Large porewater-derived carbon outwelling across two mangrove-seascapes revealed by radium isotopes

Mangroves have high CO₂ sequestration capacity, storing large amounts of carbon on their biomass and sediments/soil. Mangrove carbon is also transported to the ocean, i.e. outwelling or lateral fluxes, where it can be stored for long time scales. Here, we used radium isotopes (²²⁴Ra and ²²³Ra) to resolve carbon and alkalinity outwelling to the ocean from two mangrove seascapes in Brazil. We sampled porewaters to define the source composition, mangrove creek waters to resolve tidal cycles, and performed transects away from the mangrove into continental shelf to trace mangrove carbon across the seascape. High-resolution observations of radium isotopes in the creek indicated that tidal pumping is the main driver of carbon exchange. Low pH (6.8 – 7.0) and high ²²⁴Ra activities (165 – 290 dpm 100L^-1) were found during low tides, indicating mangrove porewater exchange. Radium mass balance models revealed porewater exchange at 20.0 ± 25.4 cm d^-1 in the tropical mangrove and 3.0 ± 2.0 cm d^-1 at the sub-tropical mangrove. Radium-derived transport rates of mangrove porewater to the continental shelf were higher in the mesotidal tropical (667 ± 313 m d^-1) than the microtidal subtropical (371 ± 168 m d^-1) seascape. Radium isotopes were positively correlated (p < 0.05) with dissolved inorganic (DIC), organic (DOC) and particulate organic (POC) carbon across the entire seascape. DIC as bicarbonate (HCO₃^-) was the main form of carbon on all scales in both mangrove seascapes, representing 57 – 82% of the total carbon pool. DOC and POC accounted for 5 – 12% and 1 – 7% of total carbon, respectively. Although mangrove waters emitted CO₂ to the atmosphere (38.4 – 142.9 mmol m^-2 d^-1), both bays and continental shelves were a CO₂ sink (-1.9 – -0.6 mmol m^-2 d^-1). Porewater-derived carbon outwelling exceeded carbon fluxes at the mangrove-bay and bay-shelf interfaces, indicating carbon transformations across the seascape continuum. Total carbon outwelled from mangroves were 3 – 4 times higher than soil carbon burial at both mangrove sites. Bicarbonate outwelling (31.0 – 71.6 mmol m^-2 d^-1) reaching the continental shelves increased mangrove soil carbon sequestration capacity by 234% in these mangrove systems. Hence, overlooking outwelling as a blue carbon sink mechanism would underestimate the role of mangroves in sequestering CO₂ and mitigating climate change.