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In addition to inorganic precipitation, carbonate cements can be formed from organically derived bicarbonate. The most dominant source of this "biogenic" bicarbonate is sulfate reduction. As bacterially produced bicarbonate increases in the pore water, precipitation of calcium carbonate can occur. Deeper in the sediment column, where sulfate has been microbially depleted, production of methane and carbon dioxide occurs from bacterial fermentation of organic matter. Methane produced below the sulfate zone can diffuse upward and is almost completely reoxidized before reaching the surface. Oxidized carbon is again introduced into the pore water as bicarbonate, thus enhancing carbonate precipitation. The resultant cement forms as a micrite crust and probably has been mistaken as a surface-precipitated marine cement.
Ideally, the sources of cement can be delineated by carbon isotope signatures that can be approximated (versus PDB) as follows, in parts per thousand: inorganic, 0; sulfate reduction, -25; fermentation, +10; and methane oxidation, -80. The origins of selected microcrystalline cements from the reef plate at Enewetak Atoll, Marshall Islands, have been determined by examining their isotope ratios. Since these ratios can be measured to better than 0.1 parts per thousand, analysis of cements from core samples is often diagnostic of the origin of the cement, although some "mixing" is common.
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