Dissolution of calcite in deep-sea sediments: pH and O 2 microelectrode results

Abstract

We present microelectrode profiles of oxygen, pH, and porosity measured at in situ depth across the sediment-water interface from stations above and below the calcite saturation horizon in the equatorial Atlantic. Resistivity electrode data indicate that the diffusion coefficient for solutes is attenuated by nearly a factor of two within the top centimeter of the sediment lattice. Oxygen and porosity data are used to estimate that the organic carbon respiration rate of the sediment is 10–15 μmol cm −2 yr −1 in these locations. Comparison of the pH data with numerical model results reveals that sedimentary calcite dissolves in response to respiration in the top few centimeters of sediment even above the saturation horizon. We calculate that the effective in situ dissolution rate constant for calcite required to explain these data is the neighborhood of 10–100% d −1. Although more data are needed to confirm this result, it appears to be at least an order of magnitude less than the rate observed in laboratory stirred-reactor experiments.