Bottom water

Mineralogy

Settling

Seafloor spreading

Carbonate

Geology

Dissolution

Oceanography

Physical chemistry

Environmental engineering

Biogeochemistry

Organic chemistry

Calcium carbonate

Upwelling

Water column

Pelagic zone

Environmental science

Chemistry

Seawater

Ocean acidification

Nature Geoscience

Calcium carbonate (CaCO3) minerals secreted by marine organisms are abundant in the ocean. These particles settle and the majority dissolves in deeper waters or at the seafloor. Dissolution of carbonates buffers the ocean, but the vertical and regional distribution and magnitude of dissolution are unclear. Here we use seawater chemistry and age data to derive pelagic CaCO3 dissolution rates in major oceanic regions and provide the first data-based, regional profiles of CaCO3 settling fluxes. We find that global CaCO3 export at 300 m depth is 76 ± 12 Tmol yr−1, of which 36 ± 8 Tmol (47%) dissolves in the water column. Dissolution occurs in two distinct depth zones. In shallow waters, metabolic CO2 release and high-magnesium calcites dominate dissolution while increased CaCO3 solubility governs dissolution in deeper waters. Based on reconstructed sinking fluxes, our data indicate a higher CaCO3 transfer efficiency from the surface to the seafloor in high-productivity, upwelling areas than in oligotrophic systems. These results have implications for assessments of future ocean acidification as well as palaeorecord interpretations, as they demonstrate that surface ecosystems, not only interior ocean chemistry, are key to controlling the dissolution of settling CaCO3 particles. About 50% of total dissolution of marine calcium carbonate occurs in the water column below 300 m depth while sinking to the seafloor, according to a reconstruction of settling fluxes of calcium carbonate in major oceanic regions from seawater observations.

Calcium carbonate dissolution patterns in the ocean