A two-dimensional model of the passive coastal margin deep sedimentary carbon and methane cycles

[摘要] We present a new geologic-time and basin-spatial scale model of thecontinental margin methane cycle. The model, SpongeBOB, is used to simulateevolution of the carbon cycle in a passive sedimentary continental margin inresponse to changing oceanographic and geologic forcing over a time scale of200 million years. The geochemistry of the sediment column is altered by theaddition of vertical high-permeability channels intended to mimic theeffects of heterogeneity in the real sediment column due to faults, andproduces results consistent with measured pore-water tracers SO₄²⁻and ¹²⁹I. Pore water dissolved inorganic carbon (DIC) concentrationsare consistent with chemical weathering (CaCO₃ formation from igneousrocks) at depth within the sediment column. The carbon isotopic compositionof the DIC is consistent with a methane production efficiency fromparticulate organic carbon (POC) of 50%, which is somewhat lower thanredox balance with the H / C of organic matter in the model. The hydrateinventory in the model is somewhat less sensitive to temperature than ourprevious results with a one-dimensional model, quite sensitive toreasonable changes in POC, and extremely sensitive to the ability of methanebubbles to rise within the sediment column, and how far gas-phase methanecan get through the sediment column before it redissolves when it reachesundersaturated conditions. Hydrate formation is also sensitive to deeprespiration of migrating petroleum. Other phenomena which we simulated hadonly a small impact on the hydrate inventory, including thermogenic methaneproduction and production/decomposition of dissolved organic carbon.

[学科分类] 地球化学与岩石