The Alkalinity Blue Carbon Pump: Tidal pumping drives long-term CO₂ sequestration as alkalinity in salt marshes

Abstract. Tidal salt marshes are increasingly recognised as significant blue carbon ecosystems. However, their total carbon sequestration capacity remains underestimated because dissolved carbon outwelling and alkalinity export are rarely considered alongside sediment burial. Here, we combine tidal creek time series measurements, open incubations, and sediment core analyses to quantify all the main carbon flux pathways in a temperate macrotidal salt marsh of the Arcachon Bay (SW France). A binary salinity mixing model coupled to direct discharge measurements allowed us to isolate the net tidal pumping fluxes of total alkalinity (TA), dissolved inorganic carbon (DIC), dissolved organic carbon (DOC) and methane (CH₄) from conservative mixing. DIC outwelling (5.3 ± 1.7 mmol m^-2 d^-1) was the largest form of dissolved carbon export but it does not fully represent net sequestration as part of it consists of CO₂ that returns to the atmosphere via air-water exchange. Net TA lateral fluxes averaged 3.9 ± 1.4 mmol m^-2 d^-1, driven by sulphate reduction coupled to pyrite formation and aerobic respiration in marsh sediments, as supported by TA:DIC ratios and δ¹³C-DIC. DOC outwelling (3.4 ± 3.3 mmol m^-2 d^-1) was on same magnitude but its sequestration potential depends on lability. High tide benthic incubations revealed significant TA and DIC fluxes from sediments to the water column, higher in vegetated than in bare mud areas, driven by benthic processes and root-network macroporosity. Organic carbon burial rates estimated from ²¹⁰Pb_xs profiles were around 80 mmol m^-2 d^-1 in vegetated zones. However, these rates likely represent an overestimated gross burial rather than net sequestration given the dynamic and erosive nature of this macrotidal system. When integrating all measured carbon flux, net TA outwelling emerges as the most chemically conservative and durable sequestration mechanism, as it is resistant to remobilisation and can remains stable over millennial timescales once exported to the coastal ocean. We propose the term Alkalinity Blue Carbon (ABC) to describe this pathway, and call for its systematic inclusion alongside sedimentary C burial in future blue carbon inventories and climate mitigation frameworks.