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 CH4, 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 CH4, SO42−, DIC, and Ca2+, and the δ13C of DIC. The model includes advection, diffusion, and two reactions that consume SO42−: 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, δ13C of DIC = –22.5‰ PDB) and Site Keathley Canyon (KC) 151–3 (Gulf of Mexico; DIC = 16 mM, δ13C of DIC = −49.6‰ PDB). The simulated profiles for CH4, SO42−, DIC, Ca2+, and δ13C 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 CH4 consumes most net SO42− at a shallow SMT, and calcium carbonate removes a portion of DIC at this horizon. However, a large flux of 13C-enriched HCO3− enters the SMT from depth at Site 1244 but not at Site KC151–3. This leads to a high concentration of DIC with a δ13C much greater than that of CH4, even though AOM causes the SMT. The addition of HCO3− 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.