An unknown culprit has been removing oxygen from our atmosphere for at least 800,000 years. Analysis of air bubbles preserved in Antarctic ice for up to 1.5 million years reveals the likely suspect.
“We know that atmospheric oxygen levels started to drop slightly at the end of the Pleistocene, and it looks like glaciers could have something to do with it,” says Yuzhen Yan, associate postdoctoral fellow in the Department of Earth Sciences, of the Environment and the Planet at Rice University and corresponding author of the article in Scientists progress.
“The glaciation has become more extensive and more intense at about the same time, and the mere fact that there is glacial crushing increases the weathering.”
Weathering refers to the physical and chemical processes that break down rocks and minerals, and the oxidation of metals is among the most important. Rusting iron is one example. Reddish iron oxide forms rapidly on iron surfaces exposed to atmospheric oxygen, or O2.
“When you expose cool crystalline surfaces of the sedimentary reservoir to O2, you get an alteration that consumes oxygen, ”Yan says.
Exposing organic carbon buried for millions of years is another way for glaciers to increase the uptake of atmospheric oxygen, says Yan.
Antarctic ice bubbles
During Yan’s doctoral studies in the laboratories of Michael Bender and John Higgins at Princeton University, he worked on a 2016 study led by Daniel Stolper, now an assistant professor at the University of California at Berkeley, who used air bubbles in ice cores to show the proportion of oxygen in Earth’s atmosphere had declined by about 0.2% over the past 800,000 years.
In the present study, Yan, Higgins and their colleagues analyzed bubbles in older ice cores to show that the decline in O2 began after the duration of the Earth’s ice cycles more than doubled about. 1 million years.
The Ice Age that the Earth is in today began around 2.7 million years ago. Dozens of glacial cycles followed. In each, the ice caps developed alternately, covering up to a third of the planet, and then retreated towards the poles. Each cycle lasted about 40,000 years until about 1 million years ago. Around the same time that atmospheric oxygen began to decline, glacial cycles began to last for about 100,000 years.
“The reason for the drop is the rate of O2 product is lower than the rate of O2 consumed, ”explains Yan. “This is what we call the source and the sink. The source is what produces O2, and the sink is what consumes or drives O2. In the study, we interpret the decline as a bigger drag on O2, which means that more is consumed.
The terrestrial biosphere did not contribute to the decline because it is balanced, drawing as much O2 from the atmosphere as it produces, says Yan. Weathering, on a global scale, is the most likely geological process capable of consuming enough excess O2 to explain the decline, and Yan and his colleagues considered two scenarios of increased weathering.
Global sea level drops as glaciers advance and rises as they retreat. As the duration of glacial cycles more than doubled, so did the magnitude of sea level fluctuations. As the coasts advanced, land previously covered with water would have been exposed to the oxidizing power of atmospheric O.2.
“We did some math to see how much oxygen it might consume and found that it could only explain about a quarter of the observed decrease,” Yan said.
Because the extent of ice cover is not precisely known for each ice cycle, there is a wider range of uncertainties about the extent of chemical weathering due to glacial erosion. But Yan says the evidence suggests he might be drawing enough oxygen to explain the decline.
“Globally, it’s very difficult to pin down,” he says. “But we did some testing on the amount of additional weathering that would be needed to explain the drop in O2, and that’s not unreasonable. Theoretically, that could explain the magnitude of what was observed.
Additional co-authors are from Oregon State University, University of Maine, and University of California San Diego. The National Science Foundation and a Poh-Hsi Pan Postdoctoral Fellowship from Rice University funded the work.
Source: Rice University