For the last 800,000 years, the amount of oxygen in the atmosphere has declined by about 0.7 percent. It's not enough to cause harm to lifeforms, but enough to puzzle scientists.

In a new study, published in Science, researchers from Princeton University looked at air bubbles trapped in ancient ice in Greenland and Antarctica. The team estimated the past atmospheric pressure of oxygen by measuring the ratio of oxygen to nitrogen in these prehistoric bubbles.

Oxygen is clearly key to life on Earth as we know it, but it also plays a role in many chemical processes around the globe. A change of 0.7 is nothing too dramatic, though; it's an equivalent drop of going from zero to about 100 meters (328 feet) above sea level.

But the puzzling question is where did it go? The researchers have two hypotheses that could explain the decline.  

"The first is that global erosion rates may have increased over the past few to tens of millions of years due to, among other things, the growth of glaciers – glaciers grind rock, thereby increasing erosion rates," lead author Daniel Stolper told Live Science.

"Alternatively, when the ocean cools, as it has done over the past 15 million years, before fossil fuel burning, the solubility of oxygen in the ocean increases. That is, the oceans can store more oxygen at colder temperatures for a given concentration of oxygen in the atmosphere," he added.

There are many materials that are also good at removing oxygen from the atmosphere. Rust itself is formed by iron bonding with oxygen. Some materials like pyrite (also known as fools’ gold) and organic carbon are particularly efficient in capturing oxygen, and these substances could be the culprits behind the decline.

There’s also another interesting finding in the research. Apart from the last 150 years, the level of carbon dioxide has been more or less constant. This was unexpected, as carbon dioxide levels tend to increase as oxygen decreases.

A potential solution is the so-called "silicate weathering thermostat", a yet-to-be tested concept suggesting an increased erosion in volcanic rock when the atmosphere becomes richer in carbon dioxide. Those broken down rocks are washed in the sea and carbon dioxide is trapped there.

More research is necessary, though, to work out how oxygen and carbon dioxide in the atmosphere interact with our planet over long periods of time.

[H/T: Live Science]