An international team of geoscientists have found that over the past 800,000 years, the Earth’s oxygen level has lost 0.7 per cent. Researchers collected ice core samples from locations in Greenland and Antarctica and measured the ratio of oxygen to nitrogen isotopes within, finding that atmospheric oxygen levels have declined by a measurable but slight amount. According to the study’s authors, the loss is “trivial” with regard to the planet’s ecosystems and human habitation.
“Every thousand years or so, all of the O2 [in our atmosphere] is turned into water and then back into O2,” says Daniel Stolper from Princeton University. “But there’s an ever so slight leak over time, in terms of extra production or consumption.”
Atmospheric oxygen is normally lost through absorption by the weathering of silicate rocks over long periods of time, but the new research shows that over the past 800,000 years, the planet’s oxygen sinks have been about two per cent larger than the oxygen producers, leading to the overall decline.
What’s caused the change? Researchers speculate that increases in global erosion rates may be to blame or possibly the slightly lower planetary temperature during the past million years may have cooled the oceans just enough for them to absorb the extra oxygen.
As for the human impact on oxygen levels, Stopler says that the past 200 years of industrial development has led to another steep decline. “We are consuming O2 at a rate a factor of a thousand times faster than before,” says Stopler, “Humankind has completely short-circuited the cycle by burning tons of carbon.”
The new study is published in the journal Science.
Going back further in the geological record, earlier this year Canadian scientists surprised the scientific community when they revealed that approximately 815 million years ago during the Neoproterozoic era, atmospheric oxygen levels were about half what they are today and five times greater than previous estimates for that time period.
The work again depended upon core samples, this time on salt crystals buried a kilometre below the surface. Researchers from Brock University in St. Catherines, Ontario, determined that during the period between 800 and 830 million years ago, oxygen levels were at about 10.9 per cent (currently, oxygen composes 20.9 per cent of the atmosphere’s gases).
The finding is important in part because it answers the question about whether animals evolved before or after oxygen levels rose to significant levels. About 3.5 billion years ago, the emergence of cyanobacteria caused atmospheric oxygen to rise, but the scientific consensus was still unclear on how concentrated oxygen levels were at the time when multicellular animals first arrived, some 580 million years ago.
“And now with our research and our result, we know there was sufficient oxygen before they arose,” says Nigel Blamey, earth sciences professor at Brock University. The new results also hint at the possibility that researchers may someday find multicellular fossils much older than those currently known. “Now paleobiologists will have reason to go looking for rocks with original traces of these first evolutionary steps,” says study co-author and Brock geochemist, Uwe Brand.
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