CALCITE DISSOLUTION IN SEDIMENTS OF THE ONTONG-JAVA PLATEAU - IN-SITUMEASUREMENTS OF PORE-WATER O-2 AND PH

We present in situ electrode measurements of sediment resistivity, por e water oxygen, and pore water pH from three stations between 2300 and 3000 m depth on the Ontong-Java Plateau in the western equatorial Pac ific. One of these stations is also the site of a concurrent benthic c hamber incubation experiment [Jahnke et al., 1994]. The pore water oxy gen data and a steady state diffusion and reaction model constrain the depth-dependent rate of oxic respiration in the sediments and imply a diffusive flux of oxygen to the sediments of 10-21 mu mol cm(-2) yr(- 1), Given these respiration rates, the pore water pH data cannot be ex plained without calcite dissolution driven by metabolically produced C O2. The dissolution necessary to explain the observations, quantified by a statistical approach, is 3.5-6 mu mol cm(-2) yr(-1), which corres ponds to at least 20-40% of the calcite rain to these sediments. Over 65% of the total dissolution is driven by metabolic CO2. Oxygen fluxes and net calcite dissolution constrained by the electrode data are com patible with the benthic chamber measurements of Jahnke et al. [1994]. The dissolution flux, while a significant part of the early diagenesi s of calcite in these sediments, is less than would be predicted by ea rlier models of dissolution, and Jahnke et al. [1994] probably could n ot distinguish it from zero with the benthic chamber technique. The di ssolution rates found in this study are lower than previous estimates because the respiration reaction is concentrated near the sediment-wat er interface, and the calcite dissolution rate constants are very smal l. The statistical evaluation of the pore water pH data and model cons train the calcite dissolution rate constant to 0.005-0.16% d(-1), foll owing the general trend of lower values determined by in situ techniqu es rather than by laboratory methods.