Carbon cycle

There are two Carbon cycles on Earth, a biological one which works on short time frames, and a longer one that works at geological time scales.

Biological carbon cycle

The natural Carbon Cycle describes the way of energetic interaction of plants and animals (or humans).

The Carbon Cycle (simplified)

The CO₂ is consumed by the food plants. This is part of the carbon cycle, assumed that all food is grown by food plants in the colony. All hydrocarbon intake is exhaled as CO₂ after digestion and metabolizing. Exactly the same amount of CO₂ is inhaled by the food plants and metabolized to hydrocarbons. The carbon is neither created nor destroyed anywhere in the cycle.

The photo synthesis basically creates glucose from CO₂ and water:

6 CO₂ + 6 H₂O → C₆H₁₂O₆

The settlers basically metabolize the glucose and release CO₂ and water:

C₆H₁₂O₆ → 6 CO₂ + 6 H₂O

Carbon is stored in the settlement atmosphere as CO2. The breathable limit is about 9000 ppm (0,9% by mass), but the amount present in the Earth atmosphere is between 300 and 400 ppm (0,03% by mass). Additional carbon is stored in plants and animals as carbohydrates, or other hydrocarbons. It can also be stored as methane, or extracted chemically as pure carbon. Stored carbon helps stabilize the cycle by adding a damping/storage mechanism to the system.

Geological carbon cycle

Over geological time scales carbon moves slowly in and out of the Earth's crust to the atmosphere / biosphere. This can be done by several processes.

...... Things that move carbon into the ground.

Vast amounts of plant matter can be buried to form peat, and if buried deeper form coal. This happened largely during the Carboniferous portion of Earth's history.
Large amounts of bio matter can land on sea floors. I buried, it can eventually be turned into oil over millions of years.
Carbon can be locked up by plants and animals to form calcium based shells. This if buried will turn into limestone.
Chemical reactions can form carbonate minerals from carbon in the air.
Carbon can be pulled down into the mantle by plate tectonics.

....... Things that move carbon into the biosphere / atmosphere.

Carbon can be released from the Earth by volcanoes.
Carbon can be released from carbonate rocks by weathering.
Peat, natural gas, coal, and oil can be dug up and burnt.

Drawing down carbon in terraforming

When terraforming Mars, we will want to increase the amount of carbon dioxide in the Martian atmosphere, as this will increase the green house warming. A partially terraformed Mars, will be better for humans and other life. But if we wish to make Mars habitable by people without air masks, the level of carbon dioxide must be reduced to trace levels. This will cool Mars which is exactly what we don't want. To make this up, we might use Super Greenhouse Gases or Greenhouse nano-particles.

Moving carbon in the air (as carbon dioxide) into the crust (as say, peat or coal) is a process that would take millions of years. Humans can speed this by cutting down forests, and burying them deep, or moving trillions of tonnes of wood outside of the biosphere (say to the top of Olympus Mons where it won't rot being above most of the atmosphere). Even so, the rate of draw down of carbon dioxide from the air will take tens of thousands of years. It takes time to grow forests and harvest them many times.

This problem is usually ignored by those who think Mars can be terraformed to a breathable atmosphere in a short time (e.g. a few thousand years).

See Terraforming for more information.