ from atmosphere)
2. Sedimentation and burial of carbon in ocean sediments (transfers carbon to lithosphere)
3. Metamorphism and volcanic outgassing (returns carbon from lithosphere to atmosphere)
4. Formation of fossil fuels (transfers carbon from biosphere to lithosphere)

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

---

Formation of Fossil Fuels

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 Formation

Coal formed primarily during the Carboniferous period (~360–300 million years ago), although coal deposits of other ages also exist.

Stage-by-stage process:

1. Accumulation — Dense tropical forests (giant ferns, club mosses, horsetails) grew in vast swampy lowlands. Dead plant material accumulated faster than it could decompose.
2. Peat formation — In waterlogged, anaerobic conditions, decomposition was inhibited. Partially decomposed plant matter accumulated as peat (60% carbon content).
3. Burial — Over millions of years, layers of sediment (sand, mud, clay) covered the peat, increasing pressure and temperature.
4. Coalification — Progressive burial transformed peat through a series of stages:

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 Natural Gas Formation

Oil and gas formed from marine microorganisms — mainly phytoplankton and zooplankton — in ancient oceans.

1. Accumulation — Dead marine organisms sank to the ocean floor and accumulated in fine-grained sediments on continental shelves and in restricted basins where bottom waters were oxygen-poor.
2. Burial — Layers of sediment buried the organic-rich mud, forming source rock (often dark-coloured shale or mudstone).
3. Catagenesis — At depths of 2–4 km and temperatures of 60–160°C, the organic matter (kerogen) was slowly "cooked" by geothermal heat, breaking down into liquid hydrocarbons (oil) and gaseous hydrocarbons (natural gas, mainly methane, CH₄).
4. Migration — Oil and gas, being less dense than water and surrounding rock, migrated upward through permeable rock layers until trapped by an impermeable cap rock (such as shale or salt) in a geological trap (anticline, fault trap, or stratigraphic trap).

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

Fossil Fuel Carbon Budget

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.

---

Carbon Sequestration in Rocks

Carbonate Rocks

Limestone (CaCO₃) is the most important geological carbon store. It forms primarily from the shells and skeletons of marine organisms:

• Foraminifera — single-celled organisms with calcium carbonate shells
• Coccolithophores — microscopic algae that produce tiny CaCO₃ plates (the White Cliffs of Dover are made of coccolithophore plates)
• Corals — colonial organisms that build CaCO₃ reef structures
• Molluscs — bivalves, gastropods and cephalopods with CaCO₃ shells

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.

Organic Carbon in Sedimentary Rocks