Modeling of subsurface calcite dissolution, including the respiration and reoxidation processes of marine sediments in the region of equatorial upwelling off Gabon

Abstract

Mineralization of organic matter and the subsequent dissolution of calcite were simulated for surface sediments of the upper continental slope off Gabon by using microsensors to measure O 2, pH, pCO 2 and Ca 2+ (in situ), pore-water concentration profiles of NO 3 −, NH 4 +, Fe 2+, and Mn 2+ and SO 4 2− (ex situ), as well as sulfate reduction rates derived from incubation experiments. The transport and reaction model CoTReM was used to simulate the degradation of organic matter by O 2, NO 3 −, Fe(OH) 3 and SO 4 2−, reoxidation reactions involving Fe 2+ and Mn 2+, and precipitation of FeS. Model application revealed an overall rate of organic matter mineralization amounting to 50 μmol C cm −2 yr −1, of which 77% were due to O 2, 17% to NO 3 − and 3% to Fe(OH) 3 and 3% to SO 4 2−. The best fit for the pH profile was achieved by adapting three different dissolution rate constants of calcite ranging between 0.01 and 0.5% d −1 and accounting for different calcite phases in the sediment. A reaction order of 4.5 was assumed in the kinetic rate law. A CaCO 3 flux to the sediment was estimated to occur at a rate of 42 g m −2 yr −1 in the area of equatorial upwelling. The model predicts a redissolution flux of calcite amounting to 36 g m −2 yr −1, thus indicating that ∼90% of the calcite flux to the sediment is redissolved.