BY DR. JOSE PALU-AY DACUDAO
(How Nature Lowered Atmospheric Carbon Dioxide in the Carboniferous Part 2)
A review of Part 1
WHAT IS the most effective way by which living organisms take out Carbon dioxide from the atmosphere?
Answer: Photosynthesis.
6 CO2 + 6 H2O → C6H12O6 + 6 O2
C6H12O6 is a six carbon carbohydrate. The often cited substance with this formula is glucose. But aside from this, photosynthetic organisms also convert carbon dioxide and water (CO2 and H2O) into three and four carbon straight or chain carbohydrates.
From these, compounds whose skeleton is made of branched chains of carbon atoms Are derived from. These compounds made of straight and/or branched chains of carbon atoms are called aliphatic compounds. Living organism make branch-chain aliphatic compounds by chemically reducing straight-chained aliphatic compounds; that is by removing oxygen from them. Thus, this makes branch-chain aliphatic compounds more carbon-rich than straight-chained aliphatic compounds.
Living organisms further chemically reduce aliphatic molecules into compounds that have closed rings of carbon atoms. These are called aromatic compounds. Since they are made by removing more oxygen (in the form of CO2 and H2O) from aliphatic molecules. Consequently, aromatics are the most carbon-rich of all organic molecules. Lignin (of which there are several varieties) is the most common aromatic compound. It is the compound that makes wood woody. Thirty percent of the sum of all photosynthesis of the biosphere goes into making lignin.
In summary, the most common substances into which carbon dioxide (and water) is chemically reduced to by photosynthetic organisms are the aliphatic cellulose and hemicellulose, and the aromatic lignin.
In the usual course of events, much of the above (except perhaps lignin which is quite hard to biodegrade) are oxidized back into carbon dioxide and water by decomposition by microbes, or by naturally occurring fires. However, a considerably amount are also buried in the earth and sea bottom under anoxic conditions. When this happens, heat and pressure underground further chemically reduce these organic compounds, eventually transforming them into fossil fuels (such as methane, petroleum, and coal). The most carbon-rich of these is coal. Further chemical reduction under the earth’s heat and pressure drives away hydrogen and oxygen (which if combined is just water H2O), thus transforming coal into graphite, the most stable form of elemental carbon. For simplicity’s sake, let the following formula represent this process:
C6H12O6 → 6 H2O + 6 C
Notice that if we sum up the above processes of photosynthesis and carbonization, 6 CO2 + 6 H2O → C6H12O6 + 6 O2 (photosynthesis) C6H12O6 → 6 H2O + 6 C (carbonization) we end up with 6 CO2 → 6 O2 + 6 C
This is what essentially happened massively during the formation of coal in the Carboniferous (specifically its latter half the Pennsylvanian around 320 to 300 million years ago) and the succeeding early Permian geological periods. Vast forests of lignified wood were buried in shallow seas and transformed into coal. In other words, carbon dioxide was fixed as elemental carbon, namely coal. The process of coalification also produced enormous amounts of free oxygen (as can be seen in the equation above). (To be continued)/PN
