GEOL 342 Sedimentation and Stratigraphy4 March 2006Assoc. Prof. A. Jay KaufmanCarbonate sedimentsCarbonate is the most abundant chemical sediment in modern and (most) ancient oceans.As we will explore later, carbonates are sensitive recorders of the global marine environment, inparticular as proxy signals for long-term and abrupt changes in the exogenic carbon cycle. Thesesoluble 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 gravel, Ca and Mgfor nutrition, and building facing stone. Carbonates are also host rock to Mississippi-type oredeposits, and when fractured can be good oil reservoir rocks. Carbonates are also the rock mostlikely to contain the globally-distributed marine fossils used in biostratigraphy.Carbonates may also monitor changes in the atmosphere. Indeed most of the carbon in theearly atmosphere (which existed as either CO2 or CH4 in high concentrations) reacted with thesilicate Earth and water to form alkalinity, and is now stored as carbonate rock in the crust. Thepresent atmosphere has a miniscule 350 ppmv CO2 and ppb levels of CH4.Are coral reefs net sources or sinks of atmospheric CO2?Carbonates form readily in most warm, shallow water environments. The mainrequirements for their formation are high concentrations of Ca2+ and HCO3- (alkalinity), which areproducts of silicate and carbonate weathering on land. 2NaAlSi3O8 + 2CO2 + 11H2O  Al2Si2O5(OH)4 + 2Na+ +2HCO3- + 4H4SiO41Since surface oceans are generally supersaturated with respect to calcite, this mineral andits sister, aragonite, form readily. Calcite has a rhombohedral structure that can substitute up to5% Mg (high Mg calcite) for Ca. On the other hand, the aragonite structure is orthorhombic andits larger cation sites allow for the incorporation of larger elements, most notably strontium. Organisms that calcify use a range of carbonate minerals to build their shells and homes. Carbonate chemistryEquilibrium constants monitor the extent of chemical reactions at set temperatures andpressures. An equilibrium constant is equal to the concentration of products over reactants.For a reaction A+B → C + D (reactants → products)Keq(1) = (C)(D)/(A)(B)The reverse of this reaction C + D → A + BKeq(2) = (A)(B)/(C)(D)If K(1) > K(2) the reaction moves to the right at equilibrium conditions.So now consider the carbonate system of equilibrium reaction and constants:1) CO2 + H2O → H2CO3 Keq = 10-1.4322) H2CO3 → H+ + HCO3- Keq = 10-6.403) HCO3- → H+ + CO32- Keq = 10-10.334) Ca2+ + CO32- → CaCO3 Keq = 10-8.33 for aragonite and 10-8.48 for calciteSo based on the kinetics of these reactions, is carbonate precipitation increased or decreased bythe removal of CO2 from seawater?The chemical and physical controls on carbonate formation in oceans and lakes include: 1. temperature2. pressure3. degree of agitation4. sediment masking 5. light availability6. oxidation stateThe effects of organic activity on CaCO3 precipitation are manifold.activity immediate effect ultimate effectCaCO3 extraction promotes skeletal growth forms allochems and mudphotosynthesis removes CO2, pH increase promotes precipitationdecay adds CO2, pH decrease hinders precipitationfeeding bioturbation generates pellets, stirs sedimentsbacterial activity removes CO2, pH increase calcifies microbial matsCarbonate classificationTwo main classification schemes have emerged for limestones. The Dunham classification isbased on the recognition of depositional textures as well as the abundance of allochthonous andautothonous components. An allochem is a carbonate particle that was formed outside of thedepositional area and transported in, hence carbonate sediments can be clastic.3In contrast, the Folk classification relies on descriptive terms for the allochems linked to thedominant matrix material, either micrite or sparite. Folk also described a classification based ontextural maturity.The most common allochems include:1. coated grains, including ooids (most smaller than 2 mm), grapestones, pisolites, andoncolites (rolled up mat)2. skeletal fragments3. intraclasts4Orthochemical components that form within the depositional area represent the rock cementand include: 1. micrite, a very fine grained component that may be an abiotic precipitate, or form due tothe photosynthetic actions of nannoplankton.2. sparUnder unusual chemical conditions (oftentimes diagenetic) a variety of other carbonateminerals can form in the marine and terrestrial environment. These include:ankerite Ca(MgFe)(CO3)2 and siderite FeCO3. These are common constituents in the Precambrianbanded iron-formations, as well as concretions in peat-rich soils and organic-rich sandstones, and dolomite CaMg(CO3)2.Most models of diagenetic dolomite formation require the kinetic barriers for theprecipitation of this highly ordered mineral to be overcome. The barriers are 1) temperature, 2)5salinity (effects of hydration on Mg2+ ion), 3) Mg/Ca ratio (seawater is 5.4 for dolomite to formmust be > 8), and 4) sulfate concentration (28 mM in seawater). Environments where thesebarriers can be removed are warm, highly evaporitic, and mixed with fresh water sources. Thesefactors promotes the precipitation of Ca and SO42- bearing minerals, which increases Mg/Ca anddecrease sulfate concentrations.One modern environment where dolomite appears to be forming as a primary precipitate isthe Coorong lakes, along the southern margin of Australia. The Coorong is an area of lagoons andalkaline lakes behind modern beach barriers, which are fed by seawater and groundwater. Thetemporary lakes are of high pH (8 to 10) and have Mg/Ca of up to 20. Dolomite forms in morelandward lakes as minute spherical