Cap carbonates are layers of distinctively textured carbonate rocks (either limestone or dolomite) that occur at the uppermost layer of sedimentary sequences reflecting major glaciations in the

geological record The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata. That is, deposits laid down by volcanism or by deposition of sediment derived from weathering detritus (clays, sand ...

Characteristics and occurrence

Cap carbonates are found on most continents. They are typically 3–30 meters thick, laminated structures. They are depleted in ¹³C compared to other carbonates. The progression of late Neoproterozoic glaciations portrayed by substantial δ¹³C deviations in cap carbonates suggest out of control ice albedo.

Formation theories

There are several different hypotheses for cap carbonate formation.

Physical stratification theory

Physical stratification results in a strong carbon isotopic gradient in the ocean. Massive carbonates will precipitate when the postglacial upwelling carries the alkalinity and isotopically light carbon to the continents. In this model, cap carbonates is the by-product of continental flooding.

Snowball formation theory

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

snowball Earth The Snowball Earth hypothesis proposes that, during one or more of Earth's Greenhouse and icehouse Earth, icehouse Climate, climates, the Earth's surface, planet's surface became entirely or nearly entirely Freezing, frozen. It is believed that ...

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 theory

A third theory for cap carbonate formation is that methane hydrate 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.

Experiments

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

Characteristics and occurrence

Formation theories

Physical stratification theory

Snowball formation theory

Methane clathrate formation theory

Experiments

See also

References

Further reading