Role of sulfate reduction and methane production by organic carbon degradation in eutrophic fjord sediments (Limfjorden, Denmark)

Abstract

The anaerobic mineralization of buried organic matter through sulfate reduction and methanogenesis was studied in 2‐m‐long piston cores of organic‐rich, silty‐clay sediment from two sites in Limfjorden, Denmark. An extended sulfate‐methane transition (SMT) zone was found at 1–1.5‐m sediment depth, accompanied by peaks in sulfide (4–6 mmol L−1) and high dissolved inorganic carbon (30–50 mmol L−1). Pore‐water acetate concentrations were 2‐10 µmol L‐1. 14C‐acetate was oxidized to 14CO2 in the sulfate zone and reduced to 14CH4 at and below the SMT. CO2 reduction was the predominant pathway of methanogenesis below the sulfate zone. Sulfate reduction rates (SRRs) determined by 35S tracer experiments decreased with depth throughout the sulfate zone from > 100 nmol cm−3 d−1 near the sediment surface to < 1 nmol cm−3 d−1 at depth. The depth trend of rates was equally well described by a power law function and a reactive continuum model. Only a small percentage of the total 35S‐SRR was due to anaerobic oxidation of methane. In contrast, diffusion‐reaction modeling of pore‐water sulfate indicated that sulfate reduction was insignificant throughout the sulfate zone and that most of the activity occurred at the SMT. A comparison of the burial flux of organic carbon below the sulfate zone and the returning flux of methane indicated that the diffusion modeling of pore‐water sulfate strongly underestimated in situ SRRs, whereas the 35S data may have overestimated the rates at depth. Modeled and measured SRR could be reconciled provided that bioirrigation reached to at least 20–40‐cm sediment depth, which appears realistic given the predominant burrowing polychaete fauna.