Kinetics of organic carbon mineralization and methane formation in marine sediments (Aarhus Bay, Denmark)

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 CH4 dynamics and the rate and depth distribution of organic matter degradation. High-resolution sulfate reduction rates determined by tracer experiments (S-35-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)) = 10(6.18) x Age(-2.17). The rate data were used to independently quantify enhanced SO42- transport by bioirrigation. Field data (SO42-, TCO2, (TCO2)-C-13, NH4+ and CH4 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 similar to 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 S-35-measured SRR data indicated active sulfate reduction throughout the SO42- zone whereas quasi-linear SO42- gradients over the same zone indicated insignificant sulfate reduction. This apparent inconsistency, observed at all stations, was reconciled by considering the transport of SO42- into the sediment by bioirrigation, which accounted for 94 +/- 2% of the total SO42- flux across the sediment-water interface. The SRR determined from the quasi-linear SO42- gradients were two orders of magnitude lower than measured rates. We conclude that models solely based on SO42- concentration gradients will not capture high SRRs at the top of the sulfate reduction zone if they do not properly account for (i) SO42- influx by bioirrigation, and/or (ii) the continuity of organic matter reactivity with sediment depth or age. (C) 2019 Elsevier Ltd. All rights reserved.