Blue carbon sequestration dynamics within tropical seagrass sediments: long-term incubations for changes over climatic scales

Determination of blue carbon sequestration in seagrass sediments over climatic time scales (>100 years) relies on several assumptions: no loss of particulate organic carbon (POC) after 1-2 years; tight coupling between POC loss and CO₂ emissions; no dissolution of carbonates; and removal of the recalcitrant black carbon (BC) contribution. We tested these assumptions via 500-day anoxic decomposition/mineralisation experiments to capture centennial parameter decay dynamics from two sediment horizons robustly dated as 2 and 18 years old. No loss of BC was detected, and decay of POC was best described for both horizons by near-identical reactivity continuum models. The models predicted average losses of 49% and 51% after 100 years of burial for the surface and 20–22 cm horizons respectively. However, the loss rate of POC was far greater than the release rate of CO₂, even after accounting for CO₂ from particulate inorganic carbon (PIC) production, possibly as siderite. The deficit could not be attributed to dissolved organic carbon or dark CO₂ fixation. Instead, evidence based on δ¹³CO₂, acidity and lack of sulphate reduction suggested methanogenesis. The results indicate the importance of centennial losses of POC and PIC precipitation and possibly methanogenesis in estimating carbon sequestration rates.