Cap carbonate

Cap carbonates are layers of distinctively textured carbonate rocks which typically form the uppermost layer of sedimentary sequences reflecting major glaciations in the geological record.

The rising temperatures, and increased oceanic surface area - due to reduced ice cover and rising sea levels - at the end of a glaciation increase the rate of precipitation.

High concentrations of greenhouse gases such as carbon dioxide (CO₂) must build up in the atmosphere to overcome the effect of the high reflectivity (albedo) of ice and allow temperatures to rise sufficiently to begin melting.

Increased precipitation dissolves carbon dioxide from the atmosphere, falling as a weak carbonic acid - acid rain.

This would weather exposed silicate and carbonate rock, including readily-attacked glacial debris, which would release large amounts of calcium. When washed into the ocean, these precipitate to form distinctively textured layers of carbonate sedimentary rock.

A heavily debated cap carbonate appears at the top of the Gaskiers glaciation, believed by many to be global in extent.

See also

• Marinoan glaciation
• Sturtian glaciation
• Boundaries of the Ediacaran Period

References

• What are Cap Carbonates? at www.snowballearth.org