Geochemical characteristics of the sediments at site G16 of the Makran accretionary wedge, the northern Arabian Sea, and their implications for gas hydrates

The Makran accretionary wedge in the northern Arabian Sea is one of the important areas with large gas hydrate potential. Sediment, headspace gas and pore water samples were collected from the sediment core at site G16 PC of the area. Comprehensive studies are conducted on organic carbon, molybdenum, methane concentrations in headspace gas, and total alkalinity, anions and cations in pore water and detrital sediments. It is revealed that concentrations of Mg²⁺ and Ca²⁺ decrease clearly with the increasing in depth, while the total alkalinity and the Mg²⁺/Ca²⁺ ratio increases sharply. Around the sulfate methane interface (SMI), the concentration of dissolved sulfate (SO₄²⁻) decreases linearly to 0.31 mmol/L, with a sharp increase in methane to 784 µmol/L. At the same time, the content of pyrite in sediments reaches its maximum together with a Mo peak. Combined with changing characteristics of total alkalinity and concentrations of Mg²⁺, Ca²⁺ and organic carbon, the concentration of sulfate decreased linearly and the high sulfate gradients suggest that there is strong anaerobic methane oxidation (AOM) occurred at the SMI, which is estimated in a depth of 4.62 mbsf. A coupling phenomenon exists between geochemical anomaly characteristics of sediment samples and pore water samples from the sediment core and the content of authigenic pyrite, which strongly indicate that gas hydrate reservoirs may occur in the deeper layer of the study area.