Biogeochemical cycles at the sulfate-methane transition zone (SMTZ) and geochemical characteristics of the pore fluids offshore southwestern Taiwan

In this study, we used pore water dissolved inorganic carbon (DIC), SO₄ ²⁻, Ca²⁺ and Mg²⁺ gradients at the sulfate-methane transition zone (SMTZ) to estimate biogeochemical fluxes for cored sediments collected offshore SW Taiwan. Net DIC flux changes (Δ_DIC-Prod) were applied to determine the proportion of sulfate consumption by organic matter oxidation (heterotrophic sulfate reduction) and anaerobic oxidation of methane (AOM), and to determine reliable CH₄ fluxes at the SMTZ. Our results show that SO₄ ²⁻ profiles are mainly controlled by AOM rather than heterotrophic sulfate reduction. Refinement of CH₄ flux estimates enhance our understanding of methane abundance from deep carbon reservoirs to the SMTZ. Concentrations of chloride (Cl⁻), bromide (Br⁻) and iodide (I⁻) dissolved in pore water were used to identify potential sources that control fluid compositions and the behavior of dissolved ions. Constant Cl⁻ concentrations throughout ∼30 m sediment suggest no influence of gas hydrates for the compositions within the core. Bromide (Br⁻) and Iodine (I⁻) concentrations increase with sediment depth. The I⁻/Br⁻ ratio appears to reflect organic matter degradation. SO₄ ²⁻ concentrations decrease with sediment depth at a constant rate, and sediment depth profiles of Br⁻ and I⁻ concentrations suggests diffusion as the main transport mechanism. Therefore diffusive flux calculations are reasonable. Coring sites with high CH₄ fluxes are more common in the accretionary wedge, amongst thrust faults and fractures, than in the passive continental margin offshore southwestern Taiwan. AOM reactions are a major sink for CH₄ passing upward through the SMTZ and prevent high methane fluxes in the water column and to the atmosphere.