Glacial erosion likely caused atmospheric oxygen levels to drop over the past 800,000 years
UNIVERSITY OF RICE
HOUSTON – (December 20, 2021) – An unidentified culprit has been removing oxygen from our atmosphere for at least 800,000 years, and an analysis of air bubbles preserved in ice in the South Poles up to 1.5 million years have revealed likely suspects.
“We know atmospheric oxygen levels started to decline slightly towards the end of the Pleistocene, and it seems likely that glaciers might have something to do with that,” Rice University said. Yuzhen Yan, the respective author of geochemical research published year Scientific advance. “The ice became more expansive and intense in the same time frame, and the simple fact is that the wear of the ice increases the weather.”
Weather Refers to the physical and chemical processes that break down rocks and minerals, and metal oxidation is one of the most important. Rusting is an example. Red iron oxide forms rapidly on iron surfaces exposed to atmospheric oxygen, or O2.
“When you expose new crystalline surfaces from the sedimentary reservoir to O2, you get O2, you get oxygen,” said Yan, a postdoctoral research associate in Rice’s Department of Earth, Environmental and Planetary Sciences. weathering consumes oxygen.
Yan said another way that glaciers can boost atmospheric oxygen consumption is by exposing organic carbon that has been buried for millions of years.
During Yan’s Ph.D. laboratory studies by Michael Bender of Princeton University and John Higgins, Yan worked on a 2016 study conducted by Daniel Stolper, now an assistant professor at the University of California, Berkeley, used air bubbles in the ice core to show The percentage of oxygen in the Earth’s atmosphere has decreased by about 0.2% over the past 800,000 years.
In the Science Advances study, Yan, Higgins and colleagues from Oregon State University, the University of Maine, and the University of California, San Diego, analyzed bubbles in older ice cores to show that the drop in O2 started after the length of the Earth’s glacial cycle more than doubled about 1 million years ago.
Today’s Earth Ice Age started about 2.7 million years ago. Dozens of frost cycles followed. Within each block, the ice caps alternately grow, cover up to a third of the planet, and then recede toward the poles. Each cycle lasted about 40,000 years until about 1 million years ago. At roughly the same time that atmospheric oxygen began to decline, glacial cycles began to last about 100,000 years.
“The reason for the drop is that the percentage of O2 produced is lower than the rate of O2 consumed,” said Yan. “That’s what we call the source and the sink. The source is what produces O2, and the sink is what consumes or pulls in O2. In the study, we interpret the decline as a stronger drag on O2, meaning more is being consumed. “
Yan said Earth’s biosphere did not contribute to the decline because it was balanced, drawing as much O2 from the atmosphere as it created. Weathering, on a global scale, is the geological process likely to consume enough excess O2 to account for the decline, and Yan and colleagues considered two scenarios for an increase in weather. .
Global sea levels drop as glaciers advance and rise as they retreat. As the length of glacial cycles doubles, so does the magnitude of sea level fluctuations. As the coastlines develop, land formerly covered with water will be exposed to the oxidizing power of O2 in the atmosphere.
“We did some calculations to see how much oxygen could be consumed and found that it only accounts for about a quarter of the reduction observed,” said Yan.
Because the extent of ice cover is not precisely known for each ice cycle, there is a broader range of uncertainty about the extent of chemical weathering due to ice erosion. But Yan says evidence suggests it can draw in enough oxygen to account for the decline.
“On a global scale, it is very difficult to define,” he said. “But we did do some tests on how much additional weathering would be needed to account for the O2 depletion, and that’s not unreasonable. Theoretically, it could account for the magnitude of what was observed. “
Other co-authors include Edward Brook of Oregon State, Andrei Kurbatov of the University of Maine, and Jeffrey Severedhaus of UC San Diego. The research was supported by the National Science Foundation (1443263, 1443276, 1443306, 0538630, 0944343, 1043681 and 1559691) and the Poh-Hsi Pan Postdoctoral Fellowship from Rice University.
Read the Science Advances paper, “Ice-core evidence of atmospheric oxygen depletion since the mid-Pleistocene transition,” at https://doi.org/10.1126/sciadv.abj9341.
High resolution IMAGES are available for download at:
CAPTION: Yuzhen Yan in Antarctica in December 2015. (Photo provided by Yuzhen Yan)
SOLUTION: Researchers studied Earth’s ancient atmosphere by capturing tiny bubbles of gas preserved in Antarctic ice for up to 1.5 million years. (Photo by Yuzhen Yan)
CAPTION: A science drilling mission to Allan Hills, East Antarctica, in 2015-16 created ice cores with trapped ancient air bubbles, including some that predate the beginning of the ice age. 2.7 million years ago. (Photo by Yuzhen Yan)
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Ice core evidence of atmospheric oxygen depletion since the mid-Pleistocene transition
ARTICLE PUBLICATION DATE
December 15, 2021