OFF LIMITS: SULFATE GENERATION BELOW THE SULFATE-METHANE TRANSITION
Classically, the sulfate methane transition in marine sediments is considered the boundary that delimits biological sulfur cycling from other metabolic processes. Two sediment cores from Aarhus Bay, Denmark reveal the constant presence of sulfate (generally 0.1 to 0.2 mM, maximum 0.97 mM) below of the sulfate-methane transition (SMT). The sulfur and oxygen isotope signature of this sulfate pool is consistent with sulfate produced by oxidative sulfur cycling. Sulfate generation in a section of the sediment column where sulfate is expected to be absent enables reductive sulfur cycling, creating the conditions under which sulfate respiration can persist in the methanogenic zone.