Controls on subsurface methane fluxes and shallow gas formation in Baltic Sea sediment (Aarhus Bay, Denmark)

Sabine Flury, Hans Roy, Andrew W. Dale, Henrik Fossing, Zsuzsanna Tóth, Volkhard Spiess, Jørn Bo Jensen, Bo Barker Jørgensen

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37 Citations (Scopus)

Abstract

Shallow gas accumulates in coastal marine sediments when the burial rate of reactive organic matter beneath the sulfate zone is sufficiently high and the methanogenic zone is sufficiently deep. We investigated the controls on methane production and free methane gas accumulation along a 400 m seismo-acoustic transect across a sharp transition from gas-free into gas-bearing sediment in Aarhus Bay (Denmark). Twelve gravity cores were taken, in which the pore water was analyzed for inorganic solutes while rates of organic carbon mineralization were measured experimentally by 35SO 4 2- radiotracer method. The thickness of organic-rich Holocene mud increased from 5 to 10 m along the transect concomitant with a shallowing of the depth of the sulfate-methane transition from >4 m to 2.5 m. In spite of drastic differences in the distribution of methane and sulfate in the sediment along the transect, there were only small differences in total mineralization, and methanogenesis was only equivalent to about 1% of sulfate reduction. Shallow gas appeared where the mud thickness exceeded 8-9 m. Rates of methanogenesis increased along the transect as did the upward diffusive flux of methane. Interestingly, the increase in the sedimentation rate and Holocene mud thickness had only a modest direct effect on methanogenesis rates in deep sediments. This increase in methane flux, however, triggered a shallowing of the sulfate-methane transition which resulted in a large increase in methanogenesis at the top of the methanogenic zone. Thus, our results demonstrate a positive feedback mechanism that causes a strong enhancement of methanogenesis and explains the apparently abrupt appearance of gas when a threshold thickness of organic-rich mud is exceeded.

Original languageEnglish
Pages (from-to)297-309
Number of pages13
JournalGeochimica et Cosmochimica Acta
Volume188
DOIs
Publication statusPublished - 1 Sep 2016

Keywords

  • S experiment
  • AOM
  • Diffusion
  • Gas accumulation
  • Marine
  • Methanogenesis
  • Organic matter mineralization
  • Seabed
  • Seismo-acoustic transect
  • Sulfate flux
  • Sulfate reduction rate

Programme Area

  • Programme Area 5: Nature and Climate

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