Blue carbon oxidation revealed by radiogenic and stable isotopes in a mangrove system

Mangroves are among the most carbon-rich ecosystems on Earth and can sequester carbon in sediments over long timescales. Here we assess whether century-old buried carbon may be remineralized and exported by measuring Δ14C in the exported dissolved inorganic carbon (DIC) as well as sediment Δ14C profiles in a subtropical mangrove. Pore water exchange released isotopically depleted, old DIC to surface waters. Keeling plots revealed that the source of DIC to surface waters had a δ13C-DIC value of −29.4 ±1.9‰ and Δ14C-DIC value of −73±9‰. The respired and exported carbon comes from an average depth of ~40 cm, equivalent to ~100 years of sediment accumulation. Therefore, century-old sequestered carbon is still susceptible to remineralization and tidal export to the coastal ocean via pore water exchange or submarine groundwater discharge. We suggest that the timescales over which blue carbon burial is assessed should consider carbon losses via pore water exchange.