The quantification of methane formation in Amazon Fan sediments (ODP Leg 155, Site 938) by hydrogeochemical modeling solid – Aqueous solution – Gas interactions

Abstract

A quantitative simulation of interdependent diagenetic reactions in deep-sea fan sediments is presented. The model scenario describes a growing sediment column and reproduces observed diagenetic signals (distribution of dissolved, mineral, and gaseous species) and predicts the amount of in situ-generated biogenic dissolved methane and methane hydrate at ODP Leg 155, Site 938 (Amazon Fan). The approach relies on thermodynamic calculations of equilibrium reactions in aqueous solutions that are constrained by the irreversible remineralization of organic carbon. Sediments of ODP Leg 155, Site 938 are located within the gas hydrate stability zone. Methanogenesis starts at a shallow depth (10 m below seafloor). The modeled amount of in situ-generated biogenic methane exceeds the solubility of the methane, with respect to methane hydrate in a depth of 115 mbsf. Moreover, anaerobic oxidation of methane (AOM) prevents the diffusion of dissolved methane to the overlying seawater. Dissolved methane and methane hydrate are stored within the sediment column.Authigenic carbonate is the most important carbon sink in the investigated sediments of the Amazon fan. Carbon mass balance calculations reveal the studied Amazon Fan sediments are a net carbon sink. Authigenic siderite precipitation, triggered by AOM, explains the overall gain in carbon within the Amazon fan sediments. Generated methane (as methane gas, hydrate and dissolved methane) is the second most important carbon species found within the sediments. This highlights the significance of organic matter-rich high accumulation areas to biogenic methane formation and storage, and the significance of these areas as possible targets for exploration.