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 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.

中文翻译：

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地下水推动的甲烷出口减少了盐沼蓝碳的潜力

沿海沼泽土壤中埋藏的“蓝色碳”被沼泽大气中的甲烷（CH ₄）通量部分抵消，但如果存在其他CH ₄出口途径，则这种抵消可能更大。在这里，我们报道盐沼也通过海底地下水排放（SGD）出口溶解的CH ₄。根据美国佐治亚州沿海地区四个研究点的保守示踪剂²²⁶ Ra的质量平衡，计算出盐沼地下水流入相邻潮汐小溪的体积通量。在为期2年的研究期内，所有站点的地下水通量在1700至105,000 m ³  day ^-1之间。溶解的CH ₄通量为27–1,200μmolCH ₄ m通过将地下水体积通量乘以地下水CH ₄浓度，并归一化为根据卫星图像估算的潮间带盐沼面积，可以计算出^-2 天^-1。以质量计，CH ₄通量的跨站点范围为1.3–5.5 g CH ₄  m ^-2  year ^-1，跨站点平均值为2.8 g CH ₄  m ^-2  year ^-1。假设CH _4的效力是CO _2的45倍，则相当于125（56-245）g CO ₂  m - ² 年^-1作为100年期限内的温室气体。这种持续通量的全球变暖潜势类似于在其他直接沼泽土壤至大气中CH ₄通量研究中计算出的138（1.1–260）g CO ₂  m - ² 年- ^1年平均值。因此，SGD可将盐沼CH _4的出口量有效增加一倍，从而抵消了土壤固碳带来的全球总冷却潜力的约30％。