Anaerobic oxidation of methane: Geochemical evidence from pore-water in coastal sediments of Qi’ao Island (Pearl River Estuary), southern China

Abstract

The concentrations of CH4 and SO 4 2− in pore-water and the carbon isotope compositions of total dissolved inorganic (ΣCO2) and CH4 were determined for three coastal sedimentary cores collected from Qi’ao Island (Pearl River Estuary), southern China. Results show that methane concentration changes dramatically at the base of the sulfate-reducing zone and sulfate concentration gradients are linear for all stations. In addition, the carbon isotope of methane becomes heavier at the sulfate-methane transition (SMT), which causes ΣCO2-δ13C to become the minimum. The geochemical profiles of pore-water render indirect evidence for anaerobic oxidation of methane (AOM). Based on numerical modeling of AOM and sulfate-reducing rates, the portion of total sulfate reduction occurring via AOM is 9.0%, 84% and 45.5%, respectively, and the percentage of ΣCO2 added to the pore-water is 4.7%, 72.4% and 29.45% correspondingly for three sites. Furthermore, it is found that the methane concentration, methane diffusive flux and the depth of SMT are controlled by the quantity and quality of sedimentary organic matter incorporated into the sediments. The great amount of organic material is favorable for rapid depletion of sulfate via sedimentary organic matter degradation, and on the other hand, causes the increase of the methane flux in the SMT, which results in a portion of sulfate reduction supported by AOM. Accordingly, the SMT was shifted towards the sediment surface.