New evidence suggests Earth's oxygen levels fell after the Great Oxidation Event

Oct 01, 2013 by Bob Yirka report
According to Emma Hammarlund these 2 billion years old fossils represent an early life form that experimented with evolving into some kind of multicellular lifeform, but did not succeed. Credit: Abder El Albani

(Phys.org) —A team of European researchers has published a paper in the journal Proceedings of the National Academy of Sciences, offering a possible explanation for the apparent drop in early Earth's oxygen levels following what has come to be known as The Great Oxidation Event. In their paper, the team suggests evidence found in ocean sediments indicates that a sudden addition of carbon into the atmosphere resulted in a relatively quick reduction in oxygen.

Scientists have found multiple sources that indicate that approximately 2.3 billion years ago, in the atmosphere (and oceans) increased dramatically—they've named it the Great Oxidation Event. Though scientists can't explain exactly why this happened, they do believe it occurred during a time of major glaciation, and was perhaps the result of the evolution of bacteria. Since that time, scientists have believed that changes in oxygen levels have been the result of a series of steps rather than great upheavals. Now, new research by the European team suggests that a change of thinking might be in store.

To try to better understand what went on with the atmosphere as the Earth was evolving from a lifeless planet to the rich biota filled environment of today, look to rocks formed millions or even billions of years ago, and to . In this latest effort, the research group looked to such samples gathered from the ocean floor off the coast of western Africa's Republic of Gabon. Because sediments collect so slowly over time, analyzing them offers a glimpse into the past, similar in many respects to ice samples taken from polar climates.

The samples they retrieved offered evidence of ocean (and therefore atmospheric) oxygen levels, before, during and after the Great Oxidation Event, and surprisingly, indicated that shortly after the large infusion of oxygen, there was a fall. The researchers attribute this fall to an increase in carbon and iron in the , part of what they call the largest positive carbon-isotope excursion in the history of the planet. The oxygen reacted, they say, with iron and carbon forming oxides that were carried to the . Though the carbon excursion can't be explained either, it is believed it was likely tied to the evolution of life in the oceans.

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More information: Oxygen dynamics in the aftermath of the Great Oxidation of Earth's atmosphere, Published online before print September 30, 2013, DOI: 10.1073/pnas.1315570110

Abstract
The oxygen content of Earth's atmosphere has varied greatly through time, progressing from exceptionally low levels before about 2.3 billion years ago, to much higher levels afterward. In the absence of better information, we usually view the progress in Earth's oxygenation as a series of steps followed by periods of relative stasis. In contrast to this view, and as reported here, a dynamic evolution of Earth's oxygenation is recorded in ancient sediments from the Republic of Gabon from between about 2,150 and 2,080 million years ago. The oldest sediments in this sequence were deposited in well-oxygenated deep waters whereas the youngest were deposited in euxinic waters, which were globally extensive. These fluctuations in oxygenation were likely driven by the comings and goings of the Lomagundi carbon isotope excursion, the longest–lived positive ?13C excursion in Earth history, generating a huge oxygen source to the atmosphere. As the Lomagundi event waned, the oxygen source became a net oxygen sink as Lomagundi organic matter became oxidized, driving oxygen to low levels; this state may have persisted for 200 million years.

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tadchem
1.6 / 5 (7) Oct 01, 2013
The answer will require addressing a single simple question: "Where was the carbon *before* it became oxidized to CO2, HCO3-, and CO3= in water and rocks?" Was it as elemental carbon, methane, or something else?
Torbjorn_Larsson_OM
not rated yet Oct 02, 2013
@tadchem: What answer?

The observation is in itself an answer to the question "what oxygen levels were present after the GOE?"

""Where was the carbon *before*".

There isn't any model of carbon sourcing specifically involved, the isotope excursion depends on istope fractional sorting which happens in many geological and biological processes when carbon is recycled through the geosphere.

The biosphere carbon has mostly been scavenged from the original CO2 atmosphere, there were never any carbon shortage. "Carbonate rocks are rare, indicating that the oceans were more acidic due to dissolved carbon dioxide than during the Proterozoic.[7]" [ http://en.wikiped.../Archean ]

Life evolved chemically as a catalyst of redox reactions, in the electron sink end as a way to rust Fe faster (reduced Fe supplied by heating crust under plate tectonics) and in the source end originally as a way to convert the atmospheric CO2 to CH4 faster (making hydrocarbons).
Torbjorn_Larsson_OM
not rated yet Oct 02, 2013
[cont] Why it evolved that way is due to free forcing of redox reactions, meaning they want to increase entropy flow to the universe vacuum by the fastest reaction channel they can evolve.

And why there were such redox reactions is due to geochemistry. We now know that we are an evolved branch of geochemistry because we clade with alkaline hydrothermal vent redox chemistry (Lane & Martin). Ultimately the driving force was the (now cooling) heated core of Earth, today supplemented by the free energy from solar radiation.