gizmodo: “We did this analysis more out of interest than any expectation,” Princeton University geologist Daniel Stolper told Gizmodo. “We didn’t know whether oxygen would be going up, down, or flat. It turns out there is a very clear trend.” by Maddie Stone
'“Every thousand years or so, all of the O2 [in our atmosphere] is turned into water and then back into O2,” Stolper said. “But there’s an ever so slight leak over time, in terms of extra production or consumption.”
'Writing today in Science, the researchers offer a few possible explanations. For one, erosion rates appear to have sped up in recent geologic history, causing more fresh sediment to be exposed and oxidized by the atmosphere, causing more oxygen to be consumed. Long-term climate change could also be responsible. Recent human-induced warming aside, our planet’s average temperature had been declining a bit over the past few million years.
'“As you cool the oceans, the solubility of oxygen goes up,” Stolper explained. “That means you oxidize more organic carbon [in the ocean] and you have less to put back in the atmosphere.”'
"A Pleistocene ice core record of atmospheric O2 concentrations" by D. A. Stolper, M. L. Bender, G. B. Dreyfus, Y. Yan, and J. A. Higgins here