Methane-related Mg-calcite cements in Recent tidal flat sediments from the Firth of Forth

Recent, intertidal, medium-coarse grained silts from the Firth of Forth have been partially cemented. This cementation is selective, restricted to planar-horizontal crusts and thalassinid burrow fills. Stained thin sections and XRD show that the cement is composed of sub-micron sized, non-ferroan calcite containing 12–14 mole% MgCO3. Stable carbon and oxygen isotope analyses of the cements give a mean d 18O of 0.57‰ PDB and a mean d 13C of -29.8‰ PDB. The d 18O value is consistent with precipitation of Mg-calcite from seawater at a temperature around 9.5°C diluted by about 17% with meteoric water. The d 13C value is not indicative of normal marine bicarbonate, or derivation from microbial sulphate reduction and is diagnostic of methane oxidation. Methane is inferred to have formed below the sulphate reduction zone by biogenic reactions. Methane migrating upwards in the pore waters was oxidised by anaerobic bacteria near the base of the sulphate reduction zone. An increase in pore water alkalinity accompanied methane oxidation, resulting in the precipitation of CaCO3. Preferential cementation of thalassinid burrows suggests that this silt was permeable, acting as a conduit for sulphate-rich pore water to diffuse into the fermentation zone, and a site for bacterial methane oxidation. Projection of the cemented silts above the present tidal flat surface shows that they are now being exhumed, but it is not certain whether cementation is still occurring deeper within the sediment.