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  1. Abstract

    The burial of “blue carbon” in coastal marsh soils is partially offset by marsh‐atmosphere methane (CH4) fluxes, but this offset may be greater if other pathways of CH4export exist. Here we report that salt marshes also export dissolved CH4via submarine groundwater discharge (SGD). The volumetric fluxes of salt marsh groundwater into adjacent tidal creeks were calculated from mass balances of the conservative tracer226Ra at four study sites in coastal Georgia, USA. Over the 2‐year study period, volumetric groundwater fluxes across all sites ranged between 1,700 and 105,000 m3 day−1. Dissolved CH4fluxes of 27–1,200 μmol CH4m−2 day−1were calculated by multiplying the volumetric groundwater flux by the groundwater CH4concentration and normalizing to the intertidal salt marsh area estimated from satellite images. On a mass basis, the cross‐site range in CH4fluxes was 1.3–5.5 g CH4 m−2 year−1with a cross‐site mean of 2.8 g CH4 m−2 year−1. This is equivalent to 125 (56–245) g CO2 m−2 year−1assuming that CH4is 45 times more potent than CO2as a greenhouse gas over a 100‐year time frame. This sustained‐flux global warming potential is similar to the 138 (1.1–260) g CO2 m−2 year−1average calculated across other studies of the direct marsh soil to atmosphere CH4flux. Therefore, SGD drives an effective doubling of salt marsh CH4export that offsets a combined total of ~30% of the global cooling potential derived from soil carbon sequestration.

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  2. Abstract

    Anaerobic oxidation of methane (AOM), a central process in the carbon cycle of anoxic environments, moderates the release of methane from soils and sediments to water bodies and, ultimately, the atmosphere. The regulation of AOM in the environment remains poorly constrained. Here we quantified AOM and sulfate reduction (SR) rates in diverse deep seafloor samples at in situ pressure and methane concentration and discovered that, in some cases, AOM exceeded SR rates by more than four times when methane concentrations were above 5 mM. Methane concentration also affected other carbon‐cycling processes (e.g., carbon assimilation) in addition to SR. These results illustrate that substantial amounts of methane may be oxidized independent of SR under in situ conditions, reshaping our view of the capacity and mechanism of AOM in methane‐rich environments, including the deep biosphere, where sulfate availability is considered to limit AOM.

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