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Exchange catalysis during anaerobic methanotrophy revealed by 12CH2D2 and 13CH3D in methane

J.L. Ash1,

1Department of Earth, Environmental and Planetary Sciences, Rice University, Houston Texas, USA

M. Egger2,#,

2Center for Geomicrobiology, Aarhus University, Aarhus, Denmark
#now at The Ocean Cleanup Foundation, Rotterdam, The Netherlands

T. Treude3,4,

3Department of Earth, Planetary, and Space Sciences, UCLA, Los Angeles CA, USA
4School of Earth and Ocean Sciences, Cardiff University, Cardiff, UK

I. Kohl3,

3Department of Earth, Planetary, and Space Sciences, UCLA, Los Angeles CA, USA

B. Cragg4,

4School of Earth and Ocean Sciences, Cardiff University, Cardiff, UK

R.J. Parkes4,

4School of Earth and Ocean Sciences, Cardiff University, Cardiff, UK

C.P. Slomp5,

5Department of Earth Sciences-Geochemistry, Utrecht University, Utrecht, The Netherlands

B. Sherwood Lollar6,

6Department of Earth Sciences, University of Toronto, Toronto Ontario, Canada

E.D. Young3

3Department of Earth, Planetary, and Space Sciences, UCLA, Los Angeles CA, USA

Affiliations  |  Corresponding Author  |  Cite as  |  Funding information

Ash, J.L., Egger, M., Treude, T., Kohl, I., Cragg, B., Parkes, R.J., Slomp, C.P., Sherwood Lollar, B., Young, E.D. (2019) Exchange catalysis during anaerobic methanotrophy revealed by 12CH2D2 and 13CH3D in methane. Geochem. Persp. Let. 10, 26–30.

The Deep Carbon Observatory

Geochemical Perspectives Letters v10  |  doi: 10.7185/geochemlet.1910
Received 07 May 2018  |  Accepted 22 February 2019  |  Published 15 April 2019
Copyright © The Authors

Published by the European Association of Geochemistry
under Creative Commons License CC BY-NC-ND 4.0




Figure 1 Geochemical profiles (±2 σ) from Bornholm Basin from 3-35 MCD. Horizontal grey bar denotes sediments deposited during lacustrine conditions overlain with sediments deposited during brackish marine conditions. Samples were not recovered above the shallow SMTZ. Pink dashed lines denote a shallow (3.3 MCD) and deep (19.8 MCD) SMTZ; only the shallow SMTZ is shown in the bottom panel for clarity. Dark grey vertical bars in e and f denote equilibrium isotopologue compositions for the subsurface temperature of 7.8 ± 0.6 °C. Modelled CH4 production, Fe-AOM and SO4-AOM rates are shown in panel d (Dijkstra et al., 2018

Dijkstra, N., Hagens, M., Egger, M., Slomp, C.P. (2018) Post-depositional vivianite formation alters sediment phosphorus records. Biogeosciences 15, 861-883.

). CH4 production (purple) and SO4-AOM (gold) rates correspond with the upper x axis and Fe-AOM rates (brown) correspond with the lower x axis. SO4-AOM rates above the upper SMTZ are calculated using bottom water SO4 concentrations of 15.0 mM. Additional porewater data shown in Supplementary Information.
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Figure 2 ξ12CH2D2 values (±2 σ) are the most negative at shallow MCD, indicating the greatest departure from equilibrium and approach zero (violet dashed line) with increasing MCD, indicating an approach towards intra-species thermodynamic equilibrium. ΔG values for SO4-AOM and Fe-AOM suggest either metabolism is capable of reversibility (see Supplementary Information).
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Figure 3 Δ12CH2D2 is plotted versus Δ13CH3D. The solid black line represents theoretical thermodynamic equilibrium abundances (with dots representing 100 °C increments from 0-1000 °C). Methane produced by thermogenesis, high temperature abiotic reactions, microbial methanogenesis and low temperature abiotic reactions inhabit unique zones in double isotopologue space. AOM, equilibrating through exchange catalysis during enzymatic back reaction, also inhibits a unique zone: low temperature intra-species thermodynamic equilibrium. Bornholm Basin data (magenta diamonds) span between the methanogenesis and AOM zones, while data from Kidd Creek Mine (black to white symbols) (Young et al., 2017

Young, E., Kohl, I., Lollar, B.S., Etiope, G., Rumble, D., Li, S., Haghnegahdar, M., Schauble, E., McCain, K., Foustoukos, D. (2017) The relative abundances of resolved 12CH2D2 and 13CH3D and mechanisms controlling isotopic bond ordering in abiotic and biotic methane gases. Geochimica et Cosmochimica Acta 203, 235-264.

) span from the abiotic to AOM zone. See Supplementary Information for additional sample description.
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