Coastal vegetated habitats, including mangroves, saltmarshes and seagrasses, mitigate climate change by storing atmospheric carbon. Previous blue carbon research has mainly focused on organic carbon stocks. However, recent studies suggest that lateral inorganic carbon export might be equally important. Lateral export is a long-term carbon sink if carbon is exported as alkalinity (TAlk) produced via sulfate reduction coupled to pyrite formation. This study evaluates drivers of pyrite formation in coastal vegetated habitats, compares pyrite production to TAlk outwelling rates, and estimates global pyrite stocks in mangroves. We quantified pyrite stocks in mangroves, saltmarshes and seagrasses along a latitudinal gradient on the Australian East Coast, including a mangrove dieback area, and in the Everglades (Florida, USA). Our results indicate that pyrite stocks were driven by a combination of biomass, tidal amplitude, sediment organic carbon, sedimentation rates, rainfall latitude, temperature, and iron availability. Pyrite stocks were three-times higher in mangroves (103 ± 61 Mg/ha) than in saltmarshes (30 ± 30 Mg/ha) and seagrasses (32 ± 1 Mg/ha). Mangrove pyrite stocks were linearly correlated to TAlk export at sites where sulfate reduction was the dominant TAlk producing process, however pyrite generation could not explain all TAlk production. We present the first global model predicting pyrite stocks in mangroves, which average 155 (range 128 – 182) Mg/ha. In mangroves, estimated global TAlk production coupled to pyrite formation (~3 mol/m2/y) is equal to ~24% of their global organic carbon burial rate, thus highlighting the importance of including TAlk export in future blue carbon budgets.

Most research evaluating the potential of mangroves as a sink for atmospheric carbon has focused on carbon burial. However, the few studies that have quantified lateral exchange of carbon and alkalinity indicate that the dissolved carbon and alkalinity export may be several-fold more important than burial. This study aims to investigate rates and drivers of alkalinity, dissolved carbon and greenhouse gas fluxes of the mangrove-dominated Shark River estuary located in the Everglades National Park in Florida, USA. Time series and spatial surveys were conducted to assess total alkalinity (TAlk), organic alkalinity (OAlk), dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). Dominant metabolic processes driving dissolved carbon and greenhouse gas dynamics varied along the estuarine salinity gradient. Dissolved carbon and greenhouse gas concentrations were strongly coupled to porewater input, which was examined using radon-222. Shark River was a source of CO2 (92 mmol/m2/d), CH4 (60 μmol/m2/d) and N2O (2 μmol/m2/d) to the atmosphere. Dissolved carbon export (DIC = 142 mmol/m2/d, DOC = 39 mmol/m2/d) was several-fold higher than burial (~28 mmol/m2/d) and represents an additional carbon sink. Furthermore, the estuary was a source of TAlk (97 mmol/m2/d, normalised to mangrove area) to the coastal ocean, potentially buffering coastal acidification. Organic alkalinity was also exported to the coastal ocean (1.9 mmol/m2/d, normalised to mangrove area). By integrating our results with previous studies, we argue that alkalinity, dissolved carbon and greenhouse gas fluxes should be considered in future blue carbon budgets.