Groundwater-Driven Methane Export Reduces Salt Marsh Blue Carbon Potential

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 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 Ra-226 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 m(3) day(-1). Dissolved CH4 fluxes of 27-1,200 mu mol 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 similar to 30% of the global cooling potential derived from soil carbon sequestration. Plain Language Summary Salt marshes are wetland ecosystems found along the coast. Scientists and policymakers are interested in salt marshes because they accumulate carbon in their soil over long time scales. This carbon would otherwise enter the atmosphere as carbon dioxide, exacerbating global climate change. However, microorganisms living in the soil break down some of this buried carbon and produce other greenhouse gases, such as methane, that contribute more to warming than carbon dioxide. Scientists have measured how much methane escapes from the soil to the atmosphere. Carbon burial more than offsets this methane export, meaning that salt marshes have a small net cooling effect on Earth's climate. In this study, we incorporate a previously unaccounted pathway for methane export from salt marshes into their radiative budget. Methane in marsh soil and surrounding sediments can be dissolved in groundwater that discharges from the pore spaces between soil and sediment particles. This groundwater flows slowly toward the marsh edge until it discharges into nearby surface waters, carrying dissolved methane with it. We estimate that this methane export from groundwater approximately doubles total methane emissions from salt marshes. This is still not enough methane export to counteract all of the climate cooling benefit of carbon burial in salt marsh soils. However, it is important for policymakers to include both pathways of methane export when calculating the climate cooling benefit of carbon burial in salt marsh soils or its economic value.