A story about when oxygen first accumulated in the atmosphere, and killed many organisms

A recent article in Nature, led by Sean Crowe from the University of British Columbia, has posited that oxygen may have first appeared in significant concentrations in the atmosphere 3 billion years ago, much further back than the accepted estimates of 2.3 billion years ago.  This article has received some coverage in the mainstream media, such as on BBC News.

I have always been fascinated by how our universe and our planet came into being, a fascination that I know I share with many people.  The composition of the Earth’s atmosphere has greatly changed in the past 4.5 billion years, and certainly in the 3.5 billion years since life began to evolve on this planet.  While none of us were there to observe it, a combination of fossil analysis and models gives us a sense of the composition of the atmosphere in that era.

(The following are portions of a lecture I gave in my analytical and environmental chemistry class at St. Francis Xavier University in 2007 and 2008; unfortunately, I do not have a list of the original sources that I used, but I collected information from several sources to put this together.)

3.5 billion years ago, at the time of abiogenesis – the first instances of life from simple organic compounds – the Earth’s atmosphere would have consisted mainly of nitrogen (N2), hydrogen (H2), water vapour (H2O), methane (CH4) and ammonia (NH3). (If you ever wondered by the atmosphere is nearly 4/5th nitrogen, read this short article.) There was no oxygen in the atmosphere at that time, and no ozone layer to shield the planet from the ultraviolet rays of the Sun.  The photons from these light rays would have sufficient energy to break most chemical bonds, initiating chemical reactions to form more complex molecules – sugars and amino acids, the building blocks of life.  Then, just as now, the formation of sugars came from photosynthesis and was kickstarted by photons from the Sun:

6 CO2(g) + 6 H2O(l) + UV light -> C6H12O6(s) + 6 O2(g)

Photosynthesis is considered a highly beneficial process for our atmosphere today, as it takes carbon dioxide out of the atmosphere, and emits oxygen.  Yet, the same oxygen that is so vital to sustaining life today was toxic to the first organisms on Earth.  The reduction of the oxygen gas would have been favoured over the fermentation reactions that the organisms needed to produce energy, eventually leading to death.  Over hundreds of millions of years, these organisms would have developed enzymatic systems to deal with the oxygen, and eventually being able to produce energy from the oxygen (aerobic respiration), a rudimentary system that evolved to our more effective cellular respiration systems.

Early photosynthesis was the initial source of oxygen gas on Earth; much of the released oxygen quickly reacted with dissolved iron in the oceans, generating iron oxide, known as rust.  The evidence of this from banded iron formations, a form of sedimentary rocks that are known to be over 2 billion years old.  As dissolved iron was depleted from the oceans, oxygen was still produced, and it started to accumulate in the atmosphere.  This accumulation of oxygen in the atmosphere, and its immediate consequences (including the oxidation of atmospheric methane into carbon dioxide), is known as the Great Oxygenation Event, thought to have occurred 2.3 billion years ago.

Crowe et al’s findings would suggest that the GOE may have begun 700 millions years earlier than originally assumed – 3 billion years ago.  These researchers investigated the ratios of chromium isotopes (53Cr to 52Cr) in 3-billion-year-old soils and sea sediments in South Africa.  Naturally-occurring chromium contains 83.8% of the 52Cr isotope, and 9.5% of the 53Cr isotope, a ratio of 8.8:1.  It is known that 53Cr is slightly more soluble in water than 52Cr when both react with oxygen.  If atmospheric oxygen was present in appreciable quantity at the time, we’d expect a smaller proportion of 53Cr in the soil, as some 53Cr would have reacted and dissolved; a larger proportion of 53Cr would be found in sea sediments, where the waters would have flowed.  This is exactly what the researchers discovered in those particular South African soils and sea sediments.  To be clear, the concentration of oxygen in the atmosphere at that time would have been approximately 0.006%, or 3/10,000ths of today’s concentration of just above 20%. (For comparison, the current concentration of CO2 in the atmosphere is close to 0.04%.)

But while oxygen was toxic to the first organisms on Earth, it also played a crucial role in shielding these organisms from the Sun’s toxic ultraviolet radiation. The accumulation of oxygen in the upper atmosphere led to the creation of the ozone layer. An oxygen molecule that receives a photon with a wavelength of 240 nm or shorter is dissociated into two oxygen atoms. Each atom attacks another oxygen molecule, and in the presence of a third molecule that can absorb the energy released by this reaction, there is formation of the O3 ozone molecule.

O2 + light -> 2 O•
O2 + O• + M -> O3 + M*

Ozone is an unstable molecule. Exposure to ultraviolet light in the 200-330 nm range causes a reaction which regenerates oxygen gas, although it has the side effect of blocking that UV light from reaching the Earth’s surface. A significant reduction in the quantity of UV light shining on the Earth, and an adaptation to the oxygen in the atmosphere, were key for organisms to thrive on land, without living as if they had a magnifying glass permanently hovering over them.

1. SA Crowe, LN Døssing, NJ Beukes, M Bau, SJ Kruger, R Frei, DE Canfield; Nature, 501, 535-538 (2011)

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