What is it?

Biocalcification & CO2 sequestration

What one calls CO2 sequestration process consists of the capture and long-term storage of carbon dioxide. This capture can occur through different natural processes that are part of the Earth’s carbon cycle. Hence, trees, the oceans, earth and the animals themselves constitute carbon sinks.

In particular, CO2 capture and sequestration can occur through microbiological processes such as bacterial and fungal metabolisms, and consists of the conversion of atmospheric CO2 and/or organic carbon into precipitated carbonates, in the presence of cations such as calcium and/or magnesium and under alkaline conditions (high pH). In nature, precipitated carbonates can be found in shelves, coccolithes, coral reefs, chalk cliffs,...

In biocalcification, a biological cell manipulation of chemical species concentrations, marine animals and plants build the shells and skeletons which protect them. Biocalcification produces cristals of calcium carbonate from dissolved chemical species in the ocean. One species is calcium ions; the other, bicarbonate ions, directly from the ocean, or by biotransformation from CO2. Bicarbonate comes from atmospheric CO2 reaction with water both on land and in the ocean, flows through the rivers into the ocean, and directly into the ocean. It is a majot source of atmospheric CO2 sink. During biocalcification, half the CO2 which was locked in bicarbonate, gets released back into the atmosphere! When calciferous organisms die, the skeletons slowly sink into the ocean, and either dissolve into the deep, or settle in shallower seas, as in those cliffs, thus locking the 50%, captured CO2 for eons.

However, there are limits where-by bicarbonate formation can help absorb atmospheric CO2 in the first place: a concentration ceiling, and also the acidification caused by the process itself when it occurs in the ocean. Then, acidification causes dissolution of calcium carbonate, which, in turns, limits somehow this acidification.

Cliffs, carbonates, animal and plant shells, their remnant, thus contribute, at their own expense, for a time, in absorbing the shock of CO2 emissions.

Whether with shells or bacteria, calcification occurs in the sea, by mineralization of bicarbonate ions which are dissolved in the ocean, as it acts as a powerful carbon sink from atmospheric CO2. In this mineralization, half of the CO2 which had been dissolved in the ocean, is released back into the atmosphere.
However, the mere presence of mineralized carbonate provides a "buffer" condition, within a limited albeit significant time-scale, which favors the uptake of atmospheric CO2 and dissolution into bicarbonate. Furthermore, over geological time scales, mineralized carbonates sink and sediment under certain conditions, and enter the earth's crust. The mechanism is complex, and involves the carbon cycle on a global basis, much summarized in the equations below. In terrestrial or industrial closed systems, carbonate mineralization processes, biological or not, must be designed so that the system provides a net carbon sink condition.