1 A model of the methane cycle , permafrost , and hydrology of the

6 A two-dimensional model of a passive continental margin was adapted to 7 the simulation of the methane cycle on Siberian continental shelf and 8 slope, attempting to account for the impacts of glacial / interglacial 9 cycles in sea level, alternately exposing the continental shelf to freezing 10 conditions with deep permafrost formation during glacial times, and 11 immersion in the ocean in interglacial times. The model is then subjected 12 to a potential future climate warming scenario. 13 Pore fluid salinity plays a central role in the model geochemical dynamics. 14 In the permafrost zone, pure water ice tolerates a higher fluid salinity 15 than methane hydrate can, eliminating hydrate as an equilibrium phase. 16 An analogous region in the ice – hydrate – brine phase diagram excludes 17 ice in favor of hydrate, but the two phases can coexist at a sub-saturated 18 methane concentration. In the permafrost zone (cold and low pressure), 19 in contrast, the dissolved methane concentration cannot be higher than 20 equilibrium with gas, so the hydrate exclusion from this zone is 21 inescapable. This thermodynamic constraint restricts methane hydrate to 22 at least 300 meters depth below the sediment surface, precluding a fast 23 hydrate dissolution response to sea-floor warming. 24 The initial salinity of the sediment column may have been affected by 25 previous hydrological forcing, because freshwater invasion driven by a 26 pressure head is probably much faster than salinity invasion due to 27 convective-diffusive processes. This has a ratcheting effect, leaving relict 28 fresh water lenses below sea level in many parts of the world. The pore 29 fluid salinity determines the relative volumes of the ice, brine, and 30 hydrate phases in the sediment column, and therefore the timing of ice 31 formation and melting, but the chemical composition, in particular the 32 salinity of the brine phase, is fixed, in equilibrium, by the local 33