Carbonate sediments and chemistry:

You may have noticed the striking omission of carbonate rocks and environments in our account of depositional environments, so far. Carbonates are so distinctive that their study requires a complex technical vocabulary and tends to attract a unique breed of geologist, including my carbonate mentor, Dr. Lynton Land of the University of Texas (right). So what's it all about?

Note: Calcium carbonate forms the minerals calcite and aragonite. Rocks made of these are called limestone regardless of how they form. (E.G.: Pedernales Falls State Park, TX. Limestones of the Central Cretaceous Seaway.) Calcium carbonate is the most abundant chemical sediment in modern and (most) ancient oceans, making up roughly 10% of sediments. As we will explore later, carbonates are sensitive recorders of the global marine environment, in particular as proxy signals for long-term and abrupt changes in the exogenic carbon cycle. These soluble minerals are also subject to diagenetic alteration by a variety of processes.

A more mundane but important aspect of the rocks is that they are sources of:

Carbonates are also host rock to:

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:

Which one forms primarily is a function of subtle variations in ocean conditions (mostly the ratio of Ca to Mg), or of the biology of the organism secreting it in its skeleton.

From Wikipedia
Calcite has a rhombohedral structure that can substitute up to 5% Mg (high Mg calcite) for Ca.

Aragonite's structure is orthorhombic and its larger cation sites allow for the easier substitution of Mg and larger ions, most notably strontium and barium.

Recrystallized brachiopod - Wikimedia Commons
  • Recrystallization: Aragonite is metastable in nature, eventually recrystallizing as calcite. (Note: When this occurs, many small-scale details of the fossil are lost, even though the same material is present.)

    Organisms that calcify use a range of carbonate minerals to build their shells and homes.

    Carbonate chemistry:

    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:

    Because they proceed in the same environment, reactants can "leak" from one reaction to the other. K3 is much greater for reaction 2 than reaction 3. Thus, H+ from 2 is able to leak into reaction 3, driving it to the left. paradoxically, precipitation of CaCO3 is facilitated by the reduction of CO2.

    Catoctin Fm. metabasalt superposed over Faquier Fm. cap carbonate
    But where, you ask, does the carbon for the CaCO3 come from? Remember that bicarb ion (HCO3-) is a product of continental weathering reactions. In fact, at times when much bare bedrock is exposed (like after glaciations), we see copious cap carbonates superposed over marine diamictites.

    Factors controlling carbonate formation:

    The principal chemical and physical controls on carbonate formation in oceans and lakes are those that control CO2 concentration:

    Exactly as you would predict from experience with sodas. These factors combine to make environments like the Bahama Banks (right) the modern world's primary carbonate factory.

    Secondary controls include:

    The effects of organic activity on CaCO3 precipitation are subtle.

    CaCO3 extraction:promotes skeletal growthforms allochems and mud
    decay:adds CO2, pH decreasehinders precipitation
    feeding:bioturbationgenerates pellets, stirs sediments
    bacterial activity:removes CO2, pH increasecalcifies 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:

    Orthochemical components: Carbonate sediments that form within the depositional area represent the rock cement or matrix and include:

    Under unusual chemical conditions (often diagenetic) a variety of other carbonate minerals can form in the marine and terrestrial environment. These include:

    These are common constituents in the Precambrian banded iron-formations, as well as concretions in peat-rich soils and organic-rich sandstones

    Carbonate classification

    Two main classification schemes have emerged for limestones:

    The Dunham System
    The Dunham classification of Robert J. Dunham is based on the recognition of depositional textures as well as the abundance of allochthonous and autothonous components. The Dunham system was developed for use with hand samples.

    The Folk System
    The Folk classification of Robert Folk relies on descriptive terms for the allochems linked to the dominant matrix material: The prefix depends on the identity of the allochem.

    Folk also described a classification based on textural maturity analogous to that of sandstones. The Folk system is typically used when examining rocks in thin section. A visit with Dr. Folk.