Control of sulphate and methane distributions in marine sediments by organic matter reactivity

The reactivity of organic matter serving as fuel for microbial activity in buried sedimentary layers imposes a strong control on the subsurface biogeochemical zonation and rates of metabolism. To understand the consequences of different organic matter degradation rates on subsurface microbial activity and sediment pore water chemistry, sulphate and methane concentration profiles were simulated using different organic matter decay models from the literature. Results show how the decay of a more reactive pool of organic matter results in a more curved sulphate profile and deeper sulphate methane transition (SMT) with decreasing apparent initial age of the organic matter. A minimum in SMT is reached if the initial age is intermediate, commonly in the order of 104–105 years. If the initial age is very large the sulphate profile is almost linear and the sulphate methane transition is located deeper with increasing apparent initial age of the organic matter. Curved versus linear shaped sulphate profiles reflect different relative contributions of organoclastic sulphate reduction versus anaerobic methane oxidation to the total consumption of sulphate. Our model results demonstrate that the SMT can move up or down depending on organic matter reactivity and its depth is not necessarily diagnostic of total organic carbon flux. The model provides a mechanistic understanding of how the shape of the sulphate profiles and the relative contributions of organoclastic sulphate reduction versus anaerobic oxidation of methane are controlled by the kinetics of organic matter degradation.