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The residence time of fluids circulating through deep‐sea hydrothermal systems influences the extent of water‐rock reactions and the flux of major and minor elements to the ocean. While the fluid residence times in numerous basaltic and gabbroic systems have been determined, those of the less studied ultramafic systems are currently unknown. Fluids that interact with mantle rocks have fundamentally different chemistries and therefore have unique influences on seawater chemistry. In this first investigation of radium isotopes in a serpentinite‐hosted system, vent fluids were discovered to contain 10–100 times greater activities of²²³Ra (half‐life = 11.4 days) than observed in high‐temperature basalt‐hosted systems. The²²³Ra activities of 10–109 dpm L⁻¹produce²²³Ra/²²⁶Ra activity ratios ranging from 9 to 109. These extremely high²²³Ra activities, which are accompanied by low activities of²²⁶Ra, place significant constraints on fluid residence times and the adsorption coefficient of radium between fluid and rock. Our data constrain the nondimensional retardation factor (R) to very low values between 1 and 4, reflecting the extent to which Ra is transported more slowly than fluids due to adsorption and other processes. These results suggest that the residence time of fluids in contact with serpentinite is less than 2 y and perhaps as low as 0.5 y. They are surprisingly similar to those of basalt‐hosted systems. Thus, fluids in hydrothermal systems share similar hydrogeological characteristics despite differences in rock types, underlying porosity, and heat sources, enabling larger‐scale models of hydrothermal biogeochemistry to be developed across systems.

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.