Big picture: The lithosphere is the largest long-term carbon store on Earth. Carbon is locked in fossil fuels and carbonate rocks over millions of years.
- Fossil fuels
- Non-renewable energy sources (coal, oil, natural gas) formed from the remains of ancient organisms over millions of years under heat and pressure.
- Coal
- Formed from terrestrial plant material deposited in swamps, buried and compressed over 300+ million years (Carboniferous period).
- Petroleum (oil) and natural gas
- Formed from marine organisms (plankton, algae) deposited on ocean floors, buried under sediment, and transformed by heat and pressure.
Conditions required for fossil fuel formation
- Rapid burial of organic matter before decomposition
- Anaerobic conditions (absence of oxygen)
- High pressure from overlying sediments
- High temperatures over millions of years
- Specific geological formations to trap hydrocarbons
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Key concept: Limestone (CaCO3) is the largest carbon store in the lithosphere. It forms from the accumulation of marine organism shells and skeletons on the ocean floor.
How limestone forms
- Marine organisms (corals, foraminifera, molluscs) build shells from calcium carbonate
- When organisms die, shells accumulate on the ocean floor
- Compaction and cementation over millions of years forms limestone
- Chemical precipitation of CaCO3 from supersaturated water also contributes
Carbon cycling through limestone
- Weathering releases CO2 back to the atmosphere (very slow)
- Volcanic activity releases CO2 from subducted carbonate rocks
- Ocean acidification dissolves carbonate, releasing stored carbon
- Cement production liberates CO2 from limestone (fast, human-driven)
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- Methane (CH₄)
- A potent greenhouse gas with approximately 80× the warming potential of CO₂ over 20 years, but a shorter atmospheric residence time (~12 years).
- Methane clathrates
- Ice-like structures on ocean floors and in permafrost that trap large quantities of methane. Also called methane hydrates.
Methane sources and feedback loops
- Permafrost thawing releases trapped methane as temperatures rise
- Warming oceans may destabilise methane clathrates on seafloor
- Wetlands produce more methane as temperatures increase
- This creates a positive feedback loop: warming → methane release → more warming
IB exam tip: Be able to explain the methane-climate positive feedback loop using systems language: stores, flows, and feedback mechanisms.