Other
Scientific paper
Mar 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991gecoa..55..777b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 55, Issue 3, pp.777-785
Other
15
Scientific paper
P CO 2 is a significant influence on the incorporation of MgCO 3 in calcite precipitated from seawater and from MgCl 2 - CaCl 2 ( a Mg 2+ / a Ca 2+ = 5.7) solutions at temperatures from 5 to 45 °C. In seawater at 25°C, the thermodynamic distribution coefficient, D Mg , increased from 0.013 at 10 -1 atm to 0.024 at 10 -4.5 atm, P co 2 · D Mg showed a similar dependence on P CO 2 at other temperatures, but trends are offset by temperature effects on D Mg . In normal seawater, variations in temperature from 5 to 45 °C, coupled with changes in p co 2 from 10 -1 to 10 -4.5 atm, produce Mg calcite compositions ranging from 6 to 18 mol% MgCO 3 · D Mg depends linearly on log P co 2 , log m HCO 3 - , and pH at any given temperature. There is no linear dependence of D Mg on log m CO 3 2- , growth rate, or calcite saturation state. Overgrowths precipitated from MgCl 2 -CaCl 2 solutions at equilibrium with different P CO 2 values showed compositional trends similar to those for seawater except that calcites were always enriched by about 2 mol% MgCO 3 relative to compositions in seawater under the same P CO 2 and temperature conditions. D Mg values also are linearly dependent on pH and log a HCO 3 - these solutions. The compositional trends in the MgCl 2 -CaCl 2 solutions correlate well with compositions of Mg-calcites precipitated by Mucci et al. (1989) from sulfate-free seawater at 25°C and 10 -2.5 atm P CO 2 . Modern marine Mg calcite cement compositions probably are influenced by local variations in porewater P CO 2 and sulfate concentration, as well as by changes in temperature. Global changes in atmospheric P CO 2 during the Phanerozoic may influence trends in compositions of ancient shallow marine Mg calcite.
Burton Elizabeth A.
Walter Lynn M.
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