Biological export production controls upper ocean calcium carbonate dissolution and CO₂ buffer capacity

Marine biogenic calcium carbonate (CaCO₃) cycles play a key role in ecosystems and in regulating the ocean's ability to absorb atmospheric carbon dioxide (CO₂). However, the drivers and magnitude of CaCO₃ cycling are not well understood, especially for the upper ocean. Here, we provide global-scale evidence that heterotrophic respiration in settling marine aggregates may produce localized undersaturated microenvironments in which CaCO₃ particles rapidly dissolve, producing excess alkalinity in the upper ocean. In the deep ocean, dissolution of CaCO₃ is primarily driven by conventional thermodynamics of CaCO₃ solubility with reduced fluxes of CaCO₃ burial to marine sediments beneath more corrosive North Pacific deep waters. Upper ocean dissolution, shown to be sensitive to ocean export production, can increase the neutralizing capacity for respired CO₂ by up to 6% in low-latitude thermocline waters. Without upper ocean dissolution, the ocean might lose 20% more CO₂ to the atmosphere through the low-latitude upwelling regions.