What drove oxygen accumulation in the earth’s atmosphere? Scientists have theories with supportive data but ultimate answers are elusive and always engender additional intriguing questions.*
What is unquestioned is that ancient oxygen atmospheres developed slowly over the eons, fluctuated before they flourished, and see-sawed before they stabilized. How atmospheric dynamism pressured various and varied organisms to adapt, evolve, dominate, die off, and otherwise reshape the barren and burning terrain of our beautiful, infant planet is both a marvelous story and germane to how we should view oxygen today.
As will become evident through future posts, evolution has equipped humans — mere blips in planetary time — with a greater capacity to withstand low oxygen levels than high oxygen levels. That’s “because” early evolution proceeded from extremely low oxygen fractions to the current 21% level with only a brief period of time spent at higher levels (a few million years) estimated to have peaked around 30% to 35% of total atmospheric pressure. In a real sense, to paraphrase the poet T.S. Eliot, “in our beginning is our end.” Not only is biology our destiny; destiny is in our biology. How true when it comes to oxygen.
As mentioned in a previous Oxygenologist entry, our late Proterozoic and early Phanerozoic eons saw oxygen levels rise enough to support complex oxygen-sustained organisms. This means life forms capable of metabolizing hydrogen sulfide, methane, and other gasses preceded, co-existed with, and probably co-operatively interacted with oxygen-dependent organisms in oxygen-rich eons and eras. It also means critical oxygen levels had to be attained before the most complex animals (metazoans) could populate and dominate our chemically volatile earth.
Planetary biogeochemical cycles saw complex interactions among minerals, water, gasses and the full spectrum of radiation (infra-red to cosmic) before a floor and a ceiling level for planetary ancient oxygen atmospheres could develop. The floor was set when saturated complex iron, carbon, sulfur, and other mineral substance “burials” permitted enough oxygen to accumulate as a measurable atmosphere in oxidative/reductive equilibrium with the volatile and volcanic surface mantle; the ceiling was set when conflagrations of organic matter (paleofires) periodically drew down maximum atmospheric oxygen levels, above which all combustibles could consumed, but below which combustibles could be cyclically restored. Between the two extremes life-forms evolved that depend on oxygen protected by a burgeoning ozone layer capable of shielding surface organisms from the highest levels of cosmic radiation capable of inducing genetic mutations.
*Technically-minded readers might be interested in reading:
Berner RA. Atmospheric oxygen over the Phanerozoic time. Proc Natl Acad Sci USA. 1999; 96:10955-7
Goldblatt C, Lenton TM, Watson AJ. Bistability of atmospheric oxygen and the Great Oxidation. Nature. 2006; 443:683-6
Plavansky NJ, Reinhard CT, Wang X, Thomson D, McGoldrick P, Rainbird RH, Johnson T, Fischer WW, Lyons TW. Low mid-proterozoic atmospheric oxygen levels and the delayed rise of animals. Science. 2014; 346:635-8
