Seagrass meadows are globally important blue carbon ecosystems, yet their climate mitigation potential remains incompletely understood because current assessments largely emphasize organic carbon burial while overlooking inorganic carbon cycling. We propose that the greatest natural CO₂ uptake potential occurs where high seagrass autotrophic production is coupled with abundant allochthonous geogenic carbonate sediments, maximizing sedimentary alkalinity generation and seawater buffering.
The South China Sea provides an exceptional natural laboratory for investigating this coupling because its extensive tropical seagrass meadows are closely associated with coral reef carbonate systems. Using the Dongsha Atoll Inner Lagoon as a representative case study, we integrated field observations of carbonate chemistry, benthic alkalinity fluxes, ecosystem metabolism, and sediment biogeochemistry. Dongsha represents an ideal end-member of this biogeochemical configuration. High seagrass productivity supplies abundant autochthonous organic carbon to the sediments, while reef-derived geogenic carbonate sediments provide a large reactive inorganic carbon reservoir. Aerobic respiration, sulfate reduction, and sulfur cycling promote carbonate dissolution, generating exceptionally high benthic alkalinity fluxes. Sediment-derived alkalinity accumulates, whereas dissolved inorganic carbon is efficiently recycled through seagrass photosynthesis, maintaining persistently low seawater pCO₂ and sustained atmospheric CO₂ uptake.
Our findings identify the combination of high seagrass autotrophic production and abundant allochthonous geogenic carbonate sediments as the optimal natural configuration for long-term CO₂ uptake. The South China Sea therefore serves as a natural analogue for understanding this process, providing a conceptual framework that can be applied to carbonate-rich seagrass ecosystems worldwide to improve blue carbon assessments and identify highly efficient natural carbon sinks.