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鈥檚 atmosphere and biosphere were like during the interval leading up to the emergence of animal life. 听
鈥淚t鈥檚 mind-boggling to think about, but this really is 鈥榝ossil鈥� atmospheric oxygen captured in minerals much in the same way that ancient atmospheric gasses get trapped as bubbles in ice cores,鈥� 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, represent the oldest measurement of atmospheric oxygen isotopes by nearly a billion years.听
鈥淚t has been suggested for many decades now that the oxygen content of the atmosphere has significantly varied through time,鈥� said Peter Crockford, who led the study as a Ph.D. student at McGill University. 鈥淲e provide unambiguous evidence that it was indeed much different 1.4 billion years ago and identify a mechanism鈥攁 much less productive biosphere鈥攖hat may help explain why.鈥�
A smaller biosphere
The study provides the oldest gauge yet of what earth scientists refer to as 鈥減rimary production,鈥� in which micro-organisms at the base of the food chain鈥攁lgae, cyanobacteria and the like鈥攑roduce organic matter from carbon dioxide and pour oxygen into the air.听
鈥淲e can see from these measurements that primary production 1.4 billion years ago was a tiny fraction of today鈥檚,鈥� said Wing, formerly of McGill University. 听鈥淭his 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鈥攐rganic carbon鈥攖o support a lot of complex macroscopic life.鈥�
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.听
The work also sheds new light on a stretch of Earth鈥檚 history known as the 鈥渂oring billion鈥� because it yielded little evidence for biological or environmental change.
鈥淪ubdued primary productivity during the mid-Proterozoic era鈥攔oughly 2 billion to 800 million years ago鈥攈as long been implied, but no hard data had been generated to lend strong support to this idea,鈥� noted Galen Halverson, a co-author of the study and associate professor of earth and planetary sciences at McGill. 听鈥淭hat 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.鈥�
The new data 鈥減rovide the direct evidence that this boring carbon cycle was due to low primary productivity.鈥�
Throughout Earth's history,听oxygen levels in the atmosphere have risen in lock-step with the emergence of complex听life. (Credit: Peter Crockford)
Exoplanet clues
The findings could also help inform astronomers鈥� search for life outside our own solar system.
鈥淔or 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,鈥� said Crockford, now a postdoctoral researcher at Princeton University and Israel鈥檚 Weizmann Institute of Science. 鈥淯nderstanding 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.鈥�
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