Abstract
Naturally occurring gas hydrates may contribute to a positive feedback for global warming because they sequester large amounts of the potent greenhouse gas methane in ice-like deposits that could be destabilized by increasing ocean/atmospheric temperatures. Most hydrates occur within marine sediments; gas liberated during the decomposition of seafloor hydrates or originating with other methane pools can feed methane emissions at cold seeps. Regardless of the origin of seep methane, all previous measurements of methane emitted from seeps have shown it to have a unique fossil radiocarbon signature, contrasting with other sources of marine methane. Here we present the concentration and natural radiocarbon content of methane dissolved in the water column from the seafloor to the sea surface at seep fields along the US Atlantic and Pacific margins. For shallower water columns, where the seafloor is not within the hydrate stability zone, we do document seep CH4 in some surface-water samples. However, measurements in deeper water columns along the US Atlantic margin reveal no evidence of seep CH4 reaching surface waters when the water-column depth is greater than 430 ± 90 m. Gas hydrates exist only at water depths greater than ~550 m in this region, suggesting that the source of methane escaping to the atmosphere is not from hydrate decomposition.
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Data availability
All data in this manuscript are available to the scientific community through the BCO-DMO database56 and through other releases15,57. Source data are provided with this paper.
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Acknowledgements
D.J.J. thanks G. Kim and the Research Institute of Oceanography, Seoul National University, for the financial support. This work was possible due to the outstanding technical support from the officers and crew members of the RV Hugh R. Sharp and the RV Rachel Carlson. This research is funded to University of Rochester by the US National Science Foundation (OCE-1851402) and US Department of Energy (DOE) through DE-FE0028980. The USGS had funding from USGS–DOE interagency agreements DE-FE0026195 and 89243320SFE000013. Any use of trade, firm or product name is for descriptive purposes only and does not imply endorsement by the US government.
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D.J.J., C.R., T.S.W. and J.D.K. designed the study, and J.S. measured 14C–CH4 using the accelerator mass spectrometry (AMS). D.J.J. and J.D.K collected and prepared samples in the fields and laboratory. All authors equally contributed to the interpretation of the data and writing of this manuscript.
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Nature Geoscience thanks Giuseppe Etiope, Jeanine Ash and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Xujia Jiang, in collaboration with the Nature Geoscience team.
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Extended data
Extended Data Fig. 2 Measurement data of 14C–CH4 for US-Atlantic margin along with mixing model results.
Plots of (a) vertical profiles of 14C–CH4 and its relationship with depth from the US Atlantic margin, (b) the measured data (blue dots) and results from the two-endmember mixing model (red line)14, and (c) the comparison between the measured and mixing modeled 14C–CH4. For the regression in (a), two data points, 570 pMC and 325 pMC from T3S3 surface and T6S1 50 m, respectively, were excluded due to the potential impact from local anthropogenic contamination from nuclear power generation. For figure (b), values in T1S2 (458 nM and 0.1 pMC) and T6S1 (3.7 nM, 123.4 pMC) were used for the bottom and surface endmembers, respectively.
Extended Data Fig. 3 Hydroacoustic observations along the US-Pacific margin.
Screen images of seafloor seeps detected on the US-Pacific margin using a hydroacoustic sensor (EK80) during the research cruise.
Extended Data Fig. 4 Surface distributions of water properties.
Surface contour plots of (a) temperature, (b) salinity, (c) dissolved oxygen, (d) CH4 concentrations, (e) Chlorophyll a, and (f) nitrate concentrations (measured via Seabird Scientific, SUNA V2) in the Pacific margin. Colored dot represents 14C–CH4, and the color scale and station ID are shown in (b).
Extended Data Fig. 5 Procedures for sample collection and preparation for measurement.
Schematic diagrams of (a) gas extraction in the field and (c) gas purification in the laboratory. Pictures of equipment (b) in the field and (d) in the laboratory are also shown. Schematic diagrams (a and c) and photograph (d) were accessed from ref. 53.
Extended Data Fig. 6 Examples of gas standards and blanks subjected to the laboratory preparation procedures.
Example diagrams for the gas standard tests monitoring the performance of the laboratory gas-purification system; High and low standards, which were customized based on the concentrations in the collected samples, were measured throughout this study, the results of which are shown in (a) and (b), respectively. Total carbon blanks (c) for the purification system were also monitored daily when samples were run. Subplots in (a) and (b) show the trapping of CO2 converted from CH4. Plots (a) and (b) were reproduced from ref. 29.
Extended Data Fig. 7 Vertical profiles of ancillary data.
Top panel shows (a) temperature, (b) salinity, and (c) dissolved oxygen in Pacific margin sites. Bottom panel represents (d) temperature, (e) salinity and (f) dissolved oxygen in Atlantic margin sites.
Supplementary information
Supplementary Information
Additional discussion about the distributions of CH4 concentration and the fate of the released CH4.
Supplementary Table 1
Data from Pacific margin
Supplementary Table 2
Data from Atlantic margin
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Joung, D., Ruppel, C., Southon, J. et al. Negligible atmospheric release of methane from decomposing hydrates in mid-latitude oceans. Nat. Geosci. 15, 885–891 (2022). https://doi.org/10.1038/s41561-022-01044-8
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DOI: https://doi.org/10.1038/s41561-022-01044-8
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