Role of carbonate burial in Blue Carbon budgets.

Calcium carbonates (CaCO ₃ ) often accumulate in mangrove and seagrass sediments. As CaCO ₃ production emits CO ₂ , there is concern that this may partially offset the role of Blue Carbon ecosystems as CO ₂ sinks through the burial of organic carbon (C _org ). A global collection of data on inorganic carbon burial rates (C _inorg , 12% of CaCO ₃ 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, CaCO ₃ burial may correspond to an offset of 30% of the net CO ₂ 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 CaCO ₃ dissolution. Hence, CaCO ₃ burial in Blue Carbon ecosystems contribute to seabed elevation and therefore buffers sea-level rise, without undermining their role as CO ₂ sinks.