Chapter 46.2: The Long-Term Carbon Cycle

Complete Carbon Cycles

Combines both biological and geological systems 

• Biology: photosynthesis, respiration, decay

• Geology: volcanoes, weathering, tectonics

• Climate: temperature, ice ages

Long-term timescales:

 Geological → More substantial changes in CO₂ levels

• Geological Cycling:  

1. Volcanos: Release CO2 into the atmosphere

2. Mountain Formation: 

3. CO2 reactions: react with water, rocks and carbonate minerals in the ocean floor 

• Chemical Weathering of rocks: 

4. Tectonic Activity: Over millions of years this activity helps return carbon back to the atmosphere  

5. Climate Change

Ancient Temperatures

Determined using oxygen isotopes from ice cores

• Process: Oxygen dissolves at different rates in water at different temperatures 

  • Cooler temperatures increase oxygen in the air 

• Pattern: temperature have cycled between 

  • CO2 and temperature change together → positive correlation 

1. A few degrees warmer than today 

2. 6-8^oC colder than today 

• Cause:

  • CO₂ gas acts as a greenhouse gas: Traps heat that is re-emitted from the land and sea so it warms up the climate  

Milankovitch Cycles

• Regular variations in the Earth’s orbit around the sun which in turn causes these periods

  • Causes: Affects how much sunlight Earth will get

    • Eccentricity of Earth’s Orbit

    • Obliquity: Tilt of Earth on its axis

    • Precession: Wobble of Earth on its Axis → Change in the Earths orientation along the rotation axis

  • The Periods: 

    • Glacial Periods (ice ages): huge ice sheets covered North America, Europe, and mountain ranges 

    • Interglacial Periods/ Glacial Retreat: Climate is milder and the ice sheets shrink  

CO₂ Over Geological Time:

Methods: Indirect estimates are uncertain but show clear long-term patterns

1. Computer Models

2. Geochemical evidence from rocks

3. Fossil indicators → types of plants and the isotopes in shells 

Variation: Due to these main goelogic process

1. Volcanic outgassing

2. Mountain building

3. Chemical weathering of rocks

Pool/Resevoir

• The reserve of an element either organic or inorganic (how much there is)

  • Like carbon dioxide in the atmosphere is the inorganic pool of carbon

  • Organic Pool: Consistent of the biomass of produces

Types of Carbon reservoirs:

1. Atmosphere

• Moderate size

• CO2 gas

2. Land Organisms/ Biosphere

• Similar to atmosphere

• Carbon in plants and animals

3. Soil

• Land organisms + atmospher combined size

• Decaying organic matter and humus

4. Biotic Ocean

• Very small

• Carbon in plankton, fish, etc.

5. Abiotic Ocean

• Very large

• Dissolved CO₂, bicarbonate (HCO₃⁻), and carbonate (CO₃²⁻) ions, especially in deep ocean

6. Sediments

• LARGEST → 1000s x bigger than the others

• Carbonate minerals (CaCO₃ in limestone) + preserved organic matter

Sensitivity and Balance of Reservoirs

• Dependent on…

  • Size of the reservoir

    • Larger reservoirs tend to change more slowly 

    • Smaller reservoirs can be more greatly impacted by smaller fluxes

  • Flux size → How much carbon can be moved in or out 

Flux

• Transfer of an element from one pool to another

Types of Carbon Flux - Caused by living organisms

1. Feeding: Eating an organism and gaining the carbon compounds from the organism

2. Photosynthesis: Process of actually absorbing the carbon dioxide from air/water sources and converting it into pure carbon that is used in compounds 

• Doesn’t have to first become pure carbon but its like it has process that dismantles the carbon dioxide and forms carbon compounds with it instead

3. Respiration: Releasing the carbon dioxide it produces from the cells into the atmosphere

4. Human Fluxes 💔: Burning fossil fuels increased oxidation rates

Types of Carbon Fluxes - Abiotic

1. Volcanism/seafloor spreading

2. Oxidation of Old Organic Matter - smallest

3. Chemical Weathering - largest

Volcanism/seafloor spreading

• Earth to the atmosphere

• CO₂ emitted at volcanoes and mid-ocean ridges

• Plate Tectonic Activity: Constant movement due to mantle convections 

  • Subduction Zones: Where one plate sinks beneath another one 

    • Carbon-rich sediments and CaCO₃ are carried into the mantle

  • Volcanic Arcs/Ridges: these carbon compounds are melted and decompose, releasing CO₂ gas through volcanoes or mid-ocean ridges

Oxidation of Old Organic Matter - smallest

• Rocks to the atmosphere

• Bacteria and fungi decompose exposed fossil carbon

Chemical Weathering - largest

• Atmosphere to the earth 

• CO₂ reacts with rocks and rainwater to form bicarbonate and carbonate ions

• Moves around 0.43 gtC/year

• Process of Chemical Weather: Negative Feedback system 

1. CO₂ dissolves in rainwater to form carbonic acid (H₂CO₃)

2. This weak acid reacts with silicate rocks on land, releasing ions like calcium (Ca²⁺) and bicarbonate (HCO₃⁻)

3. These ions are carried by rivers to the ocean, where they combine to form calcium carbonate (CaCO₃), creating limestone and shells

4. This transfers carbon from the atmosphere to sediments, where it can be stored for millions of years