Sulfate-methane transition zone

The sulfate-methane transition zone (SMTZ) is a zone in oceans, lakes, and rivers typically found below the sediment surface in which sulfate and methane coexist. The formation of a SMTZ is driven by the diffusion of sulfate down the sediment column and the diffusion of methane up the sediments.

Source: Wikipedia — Sulfate-methane transition zone (CC BY-SA 4.0)