Sulfur and oxygen isotope fractionation during sulfate reduction coupled to anaerobic oxidation of methane is dependent on methane concentration

Deusner, Christian, Holler, Thomas, Arnold, Gail L., Bernasconi, Stefano M., Formolo, Michael J. and Brunner, Benjamin (2014) Sulfur and oxygen isotope fractionation during sulfate reduction coupled to anaerobic oxidation of methane is dependent on methane concentration Earth and Planetary Science Letters, 399 . pp. 61-73. DOI 10.1016/j.epsl.2014.04.047.

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Supplementary data:

Abstract

Highlights

• Naturally enriched AOM biomass was studied in high-pressure continuous incubation.
• We report the first S- and O-isotope fractionation values by sulfate reduction coupled to AOM from culture studies.
• There is a tight link between methane concentration and S- and O-isotope fractionation.
• S- and O-isotope fractionation values indicate reversibility of energy limited microbial processes.
• The wide range of environmental S- and O-isotope signatures can be explained.

Abstract

Isotope signatures of sulfur compounds are key tools for studying sulfur cycling in the modern environment and throughout earth's history. However, for meaningful interpretations, the isotope effects of the processes involved must be known. Sulfate reduction coupled to the anaerobic oxidation of methane (AOM-SR) plays a pivotal role in sedimentary sulfur cycling and is the main process responsible for the consumption of methane in marine sediments − thereby efficiently limiting the escape of this potent greenhouse gas from the seabed to the overlying water column and atmosphere. In contrast to classical dissimilatory sulfate reduction (DSR), where sulfur and oxygen isotope effects have been measured in culture studies and a wide range of isotope effects has been observed, the sulfur and oxygen isotope effects by AOM-SR are unknown. This gap in knowledge severely hampers the interpretation of sulfur cycling in methane-bearing sediments, especially because, unlike DSR which is carried out by a single organism, AOM-SR is presumably catalyzed by consortia of archaea and bacteria that both contribute to the reduction of sulfate to sulfide.

We studied sulfur and oxygen isotope effects by AOM-SR at various aqueous methane concentrations from 1.4±0.6 mM1.4±0.6 mM up to 58.8±10.5 mM58.8±10.5 mM in continuous incubation at steady state. Changes in the concentration of methane induced strong changes in sulfur isotope enrichment (View the MathML sourceεS34) and oxygen isotope exchange between water and sulfate relative to sulfate reduction (θOθO), as well as sulfate reduction rates (SRR). Smallest View the MathML sourceεS34 (21.9±1.9‰21.9±1.9‰) and θOθO (0.5±0.20.5±0.2) as well as highest SRR were observed for the highest methane concentration, whereas highest View the MathML sourceεS34 (67.3±26.1‰67.3±26.1‰) and θOθO (2.5±1.52.5±1.5) and lowest SRR were reached at low methane concentration. Our results show that View the MathML sourceεS34, θOθO and SRR during AOM-SR are very sensitive to methane concentration and thus also correlate with energy yield. In sulfate–methane transition zones, AOM-SR is likely to induce very large sulfur isotope fractionation between sulfate and sulfide (i.e. >60‰>60‰) and will drive the oxygen isotope composition of sulfate towards the sulfate–water oxygen isotope equilibrium value. Sulfur isotope fractionation by AOM-SR at gas seeps, where methane fluxes are high, will be much smaller (i.e. 20 to 40‰).

Document Type: Article
Keywords: anaerobicoxidationofmethane(AOM) ; sulfatereduction ; sulfurdisproportionation ; sulfurisotopefractionation ; oxygenisotopefractionation ; thermodynamicsandreversibility
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-MG Marine Geosystems
Refereed: Yes
DOI etc.: 10.1016/j.epsl.2014.04.047
ISSN: 0012-821X
Projects: GEOTECHNOLOGIEN, MUMM II
Expeditions/Models:
Date Deposited: 05 Dec 2014 09:48
Last Modified: 06 Mar 2017 13:44
URI: http://eprints.uni-kiel.de/id/eprint/26249

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