Aarhus University Seal / Aarhus Universitets segl

Bo Barker Jørgensen

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

Research output: Contribution to journal/Conference contribution in journal/Contribution to newspaperJournal articleResearchpeer-review

  • A. W. Dale, Helmholtz-Zentrum für Ozeanforschung Kiel (GEOMAR)
  • ,
  • S. Flury
  • ,
  • H. Fossing
  • ,
  • P. Regnier, Université Libre de Bruxelles
  • ,
  • H. Røy
  • C. Scholze
  • ,
  • B. B. Jørgensen

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 ( 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 SO 4 2− transport by bioirrigation. Field data (SO 4 2– , TCO 2 , T 13 CO 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 35 S-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.

Original languageEnglish
JournalGeochimica et Cosmochimica Acta
Pages (from-to)159-178
Number of pages20
Publication statusPublished - May 2019

    Research areas

  • Bioirrigation, Gas accumulation, Marine, Methanogenesis, Model, Organic matter mineralization kinetics, Seabed, Sulfate reduction

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