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Methane leakage through the sulfate–methane transition zone of the Baltic seabed

Nature Geoscience volume 17, pages 1277–1283 (2024)Cite this article

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Abstract

Anaerobic oxidation of methane at the sulfate–methane transition in marine sediments is generally considered to be a near-perfect barrier against methane release from the seabed, but the mechanisms involved are not well understood. On the basis of a survey of Baltic Sea sediments we show that a highly variable amount (0–100%) of subseafloor methane leaks through the sulfate–methane transition. The diffusive methane flux to the sediment–water interface is often high, reaching over 2 mmol m−2 d−1. Even though anaerobic methane oxidation is thermodynamically and kinetically favoured where methane fluxes are high, there is no evidence of methane oxidation in concentration, isotope and modelling results. Cores that lacked anaerobic methane oxidation had high modelled organic matter mineralization rates, suggesting that a possible mechanism could be high electron donor availability due to elevated H2 concentrations, as has been predicted by laboratory studies. We show that methane leakage across the sulfate–methane transition is widespread in organic-rich marine sediments.

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Fig. 1: Acoustic transect across Gotland Basin showing sulfate and methane profiles.
Fig. 2: Methane flux to the SWI calculated from this study and from other published Baltic Sea data.
Fig. 3: AOM efficiency in the Baltic Sea and in other marine sediments around the world.
Fig. 4: Comparison of geochemical and reaction rate profiles.
Fig. 5: Relationship between methane flux to the bottom water and methane flux to the SWI.

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Data availability

Sediment porewater methane concentration, sulfate concentration, methane stable carbon isotope ratios, DIC stable carbon isotope ratios, total organic carbon stable carbon isotope ratios and porosity are freely available in the database PANGAEA, https://doi.org/10.1594/PANGAEA.935085. Water column salinity, temperature and dissolved oxygen profiles are also freely available in PANGAEA, https://doi.org/10.1594/PANGAEA.780460.

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Acknowledgements

We thank the captain and crew of the research vessel RV Maria S. Merian. We also thank J. Pedersen and K. Bomholt Oest for analytical expertise. This work was part of the BALTIC GAS project, funded by the European Community’s Seventh Framework Programme (FP/2007-2013) with the joint Baltic Sea research and development programme, BONUS (grant 217246, B.B.J.). Additional funding came from the Independent Research Fund Denmark (grant 1026-00159B, B.B.J.) and from the US Department of Energy (grant DE-SC0020369, K.G.L.). This is UMCES contribution 6394.

Author information

Author notes

  1. Laura L. Lapham

    Present address: Chesapeake Biological Laboratory, University of Maryland Center for Environmental Science, Solomons, MD, USA

Authors and Affiliations

  1. Department of Biology, Aarhus University, Aarhus, Denmark

    Laura L. Lapham, Henrik Fossing, Sabine Flury & Bo Barker Jørgensen

  2. Microbiology Department, University of Tennessee, Knoxville, TN, USA

    Karen G. Lloyd

  3. Department of Earth Sciences, University of Southern California, Los Angeles, CA, USA

    Karen G. Lloyd

  4. Department of Ecoscience, Aarhus University, Aarhus, Denmark

    Henrik Fossing

  5. Department of Construction and Justice, Office of the Environment, Solothurn, Switzerland

    Sabine Flury

  6. Geological Survey of Denmark and Greenland, Copenhagen, Denmark

    Jørn Bo Jensen

  7. Department of Marine Sciences, University of North Carolina, Chapel Hill, NC, USA

    Marc J. Alperin

  8. Department of Marine Chemistry, Leibniz Institute for Baltic Sea Research Warnemünde, Rostock, Germany

    Gregor Rehder & Wanda Holzhueter

  9. Department of Biogeochemistry, Max Planck Institute for Marine Microbiology, Bremen, Germany

    Timothy Ferdelman

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Contributions

Conceptualization: L.L.L., H.F., J.B.J., B.B.J. Data curation: L.L.L. Methodology: L.L.L., H.F., S.F., J.B.J. Formal analysis: L.L.L., J.B.J., M.J.A., W.H. Investigation: L.L.L., J.B.J., M.J.A., K.G.L., B.B.J. Visualization: L.L.L., B.B.J., J.B.J. Funding acquisition: H.F., T.F., B.B.J. Project administration: H.F., G.R., B.B.J. Supervision: H.F., T.F., B.B.J. Writing—original draft: L.L.L., K.G.L., B.B.J. Writing—review & editing: all authors.

