Carbonate chemistry in marine pore waters: MANOP sites C and S

Mathematics – Logic

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Scientific paper

Calculations of the alkalinity and total CO2 response to organic matter diagenesis in a closed system containing CaCO3 indicate that the traditional stoichiometric reactions for organic matter degradation and the accompanying CaCO3 reaction are approximations at the pH of seawater. These approximations are different from the true values by about 10% during oxygen and MnO2 reduction and about 40% during denitrification.
Intercalibration of in situ and box core methods of pore water sampling from deep-sea carbonate-rich and carbonate-deficient sediments indicates that the sampling artifact on carbonate system measurements due to pressure change is variable in magnitude and apparently related to the carbonate content of the sediments. Previously proposed methods for predicting the effect are not general, implying that there is no alternative to in situ sampling for the evaluation of the carbonate system parameters.
In situ results from a carbonate ooze sediment at MANOP site C reveal that the pore waters are slightly supersaturated with respect to calcite. Ion activity products fall within the range predicted for equilibrium with aragonite. The diagenetic model applied to an open system using molecular diffusion coefficients adequately predicts the pore water alkalinity and total CO2 changes in response to organic matter degradation. This result provides clear evidence that the transport mechanism for dissolved species near the sediment-water interface at this location is by molecular diffusion and is not enhanced by biological or physical processes. At a siliceous ooze site, MANOP site S, the alkalinity increases in the pore waters in response to CaCO3 rain to the sediment-water interface. Calcium carbonate is not preserved, and the stoichiometric model predicts a particulate CaCO3/organic carbon rain ratio of 0.4-1.2. This ratio is in the same range as that measured in sediment traps deployed at this site as part of the MANOP program.

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