In situ electrode measurements of porewater oxygen and pH from four sites on the Ceara Rise in the western Tropical Atlantic unequivocally demonstrate that calcite dissolution driven by metabolic CO2 produced within the sediments is a significant part of the diagenesis of sedimentary calcite, dissolving at least 20% of the calcite rain to the seafloor. The first ever in situ absorbance-based measurements of porewater CO2(aq) support the dissolution estimates based on models of the observed pH distributions and are incompatible with scenarios excluding metabolic CO2-driven dissolution. Our dissolution estimates are consistent with those estimated by an independent study of porewater calcium and alkalinity profiles (Martin and Sayles, 1996) at this location. The consistency of these disparate porewater measurements and our ability to interpret them with simple models of respiration and dissolution implies that no special mechanism is required to describe this process.The calcite dissolution rate constant in the sediments of the deepest station in this study is significantly higher than determined by in situ studies at other locations; values approach those determined by early laboratory measurements. The dissolution rate constant here must be at least two orders of magnitude higher than for calcite in sediments on the Ontong-Java Plateau, based on in situ electrode-measurements there (Hales and Emerson, 1996). The calcite solubility consistent with these data is not compatible with that consistent with the Ontong-Java data. The reason for this difference is currently unresolved, but may be due to the high order of the empirical dissolution rate law assumed in this study. Copyright (C) 1997 Elsevier Science Ltd.
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