Large Porewater-Derived Carbon Outwelling Across Mangrove Seascapes Revealed by Radium Isotopes

Mangrove-dominated coastlines have high carbon sequestration capacity, but it remains unclear whether tidally outwelled carbon is transformed within the coastal ocean or exported offshore. Here, we used radium isotopes (²²⁴Ra and ²²³Ra) to investigate carbon outwelling in two mangrove seascapes in Brazil across multiple spatial scales. We sampled porewaters to define the source composition, mangrove creek waters to resolve tidal cycles, and cross-shelf transects to trace outwelling in coastal seascapes. Radium isotopes were positively correlated with dissolved inorganic (DIC), organic (DOC) and particulate organic (POC) carbon across the seascapes. DIC was the primary form of carbon (mean ± SD), representing 85% of the total carbon pool as bicarbonate (75 ± 11%), carbonate (6 ± 5%), and CO₂ (4 ± 9%). DOC and POC accounted for 10 ± 6% and 5 ± 6% of total carbon, respectively. Although mangrove waters emitted CO₂ to the atmosphere (38–143 mmol m⁻² d⁻¹), both bays and continental shelves were a CO₂ sink (−2.5 to −0.5 mmol m⁻² d⁻¹) associated to chlorophyll-a enrichments (r² = 0.86). Total carbon outwelled from mangroves were 3–4 times higher than soil carbon burial at both mangrove sites. Bicarbonate export (27–72 mmol m⁻² d⁻¹) to the continental shelf was the major fate of carbon outwelling, more than doubling the perceived capacity of mangrove soil to sequester carbon. Hence, disregarding outwelling as a blue carbon sink mechanism would lead to underestimated assessments of how mangroves capture CO₂ and help to mitigate climate change.