Carbonate sediments and chemistry:
A more mundane but important aspect of the rocks is that they are sources of:
- Ca and Mg for nutrition
- Building facing stone.
- Mississippi-type ore deposits
- When fractured can be good petroleum reservoir rocks
- The globally-distributed marine fossils used in biostratigraphy.
Carbonates may also monitor changes in the atmosphere. Indeed most of the carbon in the early atmosphere (which existed as either CO2 or CH4 in high concentrations) reacted with the silicate Earth and water to form alkalinity, and is now stored as carbonate rock in the crust. The present atmosphere has a 400 ppm CO2 (miniscule compared to the Archaean but high by Holocene standards) and ppb levels of CH4.
Carbonate formation is limited by concentrations of Ca2+ and HCO3-. These are produced by weathering of silicates on land, for instance, in:
2NaAlSi3O8 + 2CO2 + 11H2O -> Al2Si2O5(OH)2 + 2Na+ + 2HCO3- + 4H4SiO4
Albite is weathered into kaolinite, hydrosilicic acid, sodium ion, and bicarb ion.
Since surface oceans are generally supersaturated with respect to CaCO3, its two polymorphs form readily:
Recrystallized brachiopod - Wikimedia Commons
Organisms that calcify use a range of carbonate minerals to build their shells and homes.
Equilibrium constants An equilibrium constant is equal to the concentration of products over reactants. In the case of carbonate system we see the following equilibrium constants:
|CO2 + H2O -> H2CO3||(K3 = 1*10-1.43)|
|H2CO3 -> H+ + HCO3-||(K3 = 1*10-6.40)|
|>HCO3- -> H+ + CO32-||(K3 = 1*10-10.33)|
|Ca2+ + CO32- -> CaCO3||(K3 = 1*10-8.33 for aragonite and 1*10-8.48 for calcite)|
Looking at this reaction series, you would think that adding more CO2 would drive the reactions toward the right and increase CaCO3 precipitation, but this is not the case:
Catoctin Fm. metabasalt superposed over Faquier Fm. cap carbonate
The principal chemical and physical controls on carbonate formation in oceans and lakes are those that control CO2 concentration:
- temperature (higher -> less CO2 -> more carbonates in warm water.)
- pressure (lower -> less CO2 -> more carbonates in shallow water.)
- degree of agitation (higher -> less CO2 -> more carbonates in agitated (and therefore well aerated) water.)
Secondary controls include:
- sediment masking Where clastic sediments accumulate rapidly, the result is mudrocks and marls, even where carbonates are actually being deposited.
- light availability Photosynthesizers remove CO2, facilitating carbonate formation
- Carbonate compensation depth
The effects of organic activity on CaCO3 precipitation are subtle.
|CaCO3 extraction:||promotes skeletal growth||forms allochems and mud|
|decay:||adds CO2, pH decrease||hinders precipitation|
|feeding:||bioturbation||generates pellets, stirs sediments|
|bacterial activity:||removes CO2, pH increase||calcifies microbial mats|
Allochems: An allochem is a carbonate particle that was formed outside of the depositional area and transported in, hence a carbonate "clast."
The most common allochems include:
- coated grains, including ooids (most smaller than 2 mm - right), grapestones, pisolites, and oncolites (rolled up mat)
- Bioclasts: skeletal fragments
- intraclasts: Fragments of previous partially hardened carbonate sediment - analogous to framework clasts of an intraformational conglomerate.
- Pellets: Fine grained (silt-sand sized) clasts of microcrystalline carbonate without internal structure. Often originating as fecal pellets.
Orthochemical components: Carbonate sediments that form within the depositional area represent the rock cement or matrix and include:
- micrite, a very fine grained component that may be an abiotic precipitate, or form due to the photosynthetic actions of nannoplankton or algae.
- spar, crystalline cement.
Under unusual chemical conditions (often diagenetic) a variety of other carbonate minerals can form in the marine and terrestrial environment. These include:
- Ankerite Ca(MgFe)(CO3)2
- Siderite FeCO3. Note: Siderite concretions are common indicators of anoxic pore fluids, as ferrous iron (Fe2+) is otherwise rapidly oxidized.
Two main classification schemes have emerged for limestones:
The Dunham System
- Allochthonous limestones: Original components were not bound together during deposition
- <10% framework (> 2 mm) by volume
- >10% framework allochems by volume > 2 mm)
- Autochthonous limestone: (aka Boundstone) Original components organically bound during deposition
The Folk System
- ...micrite: with calcareous mud as > 2/3 of matrix
- ...sparite: with sparry calcite or aragonite as > 2/3 of matrix