How Did Earth Get Oxygen? 2-Billion-Year-Old Rock Salt Reveals the Surprising Truth, Says Geochemist
The Earth wasn't always the cozy, oxygen-rich planet we know and love—once, it was covered in giant oceans filled with anaerobic bacteria, who survived without the need for oxygen. All of that changed with the rise of cyanobacteria, a photosynthetic type of blue-green algae that consumed sunlight and produced oxygen as waste.
Cyanobacteria ended up producing so much oxygen that they changed the composition of the atmosphere, but new evidence found in ancient rock salt has revealed that the process may not have been gradual—according to Clara Blättler of Princeton University, "Instead of a trickle, it was more like a firehose."
The rock salt was discovered in a two-kilometer-deep shaft drilled in northwestern Russia and is estimated to be 2.3 billion years old. The salt contained high amounts of sulfate, which can form when oxygen combines with sulfur in seawater and shed more light on the composition of the ancient oceans.
According to Aivo Lepland, one of the authors on the new study, "This is the strongest ever evidence that the ancient seawater from which those minerals precipitated had high sulfate concentrations reaching at least 30 percent of present-day oceanic sulfate as our estimations indicate. This is much higher than previously thought and will require considerable rethinking of the magnitude of oxygenation of Earth's 2-billion-year-old atmosphere-ocean system."
Scientists have called the dramatic transformation of Earth into an oxygen-rich planet "The Great Oxygenation Event," but there are still disagreements over when it began and how quickly it happened.
The discovery of this rock salt may transform our knowledge of that period and give a better picture of how Earth became hospitable to oxygen-based lifeforms.
As Phil Plait notes, the oxygenation of Earth paradoxically led to the first mass extinction event Earth had ever seen, almost killing the cyanobacteria that kickstarted it.
It's a fascinating time in Earth's history, and one that could help us understand how life develops on other planets.