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 CO₂sinks through the burial of organic carbon (C_org). A global collection of data on inorganic carbon burial rates (C_inorg, 12% of CaCO₃ mass) revealed global rates of 0.8 TgC_inorg yr⁻¹ and 15–62 TgC_inorg yr⁻¹ in mangrove and seagrass ecosystems, respectively. In seagrass, CaCO₃burial may correspond to an offset of 30% of the net CO₂ sequestration. However, a mass balance assessment highlights that the C_inorg 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.