OFF LIMITS: SULFATE GENERATION BELOW THE SULFATE-METHANE TRANSITION

One of the most intriguing recent discoveries in biogeochemistry is the ubiquity of cryptic sulfur cycling – from subglacial lakes over marine oxygen minimum zones to marine sediments. Cryptic sulfur cycling, the simultaneous sulfate consumption and production, is fascinating because it does not leave an imprint in the sulfur budget of the ambient environment – thus the term cryptic – but may have a massive impact on other element cycles and may fundamentally change our understanding of biogeochemical processes in the subsurface.

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.