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Correspondence to Laura L. Lapham.

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Extended data

Extended Data Fig. 1 Map of the Baltic Sea region with coring sites.

a) Details of the framed areas are shown in subsequent figures as numbered: Extended data figure (EDF) 3 covers Arkona Basin, Extended Data Fig. 4 covers Bornholm Basin, Extended Data Fig. 5 covers Bothnian Sea, and Extended Data Fig. 6 covers Bothnian Bay. Brown areas represent organic-rich, Holocene mud (Geological Survey of Denmark and Greenland). b) Detail of coring sites in the Gotland Basin showing coring locations across two transects with details shown in Fig. 1 and Extended Data Fig. 2. The bathymetry basemap was acquired from HELCOM (https://helcom.fi/baltic-sea-trends/data-maps/) and is used also in subsequent figures. Credit: basemap, ESRI, GEBCO, NOAA, National Geographic, Garmin, HERE, Geonames.org, and other contributors.

Extended Data Fig. 2 Gotland Basin Northern transect.

a) Acoustic transect and core locations (black arrows). The stratigraphy is same as in Fig. 1 caption. Inset profiles are from Stations 73, 57, and 56 in full CH4 scale. Methane data that may be compromised by degassing are indicated by (+) symbols. b) Sulfate and methane profiles from stations along the transect. Methane profiles are shown with expanded scale. The black dashed lines indicate the SMT depth. Values of RSWI/SMT show how much methane leaks from the SMT to the SWI. In panel for Stations 73 and 56, the faint red line connects the deeper methane concentration which is off scale, but can be seen in panel A inset.

Extended Data Fig. 3 Arkona Basin coring sites.

a) Location map of stations. The island of Rügen is shown for orientation. The brown areas mark organic-rich Holocene mud while the cross-hatched areas mark shallow gas identified by acoustics71. Note that the offset between shallow gas (cross hatch) and mud layers (brown) is likely a spatial resolution error between the techniques. b) Sulfate and methane profiles are shown for the four stations (station labels in bottom right of frames together with the RSWI/SMT values). Methane data are presented with expanded scale of 0-1 mM to demonstrate tailing up through the sulfatic zone. The black dashed lines indicate the SMT depth. Values of RSWI/SMT show how much methane leaks from the SMT to the SWI.

Extended Data Fig. 4 Bornholm Basin coring sites.

a) Location map of stations. The island of Bornholm is shown for orientation. Brown and cross-hatched areas: see Extended Data Fig. 3 legend. Stations 101–103 are not labeled but are located between the transition of a gas site (Station 31) and non-gas site (Station 32) as detailed in23. b) Sulfate and methane profiles from the southeastern Stations (24, 25 from this study and 374190 from Hilligsøe et al. 21). c) Sulfate and methane profiles from northwestern stations. The black dashed lines indicate the SMT depth. Values of RSWI/SMT show how much methane leaks from the SMT to the SWI.

Extended Data Fig. 5 Bothnian Sea coring sites.

a) Location map of stations. Sulfate and methane profiles from b) the northern area, and c) the southern area; d) shows the methane data at Station 78 in full scale. Station 78 is not shown on the map because it falls right under Station 81. The black dashed lines indicate the SMT depth. Methane data that may be compromised by degassing are indicated by (+) symbols. Values of RSWI/SMT show how much methane leaks from the SMT to the SWI.

Extended Data Fig. 6 Bothnian Bay coring sites.

a) Location map of stations. b) Sulfate and methane profiles from all stations. c) Full methane profile from Station 92. The black dashed lines indicate the SMT depth. Methane data that may be compromised by degassing are indicated by (+) symbols. Values of RSWI/SMT show how much methane leaks from the SMT to the SWI.

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Lapham, L.L., Lloyd, K.G., Fossing, H. et al. Methane leakage through the sulfate–methane transition zone of the Baltic seabed. Nat. Geosci. 17, 1277–1283 (2024). https://doi.org/10.1038/s41561-024-01594-z

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