Astrophysics (Index)About

carbonate-silicate cycle

(long-term cycle that sequesters Earth carbon)

The carbonate-silicate cycle is a naturally-occurring geochemical cycle that sequesters Earth carbon over geological time periods. Key are chemical reactions such as this one:

CaSiO3 + CO2 ⇔ CaCO3 + SiO2


calcium silicate + carbon dioxide ⇔ calcium carbonate + silicon dioxide

The reaction can go either way, but temperature determines which direction dominates. At the surface of the Earth, this reaction's balance is in favor of carbonates (dissipating carbon dioxide), but at 300°C, the balance is toward silicates. The surface of Venus tends toward the latter as do subsurface conditions of Earth.

Carbon dioxide in Earth's rain and the rock material that the rain strikes undergo the reaction (weathering), which dissipates carbon dioxide, and the product is carried with its carbon into the ocean. Living organisms incorporate it in shells, and after dying, are incorporated into the under-ocean strata, which eventually reach the depth where the reaction tends toward the silicates and carbon dioxide again. Volcanic eruptions eventually send the resulting CO2 back into the atmosphere. The cycle reaching into the mantle (and potentially core) is termed the deep carbon cycle.

The cycle helps stabilize Earth climate and the ability to complete this cycle affects an extra-solar planet's ability to maintain liquid water. Thus signs suggesting a planet has this ability suggest habitability and are of interest in astrobiology.

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