Cap carbonates are layers of distinctively textured

carbonate A carbonate is a salt of carbonic acid, (), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate group ...

rocks (either

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

or dolomite) that occur at the uppermost layer of sedimentary sequences reflecting major glaciations in the geological record.

Characteristics and occurrence

Cap carbonates are found on most

continent A continent is any of several large geographical regions. Continents are generally identified by convention (norm), convention rather than any strict criteria. A continent could be a single large landmass, a part of a very large landmass, as ...

s. They are typically 3–30 meters thick, laminated structures. They are depleted in ¹³C compared to other carbonates. The progression of late

Neoproterozoic The Neoproterozoic Era is the last of the three geologic eras of the Proterozoic geologic eon, eon, spanning from 1 billion to 538.8 million years ago, and is the last era of the Precambrian "supereon". It is preceded by the Mesoproterozoic era an ...

glaciations portrayed by substantial δ¹³C deviations in cap carbonates suggest out of control ice

albedo Albedo ( ; ) is the fraction of sunlight that is Diffuse reflection, diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects ...

. Experiments have been performed to see if the massive abiotic carbonate is possible in extreme environments.

Formation theories

There are several different hypotheses for cap carbonate formation.

Physical stratification

Physical stratification results in a strong carbon isotopic gradient in the ocean. Massive carbonates will precipitate when the postglacial

upwelling Upwelling is an physical oceanography, oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted sur ...

carries the

alkalinity Alkalinity (from ) is the capacity of water to resist Freshwater acidification, acidification. It should not be confused with base (chemistry), basicity, which is an absolute measurement on the pH scale. Alkalinity is the strength of a buffer s ...

and isotopically light carbon to the continents. In this model, cap carbonates is the by-product of continental flooding.

Snowball Earth

The short-lived change in carbon isotopic composition is the foundation for this theory. In the

snowball Earth The Snowball Earth is a historical geology, geohistorical hypothesis that proposes that during one or more of Earth's greenhouse and icehouse Earth, icehouse climates, the planet's planetary surface, surface became nearly entirely freezing, fr ...

episode, the surface ocean of Earth is covered by the sea ice that separates the ocean and the atmospheric CO₂ reservoirs. The atmospheric CO₂ then built up to ~100,000 ppm and triggered the rapid deglaciation and melting of the sea ice, which reconnects the ocean and the atmosphere and provides excess alkalinity to the ocean. The transport of carbon dioxide from that atmosphere to the ocean will lead to carbonate precipitation. This is caused by mixing upwelling, isotopically-depleted, alkaline bottom water and calcium-rich surface water.

Methane clathrate formation

A third theory for cap carbonate formation is that

methane hydrate Methane clathrate (CH4·5.75H2O) or (4CH4·23H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large am ...

destabilization results in the formation of cap carbonate and strongly negative carbon anomalies The unusual fabrics within the cap carbonate is similar to carbonate fabrics as from cold methane seeps.

Characteristics and occurrence

Formation theories

Physical stratification

Snowball Earth

Methane clathrate formation

See also

References

Further reading