pH and pCO2 microelectrode measurements and the diffusive behavior of carbon dioxide species in coastal marine sediments

The performance of a polymeric membrane-based pH microelectrode and a new pCO2 microelectrode in marine sediments is evaluated for the purpose of collecting fine-scale and undisturbed porewater profiles of carbon dioxide parameters. Both microelectrodes have a precision of 0.01–0.02 logarithmic units. A fine-scale porewater profile of total dissolved inorganic carbon (TCO2) is calculated from the pH and pCO2 data and is in good agreement with coarse-scale TCO2 profiles measured in squeezed porewater. The measuring uncertainty for TCO2 is within 5%. The measurements of the pH microelectrode also agree in general with those of a glass mini-electrode. The mini-electrode, however, measures smaller pH changes as a result of mixing the sediments. Very low pH values (<6.0) are measured in this salt marsh-influenced sediment and are correlated to the oxidation of Mn2+, Fe2+, and possibly, solid sulfides. Concentration profiles and fluxes of TCO2, CO2, HCO3− and CO32− are calculated from the pH and pCO2 data. These millimeter-scale profiles demonstrate that the calculation of benthic flux of TCO2 based on porewater TCO2 profiles and on a HCO3− (or a concentration “weighted”) diffusion coefficient can be erroneous. When porewater pH is very low (<7.0) in the near-surface sediment, as is the case in many organic-rich coastal sediments, TCO2 transport is accomplished significantly through the diffusion of CO2 and can be greatly enhanced due to a sharp concentration gradient and a large diffusion coefficient of CO2.