Effect of the diffusive boundary layer on benthic mineralization and O2 distribution: A theoretical model analysis

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On the basis of a dynamic diagenetic model, we evaluate and discuss the effect of the diffusive boundary layer (DBL) on benthic O2 exchange and O2 consuming pathways. The analysis documents that the DBL has only minor importance for the annual O2 uptake of coastal cohesive sediments. Imposing static DBL thicknesses of 300‐900 mm decreased the annual O2 uptake by only 2–10% in comparison to a situation without any DBL. Lower O2 availability as imposed by a thicker DBL, however, markedly reduced the aerobic heterotrophic respiration but enhanced aerobic reoxidation of solutes released by the stimulated anaerobic respiration. The 2–10% decrease in the annual O2 uptake was caused mainly by higher benthic release rates of NH4+ and Mn2+. The overall carbon degradation rate—and thus the carbon preservation—remained unaffected by the DBL thickness. Dynamic modeling revealed that abrupt changes in the DBL thickness caused an instantaneous change in the interstitial O2 distribution and in the benthic O2 uptake rate. However, conditions quickly reversed as the porewater profiles of reduced solutes and distribution of reduced solids readjusted even though full steady state was obtained only after several months. In nature the DBL is constantly changing, and thus in situ O2 microprofiles are transient by nature. Dynamic modeling showed that the benthic O2 concentration and the O2 uptake in Aarhus Bay could vary by 30% or more on time scales of a few hours or days solely because of changes in the DBL thickness. The integrated annual O2 uptake remained unaffected by these fluctuations.