Published: July 18, 2018

Scientists probe the chemistry of Earth's ancient atmosphere by analyzing oxygen atoms trapped in sulfate salts, such as the white layers of gypsum and anhydrite minerals seen here. (Credit: Peter Crockford)

A sample of ancient oxygen, teased out of a 1.4-billion-year-old evaporative lake deposit in Ontario, Canada, provides fresh evidence of what the Earth’s atmosphere and biosphere were like during the interval leading up to the emergence of animal life.  

“It’s mind-boggling to think about, but this really is ‘fossil’ atmospheric oxygen captured in minerals much in the same way that ancient atmospheric gasses get trapped as bubbles in ice cores,” said Boswell Wing, senior co-author of the new research and an associate professor in the Department of Geological Sciences at CU Boulder.

The findings, published in the journal Nature, represent the oldest measurement of atmospheric oxygen isotopes by nearly a billion years. 

“It has been suggested for many decades now that the oxygen content of the atmosphere has significantly varied through time,” said Peter Crockford, who led the study as a Ph.D. student at McGill University. “We provide unambiguous evidence that it was indeed much different 1.4 billion years ago and identify a mechanism—a much less productive biosphere—that may help explain why.”

A smaller biosphere

The study provides the oldest gauge yet of what earth scientists refer to as “primary production,” in which micro-organisms at the base of the food chain—algae, cyanobacteria and the like—produce organic matter from carbon dioxide and pour oxygen into the air. 

“We can see from these measurements that primary production 1.4 billion years ago was a tiny fraction of today’s,” said Wing, formerly of McGill University.  “This tells us that the biosphere, the sum total of all life on earth, had to be smaller as well. There just may not have been enough food—organic carbon—to support a lot of complex macroscopic life.”

To come up with these findings, Crockford teamed up with colleagues from Yale University, University of California Riverside and Lakehead University in Thunder Bay, Ontario, who had collected pristine samples of ancient sulfate salts like gypsum, found in a sedimentary rock formation north of Lake Superior. Crockford shuttled the samples to Louisiana State University where he worked closely with co-authors Huiming Bao, Justin Hayles and Yongbo Peng, whose lab is one of handful in the world using a specialized mass-spectrometry technique capable of probing such materials for rare oxygen isotopes within sulfate salts. 

The work also sheds new light on a stretch of Earth’s history known as the “boring billion” because it yielded little evidence for biological or environmental change.

“Subdued primary productivity during the mid-Proterozoic era—roughly 2 billion to 800 million years ago—has long been implied, but no hard data had been generated to lend strong support to this idea,” noted Galen Halverson, a co-author of the study and associate professor of earth and planetary sciences at McGill.  “That left open the possibility that there was another explanation for why the middle Proterozoic ocean was so uninteresting, in terms of the production and deposit of organic carbon.”

The new data “provide the direct evidence that this boring carbon cycle was due to low primary productivity.”

Oxygen graphicThroughout Earth's history, oxygen levels in the atmosphere have risen in lock-step with the emergence of complex life. (Credit: Peter Crockford)

Exoplanet clues

The findings could also help inform astronomers’ search for life outside our own solar system.

“For most of Earth history our planet was populated with microbes, and projecting into the future they will likely be the stewards of the planet long after we are gone,” said Crockford, now a postdoctoral researcher at Princeton University and Israel’s Weizmann Institute of Science. “Understanding the environments they shape not only informs us of our own past and how we got here, but also provides clues to what we might find if we discover an inhabited exoplanet.”

This story was modified from a version by McGill University.