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Abstract Sediments were sampled at nine stations on a transect across a 7–10 m thick Holocene mud layer in Aarhus Bay, Denmark, to investigate the linkages between CH 4 dynamics and the rate and depth distribution of organic matter degradation. High-resolution sulfate reduction rates determined by tracer experiments (35S-SRR) decreased by several orders of magnitude down through the mud layer. The rates showed a power law dependency on sediment age: SRR (nmol cm−3 d−1) = 106.18 × Age−2.17. The rate data were used to independently quantify enhanced SO 4 2− transport by bioirrigation. Field data (SO 4 2–, TCO 2 , T13CO 2 , NH 4 + and CH 4 concentrations) could be simulated with a reaction-transport model using the derived bioirrigation rates and assuming that the power law was continuous into the methanogenic sediments below the sulfate-methane transition zone (SMTZ). The model predicted an increase in anaerobic organic carbon mineralization rates across the transect from 2410 to 3540 nmol C cm−2 d−1 caused by an increase in the sediment accumulation rate. Although methanogenesis accounted for only ∼1% of carbon mineralization, a large relative increase in methanogenesis along the transect led to a considerable shallowing of the SMTZ from 428 to 257 cm. Methane gas bubbles appeared once a threshold in the sedimentation accumulation rate was surpassed. The 35S-measured SRR data indicated active sulfate reduction throughout the SO 4 2− zone whereas quasi-linear SO 4 2− gradients over the same zone indicated insignificant sulfate reduction. This apparent inconsistency, observed at all stations, was reconciled by considering the transport of SO 4 2− into the sediment by bioirrigation, which accounted for 94 ± 2% of the total SO 4 2− flux across the sediment-water interface. The SRR determined from the quasi-linear SO 4 2− gradients were two orders of magnitude lower than measured rates. We conclude that models solely based on SO 4 2− concentration gradients will not capture high SRRs at the top of the sulfate reduction zone if they do not properly account for (i) SO 4 2− influx by bioirrigation, and/or (ii) the continuity of organic matter reactivity with sediment depth or age. [ABSTRACT FROM AUTHOR]