1.4 The Carbon Cycle

Carbon reservoirs and fluxes

Carbon moves between major reservoirs: the atmosphere (CO2), oceans (dissolved CO2, carbonate), biosphere (organisms), soil, and rocks/fossil fuels (long-term storage). Movement between reservoirs is called a flux. Fossil fuels and sedimentary rock hold carbon for millions of years, making them the slowest-cycling reservoirs, while the atmosphere and living organisms cycle carbon rapidly.

Biological and chemical processes

Photosynthesis removes CO2 from the atmosphere to build organic matter; respiration and decomposition return it. In water, CO2 dissolves to form carbonic acid, which can precipitate as calcium carbonate (shells, limestone) — a long-term carbon sink. Combustion of fossil fuels and biomass releases stored carbon quickly back into the atmosphere.

Human impacts on the carbon cycle

Burning fossil fuels and deforestation release carbon faster than natural processes can absorb it, increasing atmospheric CO2. This disrupts the natural balance between carbon sources and sinks, contributing to the enhanced greenhouse effect and climate change discussed in later units.

Key terms

Carbon reservoir

A location where carbon is stored, such as the atmosphere, oceans, biosphere, soil, or rocks/fossil fuels.

Flux

The movement/transfer of carbon between reservoirs.

Photosynthesis (carbon cycle role)

Removes CO2 from the atmosphere and converts it into organic matter.

Combustion

Burning organic matter or fossil fuels, releasing stored carbon as CO2 into the atmosphere.

Calcium carbonate

A compound formed from dissolved CO2 in water; forms shells and limestone, acting as a long-term carbon sink.

Try an AP-style question

What is the largest reservoir of carbon on Earth?

1. Living plants
2. The atmosphere
3. Sedimentary rock
4. Fossil fuels in use