The Day The Anaerobes Died

A little over 2 million years ago the atmosphere filled with oxygen, and quite rapidly by geological times. The world went through the Great Oxygenation Event (GOE), or if you’re feeling saucy the Oxygen Catastrophe. Cyanobacteria had been chugging away 200 million years prior, going through photosynthesis and releasing oxygen. Before the GOE their released oxygen was trapped by organic matter or dissolved iron (FeS2 and other easily oxygenated molecules). The majority of life, living around these few cyanobacteria, were anaerobic—they didn’t need oxygen. In fact, oxygen was poison to them. The anaerobic lifeforms depended on these oxygen traps, so I’m sure they weren’t pleased to find out that the cyanobacteria had filled up the traps. The cyanobacteria kept producing oxygen and it went straight into the atmosphere. Anaerobic life died. It was one of the biggest extinctions our planet has known. A catastrophe. A second oxygen jump, up to the levels we know today, happened a couple million years later when the deep ocean was oxygenated. This seems completely reasonable, so much so that it’s accepted almost universally as a scientific truth.

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Oxygen in the atmosphere increases over time. The red line show the traditional two-step oxygenation event, where everything happened in big jumps. The blue line shows the new proposed model, where things are a little more gradual. Arrows are poorly understood “whiffs” of oxygen arising from anomalous metal enrichments in Australian shale.

But a new model, published in Nature Reviews, suggests the GOE isn’t as step-wise as previously thought. Proponents of the model say that the oxygen level spiked, then decreased. That the one billion or so years of atmospheric consistency had relatively low levels of oxygen. Large pockets of hydrogen sulfide in the oceans harbored life during this time, and it wasn’t until the final oxygen jump that most of life on Earth died. To them, the GOE is less of an event and more of a process.

The only way to test these kinds of theories is to look at rocks. For example, by looking at the sulfur isotope fractions, which are greatly affected by the amount of oxygen present. But this, they claim, may be tainted by rivers flushing their sulfur to the ocean, creating a sort of false positive. If this did, in fact, occur then oxygen levels may have risen more gradually than previously assumed—tens of millions of years more gradually. To support the challenge to current theory, they tied in the Archaean-Proterozoic boundary (defined in time and geology), where tectonic plates reorganized into the first volcanoes, and the “Snowball Earth,” the first of the global cooling cycles (or glaciations), saying these events contributed to both the sulfur isotope fractions (further obscuring the data) and accumulation of atmospheric oxygen.

But even before the GOE (or, as they suggest, the GOT for transition) animal life was rare—estimated at less than 1% of today’s diversity. Some research suggests that, instead of the rise in animal life being a coincidence, it was the emergence of animals that triggered the GOE. Others suggest that without the GOE life as we know it would not have abounded. Either way, this is one catastrophe we should feel good about.

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