Pore‐water exchange flushes blue carbon from intertidal saltmarsh sediments into the sea

We quantified whether pore‐water exchange flushes out saltmarsh sediment carbon, driving carbon outwelling into the ocean and outgassing into the atmosphere. Radon‐derived pore‐water exchange released 1.8 times more sediment carbon in the wet than in dry season. Both crab burrow flushing and delayed seepage of surface water infiltrating sediments during the spring tide released sediment carbon to surface waters. The outwelling flux of dissolved inorganic carbon exceeded dissolved organic carbon. Carbon dioxide and methane emissions were 169 and 0.25 mmol m−2 d−1, respectively. Pore‐water carbon fluxes exceeded carbon outwelling. This requires some carbon processing within the saltmarsh (e.g., degradation or outgassing to the atmosphere) before pore‐water carbon is exported to the ocean. Overall, pore‐water exchange and outwelling are key components of saltmarsh carbon budgets and should be considered when assessing their carbon sequestration potential and strategies to mitigate climate change.