Rates of carbonate cementation associated with sulphate reduction in DSDP/ODP sediments: implications for the formation of concretions

Abstract

DSDP/ODP porewater profiles in organic carbon-bearing (<5% org. C) sediments commonly show decreases in Ca 2+ concentrations and increases in alkalinity over depths where sulphate is being removed by microbial reduction. These Ca 2+ depletion profiles represent the combined effect of diffusion, advection and reaction (addition by ion exchange and removal by precipitation mainly as CaCO 3 and/or dolomite). A diagenetic model has been used to estimate the rate constant ( k) for Ca 2+ removal by precipitation during sulphate depletion over depths of 15–150 m, assuming first order kinetics. The rate constants for Ca 2+ removal range from 10 −14 to 10 −11 s −1 in 19 DSDP/ODP sediments, which span a range of bottom water temperatures (0–10 °C), lithologies (calcareous to clastic) and sedimentation rates (0.001–0.4 cm year −1). Values of k correlate with sedimentation rate ( ω) such that log k=1.16 log ω−10.3, indicating that faster rates of Ca 2+ removal occur at higher sedimentation rates where there are also higher degrees of saturation with respect to CaCO 3 and dolomite. Depth-integrated masses of Ca 2+ removed (<100 μmol cm −2) during sulphate depletion over these depth ranges are equivalent to a dispersed phase of approximately 1.5 wt.% CaCO 3 or 3 wt.% dolomite in a compacted sediment. The complete occlusion of sediment porosity observed in concretions with isotopic signatures suggesting carbonate sourced from sulphate reduction therefore requires more time (a depositional hiatus), more rapid sulphate reduction (possibly by anaerobic methane oxidation) and/or the continued transport of isotopically light carbonate to the concretion site after sulphate reduction has ceased.