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Kinetics of organic carbon mineralization and methane formation in marine sediments (Aarhus Bay, Denmark)
Dale, A.W.; Flury, S.; Fossing, H.; Regnier, P.; Røy, H.; Scholze, C.; Jørgensen, B.B. (2019). Kinetics of organic carbon mineralization and methane formation in marine sediments (Aarhus Bay, Denmark). Geochim. Cosmochim. Acta 252: 159-178. https://hdl.handle.net/10.1016/j.gca.2019.02.033
In: Geochimica et Cosmochimica Acta. Elsevier: Oxford,New York etc.. ISSN 0016-7037; e-ISSN 1872-9533, more
Peer reviewed article  

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Keyword
    Marine
Author keywords
    Marine; Seabed; Gas accumulation; Methanogenesis; Sulfate reduction;Organic matter mineralization kinetics; Bioirrigation; Model

Authors  Top 
  • Dale, A.W.
  • Flury, S.
  • Fossing, H.
  • Regnier, P., more
  • Røy, H.
  • Scholze, C.
  • Jørgensen, B.B.

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 (<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 SO42− transport by bioirrigation. Field data (SO42–, TCO2, T13CO2, 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 carbonmineralization 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 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.

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