WHEN eukaryotic organisms evolved in the seas, some of them began incorporating calcium carbonate (CaCO3) into their bodies as skeleton and armor, making it ultimately from calcium oxides and carbon dioxide.
They only began doing so in a really massive and abrupt scale just over half a billion years ago, which is relatively recent in our planet’s geological history.
The most obvious ones we see today are the shelled mollusks, arthropods, echinoderms, brachiopods, sponges, and corals; but massive amounts were also made by planktonic microorganisms such as Coccolithophores and Foraminiferans.
When they died, they sank into the seafloor and transformed into hard-to-dissolve carbonate minerals, each molecule of which represents one CO2 molecule sequestered out of the atmosphere. The net reaction would be:
CO2 (Carbon dioxide) + CaSiO3 (Calcium silicate, a component of many rocks) → SiO2 (Silicon dioxide, the main constituent of sand and quartz) + CaCO3 (Calcium carbonate, a component of carbonate minerals).
Another reason for CO2 sequestration is more long-term. The naturally occurring long-lived radioactive nuclides in the planet’s interior have been steadily decaying over billions of years.
Heat from their radioactive decay, which originally was being produced at least two magnitudes greater than that of the present rate, ultimately causes much of Terra’s volcanic activity. As the planet’s primordial radionuclides decayed, corresponding volcanic activity has also steadily diminished.
Carbon sequestered into the crust by photosynthesizing organisms as fossil fuels (and also by the carbonate-producing organisms as carbonate minerals), instead of being rapidly released back into the atmosphere with each volcanic event (which would combust the fossil fuels and calcine carbonate minerals thus releasing CO2 back into the atmosphere), tended to stay there for longer periods of geological time.
In other words, the geological carbon cycle (in particular the combustion of buried fossil fuels and the second part of the carbonate-silicate cycle CaCO3 + SiO2 → CaSiO3 + CO2) was much slowed down. More and more of the carbon tended to remain in the crust as elemental Carbon and carbonate minerals (mainly Calcium carbonate and Magnesium carbonate) than in the atmosphere over the eons.
While this was happening, land plants began to get more efficient in extracting the decreasing amounts of CO2 in the air by evolving the C4 and CAM carbon-concentrating mechanisms for carbon fixation and photosynthesis, just 35 million years ago. This lowered the atmospheric CO2 level even more.
By pre-industrial times, CO2 level had sunk to starvation levels for plants. They had gobbled up most of the CO2 that could be gobbled up.
Then something new came up. During the industrial era starting in the mid-1800s, but especially significant only after the 1950s, humanity has been pumping out Carbon dioxide in massive quantities, leading to an ever-rising excess of this gas in the atmosphere as seen from the Mona Loa Observatory survey above. (For comments and suggestions please email to mabuhibisaya2017@gmail.com)/PN
