POSTECH: Substantial Dissolution of Calcium Carbonate in the Upper Ocean

The process of calcium carbonate formation by microorganisms (pteropods, foraminifera and coccolithophores) living in the euphotic zone – the layer closest to the ocean surface – acts as one of the key mechanisms by which the ocean absorbs carbon dioxide from the atmosphere. However, the industrial revolution has rapidly increased the carbon dioxide in the atmosphere with burning of fossil fuels and has increased its absorption into the ocean. This in turn has caused ocean acidification worldwide, which ultimately caused the dissolution of marine carbonate minerals. Recently, a study analyzing the effects of the death of the microorganisms that form the calcium carbonate and the dissolution of calcium carbonate on the carbon absorption capacity of the ocean was introduced in Nature Geoscience.

The Carbon Cycle Lab led by Professor Kitack Lee of POSTECH’s Division of Environmental Science and Engineering and the research team of Dr. Richard A. Feely of the Pacific Marine Environmental Laboratory (PMEL) of the U.S. National Oceanic and Atmospheric Administration (NOAA) have together analyzed the global ocean carbon data to confirm that the calcium carbonate particles dissolve in the shallow waters of the ocean. In particular, this study identified evidence for something that was not clearly revealed in a paper published by the two teams in Science in 2004.

It has been known that when calcium carbonate-forming microorganisms die, calcium carbonate particles sink and dissolve at the bottom of the deep sea or ocean. However, this study, based on analysis of global carbonate chemistry and water mass age information, shows that about 50% of calcium carbonate that has fallen from the surface is already dissolved before falling into the deep sea.

When a large amount of calcium carbonate is dissolved in the upper water column less than 1000m deep, the dissolved inorganic carbon components can return to the surface of the ocean within a relatively short period of time (several decades) through the circulation of seawater. This makes it possible to recover the concentration of inorganic carbon components in the ocean surface much faster than when calcium carbonate particles are dissolved in the deep sea.

The research team concluded that the recovery of the inorganic carbon in the ocean surface can increase its ability to remove the carbon dioxide from the atmosphere and restore the ocean’s carbon absorption capacity – which has been lowered due to acidification – faster than expected. According to the findings from this study, although the ocean acidification caused by increased carbon dioxide concentration dissolves the calcium carbonate formed by marine microorganisms, the calcium carbonate particles of dead microorganisms dissolve before they sink to the deep sea, thereby mitigating the decrease in the carbon removal capacity of the ocean surface. The ocean is recovering its ability to absorb carbon on its own, even though it has been acidified by the saturation of already dissolved carbon dioxide.

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