Kinetics of organic carbon mineralization and methane formation in marine sediments (Aarhus Bay, Denmark). (1st May 2019)

• Record Type: Journal Article
• Authors: Dale, A.W.; Flury, S.; Fossing, H.; Regnier, P.; Røy, H.; Scholze, C.; Jørgensen, B.B.
• Is Part Of: Geochimica et cosmochimica acta. Volume 252(2019)
• Page Start: 159
• Page End: 178
• Publication Date: 2019-05-01
• Subjects: Marine -- Seabed -- Gas accumulation -- Methanogenesis -- Sulfate reduction -- Organic matter mineralization kinetics -- Bioirrigation -- Model; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2019.02.033
• Languages: English
• ISSNs: 0016-7037

Abstract

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 CH4 dynamics and the rate and depth distribution of organic matter degradation. High-resolution sulfate reduction rates determined by tracer experiments ( 35 S-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 × Age −2.17 . The rate data were used to independently quantify enhanced SO4 2− transport by bioirrigation. Field data (SO4 2–, TCO2, T 13 CO2, 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 ∼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 35 S-measured SRR data indicated active sulfate reduction throughout the SO4 2− zone whereas quasi-linear SO4 2− gradients over the same zone indicated insignificant sulfate reduction. This apparent inconsistency, observed at all stations, was reconciled by considering the transport of SO4 2− into the sediment by bioirrigation, which accounted for 94 ± 2% of the total SO4 2− flux across the sediment-water interface. The SRR determined from the quasi-linear SO4 2− gradients were two orders of magnitude lower than measured rates. We conclude that models solely based on SO4 2− concentration gradients will not capture high SRRs at the top of the sulfate reduction zone if they do not properly account for (i) SO4 2− influx by bioirrigation, and/or (ii) the continuity of organic matter reactivity with sediment depth or age.