Up until the last 250 years, concentrations of carbon dioxide also remained lower than a million years ago. The reasons for the long-lasting drop in concentrations of these gases are probably the same but rather unexpected.
Scientists from the U.S. have studied the air bubbles in the samples of "blue ice" from East Antarctica age 1.5 million years and found that in the last 0.8 million years, the concentration of oxygen on our planet was falling. The paper has been published in Science Advances.
Oxygen content in the "blue ice" after 0.8 million years decreased by 1.02 percent per million years, but before that time, there is no statistically significant change in its concentration. The same 0.8 million years ago, the Earth's average temperature dropped markedly and glacial periods became more prolonged than before. The authors of the paper point out that the biosphere may not have been responsible for the drop in oxygen levels during these cool 800,000 years.
Indeed, during ice ages, oxygen production inevitably declines as photosynthesis on the planet declines. However, this reduction cannot be too great per unit of time: the less oxygen plants produce, the less is consumed by animals and the plants themselves (the latter mostly at night). The maximum possible drop in oxygen concentration due to biogenic reasons, according to a number of other works, is 0.1 per cent over the same million years, i.e. ten times less.
There is an additional factor that prevents one from believing that the biosphere is the cause of the "oxygen depletion". If the biogenic production of oxygen is decreasing, there must be more carbon dioxide in the atmosphere (plants will use less of it, because photosynthesis will be weaker). However, nothing like this has happened in the last 0.8 million years, the concentration of CO2 has not increased over long stretches of time (before humans started burning coal).
Air bubbles from the blue ice of East Antarctica indicate that atmospheric oxygen uptake in the last million years has been around two percent greater than from photosynthesis and other sources. This is a notable figure, surpassing, for example, the current mass of human CO2 emissions. Consequently, a rather large-scale process must be responsible for the deoxygenation of the Earth.
The question arises as to where this oxygen has gone. The authors of a new paper point out that, in theory, cooling of the oceans during the glaciation epoch may have absorbed some of this gas. The colder the water, the better the gases dissolve in it, and since the last million years are unusually cold by the standards of Earth's history, this hypothesis looks sensible. But it lacks confirmation: there is no clear understanding of the change in ocean temperatures across its entire depth over the last million years.
Two other possible explanations are a dramatic increase in the rate at which rocks containing iron have been exposed. Firstly, glaciations in the last million years have lowered sea levels by as much as 100 metres or more. During the process vast areas previously covered by water have been exposed. There is not enough dissolved oxygen in the water, so ferrous minerals on the seabed could not bind it actively. But after the sea receded, the process began fairly quickly. Second, mountain glaciers during the advance and retreat seriously strip away the rocks from the layers covering them. There are also many materials containing iron that can actively oxidize during the interglacial period, after they are no longer covered by ice.
