Gas hydrates dominated by methane naturally occur in deep marine sediment along continental margins. These compounds form in pore space between the seafloor and a sub-bottom depth where appropriate stability conditions prevail. However, the amount and distribution of gas hydrate within this zone, and free gas below, can vary significantly at different locations. To understand this variability, we develop a one-dimensional numerical model that simulates the accumulation of gas hydrates in marine sediments due to upward and downward fluxes of methane over time. The model contains rigorous thermodynamic and component mass balance equations that are solved using expressions for fluid flow in compacting sediments. The effect of salinity on gas hydrate distribution is also included.The simulations delineate basic modes of gas hydrate distribution in marine sediment, including systems with no gas hydrate, gas hydrate without underlying free gas, and gas hydrate with underlying free gas below the gas hydrate stability zone, for various methane sources. The results are scaled using combinations of dimensionless variables, particularly the Peclet number and Damkohler number, such that the dependence of average hydrate saturation on numerous parameters can be summarized using two contour maps, one for a biogenic source and one for upward flux from a deeper source. Simulations also predict that for systems at steady state, large differences in parameters like seafloor depth, seafloor temperature and geothermal gradient cause only small differences in average hydrate saturation when examined with scaled variables, although important caveats exist. Our model presents a unified picture of hydrate accumulations that can be used to understand well-characterized gas hydrate systems or to predict steady-state average hydrate saturation and distribution at locations for which seismic or core data are not available.
[1] Both the concentration and the carbon isotope composition of dissolved inorganic carbon (DIC) vary considerably across the sulfate-methane transition (SMT) in shallow marine sediment at locations with gas hydrate. This variability has led to different interpretations for how carbon, including CH 4 , cycles within gas-charged sediment sequences over time. We extend a one-dimensional model for the formation of gas hydrate to account for downhole changes in dissolved CH 4 , SO 4 2− , DIC, and Ca 2+ , and the d 13 C of DIC. The model includes advection, diffusion, and two reactions that consume SO 4 2− : degradation of particulate organic carbon (POC) and anaerobic oxidation of methane (AOM). Using our model and site-specific parameters, steady state pore water profiles are simulated for two sites containing gas hydrate but different carbon chemistry across the SMT: Site 1244 (Hydrate Ridge; DIC = 38 mM, d13 C of DIC = -22.5‰ PDB) and Site Keathley Canyon (KC) 151-3 (Gulf of Mexico; DIC = 16 mM, d13 C of DIC = −49.6‰ PDB). The simulated profiles for CH 4 , SO 4 2− , DIC, Ca 2+ , and d 13 C of DIC resemble those measured at the sites, and the model explains the similarities and differences in pore water chemistry. At both sites, an upward flux of CH 4 consumes most net SO 4 2− at a shallow SMT, and calcium carbonate removes a portion of DIC at this horizon. However, a large flux of 13 C-enriched HCO 3 − enters the SMT from depth at Site 1244 but not at Site KC151-3. This leads to a high concentration of DIC with a d 13 C much greater than that of CH 4 , even though AOM causes the SMT. The addition of HCO 3 − from depth impacts the slope of certain concentration crossplots. Crucially, neither the DIC concentration nor its carbon isotope composition at the SMT can be used to discriminate between sulfate reduction pathways.
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