Factors controlling the carbon isotope composition of dissolved inorganic carbon and methane in marine porewater: an evaluation by reaction-transport modelling

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  • Patrick Meister, University of Vienna
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  • Bo Liu, Geochemistry & Isotope Biogeochemistry Group, Leibniz Institute for Baltic Sea Research Warnemünde
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  • Arzhang Khalili, Jacobs University Bremen, Max Planck Institute for Marine Microbiology
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  • Michael E. Böttcher, Leibniz Institute for Baltic Sea Research Warnemünde, Ernst-Moritz-Arndt-Universitat Greifswald, Faculty of Agricultural and Environmental Sciences
  • ,
  • Bo Barker Jørgensen

Carbon isotope compositions of dissolved inorganic carbon (DIC) and methane (CH4) in porewater of marine sediments at seafloor temperatures show very large variation covering a δ13C range from −100‰ to +35‰. These extreme values are the result of isotope fractionation during microbial carbon metabolism, but the combined effect of all factors controlling the isotope distributions is still not completely understood. We used a model approach to evaluate the effects of reaction and transport on carbon isotope distributions in modern sediment porewater under steady state. Simulated δ13CDIC profiles typically show negative values in the sulphate reduction zone and more positive values in the methanogenic zone. With increasing depth in the methanogenic zone, δ13C values approach a distribution where the offset of δ13CDIC from δ13C of total organic carbon (TOC) to more positive values is similar to the offset of δ13CCH4 to more negative values (δ13CDIC and δ13CCH4 approach a symmetric distribution relative to δ13CTOC). The model never exceeds this symmetry of the DIC-CH4 couple towards more positive values under steady-state conditions in a purely diffusive system. Our model shows that to reach an offset in δ13C between DIC and CH4 in the order of 70‰, as frequently observed in methanogenic zones, a larger fractionation than reported from culture experiments with acetoclastic or autotrophic methanogens would be required. In fact, the observed isotope offset in natural systems would be consistent with the known inorganic equilibrium fractionation factor at in-situ temperature, which may suggest isotope exchange via a microbial pathway, during methanogenesis. Furthermore, the model reproduces strongly negative δ13CCH4 values at the sulphate methane transition (SMT) as result of a reverse flux of carbon from DIC to CH4 during AOM. Such a reverse AOM has no influence on the δ13CDIC at the SMT as methane is almost completely consumed. Only at high sedimentation rate combined with low porosity, δ13CDIC values significantly more negative than δ13CTOC occur at the SMT.

Original languageEnglish
Article number103227
JournalJournal of Marine Systems
Volume200
ISSN0924-7963
DOIs
Publication statusPublished - Dec 2019

    Research areas

  • Anaerobic methane oxidation, Carbon isotopes, Marine porewater, Methane, Methanogenesis, Reaction-transport model

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