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Cap carbonates are layers of distinctively textured carbonate rocks (either
limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms when ...
or dolomite) that occur at the uppermost layer of sedimentary sequences reflecting major
glaciations A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betwe ...
in the geological record.


Characteristics and occurrence

Cap carbonates are found on most
continent A continent is any of several large landmasses. Generally identified by convention rather than any strict criteria, up to seven geographical regions are commonly regarded as continents. Ordered from largest in area to smallest, these seven ...
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 unit of geologic time from 1 billion to 538.8 million years ago. It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is pre ...
glaciations portrayed by substantial δ13C deviations in cap carbonates suggest out of control ice
albedo Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...
.


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 Upwelling is an 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 surface water. The nutr ...
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 icehouse climates, the planet's surface became entirely or nearly entirely frozen. It is believed that this occurred sometime before 650 M.Y.A. (million years ago) du ...
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 Methane clathrate (CH4·5.75H2O) or (8CH4·46H2O), 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 amou ...
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.


See also

*
Marinoan glaciation The Marinoan glaciation, sometimes also known as the Varanger glaciation, was a period of worldwide glaciation that lasted from approximately 650 to 632.3 ± 5.9 Ma (million years ago) during the Cryogenian period. This glaciation possibly cove ...
*
Sturtian glaciation The Sturtian glaciation was a Snowball Earth glaciation, or perhaps multiple glaciations, during the Cryogenian Period when the Earth experienced repeated large-scale glaciations. The duration of the Sturtian glaciation has been variously defined, ...
* Boundaries of the Ediacaran Period *
Snowball Earth The Snowball Earth hypothesis proposes that, during one or more of Earth's icehouse climates, the planet's surface became entirely or nearly entirely frozen. It is believed that this occurred sometime before 650 M.Y.A. (million years ago) du ...
* Katanga Supergroup


References


Further reading

What are Cap Carbonates?
at www.snowballearth.org Carbonate rocks Glaciology {{geology-stub