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

Marine biogenic calcium carbonate (CaCO) cycles play a key role in ecosystems and in regulating the ocean's ability to absorb atmospheric carbon dioxide (CO). However, the drivers and magnitude of CaCO 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 CaCO particles rapidly dissolve, producing excess alkalinity in the upper ocean. In the deep ocean, dissolution of CaCO is primarily driven by conventional thermodynamics of CaCO solubility with reduced fluxes of CaCO 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 CO by up to 6% in low-latitude thermocline waters. Without upper ocean dissolution, the ocean might lose 20% more CO to the atmosphere through the low-latitude upwelling regions.