Molecular Oxygen (O₂)

The atmospheric concentrations of Oxygen have had - and continue to - exert the greatest effect and influence to life on this planet. Today the present O₂ levels are 20.946%, however, the concentrations have previously been considerably less and have only remained relatively constant for the last 60 million years. Several hundred million years ago, before the advent of water oxidation by Photosystem II, the earth’s atmosphere contained minimal O₂and instead was a reducing atmosphere containing CO₂, N₂ CO, H₂O and smaller amounts of H₂, H₂S, NH₃, HCN (3). During this era the reducing atmosphere equilibrated with the ocean (at redox levels several hundred mV more negative than today) and sustained a diversity of bacteria and archea. These life forms operated happily under this reducing environment and performed a variety of chemistries that were catalyzed by a variety of enzymes.

From 2,200 to 1,000 million years ago O₂ levels increased slowly generated by cyanobacterial photosynthesis. Considerable uncertainty exists in this time region about the composition of the oceans and atmosphere. About 1,000 million years ago the primordial soup thickened and cells began incorporating each other in symbiotic relationships (phagocytosis) that ultimately led to additional photosynthetic organisms that included the green and red algae (4). At this point oxygenic photosynthetic organisms had increased density and the amounts of net primary photosynthesis on earth increased and O₂ began to accumulate at an increased rate.

About 300 million years ago, O₂ concentrations peaked with the appearance of large vascular plants. The extra photosynthesis from larger plant leaf areas, combined with swampy global terrain and reduced lignin degradation, resulted in increased carbon burial. The high amounts of coal from this period support the burial idea and are an example of how nature sequestrated carbon. The high oxygen concentrations have also been suggested to provide for significantly increased density and intensity of forest fires during this period. The fires may have provided a natural feedback mechanism. Another feedback mechanism that has not been considered is the O₂ feedback on the water splitting mechanism from photosystem II , which means the higher [O₂] may simply slow the reaction.

These distributions remain common for many things but there are chemical processes that cause deviations in these numbers. Mass spectrometers have been able to measure these differences and current instrumentation will discern differences of as small as 0.001%. The small differences in isotopic abundance is present in all chemistry on earth and arises due to small differences in reaction rate or diffusion rate of chemical substrates. The difference in isotopic composition for a chemical process is termed isotopic fractionation or isotopic discrimination and is usually expressed at a per thousand (per mil) as ‰. Typical fractionation factors are ~2-20‰.

The difference reason for the changes in fractionation are due to two processes dependent on the mass of the isotope.

1. Diffusion rates are faster for lighter molecules. At a given temperature molecules of different mass (M) have the same kinetic energy (K). As K = 1/2 (MV)² the velocity of the diffusion rate must be faster for the lighter isotope. This therefore establishes a isotopic fractionation factor based on diffusion.
2. Chemical reaction rates are slightly faster for lighter molecules. The reason for this is nuclear mass differences between isotopes reflect a change in bond energy. The bond energy for the heavier isotopes correspond to a lower zero point energy (ZPE), due a lower vibrational energy. A chemical bond with a lower ZPE corresponds to a higher thermodynamic barrier, and hence is manifested a slightly slower reaction rate.