Figure 5: Simulation of non-steady-state porewater profiles for the timing of methane seepages.

We applied the reduced model on 911GC (a,b), 940GC (c,d) and 1520GC (e,f) to estimate the timing of seepage. Orange lines in a, (c,e) are initial sulfate concentrations used in the model which are identical to the black lines in b, (d,f) for the corresponding depth. These initial conditions were constrained by the shallow part of the measured sulfate profiles (orange dots in all panels) where concentration gradients are small. Blue lines in a, (c,e) are initial profiles for methane whereas the blue lines in b, (d,f) are model output for methane, which were also constrained by the first appearance of gas hydrates (yellow bars). Evolution of the modelled sulfate profiles (tortuosity equals to 1.5) at each site were presented in b, (d,f). The rate of evolution largely depends on the rate of sulfate consumption through anaerobic oxidation of methane, which is fastest at 911GC and slowest at 940GC. Model results reflect differences in the timescales of methane seepage among the three coring sites that are in geographical vicinity.