Figure 2: Schematic diagram showing geochemical reactions within marine sediments bathed beneath ferruginous and sulphidic seawaters.
From: Molar tooth carbonates and benthic methane fluxes in Proterozoic oceans
(a) Under ferruginous conditions, the MSR and methanogenesis zones are separated. Methanogens are outcompeted by sulphate-reducing bacteria, if both use competitive substrates (CH2O)n. Within the MSR zone, reaction between H2S and Fe2+ precipitates pyrite and generates H+, which lowers porewater pH. Most CH4 produced within methanogenesis zone is oxidized by sulphate at the base of MSR zone where AOM occurs. Thus, there is little benthic CH4 flux from marine sediments. (b) Under sulphidic conditions, methyl sulphides are produced within both water column and sediments. In sediments, methyl sulphides serve as a non-competitive substrate for methanogens, allowing MSR and methanogenesis to take place concurrently in the MSR-methanogenesis convergence zone. H2S and Fe2O3 react to produce pyrite and generate OH–, thus favouring CaCO3 precipitation. AOM is prohibited by methanethiol, allowing CH4 accumulation in sediments and significant benthic CH4 fluxes into atmosphere.
