Princeton University researchers have compiled 30 years of data to construct the first ice core-based record of atmospheric oxygen concentrations spanning the past 800,000 years, according to a paper in the journal Science.
The record shows that atmospheric oxygen has declined 0.7 percent 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 percent because of the burning of fossil fuels, which consumes oxygen and produces carbon dioxide.
Curiously, the decline in atmospheric oxygen over the past 800,000 years 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. To explain this apparent paradox, the researchers called upon a theory for how the global carbon cycle, atmospheric carbon dioxide and Earth's temperature are linked on geologic timescales.
"The planet has various processes that can keep carbon dioxide levels in check," said first author Daniel Stolper, a postdoctoral research associate in Princeton's Department of Geosciences. The researchers discuss 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 hypothesized to increase and remove carbon dioxide from the atmosphere faster.
Stolper and his co-authors suggest that the extra carbon dioxide emitted due to declining oxygen concentrations in the atmosphere stimulated silicate weathering, which stabilized 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 co-author John Higgins, Princeton assistant professor of geosciences. "However, atmospheric carbon dioxide levels aren't changing because the Earth has had time to respond via increased silicate-weathering rates.