New clues into how Earth got its oxygen

The innovative technique analyzes ancient rocks to understand the role of iron.

Up until four and a half billion years, there was no oxygen in Earth’s air, i.e., the planet was barren and inhospitable.

For decades, scientists have understood how and why the first oxygen was pumped into the air. They have long suspected that life itself was responsible for creating the air that we breathe.

Nicolas Dauphas, the Louis Block Professor of Geophysical Sciences at the University of Chicago, said, “This is the most important change that our planet experienced in its lifetime, and we are still not sure exactly how this happened. Any progress you can make toward answering this question is significant.”

Using a pioneering technique, UChicago graduate student Andy Heard, Dauphas, and their colleagues uncovered new information about oceanic iron’s role in the rise of Earth’s atmosphere. Their research revealed more about Earth’s history.

This study also can shed light on the search for habitable planets in other star systems.

Scientists have carefully reproduced a timeline of the ancient Earth by analyzing antiquated rocks; the chemical makeup of such rocks changes as per the conditions they formed under.

Heard said, “The interesting thing about it is that before the permanent Great Oxygenation Event that happened 2.4 billion years ago, you see evidence in the timeline for these tantalizing little bursts of oxygen, where it looks like Earth was trying to set the stage for this atmosphere. But the existing methods weren’t precise enough to tease out the information we needed.”

“It all comes down to a puzzle.”

“As bridge engineers and car owners know, if there’s water around, oxygen and iron will form rust. In the early days, the oceans were full of iron, which could have gobbled up any free oxygen hanging around. Theoretically, rust formation should consume any excess oxygen, leaving none to form an atmosphere.”

Scientists wanted to test a way to explain how oxygen could have accumulated despite this apparent problem: they knew that some of the iron in the oceans was combining with sulfur coming out of volcanoes to form pyrite (better known as fool’s gold).

Using state-of-the-art facilities in Dauphas’ Origins Lab, scientists developed a new technique to measure tiny iron isotope variations to determine which route the iron was taking. Along with world experts at the University of Edinburgh, they flesh out a fuller understanding of how the iron-to-pyrite pathway works.

Heard said, “To make sulfide and run these experiments, you need understanding colleagues because you make labs smell like rotten eggs. Then, the scientists used the technique to analyze 2.6 to 2.3 billion-year-old rocks from Australia and South Africa.”

Their analysis showed that, even in oceans that should have tucked away a lot of oxygen into rust, certain conditions could have fostered the formation of enough pyrite to allow oxygen to escape the water and potentially form an atmosphere.

Dauphas said, “It’s a complicated problem with many moving parts, but we’ve been able to solve one part of it.”

“Progress on a problem is enormous precious to the community. Especially as we’re starting to look for exoplanets, we need to understand every detail about how our Earth became habitable.”

“By learning more about the way that Earth became habitable, they can look for evidence of similar processes on other planets.”

“The way I like to think about it is, Earth before the rise of oxygen is the best laboratory we have for understanding exoplanets.”

Journal Reference:
  1. Andy W. Heard et al. Triple iron isotope constraints on the role of ocean iron sinks in early atmospheric oxygenation, Science (2020). DOI: 10.1126/science.aaz8821

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