Research ArticleCLIMATOLOGY

Limited contribution of ancient methane to surface waters of the U.S. Beaufort Sea shelf

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Science Advances  17 Jan 2018:
Vol. 4, no. 1, eaao4842
DOI: 10.1126/sciadv.aao4842
  • Fig. 1 Surface water 14C-CH4 data and potential CH4 endmembers in the U.S. Beaufort Sea shelf study area.

    (A) Station map showing both the 14C-CH4 data in units of percent Modern Carbon (pMC), with the atmosphere in 1950 defined as 100 pMC (33, 34), as well as the calculated fraction of ancient C–sourced CH4 (fs) (Eqs. 1 to 5) in surface waters at each station. The white curve is the bulk sediment velocity contour (2000 m/s) used to delineate the seaward boundary of the sedimentary section that contains substantial (up to 29%) ice-bearing permafrost in the upper ~600 m (21). White circles and triangles respectively show boreholes (hundreds of meters deep) and geotechnical borings (<100 m) that contain permafrost based on an analysis of well logs and recovery of permafrost samples, respectively (22). Black circles and triangles respectively indicate no permafrost inferred or found in deep boreholes and geotechnical borings (22). (B) System schematic showing 14C values of dissolved CH4 (stations 5 to 8) and possible ancient and modern endmembers that were also measured here. SGD, submarine groundwater discharge; OC, organic carbon; DOC, dissolved organic carbon.

  • Fig. 2 14C-CH4 data from each station and Keeling plot analysis.

    (A) Dissolved 14C-CH4 data for stations 1 to 8, plotted by the water depth of the station. The data include lagoon samples (×), surface samples (white circles), and near-seafloor samples (black circles). Error bars that are not visible are smaller than the markers. Uncertainty for 14C-CH4 data incorporates the collection, preparation, and measurement uncertainties (20). (B) A Keeling plot (Eq. 3) incorporating [CH4] and 14C-CH4 measurements from stations 1 to 8 suggests that the system can be viewed as a pseudo–two-component mixture and that the 14C-CH4 source signature (14Cs) likely ranges from 0 to 5.62 pMC.

  • Table 1 Calculated fractions of ancient and modern C–sourced CH4 in each sample.
    StationWater depth
    (m)
    Distance
    offshore (km)
    Sample typeAncient C–sourced
    CH4 fraction, fs
    Atmospheric-sourced
    CH4 fraction, fa
    In situ produced
    CH4 fraction, fi
    123Lagoon0.18 ± 0.060.47 ± 0.180.35 ± 0.25
    232Lagoon0.50 ± 0.040.23 ± 0.120.27 ± 0.17
    31412Surface0.26 ± 0.060.37 ± 0.180.37 ± 0.24
    Near-seafloor0.60 ± 0.040.18 ± 0.100.22 ± 0.13
    41510Surface0.39 ± 0.050.29 ± 0.150.33 ± 0.20
    Near-seafloor0.45 ± 0.050.25 ± 0.140.30 ± 0.18
    51318Surface0.42 ± 0.050.27 ± 0.140.31 ± 0.19
    Near-seafloor0.58 ± 0.040.19 ± 0.100.23 ± 0.14
    61927Surface0.53 ± 0.040.21 ± 0.120.26 ± 0.16
    Near-seafloor0.83 ± 0.020.07 ± 0.040.10 ± 0.06
    72848Surface0.10 ± 0.030.72 ± 0.100.18 ± 0.13
    Near-seafloor0.86 ± 0.020.06 ± 0.040.08 ± 0.05
    83869Surface0.07 ± 0.030.79 ± 0.070.14 ± 0.10
    Near-seafloor0.61 ± 0.030.17 ± 0.100.22 ± 0.13

Supplementary Materials

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Dissolved CH4 concentration and isotopic data plotted by station depth.
    • table S1. Dissolved 14C-CH4, δ13C-CH4, and CH4 data with relevant sample information.

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    Correction (7 May 2018): Through no fault of the authors, table S1 in the Supplementary Materials was wrongly replaced with a duplicate of the main text's Table 1 due to a typesetting error. The Supplementary Materials were corrected with the intended table S1 in the PDF version of the paper.

    The original version is accessible here.

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