Submillimeter depth distributions of total dissolved inorganic carbon (DIC) were derived from pH and PcO, profiles measured with microelectrodes in an organic-rich, laboratory-maintained sediment. The DTC profiles were used to calculate diffusive fluxes of DIC across the sediment-water interface. In two experiments, the calculated diffusive fluxes fell within 250% of the total flux of DIC determined by core incubation. An assessment of errors suggests that the microelectrode-derived estimates are not significantly different from measured total DIC fluxes (P = 0.05). It is concluded, therefore, that pH and PcO, microelectrode measurements can be paired to determine finescale pore-water DIC profiles and DIC diffusive fluxes. Problems will arise only in situations in which pH and P,,, gradients are extremely steep or spatially heterogeneous; this is because these conditions can cause mismatching of pH and PcoZ measurements or CO, system disequilibrium.Microelectrodes have been used to measure in situ oxygen and pH pore-water profiles in a wide variety of marine environments in recent years. These new data are being used to calculate diffusive benthic fluxes of 0, (Reimers et al. 1986(Reimers et al. , 1992Archer and Devol 1992;Glud et al. 1994a) and applied as constraints for diagenetic models that estimate rates of organic matter degradation and calcium carbonate dissolution (Archer et al. 1989;Cai et al. 1995; Hales and Emerson 1996, 1997). However, one shortcoming of these studies is that 0, and pH data need to be accompanied by an additional measurable parameter of the CO, system to fully describe this system in pore solutions. P,.,, is the only other parameter of the CO, system that is readily measurable by microelectrodes. Therefore, Cai and Reimers (1993) and Cai et al. (1995) attempted to use a potentiometric Pco2 microelectrode to measure the first in situ PVo2 profiles in marine sediments. These PcO, profiles, however, did not compare well with models constrained by the pH and oxygen microelectrode data at several key depths within the sediment. Cai and Reimers (1993) and Cai et al. (1995) attributed these deviations to the slow response characteristics of the potentiometric P,,, sensor.The goal of this study was to verify that potentiometric PH and PO, microelectrodes, given adequate response times, produce pH and Pcoz data that can be used to derive (1) reliable total dissolved inorganic carbon (DIC) concentrations in pore waters and (2) accurate DIC fluxes across the sediment-water interface. Diffusive fluxes of DIC across the sediment-water interface were calculated using pH and Pco, microelectrode profiles and compared with total fluxes determined directly using laboratory core incubations as the primary test of microelectrode performance. It was assumed
AcknowledgmentsWe thank Wei Wang for the construction of oxygen microelectrodes and assistance in the field. S. Seitzinger and G. Taghon helped improve early versions of the manuscript. We also thank B. Boudreau, B. Hales, and W. Martin for i...