EARTH'S OXYGEN

The amount of oxygen in the Earth’s atmosphere makes it a habitable planet. Twenty-one percent of the atmosphere consists of this life-giving element. But in the deep past as far back as the Neoarchean era 2.8 to 2.5 billion years ago -This oxygen was almost absent.

2. The Archean Earth

• The Archean eon represents one-third of our planet’s history, from 2.5 billion years ago to four billion years ago. This alien Earth was a water world, covered in green oceans shrouded in a methane haze and completely lacking multi-cellular life. Another alien aspect of this world was the nature of its tectonic activity.
• On modern Earth, the dominant tectonic activity is called plate tectonics, where oceanic crust -the outermost layer of the Earth under the oceans -sinks into the Earth’s mantle (the area between the Earth’s crust and its core) at points of convergence called subduction zones.
• However, there is considerable debate over whether plate tectonics operated back in the Archean era.
• One feature of modern subduction zones is their association with oxidized magmasThese magmas are formed when oxidized sediments and bottom waters- cold, dense water near the ocean floor -are introduced in the earth's mantle. This produces magma with high oxygen and water content.

3. Creating Oxygen from Water

• It is found that magma sulfur content, which was initially around Zero, increased to 2000 parts per million around 2705 million years. This indicated the magma had become more sulfur-rich.
• These new findings indicate that Oxidized magmas did form in the Neoarchean era 2.7 billion years ago. The data show that the lack of dissolved oxygen in the Archean ocean reservoirs did not prevent the formation of sulfur-rich, oxidized magmas in the subduction zones.
• The oxygen in these magma must have come from another source and was ultimately released into the atmosphere during volcanic eruptions.
• The implications of these oxidized magmas go beyond the understanding of early earth geodynamics. Previously, it was thought unlikely that Archean magmas could be oxidized, when the ocean water and ocean floor rocks or sediments were not.
• While the exact mechanism is unclear, the occurrence of these magmas suggests that the process of subduction, where ocean water is taken hundreds of kilometers into our planet, generates free oxygen. This then oxidizes the overlying mantle.
• Study shows that Archean subduction could have been a vital, unforeseen factor in the oxygenation of the Earth, the early whiffs of oxygen 2.7 billion years ago and also the Great Oxidation Event, which marked an increase in atmospheric oxygen by two percent 2.45 to 2.32 billion years ago.
• As far as we know, the Earth is the only place in the solar system-past that is present-with plate tectonics and active subduction. This suggests that this study could partly explain the lack of oxygen, and ultimately, life on the other rocky planets in the future as well.

For Prelims