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 Earthâs atmosphere and biosphere were like during the interval leading up to the emergence of animal life. Ìę
âItâs mind-boggling to think about, but this really is âfossilâ atmospheric oxygen captured in minerals much in the same way that ancient atmospheric gasses get trapped as bubbles in ice cores,â 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.Ìę
âIt has been suggested for many decades now that the oxygen content of the atmosphere has significantly varied through time,â said Peter Crockford, who led the study as a Ph.D. student at McGill University. âWe provide unambiguous evidence that it was indeed much different 1.4 billion years ago and identify a mechanismâa much less productive biosphereâthat may help explain why.â
A smaller biosphere
The study provides the oldest gauge yet of what earth scientists refer to as âprimary production,â in which micro-organisms at the base of the food chainâalgae, cyanobacteria and the likeâproduce organic matter from carbon dioxide and pour oxygen into the air.Ìę
âWe can see from these measurements that primary production 1.4 billion years ago was a tiny fraction of todayâs,â said Wing, formerly of McGill University. ÌęâThis 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 foodâorganic carbonâto support a lot of complex macroscopic life.â
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.Ìę
The work also sheds new light on a stretch of Earthâs history known as the âboring billionâ because it yielded little evidence for biological or environmental change.
âSubdued primary productivity during the mid-Proterozoic eraâroughly 2 billion to 800 million years agoâhas long been implied, but no hard data had been generated to lend strong support to this idea,â noted Galen Halverson, a co-author of the study and associate professor of earth and planetary sciences at McGill. ÌęâThat 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.â
The new data âprovide the direct evidence that this boring carbon cycle was due to low primary productivity.â
Throughout Earth's history,Ìęoxygen levels in the atmosphere have risen in lock-step with the emergence of complexÌęlife. (Credit: Peter Crockford)
Exoplanet clues
The findings could also help inform astronomersâ search for life outside our own solar system.
âFor 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,â said Crockford, now a postdoctoral researcher at Princeton University and Israelâs Weizmann Institute of Science. âUnderstanding 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.â
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