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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 63 (1979)

Issue: 3. (March)

First Page: 490

Last Page: 490

Title: Possible Biogenic Origin for Some Sedimentary Dolomite: ABSTRACT

Author(s): Charles F. Mansfield

Article Type: Meeting abstract


A rather simple biochemical system comprising seawater, algae, and urease-producing bacteria could form microenvironments favoring the formation of sedimentary dolomite. Calcium and magnesium ions are supplied by seawater. Algae, like most plants, produce urea in the ornithine cycle; the urea [(NH2)2CO] is, in turn, hydrolyzed, producing abundant CO2 and NH3. This hydrolysis of urea is catalyzed at ambient temperatures by the enzyme urease, which is produced by several bacterial species as well as by certain plants. Urease-producing bacteria have been found in the world's oceans and in bottom sediments.

Although other biochemical reactions may also favor dolomite formation, the hydrolysis of urea appears to be preeminently important. This is because hydrolysis ultimately produces 1 mole of H2CO3 and 2 moles of NH4OH per mole of urea. Consequently, the solution becomes increasingly basic as CO2 continues to be produced, which means that the alkalinity also increases. Because of the increased basicity and carbonate content, as HCO3- or CO3= depending on pH, carbonate minerals, especially dolomite, may form. Experiments by Gebelein and Hoffman illustrated the importance of NH3 and CO2, and experiments by Medlin at elevated temperatures and pressures demonstrated the importance o urea hydrolysis.

Thus, the burial of dead, proteinaceous marine organisms, especially algae, and associated urease-producing bacteria may produce conditions favoring the formation of dolomite or at least promoting dolomitization. Stromatolites are a case in point; their organic-rich laminae commonly contain dolomite, and the inorganic interlayers commonly contain calcite. Furthermore, the feasibility of this, or a similar, system was recently demonstrated (unintentionally) by a male dalmatian who produced uroliths of ordered dolomite in his urinary bladder.

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Copyright 1997 American Association of Petroleum Geologists