The amount and formation rate of methane in marine sediments is a crucial aspect when determining both the methane fluxes towards the ocean-atmosphere system and the potential for dissolved methane to form other phases (e.g. free gas and hydrate). Nevertheless, due to the low methane solubility at ambient pressure (ca. 1.5 mM for average seawater temperature and salinity) methane degassing upon core retrieval limits the accuracy of methane concentrations measured above 1.5 mM. While several tools have been developed that may tackle this problem (e.g. autoclave coring), these methods may not be readily available nor practical for the entire scientific community. Nevertheless, diagenetic models which simulate both δ13C-methane and δ13C-DIC (dissolved inorganic carbon) can not only be used to better infer methane concentration profiles, but may also provide a comprehensive method for constraining methanogenic and methanotrophic rates in a cost-efficient manner. Here, we demonstrate the application of these carbon isotope, reaction-transport models to both steady-state and transient geochemical conditions in sediments located both in the shelf and in the slope.