Martin, W.R., and F.L. Sayles

Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA, E-mail: wmartin@whoi.edu

Metabolically driven calcite dissolution in deep-sea sediments

The purpose of this work is to examine the potential importance of metabolic calcite dissolution in deep-sea sediments. The work is composed of 3 parts. First, we examine new, high-resolution profiles of TCO₂, Alkalinity, NO₃^- and O₂ in pore waters, finding that they are consistent with the occurrence of net CaCO₃ dissolution in carbonate-rich sediments lying above the calcite lysocline. These profiles are used to determine kinetic parameters for calcite dissolution, which are used in subsequent modeling exercises. Second, we use profiles of O₂ and NO₃^- from sediments from 4 distinct oceanic regions to characterize the variability of the distribution of oxic organic matter decomposition within the sediment column, as a function of the rain rate of organic carbon to the sea floor. Oxidation occurs increasingly close to the sediment-water interface as the rain rate increases form about 5 µmol/cm²/y to about 20 µmol/cm²/y, and is essentially invariant at rain rates of 20 or greater. Subsequent modeling exercises show that when a larger portion of organic matter oxidation occurs close to the interface, more of the metabolic acid released to pore waters is neutralized by reaction with dissolved carbonate ion in bottom waters, resulting in less dissolution of sedimentary calcite, other factors being equal. When the organic matter and CaCO₃ rain rates are allowed to covary, as results from deep sediment traps suggest they may, metabolic dissolution remains important, resulting in dissolution of 20-50% of the calcite rain at DCO₃^2- levels of + 30 to 0 µmol/kg. However, the effect of shoaling oxidation within the sediment column as the organic rain rate increases is counterbalanced by the increasing sedimentary CaCO₃ concentrations, so that metabolic dissolution efficiency remains constant at a given supersaturation state. The total metabolic dissolution rate does increase as the organic matter rain rate increases at a given supersaturation state.