CALCITE DISSOLUTION IN SEDIMENTS OF THE CEARA RISE - IN-SITU MEASUREMENTS OF POREWATER O-2, PH, AND CO2(AQ)

In situ electrode measurements of porewater oxygen and pH from four si tes on the Ceara Rise in the western Tropical Atlantic unequivocally d emonstrate that calcite dissolution driven by metabolic CO2 produced w ithin the sediments is a significant part of the diagenesis of sedimen tary calcite, dissolving at least 20% of the calcite rain to the seafl oor. The first ever in situ absorbance-based measurements of porewater CO2(aq) support the dissolution estimates based on models of the obse rved pH distributions and are incompatible with scenarios excluding me tabolic CO2-driven dissolution. Our dissolution estimates are consiste nt with those estimated by an independent study of porewater calcium a nd 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 i mplies 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 s itu studies at other locations; values approach those determined by ea rly laboratory measurements. The dissolution rate constant here must b e at least two orders of magnitude higher than for calcite in sediment s on the Ontong-Java Plateau, based on in situ electrode-measurements there (Hales and Emerson, 1996). The calcite solubility consistent wit h these data is not compatible with that consistent with the Ontong-Ja va data. The reason for this difference is currently unresolved, but m ay be due to the high order of the empirical dissolution rate law assu med in this study. Copyright (C) 1997 Elsevier Science Ltd.