Groundwater‐Driven Methane Export Reduces Salt Marsh Blue Carbon Potential

Abstract

AbstractThe 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 CH4 export exist. Here we report that salt marshes also export dissolved CH4 via submarine groundwater discharge (SGD). The volumetric fluxes of salt marsh groundwater into adjacent tidal creeks were calculated from mass balances of the conservative tracer 226Ra 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 CH4 fluxes of 27–1,200 μmol CH4 m−2 day−1 were calculated by multiplying the volumetric groundwater flux by the groundwater CH4 concentration and normalizing to the intertidal salt marsh area estimated from satellite images. On a mass basis, the cross‐site range in CH4 fluxes was 1.3–5.5 g CH4 m−2 year−1 with 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−1 assuming that CH4 is 45 times more potent than CO2 as 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−1 average calculated across other studies of the direct marsh soil to atmosphere CH4 flux. Therefore, SGD drives an effective doubling of salt marsh CH4 export that offsets a combined total of ~30% of the global cooling potential derived from soil carbon sequestration.