BY DR. JOSE PALU-AY DACUDAO
WE SHALL review the concepts in the previous article.
When discussing carbon sinks, it is always a good thing to remind ourselves of why the Earth has such low levels of atmospheric carbon dioxide (and plentiful oxygen) compared to other terrestrial planets with sizable atmospheres, such as Venus and Mars.
The first atmosphere of our planet was mostly hydrogen from the solar nebula. This did not last long as hydrogen and the compounds it forms are mostly volatile, so that the then lightweight Earth could not hold on to them as impactors up to the size of Mars rained down on the surface. As the early Earth gained mass from these impactors, its gravity grew stronger.
Coupled with the cessation of the early gigantic impactors which would literally explode atmospheric gases into space, Earth could start retaining carbon dioxide. Carbon is the fourth most abundant element in the solar system (after hydrogen, helium, and oxygen) and subsequently, most of the elemental carbon left behind combined with oxygen to form second atmosphere.
Methane, another carbon compound, may also have been abundant early on. Venus and Mars, terrestrial planets quite similar to Earth still have atmospheres mostly made up of carbon dioxide. So again, why does the Earth have such low levels of atmospheric carbon dioxide (and plentiful oxygen) compared to other terrestrial planets with sizable atmospheres, such as Venus and Mars.
The fundamental answer is photosynthesis:
6 CO2 (carbon dioxide taken from the atmosphere in a geological instant of a few decades or less) + 6 H2O (water) → C6H12O6 (organic substance) + 6 O2 (oxygen)
Photosynthetic living organisms extracted CO2 and released molecular oxygen. The free oxygen oxidized any remaining methane (CH4) and ammonia (NH3) in the second atmosphere into CO2, free diatomic nitrogen (N2), and water (H2O). Thus, our planet’s third atmosphere came into existence – nitrogen, oxygen, argon (from the decay of radioactive Potassium-40 isotope over billions of years), and a bit of carbon dioxide.
Photosynthesis in earnest may have begun about 3 billion years ago. Around 2.5 billion years ago, after most of the reduced compounds of the air, sea, and land got oxidized, molecular oxygen began to accumulate in the atmosphere. The CO2-depleting effect caused by steady photosynthetic activity added up over the eons.
On the other hand, with an atmosphere rich in oxygen, we need to remind ourselves what happens in aerobic biological decomposition, one that occurs in an environment with lots of oxygen.
For simplicity’s sake let us assume the decomposition of glucose by microbes to represent the aerobic decomposition of organic material in general.
C6H12O6 (organic substance) + 6 O2 (free molecular oxygen in the atmosphere) → 6 CO2 + 6 H2O (water) [in a geological instant, of days to a few years]
Wildfires also follow the same formula above. However, most of the decomposition of organic matter is done by microbial living organisms. (To be continued)/PN
