1-Jan-2017: Large scale evolution of animals during Cambrian explosion(500 Mya) led to spike in atmospheric oxygen.

Photosynthesis in green plants separates carbon dioxide into molecular oxygen(released into atmosphere) and carbon is stored as carbohydrates.

Photosynthesis in plants is already there for 2.5 billion years, but what led to the sudden spike in oxygen during the Cambrian(first period in the Paleozoic era)?

The sudden rise in oxygen was a result of rapid increase in the burial of sediment containing large amounts of carbon-rich organic matter. Sediment storage blocked the oxidation of carbon.

Stored sediment contains organic matter that was formed by photosynthesis, which converted carbon dioxide into biomass and released oxygen into the atmosphere. Burial removes the carbon from Earth's surface, preventing it from bonding molecular oxygen pulled from the atmosphere.

Surges in sediment burial coincided with the formation of vast fields of fossil fuel that are still mined today.

Today, burning billions of tons of stored carbon in fossil fuels is removing large amounts of oxygen from the atmosphere, reversing the pattern that drove the rise in oxygen. And so the oxygen level in the atmosphere falls as the concentration of carbon dioxide rises.

The ultimate geological cause for the accelerated sediment storage that promoted the two surges in oxygen remains murky. There are many ideas to explain the different phases of oxygen concentration. Deep-rooted changes in the movement of tectonic plates or conduction of heat or circulation in the mantle may be in play, but there is no explanation at this point.

The secret to having more oxygen in the atmosphere is to remove a portion of the present biomass and sequester it in sedimentary deposits. That's what happened when fossil fuels were deposited.