Atmospheric oxygen declined over last 800,000 years: study

Washington: Oxygen has been gradually disappearing from the Earth's atmosphere over the last 800,000 years, with the decline speeding up during the last century due to the burning of fossil fuels, a new study has found.

Researchers at the Princeton University in the US compiled 30 years of data to construct the first ice core-based record of atmospheric oxygen concentrations spanning the past 800,000 years. The record shows that atmospheric oxygen has declined 0.7 per cent relative to current atmospheric-oxygen concentrations, a reasonable pace by geological standards, the researchers said.

During the past 100 years, however, atmospheric oxygen has declined by a comparatively speedy 0.1 per cent because of the burning of fossil fuels, which consumes oxygen and produces carbon dioxide. The researchers built their history of atmospheric oxygen using measured ratios of oxygen-to-nitrogen found in air trapped in Antarctic ice.

Since oxygen is critical to many forms of life and geochemical processes, numerous models and indirect proxies for the oxygen content in the atmosphere have been developed over the years, but there was no consensus on whether oxygen concentrations were rising, falling or flat during the past million years (and before fossil fuel burning). The decline in atmospheric oxygen was not accompanied by any significant increase in the average amount of carbon dioxide in the atmosphere, though carbon dioxide concentrations do vary over individual ice age cycles.

"The planet has various processes that can keep carbon dioxide levels in check," said Daniel Stolper, a postdoctoral research associate in Princeton. The researchers described a process known as "silicate weathering" in particular, wherein carbon dioxide reacts with exposed rock to produce, eventually, calcium carbonate minerals, which trap carbon dioxide in a solid form.

As temperatures rise due to higher carbon dioxide in the atmosphere, silicate-weathering rates are hypothesised to increase and remove carbon dioxide from the atmosphere faster. Researchers suggest that the extra carbon dioxide emitted due to declining oxygen concentrations in the atmosphere stimulated silicate weathering, which stabilised carbon dioxide but allowed oxygen to continue to decline.

"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," said John Higgins, assistant professor at Princeton. "However, atmospheric carbon dioxide levels aren't changing because the Earth has had time to respond via nincreased silicate-weathering rates," Higgins said. The research was published in the journal Science.