Enhanced CO2 uptake at a shallow Arctic Ocean seep field overwhelms the positive warming potential of emitted methane
Saturday, 2017/05/27 | 06:06:27
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John W. Pohlman, Jens Greinert, Carolyn Ruppel, Anna Silyakova, Lisa Vielstädte, Michael Casso, Jürgen Mienert, and Stefan Bünz ENVIRONMENTSignificanceMethane released from the seafloor and transported to the atmosphere has the potential to amplify global warming. At an arctic site characterized by high methane flux from the seafloor, we measured methane and carbon dioxide (CO2) exchange across the sea−air interface. We found that CO2 uptake in an area of elevated methane efflux was enhanced relative to surrounding waters, such that the negative radiative forcing effect (cooling) resulting from CO2 uptake overwhelmed the positive radiative forcing effect (warming) supported by methane output. Our work suggests physical mechanisms (e.g., upwelling) that transport methane to the surface may also transport nutrient-enriched water that supports enhanced primary production and CO2 drawdown. These areas of methane seepage may be net greenhouse gas sinks. AbstractContinued warming of the Arctic Ocean in coming decades is projected to trigger the release of teragrams (1 Tg = 106 tons) of methane from thawing subsea permafrost on shallow continental shelves and dissociation of methane hydrate on upper continental slopes. On the shallow shelves (<100 m water depth), methane released from the seafloor may reach the atmosphere and potentially amplify global warming. On the other hand, biological uptake of carbon dioxide (CO2) has the potential to offset the positive warming potential of emitted methane, a process that has not received detailed consideration for these settings. Continuous sea−air gas flux data collected over a shallow ebullitive methane seep field on the Svalbard margin reveal atmospheric CO2 uptake rates (−33,300 ± 7,900 μmol m−2⋅d−1) twice that of surrounding waters and ∼1,900 times greater than the diffusive sea−air methane efflux (17.3 ± 4.8 μmol m−2⋅d−1). The negative radiative forcing expected from this CO2 uptake is up to 231 times greater than the positive radiative forcing from the methane emissions. Surface water characteristics (e.g., high dissolved oxygen, high pH, and enrichment of 13C in CO2) indicate that upwelling of cold, nutrient-rich water from near the seafloor accompanies methane emissions and stimulates CO2 consumption by photosynthesizing phytoplankton. These findings challenge the widely held perception that areas characterized by shallow-water methane seeps and/or strongly elevated sea−air methane flux always increase the global atmospheric greenhouse gas burden.
See: http://www.pnas.org/content/114/21/5355.abstract.html?etoc PNAS May 23 2017; vol.114; no.21: 5355–5360
Fig. 1. Site map and water column methane offshore of western Svalbard during the CAGE 14-1 cruise. (A) Gas and gas hydrate sectors surveyed for methane flux, CO2 flux, and water chemistry. Red lines are the survey tracklines. (B) Distribution of dissolved methane along transect A−A’ (see A). (C) Hydroacoustic evidence for gas flares and bubbles overlying the main seep area along transect B−B’ (see B). PKF, Prins Karls Forland. |
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