Carbon Sequestration in Land and Water

Carbon sequestration is the process by which carbon dioxide is removed from the atmosphere and stored in terrestrial or aquatic systems. Sequestration occurs across multiple time scales — from decades to millions of years — and plays a central role in regulating atmospheric CO$_2$ concentrations.

The Carbon Cycle — Background

Carbon cycles continuously between the atmosphere, biosphere, hydrosphere and lithosphere. Key fluxes include:

Short-Term Carbon Sequestration (Less Than 100 Years)

Short-term sequestration involves storing carbon in living biomass, soils and dissolved in surface waters.

Forest Biomass and Vegetation

• Growing forests are net carbon sinks — photosynthesis exceeds respiration and decomposition
• Mature forests can store 150–400 tonnes of carbon per hectare (above and below ground)
• Reforestation and afforestation are key short-term sequestration strategies
• Limitation: Forests can be destroyed by fire, disease or logging, releasing stored carbon rapidly

Soil Organic Carbon

• Organic matter in topsoil stores significant carbon (globally: ~1,500–2,000 Gt C in top 1 m)
• Practices that increase soil organic carbon: no-till farming, compost addition, cover crops, retained vegetation
• Limitation: Soil carbon is relatively easily released by tillage, warming or drought

Wetlands and Peatlands

• Wetlands (including mangroves, seagrasses, tidal marshes) are highly efficient carbon stores
• Blue carbon: Carbon stored in coastal and marine ecosystems
• Mangroves store 3–5× more carbon per unit area than tropical forests
• Limitation: Drained or cleared wetlands release stored carbon rapidly

Ocean Surface Uptake

• The upper ocean absorbs CO$_2$ through physical dissolution and biological uptake by phytoplankton
• Phytoplankton fix carbon through photosynthesis: $$6CO_2 + 6H_2O \xrightarrow{\text{light}} C_6H_{12}O_6 + 6O_2$$
• When phytoplankton die, some carbon sinks to deeper water (biological pump)
• Oceans currently absorb ~25–30% of annual anthropogenic CO$_2$ emissions

Long-Term Carbon Sequestration (More Than 1,000 Years)

Long-term sequestration involves storing carbon in geological reservoirs that exchange carbon with the atmosphere very slowly.

Peat Accumulation

• In waterlogged, oxygen-poor conditions (bogs, fens), dead plant material does not fully decompose
• Over centuries to millennia, layers of compressed, partially decomposed plant matter (peat) accumulate
• Peatlands cover only ~3% of Earth’s land surface but store twice as much carbon as all forests combined (~650 Gt C)
• Peat can accumulate for thousands of years

Marine Carbonate Formation

• Marine organisms (corals, molluscs, foraminifera) extract dissolved CO$_2$ and calcium from seawater to form calcium carbonate (CaCO$_3$) shells
• $$Ca^{2+} + 2HCO_3^- \rightarrow CaCO_3 + H_2O + CO_2$$
• After death, shells sink and accumulate as carbonate sediments → eventually form limestone
• This process removes carbon from the ocean–atmosphere system for millions of years

Fossil Fuel Formation

• Over millions of years, buried organic matter (ancient marine organisms, swamp vegetation) is transformed into coal, oil and natural gas under heat and pressure
• This represents geological-scale sequestration
• Burning fossil fuels reverses hundreds of millions of years of sequestration in decades — an extremely rapid flux

Sequestration and Climate Management

Enhancing sequestration is one component of climate mitigation:

EXAM TIP: When explaining carbon sequestration time scales, always identify the specific storage mechanism (e.g. peat formation, carbonate precipitation, forest biomass) rather than just saying ‘stored underground’. VCAA expects you to distinguish clearly between short-term biological sequestration and long-term geological processes.