Rates of methanogenesis and methanotrophy in deep‐sea sediments

ABSTRACT We use the carbon isotopic composition (δ 13 C) of the dissolved inorganic carbon (DIC) of pore fluids from Leg 175 of the Ocean Drilling Program (ODP) along the West African Margin to quantify rates of methane production (methanogenesis) and destruction via oxidation (methanotrophy) in deep‐sea sediments. Results from a model of diffusion and reaction in the sedimentary column show that anaerobic methane oxidation (AOM) occurs in the transition zone between the presence of sulfate and methane, and methanogenesis occurs below these depths in a narrow confined zone that ends at about 250 m below the sea‐sediments surface in all sediment profiles. Our model suggests that the rates of methanogenesis and AOM range between 6 · 10 −8 and 1 · 10 −10 mol cm −3 year −1 at all sites, with higher rates at sites where sulfate is depleted in shallower depths. Our AOM rates agree with those based solely on sulfate concentration profiles, but are much lower than those calculated from experiments of sulfate reduction through AOM done under laboratory conditions. At sites where the total organic carbon (TOC) is less than 5% of the total sediment, we calculate that AOM is the main pathway for sulfate reduction. We calculate that higher rates of AOM are associated with increased recrystallization rates of carbonate minerals. We do not find a correlation between methanogenesis rates and the content of carbonate or TOC in the sediments, porosity, sedimentation rate, or the C:N ratio, and the cause of lack of methanogenesis below a certain depth is not clear. There does, however, appear to be an association between the rates of methanogenesis and the location of the site in the upwelling system, suggesting that some variable such as the type of the organic matter or the nature of the microbiological community may be important.