Analysis of global ocean carbonate chemistry and water mass age information confirms the substantial in situ dissolution of calcium carbonate particles in the upper water column.
Rising atmospheric carbon dioxide (CO2) concentrations from burning of fossil fuels and other anthropogenic sources over the past two and a half centuries have led to an oceanic uptake of about 623 PgCO2 (ref. 1) from the atmosphere, causing ocean acidification worldwide. Marine biogenic carbonate minerals produced in the euphotic zone, the layer closest to the ocean surface, ultimately neutralize the excess CO2, leading to the dissolution of carbonate minerals in the water column or within the sediments. As the influx of anthropogenic CO2 progressively acidifies the oceans, the extent of dissolution will increase within the ocean interior. In previous studies, it had overwhelmingly been thought that calcium carbonate in the open ocean primarily dissolves at the ocean bottom. Writing in Nature Geoscience, Sulpis et al.2, however, find that as much as 47% of the calcium carbonate exported from the ocean surface dissolves in the water column before reaching the seafloor. This in situ dissolution of calcium carbonate particles has a positive effect on absorbing atmospheric CO2.
Calcium carbonate minerals are produced from marine plankton (Fig. 1). Upon the death of these organisms, mineral particles fall rapidly through the water column and a portion is likely to be dissolved while sinking. A parcel of water in the ocean interior today was near the surface at sometime in the past. As the calcium carbonate particles fall through the water column and dissolve, the alkalinity of the seawater in the water column would begin to increase. Therefore, the dissolution rate of calcium carbonate minerals for a certain depth can be estimated using both the changes in alkalinity and the age of the water mass since it was last at the surface.
Lee K. & Feely R. A., 2021. Dissolution resolution. Nature Geoscience 14: 356–357. Article.