Baltic Gas

Methane is produced in vast quantities in marine sediments at several meters sub-surface depth, below the sulphate zone. Most of the methane is continuously degraded sub-surface by the process of anaerobic methane oxidation (AOM). The methane is oxidized to carbon dioxide by microorganisms, probably by a metabolic co-operation between archaea and bacteria.

Anaerobic methane oxidation is mostly confined to the lower sulphate zone – the depth to which seawater sulphate penetrates down into the sediment. Here methane and sulphate coexist at low concentrations across a distinct depth interval where methane is consumed by microorganisms which couple the process to a dissimilatory sulphate reduction to hydrogen sulphide. The sulphate-methane transition (SMT) constitutes a, generally very effective, barrier against methane escape from deep sediment strata. On a global scale, more than 90% of all methane produced in marine sediments is retained at this methane barrier and thus does not reach the sea floor.
 

In spite of its effectiveness, anaerobic oxidation of methane is surprisingly sluggish compared to other microbiologically catalyzed processes in marine sediment and it allows a broad coexistence of sulphate and methane at turnover times of months to years. Only under special conditions, for example by advective pore water flow or through gas ebullition by excessive methane accumulation, does physical transport effectively enable the methane to escape up through the methane barrier.

The methane oxidation in the sulphate-methane transition is a key process among global biosphere-geosphere interactions as the methane flux integrates the organic carbon degradation deeper in the seabed. As methane is an end-product of organic matter degradation below the sulphate zone, and as the methane accumulation in the sediment constitutes only a small fraction of the entire methane production over time, the major part of methane diffuses upward until it reaches the methane barrier at the lower sulphate boundary.