Billion-year-old lake deposit yields clues to Earth鈥檚 ancient biosphere
A sample of ancient oxygen, teased out of a 1.4 billion-year-old evaporative lake deposit in Ontario, 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.聽
The findings, published in the journal Nature, represent the oldest measurement of atmospheric oxygen isotopes by nearly a billion years. The results support previous research suggesting that oxygen levels in the air during this time in Earth history were a tiny fraction of what they are today due to a much less productive biosphere.
鈥淚t has been suggested for many decades now that the composition of the atmosphere has significantly varied through time,鈥� says Peter Crockford, who led the study as a PhD student at 平特五不中. 鈥淲e provide unambiguous evidence that it was indeed much different 1.4 billion years ago.鈥�
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� algae, cyanobacteria, and the like 鈥� produce organic matter from carbon dioxide and pour oxygen into the air.
聽A smaller biosphere
鈥淭his study shows that primary production 1.4 billion years ago was much less than today,鈥� says senior co-author Boswell Wing, who helped supervise Crockford鈥檚 work at 平特五不中. 鈥淭his means that the size of the global biosphere had to be smaller, and likely just didn鈥檛 yield enough food 鈥� organic carbon 鈥� to support a lot of complex macroscopic life,鈥� says Wing, now an associate professor of geological sciences at University of Colorado at Boulder.
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 salts, known as sulfates, 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 a handful in the world using a specialized mass-spectrometry technique capable of probing such materials for rare oxygen isotopes within sulfates.
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 apparent biological or environmental change.
鈥淪ubdued primary productivity during the mid-Proterozoic era 鈥� roughly 2 billion to 800 million years ago 鈥� has long been implied, but no hard data had been generated to lend strong support to this idea,鈥� notes Galen Halverson, a co-author of the study and associate professor of earth and planetary sciences at 平特五不中.聽 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� Crockford鈥檚 data 鈥減rovide the direct evidence that this boring carbon cycle was due to low primary productivity.鈥�
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� says 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�
鈥淭riple oxygen isotope evidence for limited mid-Proterozoic primary production,鈥� Peter W. Crockford et al., Nature, published online July 18, 2018. DOI: 10.1038/s41586-018-0349-y
The research was supported by funding from the Natural Sciences and Engineering Research Council of Canada, the Fonds de recherche du Qu茅bec 鈥� Nature et Technologies, and the University of Colorado Boulder.
