Researchers from Princeton University have recently completed analyzing Pleistocene Epoch ice cores from Antarctica and Greenland. From thirty years of data, they have created a record of atmospheric oxygen levels spanning the last 800,000 years.
They built their research timeline by using measured rates of oxygen-to-nitrogen found trapped in small air bubbles trapped in the ice, using a method developed by Michael Bender, Princeton professor of geosciences, emeritus.
Study co-author John Higgins, assistant professor of geosciences at Princeton, says,
This record represents an important benchmark for the study of the history of atmospheric oxygen. Understanding the history of oxygen in Earth’s atmosphere is intimately connected to understanding the evolution of complex life. It’s one of these big, fundamental ongoing questions in Earth science.
The results reveal a 0.7- percent decline of atmospheric oxygen. However, this has been a slow decline. Prior to this study, scientists have suspected that atmospheric oxygen levels have dropped over the course of millions of years but couldn’t show proof.
The oxygen record is telling us there’s also a change in the amount of carbon dioxide (that was created when oxygen was removed) entering the atmosphere and ocean. However, atmospheric carbon dioxide levels aren’t changing because the Earth has had time to respond via silicate-weathering rates.
Silicate weathering is a process when carbon dioxide reacts with exposed rock over time to produce calcium carbonate minerals that trap carbon dioxide in solid form. When temperatures rise due to carbon dioxide in the atmosphere, the silicate-weathering rates are hypothesized to increase and remove carbon dioxide from the atmosphere faster. The carbon dioxide stabilizes but oxygen continues to decline.
The Earth can take care of extra carbon dioxide when it has hundreds of thousands or millions of years to get its act together. In contrast, humankind is releasing carbon dioxide today so quickly the silicate-weathering can’t possibly respond fast enough. The Earth has these long processes that humankind has short-circuited.
The burning of fossil fuels, which consumes oxygen and produces carbon dioxide is blamed. However, the decline of oxygen levels through the years has been attributed to two other causes. First, erosion around the world has exposed more pyrites and organic compounds that react with oxygen and draw it out of the atmosphere. Second, the cooling oceans promote more activity by oxygen-consuming microbes. Of course, there may be other causes.
The researchers acknowledge that additional research is need. Their ultimate goal is to determine what made the Earth habitable in the first place. This could show scientists what to look for in other planets when searching for signs of life.