1.4 Carbon Cycle

What It Is

  • The carbon cycle is how carbon moves through:

    • Atmosphere (air)

    • Biosphere (living things)

    • Hydrosphere (oceans, water)

    • Geosphere (rocks and fossil fuels)

  • Earth is a closed system: carbon is recycled, not created or destroyed

  • Carbon is essential for life: found in DNA, proteins, carbohydrates, and fats

Major Carbon Reservoirs

  • Short-term reservoirs (days–decades)

    • Atmosphere (CO₂, CH₄)

    • Plants and animals (organic molecules)

    • Surface ocean and marine life

  • Long-term reservoirs (thousands–millions of years)

    • Fossil fuels (coal, oil, natural gas)

    • Sedimentary rock (limestone)

    • Deep ocean and marine sediments

  • Sinks vs Sources

    • Carbon sinks: absorb more carbon than they release (e.g., forests, oceans)

    • Carbon sources: release carbon (e.g., respiration, combustion, decay)

Key Processes

  • Photosynthesis

    • Plants, algae, and some bacteria absorb CO₂ and use sunlight to make glucose and oxygen

    • Moves carbon from atmosphere → biosphere

    • Removes CO₂ from atmosphere and stores carbon in biomass

  • Cellular Respiration

    • All living organisms break down glucose to release energy, producing CO₂ and water

    • Moves carbon from biosphere → atmosphere or water

    • Opposite of photosynthesis

  • Decomposition

    • Decomposers break down dead plants, animals, and waste

    • Returns carbon to soil or releases it as CO₂ or methane (CH₄)

    • Creates soil organic matter (humus)

    • In anaerobic conditions (waterlogged soils or landfills), decomposition produces methane

  • Combustion

    • Rapid burning of biomass or fossil fuels releases stored carbon as CO₂

    • Natural sources: wildfires, volcanic eruptions

    • Human sources: burning coal, oil, gas for energy or transportation

  • Ocean-Atmosphere Exchange (Gas Exchange)

    • CO₂ dissolves in the ocean surface and moves back into the atmosphere

    • Cold water absorbs more CO₂ than warm water

    • Marine organisms use dissolved CO₂ for photosynthesis and to build calcium carbonate (CaCO₃) shells

    • Some CO₂ forms carbonic acid (H₂CO₃), contributing to ocean acidification

  • Sedimentation and Burial

    • Carbon in dead marine organisms or shells sinks to the ocean floor

    • Over millions of years, forms sedimentary rock (limestone) or fossil fuels

    • Stores carbon for long periods (slow cycle)

Fast vs. Slow Carbon Cycle

  • Fast cycle (days–years)

    • Involves plants, animals, microbes, and short-term atmospheric interactions

    • Includes:

      • Photosynthesis

      • Respiration

      • Decomposition

      • Ocean surface exchange

  • Slow cycle (thousands–millions of years)

    • Involves carbon stored in rocks, ocean sediments, and fossil fuels

    • Includes:

      • Sedimentation

      • Fossil fuel formation

      • Tectonic activity and volcanic release

  • Human impact

    • Moves carbon from slow → fast cycle too quickly, increasing atmospheric CO₂

Ocean’s Role

  • CO₂ dissolves into/out of surface waters (gas exchange)

  • Phytoplankton photosynthesis removes CO₂ from the ocean

  • CO₂ reacts with calcium to form calcium carbonate (CaCO₃) shells

  • Dead shells settle as marine sediment → long-term carbon storage

Human Impacts

  • Fossil fuel combustion: adds ancient carbon rapidly

  • Deforestation: removes carbon sinks and releases stored CO₂

  • Agriculture and livestock:

    • Disturbs soil carbon

    • Produces methane (CH₄)

  • Humans have tipped the carbon cycle, making CO₂ enter the atmosphere faster than natural systems can remove it

Quick Exam Tips

  • Photosynthesis removes CO₂ from the atmosphere (carbon sink)

  • Respiration, decomposition, and combustion release CO₂ (carbon sources)

  • Know the chemical equations:

    • Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

    • Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy