The 3D version applies the competitive-inhibition formula once, to a single bulk sulfate value, and animates a particle population to show the result. This 2D version instead solves the reaction-diffusion PDE on a real depth grid — a genuinely distinct, spatially-resolved computation of the same kinetics, not a flattened camera view:
∂[SO4]/∂t = D_SO4·∂²[SO4]/∂z² − R(z)·(1−f_CH4(z))
∂[CH4]/∂t = D_CH4·∂²[CH4]/∂z² + R(z)·f_CH4(z) − k_eb·max(0,[CH4]−C_sat)
f_CH4(z) = 1 / (1 + [SO4](z) / Ki) Ki = 1.0 mM
R(z) = C(z)·0.7·Q10^((T−20)/10), Q10 = 2.5, C(z) = carbon% · e^(−z/0.6m)
[SO4] boundary: surface = 0.8·salinity (mM), no-flux at depth
[CH4] boundary: surface = 0 (free exchange), no-flux at depth
Because sulfate is depleted fastest near the surface (where organic carbon and respiration are highest) and resupplied there by tidal exchange, a real vertical sulfate gradient forms — something the 3D box model can't show since it only ever holds one number for "the" sulfate concentration. The right-hand phase diagram plots every depth cell's (sulfate, f_CH4) pair directly on the theoretical inhibition curve, colour-coded from surface (teal) to depth (violet): as you raise salinity the whole cloud of points slides left along the curve toward f_CH4→0, visualising the depth-resolved competitive exclusion instead of just a single bar-split percentage.
- Salinity — sets the surface sulfate boundary condition (0.8·ppt mM), driving the depth-profile shape.
- Organic carbon supply — scales the carbon-decay-with-depth source term R(z) feeding both pathways.
- Sediment temperature — Q10 ≈ 2.5 scales R(z) uniformly with depth.
- Tidal flooding — sets how deep the oxic (aerobic, non-reactive) skin extends: 5 cm flooded vs. 40 cm drained, directly shrinking the anaerobic reaction zone rather than just multiplying a single total.
Verified numerically: in the well-mixed limit (very high D_SO4) the depth-resolved f_CH4 converges on the 3D model's bulk closed-form value: a check run at salinity=20 gave 0.0589 depth-resolved vs. 0.0588 bulk. At quasi-steady state, total CH₄ production and total CH₄ outflow (ebullition + diffusive escape) converge to a 1:1 ratio, confirming mass conservation.