Role of carbonate burial in Blue Carbon budgets

Calcium carbonates (CaCO₃) often accumulate in mangrove and seagrass sediments. As CaCO₃ production emits CO₂, there is concern that this may partially offset the role of Blue Carbon ecosystems as CO₂ sinks through the burial of organic carbon (Corg). A global collection of data on inorganic carbon burial rates (Cinorg, 12% of CaCO₃ mass) revealed global rates of 0.8 TgCinorg yr⁻¹ and 15-62 TgCinorg yr⁻¹ in mangrove and seagrass ecosystems, respectively. In seagrass, CaCO₃ burial may correspond to an offset of 30% of the net CO₂ sequestration. However, a mass balance assessment highlights that the Cinorg burial is mainly supported by inputs from adjacent ecosystems rather than by local calcification, and that Blue Carbon ecosystems are sites of net CaCO₃ dissolution. Hence, CaCO₃ burial in Blue Carbon ecosystems contribute to seabed elevation and therefore buffers sea-level rise, without undermining their role as CO₂ sinks.