In the beginning, Earth’s atmosphere had no oxygen. Then photosynthesis made life as we know it possible, produced much of our mineable iron, and caused an ice age.
The Great Oxidation Event
Today’s atmosphere is 21% oxygen, but during the early Archean Eon (3.8 billion to 2.5 billion years ago) the atmosphere was anoxic – that is, until the Great Oxidation Event, the world’s first appearance of oxygen in the atmosphere. This event changed Earth’s environment forever, paving the way for life to flourish.
While there is not yet a complete consensus on what started the Great Oxidation Event, one prominent theory points to the appearance of photosynthetic cyanobacteria. Cyanobacteria, or blue-green algae, are organisms that produce oxygen as a byproduct of photosynthesis. Though single-celled, they can live together as a mat of sediments and cells on the ocean floor. These structures, referred to as stromatolites, are common fossils in Archean rock worldwide. Some of these fossils contain microbial filaments with evidence of phototaxis, meaning they followed the sun and carried out photosynthesis. Although this fossil evidence strongly supports the role of cyanobacteria, their presence has not been definitively proven.
Boom and Bust
If cyanobacteria are responsible for oxygen in the atmosphere during the Archean Eon, their population growth was not without interruption. The presence of oxygen proved deadly for cyanobacteria, meaning they were poisoning themselves over time. Oxygen would react with iron in the water, which the cyanobacteria likely used as a catalyst for nitrogen fixation. Saturation of the environment with oxygen led to the near-extinction of cyanobacteria when all the iron near the cyanobacteria had been “soaked up” by the oxygen they produced. Over time, iron would reenter the water column via weathering and erosion from land as well as from volcanoes and mid-ocean ridges underwater. When iron was present, oxygen production was no longer deadly, allowing the few remaining cyanobacteria to reproduce exponentially again. Thus the “Great Oxidation Event” may be a slight misnomer as Archean oxygenation was a drawn-out, cyclical process involving hundreds of recolonizations and near-extinctions rather than a single event.
Banded Iron Formations
The effects of this cycle of life and death can be observed in banded iron formations, thick geologic deposits that formed starting 2.7 billion years ago during the Late Archean and Early Proterozoic. Banded iron formations consist of gray hematite or magnetite containing a high abundance of Fe2O3 layered with red, iron-poor chert. They formed in shallow marine environments with high dissolved iron that reacted with oxygen and precipitated into iron-rich layers. Between the gray, iron-rich layers lie red iron-poor layers of chert, which reflect cyanobacteria population decline caused by either a lack of iron or an excess of oxygen. These formations are widespread and can be found globally. Besides their usefulness in investigating ancient history, banded iron formations are the world’s primary source of iron today, making up 60% of iron reserves worldwide.
In addition to leaving behind minable iron, the Great Oxidation Event caused atmospheric change which is at least partially responsible for the Huronian glaciation, around 2.29-2.25 billion years ago. This event covered much of Earth with snow and ice and was caused by oxygen from the ocean entering the atmosphere and reacting with methane to produce carbon dioxide. Methane is a more potent greenhouse gas than carbon dioxide, so this reaction decreased the greenhouse effect of the atmosphere, partnering with a faint young Sun (70-80% as bright as it is today) to cause global cooling and a global glaciation event.
The Great Oxidation Event most likely marks the advent of photosynthesis and biological oxygen production in Earth’s history, a landmark for understanding evolution and the environment. While O2 levels in the atmosphere probably didn’t rise all the way to today’s levels by the end of the Great Oxidation Event, the appearance of detectable O2 was a dramatic change and served as a stepping stone to our richly oxygenated atmosphere.
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