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Methane hydrate-bearing seeps as a source of aged dissolved organic carbon to the oceans

Abstract

Marine sediments contain about 500–10,000 Gt of methane carbon1,2,3, primarily in gas hydrate. This reservoir is comparable in size to the amount of organic carbon in land biota, terrestrial soils, the atmosphere and sea water combined1,4, but it releases relatively little methane to the ocean and atmosphere5. Sedimentary microbes convert most of the dissolved methane to carbon dioxide6,7. Here we show that a significant additional product associated with microbial methane consumption is methane-derived dissolved organic carbon. We use Δ14C and δ13C measurements and isotopic mass-balance calculations to evaluate the contribution of methane-derived carbon to seawater dissolved organic carbon overlying gas hydrate-bearing seeps in the northeastern Pacific Ocean. We show that carbon derived from fossil methane accounts for up to 28% of the dissolved organic carbon. This methane-derived material is much older, and more depleted in 13C, than background dissolved organic carbon. We suggest that fossil methane-derived carbon may contribute significantly to the estimated 4,000–6,000 year age of dissolved organic carbon in the deep ocean8, and provide reduced organic matter and energy to deep-ocean microbial communities.

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Figure 1: Radiocarbon (14C) and 13C properties of DOC plotted versus DOC concentration from deep sea water overlying northern Cascadia margin seeps.
Figure 2: Sediment depth profiles from Barkley Canyon showing the AOM zone and associated production of DOC.
Figure 3: Carbon contributions to deep-seawater DOC at the Barkley Canyon seep, determined by three mass-balance calculations.

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Acknowledgements

We thank the captain and crew of the CCGV J. P. Tully and ROV ROPOS, E. Kessee, B. Green and L. Lapham for assistance with sample collection and analysis, and the staffs of the University of Arizona and National Ocean Science AMS facilities for radiocarbon analyses. J. Seewald, V.B. Heuer and D.L. Valentine are thanked for constructive comments on previous versions of the manuscript. This research was supported by financial support from the Natural Sciences and Engineering Research Council (NSERC) of Canada, the Chemical Oceanography and Integrated Carbon Cycle Research Programs of the National Science Foundation and the USGS Mendenhall Research Postdoctoral Fellowship Program. Any use of trade names is only for descriptive purposes and does not imply endorsement by the US Government.

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J.W.P. led and coordinated the design and execution of the field study, collection and analyses of samples, and writing of the manuscript. J.E.B. and W.F.W. contributed to the study design, sample analyses, development of the mass-balance model and writing of the manuscript. All other authors made essential, substantive contributions to the original and revised versions of the manuscript. In addition, C.L.O. assisted with pore-water DOC δ13C analyses and N.R.C. was Chief Scientist and contributed to the study design and sampling at sea.

Corresponding author

Correspondence to John W. Pohlman.

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The authors declare no competing financial interests.

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Pohlman, J., Bauer, J., Waite, W. et al. Methane hydrate-bearing seeps as a source of aged dissolved organic carbon to the oceans. Nature Geosci 4, 37–41 (2011). https://doi.org/10.1038/ngeo1016

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